The new worldwide collapse caldera database (CCDB): A tool for studying and understanding caldera processes

Journal of Volcanology and Geothermal Research 175 (2008) 334–354

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Journal of Volcanology and Geothermal Research j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j vo l g e o r e s

The new worldwide collapse caldera database (CCDB): A tool for studying and understanding caldera processes A. Geyer ⁎, J. Martí Department of Geophysics and Geohazards. Institute of Earth Sciences “Jaume Almera”; CSIC, c/Lluis Solé Sabaris s/n, 08028 Barcelona, Spain

A R T I C L E

I N F O

Article history: Received 29 August 2007 Accepted 27 March 2008 Available online 10 April 2008 Keywords: caldera collapse database

A B S T R A C T Collapse calderas are one of the most important volcanic structures not only because of their hazard implications, but also because of their high geothermal energy potential and their association with mineral deposits of economic interest. The objective of this work is to describe a new general worldwide Collapse Caldera DataBase (CCDB), in order to provide a useful and accessible tool for studying and understanding caldera collapse processes. The principal aim of the CCDB is to update the current field based knowledge on calderas, merging together the existing databases and complementing them with new examples found in the bibliography, and leaving it open for the incorporation of new data from future studies. This database does not include all the calderas of the world, but it tries to be representative enough to promote further studies and analyses. We have performed a comprehensive compilation of published field studies of collapse calderas including more than 200 references, and their information has been summarized in a database linked to a Geographical Information System (GIS) application. Thus, it is possible to visualize the selected calderas on a world map and to filter them according to different features recorded in the database (e.g. age, structure). The information recorded in the CCDB can be grouped in seven main information classes: caldera features, properties of the caldera-forming deposits, magmatic system, geodynamic setting, pre-caldera volcanism, caldera-forming eruption sequence and post-caldera activity. Additionally, we have added two extra classes. The first records the references consulted for each caldera. The second allows users to introduce comments on the caldera sample such as possible controversies concerning the caldera origin. A further purpose of this work is to construct the CCDB web page. In this web page where registered users can acquire the current database version, as well as to propose corrections or updates and to exchange information with other registered members also involved in the study of caldera collapse processes. Additionally, the CCDB includes a formulary that will facilitate the incorporation of new calderas into the database. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Collapse calderas are defined as the volcanic depressions that result from the disruption of the magma chamber roof due to down faulting during the course of an eruption. The diameter of these volcanic depressions, usually more or less circular or elliptical in form, is many times greater than the diameter of the eruptive vents (Lipman, 1997, 2000). Despite their low frequency of occurrence, large pyroclastic eruptions and associated caldera collapse structures represent one of the most catastrophic geologic events that have occurred on the Earth's surface during Phanerozoic times, causing considerable impacts on the environment (e.g. climate) and on human society (e.g. Tambora, 1815 (Self et al., 1984; Newhall and Dzurisin, 1988), Krakatau, 1883 (Self and Rampino, 1981; Simkin and Fiske, 1983; Newhall and Dzurisin, 1988) and Pinatubo, 1991 (Hattori, 1993;

⁎ Corresponding author. Fax: +34 93 401 11 12. E-mail address: [email protected] (A. Geyer). 0377-0273/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2008.03.017

Lipman, 2000; Dartevelle et al., 2002)). Additionally, collapse calderas have received considerable attention due to their link to ore deposits and geothermal energy resources (Lipman, 2000). Calderas have been analysed through field studies, analogue models and numerical simulations (e.g. Druitt and Sparks, 1984; Komuro et al., 1984; Martí et al., 1994, 2000, in press-a; Bower and Woods, 1997; Gudmundsson et al., 1997; Gudmundsson, 1998; Burov and Guillou-Frottier, 1999; Acocella et al., 2000, 2001, 2004; GuillouFrottier et al., 2000; Roche et al., 2000; Roche and Druitt, 2001; Folch and Martí, 2004; Gray and Monaghan, 2004; Lavallée et al., 2004; Geyer et al., 2006). However, some important aspects on caldera dynamics and structure still remain uncertain and controversial. Traditionally, field studies have constituted the most important way to investigate and understand volcanic processes. Structural, stratigraphic, sedimentological, petrological and geochemical investigations have been necessary to understand processes from magma generation to volcanic eruption. In the case of caldera-forming eruptions, field studies have provided also valuable information concerning the calderaforming deposits and the related collapse structures. The reconstruction

A. Geyer, J. Martí / Journal of Volcanology and Geothermal Research 175 (2008) 334–354

of past collapse calderas and their comparison with historical examples (e.g. Fernandina, Galapagos, Chadwick and Howard, 1991; Katmai, Alaska, Hildreth and Fierstein, 2000; Hildreth et al., 2003; Miyakejima, Japan, Geshi et al., 2002; Nakada et al., 2005) is a powerful tool to understand caldera mechanisms. There exists a large number of scientific papers compiling field data from hundreds of collapse calderas distributed worldwide (e.g. Steven and Lipman, 1976; Aramaki, 1984; Rytuba and McKee, 1984; Eyal and Peltz, 1994; Geshi et al., 2002). Although dealing with such a large amount of information from original sources is usually unfeasible, easier-to-use databases compiling the existing information on field studies of collapse calderas are scarce and incomplete. The International Association of Volcanology and Chemistry of the Earth's Interior (IAVCEI) offers at its website (http://www.iavcei.org) a summary of the largest calderas of the world recording the most important features like the caldera age, diameter, magma composition and the volume of extruded deposits. Also the Smithsonian National Museum of Natural History (http://www.volcano.si.edu/) offers a database of those volcanoes active during the last 10,000 years including caldera-forming events. The information recorded in this database is similar to that offered by the IAVCEI but it also includes a detailed description of the caldera geographical location (e.g. world region, subregion, latitude, longitude, elevation), the volcano number according to the Catalogue of Active Volcanoes of the World (CAVW), a brief description of the geological history and a photograph. Newhall and Dzurisin (1988) compiled worldwide historical unrest at large calderas. In their database, the authors incorporate jointly with the abovementioned information a short description of the associated tectonic setting, the type of pre-caldera edifice and the historical unrest. Other smaller compilations like those of Spera and Crisp (1981) and Walker (1984) provide additional information regarding caldera areas and volumes of extruded deposits, and the distribution of postcaldera vents, respectively. Here we present a new database of collapse calderas: “The Collapse Caldera DataBase (CCDB)”. The rationale behind constructing this new database is to create a comprehensive catalogue including all known or identified collapse calderas produced by either explosive eruptions or effusive basaltic activity. The availability of accurate and comprehensive statistics is vital to understand these volcanic structures. Thus, this extensive data compilation should be an accessible and useful tool for studying and understanding caldera collapse processes. The final aim of the CCDB is to update the current field based knowledge on calderas, by merging together the abovementioned databases and complementing them with the existing peer-reviewed articles on calderas. This database does not include all the calderas of the world but tries to be representative and will remain open for further updates. In this paper, we present an example of the CCDB applicability. For instance, we show how the information included in the CCDB allows us to find out if there exists any correlation between the different characteristics of collapse calderas (e.g. morphology, age, dimensions, etc), in order to establish similarities and differences among the different calderas recorded in the database. Table 1 is a simplified version of the CCDB. For compactness, this version does not show all caldera samples included in the database. The full version of the CCDB will be published online at the website of the CSIC Group of Volcanology of Barcelona (http://www.GVB-csic.es/ CCDB.htm). In this website registered users will be able to acquire the current database version, to propose possible corrections or updates, and to exchange information with other registered members. 2. CCDB architecture Data on collapse calderas were compiled from a wide range of primary and secondary sources of information, which are referenced for each database entry. These information sources include the IAVCEI and the Smithsonian Museum of Natural History databases (http://

335

www.iavcei.org/ and http://www.volcano.si.edu/, respectively) and the works of Spera and Crisp (1981), Walker (1984) and Newhall and Dzurisin (1988). Additionally, we have performed a comprehensive compilation of more than 200 published field studies on calderas. We have tried to review the most representative publications, and wherever possible more than one source is used for each caldera sample. The collapse caldera database (CCDB) is designed with Microsoft Access®. This first version of the CCDB includes around 280 calderas worldwide. However, we have designed also a formulary that will facilitate the incorporation of new calderas into the CCDB (Fig. 1), thus allowing for continuous updating. Furthermore, the CCDB may be linked to a GIS application that permits users to represent the included calderas on a world map, in order to study the spatial distribution of specific features. The CCDB architecture is based on the principle that all the information concerning the different calderas included in the database has to be comparable and consistent enough for future comparisons and data analyses. In order to have a good and accurate characterization of a collapse caldera, we consider necessary to have information on the collapse depression (e.g. dimensions, morphology, age), the caldera-forming deposits (e.g. volume and thickness of the deposits), the associated magmatic system (e.g. magma composition), the geodynamic setting where the caldera is located (e.g. crustal type, plate tectonic setting), the type of pre-caldera volcanism, the calderaforming eruption sequence (deduced from the sequence of deposits) and the post-caldera evolution (e.g. post-caldera volcanism, resurgence, caldera erosion). Thus, the information included in the CCDB is classifiable into the latter information classes. Additionally, we have added two extra classes. The first records the references consulted for each caldera sample. The second allows comments about each particular caldera sample such as possible controversies concerning the caldera origin. The information included in each of these main classes is recorded in individual fields that constitute the elemental units of the database, corresponding to the individual columns of Table 1. Table 2 reproduces the CCDB architecture including the information classes, the individual fields and a short description of all of them. Throughout the next sections we offer a brief description of the type of data included in each of the information classes. 2.1. Caldera features This class includes the fields: caldera identification number, corresponding to the Smithsonian Institution volcano number; caldera name; latitude; longitude; world region and subregion, caldera age; maximum and minimum diameter of the caldera; caldera area; total caldera subsidence; collapse volume; subsidence type/ geometry; and a picture of the caldera. The Smithsonian Institution volcano numbers are assigned according to the volcano numbering system developed in the Catalogue of Active Volcanoes of the World, which is geographic and hierarchical. In short, the first two numerals identify region, the next two identify subregion, and the last two or three (after the hyphen) identify individual volcanoes in that subregion. Wherever possible, original CAVW volcano numbers have been retained, but for the many volcanoes it has been necessary to introduce some modifications. More details about the collapse caldera numbering system can be found at the Smithsonian Museum of Natural History and the CCDB web pages (http://www.volcano.si.edu/ and http://www.GVB-csic.es/ CCDB.htm, respectively). For assignment of the different world region and subregions where calderas are located, we have followed the definitions proposed by Simkin et al. (1981), latter modified by Newhall and Dzurisin (1988). The world is divided in 19 main regions that are subdivided, in turn, in several subregions (see Fig. 1 in Newhall and Dzurisin, 1988) (Appendix A: (A) and (B)).

336

IDCaldera CALDERA number

0201-19 0601-09

Fantale Toba

LAT

8.984 2.58

LONG

39.907 98.83

WR WSR AGE

Max. Min. AREA D. D. (km2) (km) (km)

2 6

201 601 74 ka

4 100

3 30

10 2000 ± 100

6

601 280 ka

22

11

215 ±20

0601-151 Maninjau

−0.33

1201-A

Kulshan

48.83

− 121.7

12

1201 1.15 Ma

8

4.5

1202-16 1203-14 1210-13

CraterLake LongValley Valles Caldera

42.93 37.7 35.87

− 122.12 − 118.87 − 106.57

12 12 12

1202 7.7 ka 1203 700 ka 1210 11200 ka

10 32 16

8 17 16

64 350 ±5 434 ±2

1210-A

Bursum

33.5

− 108.67

12

1210 29–28 Ma

40

30

1500 ± 45

1402-06 1505-A 1505-B

Atitlan Soledad LaPacana

14.58 − 17.67 −23.17

− 91.185 14 −68 15 − 67.417 15

1402 84 ka 1505 54000 1505 4 Ma

20 22 60

17 14 35

125 240

1507-021 Diamante 1507-09 Copahue

−34.2 −37.85

−69.8 − 71.17

15 15

20 10

15 10

1507-111 Sollipulli 1507-A Cerro Galán

−38.97 −26

− 71.52 − 67

15 15

1507 450 ka 1507 Pleistocene. b 1.64 Ma 1507 N 2.8 ka 1507 2.2 Ma

4 35

4 20

17

1703 5 ka

9

9

170303BI 1703-11 1802-08 1802-09 1802-10

100.18

SUB (km)

VOL SUB DEP NAME (km3) GEO

? ? 100

? PI

1.2? 2–3

3

Fantale Tuff Young Toba Tuff Maninjau Tuff Swift Creek Ignimbrite

50–60 PaPC N 500 PI Bishop Tuff 150 T_PC Upper Bandelier Tuff 2000 PI Apache Spring Tuff Bloogood Tuff ? Los Chokoyos ? Soledad Tuff 2541 T AtanaIgnimbrite Toconao Ignimbrite ? ?

N 0.55 6

T VOL DEP DEP (km) (km3)

N1

VOL MG (DRE, km3)

MG ROCK COMP SUITE

MC_D R (km)

PTS

CT TF

PCD PCV

TCO PCVA PCR CPR

N2 N1.5 1500–2000 2800

P_T R

CALCOf CALCOf

CR SC

Cd EXT Cd Stens

VE ?

? B

? ?

100–250

R

CALCOf

SC

Cd Stens

STR

?

?

WP

R_D

CALCOf

SC

Cd ?

?

B

?

CD

50

N 30

Τ

WP WP

200–300

42 RD 750 R 100–200/300 R

CALCOf 5–8 CALCOf 4–7 CALCOf 4.5

0.5–1 SC 0.13–0.41 CR 0.28 CR

Cd ? Cd EXT Cd EXT

STR ? VC

B B B

? S R

Τ Τ

WP PD WP

1400

1050

R_D

CALCOf 8

0.2–0.27

CR

Cd EXT

?

?

?

Τ

CD

150

R D R_D

CALCOf CALCOf CALCOf 6

0.1–0.17

SC SC SC

T C C

Stens EXT Stens

STR VC ?

? B A

Ms ? S

Τ

PD CD PD

R S

CALCOf CALCOf

SC SC

C C

EXT IF

? STR

? ?

? C

? D

? CALCOf 3.5–5?

SC SC

C C

EXT ?

? VC

A ?

? RS

P-D

RaD

CALCOf

HOC O

EXT

LLSTR ?

?

WP

P? T

Cerro Galán Ignimbrite

1.5 1.1 1

N2

2700

N1.4 N 1000

1600

PD PD

Τ

PD PD

Askja

65.05

− 16.8

Krafla Sete Cidades Agua de Pau Furnas

65.73 37.87 37.77

− 16.783 17 −25.78 18 −25.47 18

1703 b 70 ka 1802 22 ka 1802 15.2 ka

10.8 10.8 5 5 7 4

? ? ?

? D D

? CALCOf CALCOf

HOC O EXT HOC Od EXT HOC Od EXT

LLSTR ? STR ? STRC ?

? R ?

WP PD

37.77

−25.32

1802 12 ka

6

?

D

CALCOf

HOC Od EXT

LPSTR ?

?

PD

18

6

A. Geyer, J. Martí / Journal of Volcanology and Geothermal Research 175 (2008) 334–354

Table 1 Simplified version of the CCDB. The meaning of the codes for the information included in the different columns is explained in Appendix A. AGE: Caldera age; AREA: Caldera area; CALDERA: Caldera name; CPR: Caldera preservation; CT: Crustal type; IDCaldera number: Caldera identification number according to the CAVW; DEP NAME: Common name of the caldera-forming deposits; LAT: Latitude; LONG: Longitude; Max. D.: Maximum caldera diameter; Min. D.: Minimum caldera diameter; MC_D: Magma chamber depth; MG COMP: Composition of the extruded magma; PCV: Type of pre-caldera volcanism; PCVA: Post-caldera volcanic activity (Fig. 6); PCD: Periods of pre-caldera doming; PCR: Post-caldera resurgence; PTS: Plate tectonic setting (Fig. 3); R: Ratio between the roof thickness (magma chamber depth) and the magma chamber width; ROCK SUITE: Rock suite classification of the magma composition; T DEP: Thickness of the caldera-forming deposits; TCO: Timing of caldera onset (Fig. 5); TF: Type of tectonic faulting (Fig. 4); SUB: Calera subsidence; SUB GEO: Subsidence type/geometry according to Lipman (1997) (Fig. 2); VOL: Caldera volume; VOL DEP: Volume of the calderaforming deposits; VOL MG: Total volume of extruded magma (Dense Rock Equivalent, DRE); WR: World region according to Simkin et al. (1981) and modified by Newhall and Dzurisin (1988); WSR: World Subregion according to Simkin et al. (1981) and modified by Newhall and Dzurisin (1988)

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337

Fig. 1. Screen-shot of the formulary included in the database.

Caldera subsidence type/geometry has been assigned according to definitions of Lipman (1997). He presented a classification of calderas relating subsidence geometry and resulting structures to a few geometrically simplified end-member processes (Fig. 2): plate/piston subsidence, trap-door subsidence, piecemeal disruption, chaotic subsidence and funnel calderas. In the CCDB we consider the five subsidence type/geometry end-members as well as possible combinations of them (Appendix A: (C)). The CCDB can include a photograph of the recorded calderas. Current caldera images included in the CCDB have been obtained with the freeware software Google Earth ® (http://earth.google.com). 2.2. Characteristics of the deposits The information class “Characteristics of the deposits” records the name, thickness and volume of the caldera-forming deposits. In fact, the thickness and volume of the caldera-forming deposits located

inside (intra-caldera deposits) and outside (extra-caldera deposits) the caldera depression may give an estimate of the erupted volume and consequently, an approximation of the eruption size (e.g. Lindsay et al., 2001). 2.3. Magmatic system In this main class, we compile all the available information concerning the caldera shallow magmatic system. The information class includes data about the volume of erupted magma (Dense Rock Equivalent, DRE), as well as its composition (Appendix A: (D)). We have assigned each of the former magma compositions to a more general rock suite in order to homogenize existing data (Appendix A: (E)). Moreover, the information class “Magmatic system” records the magma chamber depth and the magma chamber roof aspect ratio R, defined as the ratio between the magma chamber roof thickness (magma chamber depth) and the magma chamber width (Roche et al., 2000).

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Table 2 CCDB architecture including the information classes, the individual fields and a short description of them. Thick.: Thickness; Vol.: Volume Class

Field

Field description

Caldera features

Caldera identification number Caldera name Latitude Longitude World region

ID number according to the CAVW, Simkin et al. (1981) and the Smithsonian Institution. When necessary, new ID numbers are defined Caldera name Latitude (in geographic coordinates) Longitude (in geographic coordinates) World region and subregion where the caldera sample is located, following the definition proposed by Simkin et al. (1981) and modified by Newhall and Dzurisin (1988).When necessary, new subregions are defined. (Appendix A: (A) and (B))

Characteristics of the deposits

Magmatic system

Subregion Caldera age Max. diameter (km) Min. diameter (km) Area (km2) Subsidence (km) Volume (km3) Subsidence type/ geometry Caldera photograph Deposit name Thickness of the deposits (km) Volume of the deposits (km3) Erupted magma volume (DRE,km3) Magma composition Rock suite Magma chamber depth (km) Roof aspect ratio R Plate tectonic setting

Caldera age Maximum caldera diameter (in km) Minimum caldera diameter (in km) Collapse area given (in km2) Collapse subsidence (in km) Collapse volume (in km3) Subsidence type/geometry according to Lipman's (1997) classification: Plate/piston, trap-door, piecemeal-chaotic, downsag and funnel (Fig. 2). (Appendix A: (C)) Images of the caldera obtained with the freeware software Google Earth (http://earth.google.es) Common name of the caldera–forming deposits Thickness of the caldera–forming deposits (in km) Volume of the caldera–forming deposits (in km3) Volume of magma (DRE) erupted during the caldera–forming eruption (in km3) Composition of the magma erupted during the caldera–forming eruption.(Appendix A: (D)) Rock suite classification of the magma erupted during the caldera–forming eruption.(Appendix A: (E)) Depth of the magma chamber during the caldera–forming event

Ratio between the magma chamber roof thickness (magma chamber depth) and the magma chamber width Plate tectonic setting: Chilean-type or Mariana-type subduction, island arc collision, back-arc rifting, ocean ridge, continental rifting, hotspot near or over an oceanic ridge and hotspot (Fig. 3). (Appendix A: (F)) Crustal type Type of crust hosting the caldera sample: Thick or standard–thin continental, transitional or oceanic crust. (Appendix A: (G)) Type of tectonic faulting Local and regional structures that may have influenced pre–caldera volcanism, caldera formation and/or the distribution of the post–caldera volcanism: Extensional, compressional, shear, transtensional, transpressional or intersection of faults (Fig. 4) (Appendix A: (H)) Regional pre-caldera Regional (large–scale) tectonic or magmatic tumescence or doming periods prior to the caldera–forming eruption doming Pre-caldera volcanism Pre-caldera volcanism Description of pre–caldera volcanism: Cones, basaltic volcanoes and simple volcanic edifices, stratovolcanoes and stratocones, shield volcanoes, lava flows and domes, no previous edifices, calderas or caldera clusters and other structures. (Appendix A: (I)) Caldera-forming eruption Timing of caldera Moment during the eruption at which caldera collapse starts: At the beginning or nearly at the beginng of the calderasequence collapse onset forming event, later on or at the end of the caldera–forming eruption (Fig. 5). (Appendix A: (J)) Post-caldera evolution Post-caldera volcanic Type of post–caldera activity following the classification performed by Walker (1984): Type–C, type–L, type–M, type–R, activity type–S, type–CaR, type–CC, type–Ms and type–Rs. (Fig. 6). (Appendix A: (K)) Post-caldera resurgence Post–caldera periods of resurgence or intra–caldera doming Caldera preservation Caldera preservation, i.e. erosion level: Well–preserved (caldera rim is recognizable), partially eroded or destroyed (caldera rim is partially recognizable), completely eroded (only caldera–forming deposits are observable).(Appendix A: (L)) References References List of consulted references.(Appendix B) Comments Comments Comments about the caldera sample (e.g. possible controversies concerning the collapse caldera origin) Geodynamic setting

2.4. Geodynamical setting The CCDB information class “Geodynamical setting” groups the fields: plate tectonic setting, crustal type, type of tectonic faulting, and whether regional pre-caldera doming (magmatic or tectonic) exists or not. On the basis of consulted references (e.g. Uyeda, 1982; Kearey and Vine, 1996; Turcotte and Schubert, 2002; Carey, 2005), we have decided to classify the plate tectonic settings hosting the collapse caldera samples included in the CCDB into the following categories (Fig. 3): Chilean- and Mariana-type subduction zones; island arc collision; back-arc rifting; ocean ridge, continental rifting; hotspot; hotspot near or over an oceanic ridge; transform boundaries; and, triple junctions (Appendix A: (F)). While the rest of tectonic settings are well-defined, there appear to be some problems when designing “Hotspots”. Some hotspots that lie in the middles of plates have been recognized as unique and dominant tectonic setting (e.g. Yellowstone, U.S.A; Spera and Crisp, 1981 and references therein). In other cases, hotspots are located at or near other structures like ocean ridges. In such situations, it is difficult to discriminate which one of the two tectonic settings is the most relevant for the area. According to well-

studied samples (e.g. Iceland), the combination of both tectonic settings (ocean ridge + hotspot) is the determining factor for the volcanism to be more voluminous than normal ocean ridge volcanism. In fact, the volcanism in Iceland results in a thick oceanic crust and the elevation of the island above the sea level (Kearey and Vine, 1996). As a direct consequence of this kind of observations, we have decided to distinguish between intraplate (isolated) hotspots and those near or at an oceanic ridge. Concerning the type of Earth crust on which caldera samples develop, we define the different crustal categories depending on crust composition and thickness hc. The first influences the composition and also the physical properties of the magma (Best and Christiansen, 2001). The second may also modify magma composition and defines, as well, the mechanical behaviour of the crust and controls the magma storage capacity (Turcotte and Schubert, 2002). From the references consulted in this review (e.g. Condie, 1993; Kearey and Vine, 1996) we propose to classify the crustal types in: Thick continental crust (hc N 30 – 35 km); standard-thin continental crust (hc ≤ 30–35 km); thick transitional crust (hc ≥ 20–25 km); standard-thin transitional crust (hc b 20–25 km); thick oceanic crust (hc ≥ 10–15 km); and

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339

Fig. 2. Models of alternative subsidence types/geometries in relation to depth and roof geometry of underlying magma chamber (Modified from Lipman, 1997).

standard-thin oceanic crust (hc b 10–15 km) (Appendix A: (G)). We assume continental crust to be silicic (granitic) and oceanic crust more mafic and primarily of basaltic composition. The transitional crust has an intermediate composition between the continental and the oceanic crust (e.g. Condie, 1993; Kearey and Vine, 1996). Information included in the field “Type of tectonic faulting”, describes those local and regional structures that may have influenced pre-caldera volcanism, caldera formation and/or the distribution of the post-caldera volcanism. We assume five different types tectonic faulting: (1) extensional (graben structures, normal faults), (2) compressional (thrust faults, horst-like structures), (3) shear (trans-

form structures), (4) transtensional and (5) transpressional (Fig. 4) (Appendix A: (H)). The latter two categories include those structures formed due to the displacement along strike–slip faults with bends or stepovers (e.g. pull-apart basins). Additionally, some authors (e.g. Nappi et al., 1991) assume that intersection between two structural families (e.g. Montefieascone–Italy) creates a favourable path for magma ascent and points of structural weakness relevant for the caldera-forming event. Thus, we consider it necessary to add the additional category: “Intersection of faults”. The field “Pre-caldera doming” records the existing information about tectonic or magmatic tumescence or doming periods prior to

Fig. 3. Sketch of the different tectonic settings where collapse calderas may take place.

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Fig. 4. Sketch showing the different types of local tectonic faulting (Modified from Vigneresse et al., 1999).

Fig. 5. Relation between the pressure evolution inside the magma chamber and the resulting sequence of deposits during a caldera-forming eruption. Field studies and numerical models (see Martí et al., in press-a,b), we can distinguish between two types of caldera-forming eruptions, here named, case A and B. In both cases, prior to the eruption, the chamber is overpressurized (magma pressure, PMPM;N lithostatic pressure, PL). The eruption starts when PM reaches a value PE = PL + ΔPSTART, where PE is the magma pressure at the magma chamber roof and ΔPSTART is the overpressure required to fracture the country rock by tension (Martí et al., 2000). In case of A-type caldera-forming eruptions, the initiation of the eruption and the onset of the caldera collapse are almost coincident in time (the collapse onset occurs exactly at the beginning of the eruption or shortly after). Therefore, the pressure at the top of the magma chamber at the collapse onset (PcA) is close to or slightly lower than PE (PE ≥ PcA), so that, the conditions for ring fault formation are achieved while the magma chamber is still overpressurized (PE ≥ PcA > PL). Thus, the whole A-type caldera-forming sequence of deposits will be composed of “syn-collapse” deposits (mostly ignimbrites), with very few or no evidence of previous pre-caldera collapse deposits. If conditions for ring fault formation are not reached, caldera collapse does not occur and the eruption progresses decreasing the pressure inside the chamber below the lithostatic pressure, i.e. the magma chamber becomes underpressurized (PM < PL). In B-type calderas, once the magma chamber is underpressurized (after a first eruptive episode), conditions for ring fault formation may be achieved if pressure at the top of the magma chamber (PM) reaches a value PcB = PL − ΔPCOLL, where PcB is defined as the pressure at the top of the magma chamber when caldera collapse starts and ΔPCOLL is the shear strength of the rock, i.e. the underpressure necessary to allow the magma chamber roof collapse under shear. In B-type caldera-forming eruptions, since the caldera collapse initiates (i.e. the conditions for ring fault formation are achieved) after a significant non-caldera eruptive episode, it is possible to find pre-caldera collapse deposits at the base of the caldera-forming sequence. The pressure ranges at which caldera collapse may occur in both situations (A-type and B-type), indicated by shaded zones, depend on several factors such as the host rock mechanics, magma physics and the geometry and depth of the magma chamber (Martí et al., 2000).

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the caldera-forming eruption. In most cases, due to the scarcity of information, we are not able to elucidate if the process has a tectonic or magmatic origin, and it is not specified in the CCDB. 2.5. Pre-caldera volcanism In many cases, the exposed geological record in the caldera area allows us to identify the existence of volcanism preceding the caldera formation. The information class “Pre-caldera volcanism” records the type pre-caldera volcanism (Appendix A: (I)). For some of the calderas included in the CCDB, available information allows to define if there were volcanic edifices, such as volcanic cones, basaltic volcanoes, stratovolcanoes or stratocones, shield volcanoes, prior to the calderaforming event. In other cases, field studies may also record pre-caldera volcanic activity associated with lava flows and domes, as well as previous caldera-forming events or caldera clusters. 2.6. Caldera-forming eruption sequence This information class compiles information concerning the calderaforming eruption sequence. Currently, this class only includes the field “Timing of caldera onset”, i.e. the precise instant during the calderaforming eruption at which caldera collapse initiates. We include in the caldera-forming eruption sequence possible plinian phases or other eruptive episodes that could decompress the magma chamber and trigger caldera subsidence. In some cases, the onset of caldera collapse is marked by a large and abrupt increase in lithic content in the deposits or changes in the eruptive mass flux may also indicate the beginning of caldera collapse, possibly due to a transition between different vent systems (e.g. Bacon, 1983; Gardner and Tait, 2000; Roche and Druitt, 2001). According to the information provided by field studies and in agreement with numerical results (e.g. Gudmundsson et al., 1997; Gudmundsson, 1998; Folch and Martí, 2004) we distinguish between two well-differentiated types of eruption sequence (Fig. 5)(Appendix A:

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(J)). In case A, the caldera eruption evolves immediately into massive proportions causing caldera collapse since the initiation of the eruption, i.e. the onset of the eruption and the beginning of the caldera collapse are coincident in time. In this case, the conditions for ring fault formation are reached in the meantime the magma chamber is overpressurized (e.g. Folch and Martí, 2004; Martí et al., in press-a,b). In case B, the caldera onset is preceded by a non-caldera eruptive episode. This first eruptive episode is responsible for causing a significant decompression of the magma chamber previous to the initiation of calderas collapse. The conditions for ring fault formation are achieved once magmatic pressure has decreased below lithostatic (e.g. Folch and Martí, 2004; Martí et al., in press-a,b. Depending on the moment of the eruption at which caldera collapse starts, the resulting sequence of caldera-forming deposits will have a completely different appearance. In case A, the whole sequence is characterized by “syn-collapse” deposits, whereas in case B, is the corresponding caldera-forming deposits will overly deposits from the preceding phases of the eruption (Fig. 5). 2.7. Post-caldera evolution A caldera collapse does not necessarily imply the end of the volcanic activity in an area. Contrarily, a great majority of calderaforming events are only an intermediate stage in the evolution of a specific volcanic system. Therefore, it is interesting to study the type of post-caldera activity that takes place after the caldera-forming event. Regarding the type of post-caldera activity, Walker (1984) investigated the distribution of post-caldera vents as a possible indicator of caldera origin performing a literature survey at 160 Quaternary calderas in various parts of the world. He made an attempt to categorize the vent distribution in the considered caldera examples and proposed the following categories of post-caldera volcanic activity (Fig. 6)(Appendix A: (K)): Type-C (A single vent occupies a central or near-central position), Type-L (Vents are distributed in a defined straight line or

Fig. 6. Sketch showing the different types of post-caldera volcanic activity. Those types indicated in bold correspond to Walker's (1984) classification, whereas those in normal-italic are new types defined for the CCDB.

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linear zone), Type-M (A single vent occurs at or near the caldera margins), Type-R (Vents occur along an arcuate line parallel to the caldera margin) and Type-S (Vents are scattered widely within the caldera). In the CCDB we have considered the abovementioned types of post-caldera volcanic activity but we have also added four further categories that allow us a more accurate description of the post-caldera activity (Fig. 6): Type-CaR (Central vent and multiple vents located at the caldera margins), Type-CC (Caldera collapse), Type-Ms (Multiple vents at o near the caldera margins) and Type-Rs (Vents controlled by welldefined regional structures). It is also interesting to indicate whether or not the caldera-forming eruptions are followed by resurgence periods. Therefore, in the field “Post-caldera resurgence” we mark those samples with evidences of further resurgence or intra-caldera doming. Concerning the post-caldera period, it is also possible to register how the collapse structure has been preserved with time. In most cases, erosion or other geomorphological processes may have destroyed part of the caldera structure restricting, for example, the possibilities for a correct interpretation of the collapse mechanism. In this sense, we also include the following categories: Well-preserved (Caldera rim is perfectly recognizable), Partially eroded or destroyed (Caldera rim is only partially observable) and Completely eroded (Not remaining structure, only caldera-forming deposits are observable) (Appendix A: (L)). 2.8. References In this information class we list for each caldera sample all the consulted references. These are commonly the most representative published field studies on each caldera. Wherever possible one reference about plate tectonics of the area where the caldera is located is also given. Notice that these works are not necessarily directly related to the caldera but to the area. All references cited in the database are listed in Appendix B. 2.9. Comments The field “Comments” offers to the users the possibility to add some comments about the caldera sample. For example, possible controversies concerning the caldera origin or collapse mechanism.

3. The CCDB data analysis As an example of the CCDB applicability, we offer here a summary of the results obtained when analysing part of the information included in the database. A more detailed description of the analysis process would be too extensive and is beyond the scope of this paper. Interested readers can found at the CCDB website (http://www.GVBcsic.es/CCDB.htm). First, we can list and study separately the most relevant features of collapse calderas such as age, dimensions, composition of the extruded deposits. Second, we are able to crossover the information concerning two or more features (e.g. age-dimensions; crustal type-plate tectonic setting). For example, since the CCDB is linked to a GIS, we are able to represent the spatial distribution of the recorded collapse caldera samples. Fig. 7 illustrates the distribution of the collapse calderas included in the CCDB. Analysing exhaustively this spatial distribution and classifying the caldera samples according to the 19 world regions, we observe that the geographical distribution of calderas has two important peaks, one in the Mariana trench and the other in North America (Fig. 8A). The European/Mediterranean region is also characterized by a relatively high number of calderas. Regions such as Africa, Melanesia or Central and South America host a moderately low number of caldera samples and extremely few examples of natural calderas are located in regions like the Indian Ocean, the Caribbean or Antarctica. Concerning the age and dimensions of calderas, data analysis confirms that these structures have occurred during all geological periods (Fig. 8B) and that they vary widely in size (Fig. 8C). The CCDB documents from 0.005 ka (Miyakejima eruption in 2000; Geshi et al., 2002; Nakada et al., 2005) to 93.6 Ma (West Fork, Alaska; Bacon et al., 1990) old calderas and also a Late Precambrian example (Ramat Yotam caldera, Israel; Eyal and Peltz, 1994). Regarding the caldera dimensions, there exist very small samples with around 0.03 km2 (Piton de la Fournaise, Reunion Island; Hirn et al., 1991) to huge ones with 4712 km2 (Blacktail, U.S.A.; Morgan et al., 1984) (Fig. 8C). However, the 91.8% of the caldera samples (i.e. 235 calderas) are smaller than 500 km2. Also, of special interest is to classify the calderas included in the CCDB according to their subsidence types or geometry (Fig. 9A).

Fig. 7. World map with the location of the collapse calderas (black circles) included in the database.

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Fig. 8. (A) Results for the worldwide distribution of the calderas included in the CCDB. Histogram showing the number of calderas located in the different world regions defined by Simkin et al. (1981) and modified by Newhall and Dzurisin (1988). NANA: Total number of calderas with information valid for the world region analysis. (B) Results for the age distribution of the calderas included in the CCDB. Each black vertical bar corresponds to one caldera sample. NANA: Total number of calderas with information valid for the age analysis. (C) Results for the area distribution of the calderas included in the CCDB. Each black vertical bar corresponds to one caldera sample. NANA: Total number of calderas with information valid for the area analysis.

Applying Lipman's (1997) terminology we observe that among all recorded caldera samples with information in this field the most common geometry is the plate/piston like, with a 35%, equivalent to 33 calderas. Furthermore, also funnel and trap-door collapse calderas are also quite frequent (20.2% each). Since information concerning the magmatic system is primordial to understand the collapse caldera dynamics, we have also analysed the volume of erupted magma during the caldera-forming event (Fig. 9B) and its composition (Fig. 9C). About the volume of erupted magma, a high number of the caldera-forming events included in the CCDB are related to magma volumes comprised between 100 and 500 km3 and there exists also an important number of samples involving less than 50 km3. Additionally, caldera-forming events involving more than 2000 km3 of extruded material are rare; the database includes only few samples (e.g. La Garita, United States; Lipman, 1976, 1984, 1997; Toba, Sumatra; Chesner, 1998). Felsic, calcalkaline magmas are among the most abundant compositions, whereas any other composition is merely represented by less than 5% of the total analysed calderas.

From the information field “Geodynamic setting” we can study the most common plate tectonic setting, crustal type and type of tectonic faulting associated with the caldera samples recorded in the CCDB. In fact, more than the 40% of the CCDB calderas with information on the tectonic setting are located in C-type subduction zones and a 25.5% in areas of continental rifting (Fig. 10A). In addition, Fig. 10B clearly illustrates that calderas are mainly (86%, equivalent to 235 caldera samples) located in areas of continental standard-thin or thick and transitional thick crust. Regarding the type of tectonic faulting, results obtained indicate that collapse calderas are mostly (64.4%, equivalent to 148 caldera samples) associated with local extensional structures (Fig. 10C). The CCDB analysis also allows us also to determine the most common types of pre-caldera volcanic activity (Fig. 11A). An important problem when dealing with this type of information is that the bibliography proposes a too extensive number of possibilities that would complicate further statistical analysis. Therefore, we have reclassified them into six main categories: (1) Volcanic edifice (incl. various types of edifices, e.g. cones, basaltic volcanoes and simple

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ones are Type-C, R and S with more than 15 calderas included in each category. Apart from the individual analyses of the different selected features, it is also useful and telltale to combine simultaneously the information coming from two or more of the individual analyses described above. We sum up in this paper the most remarkable results and relevant observations. For example, large calderas (caldera area N500 km2) are uniquely felsic cal-alkaline (Fig. 12A), mostly rhyolitic (e.g. Emory, U.S.A.; Spera and Crisp, 1981; Elston, 1984; Lipman, 1984). Additionally, these large structures are mostly located in areas of extensional tectonic faulting and occasionally, they are also associated with transtensional structures, but no with compressional ones (Fig. 12B). Concerning

Fig. 9. (A) Results for the subsidence types/geometries of the calderas included in the CCDB. NANA: Total number of calderas with information valid for the subsidence type/ geometry analysis. (B) Results for the distribution of values of total erupted volume of magma of the calderas included in the CCDB. Each black vertical bar corresponds to a caldera sample. NANA: Total number of calderas with information valid for the volume of erupted magma analysis. (C) Results for the analysis of the composition of the magma extruded during the caldera-forming events included in the CCDB. NANA: Total number of calderas with information valid for the magma composition analysis.

volcanic edifices), (2) Stratovolcanoes and stratocones, (3) Shield volcanoes, (4) Lava flows and domes, (5) No previous edifices, calderas or caldera clusters and (6) Other structures not included in the other five groups like pyroclastic plateaus. From Fig. 11A it is evident that frequently (53.3%), pre-caldera volcanic activity involves the development of stratovolcanoes and stratocones. By contrast, the precaldera volcanic activity related to lava flows and domes structures or the absence of a previous edifice are less common. Also of special interest is to classify the recorded caldera samples according to the timing of the collapse onset, i.e. the moment at which the collapse starts compared to the whole caldera-forming sequence. Based on the information compiled in Fig. 11B, we can say that the collapse starts commonly after a first eruptive episode responsible for the magma chamber decompression. Finally, we have analysed which are the most abundant types of post-caldera volcanic activity. Fig. 11C illustrates that the most usual

Fig. 10. (A) Results for the tectonic setting associated with the calderas included in the CCDB. NANA: Total number of calderas with information valid for the plate tectonic setting analysis. (B) Results for the crustal type associated with the calderas included in the CCDB. NANA: Total number of calderas with information valid for crustal type analysis. (C) Results for the type of tectonic faulting associated with the calderas included in the CCDB. NANA: Total number of calderas with information valid for the type of tectonic faulting analysis.

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all possible comparisons between the different fields included in the CCDB is beyond the scope of this paper. As any other methodology, the CCDB has its own restrictions directly related to the quality and quantity of the recorded data. Obviously, field data are not exempt from important restrictions. Information recorded in the field is strongly influenced by erosion and reworking processes, as well as subsequent post-caldera volcanic activity or post-caldera deformation processes that may destroy or bury the original materials and structures. Also, there appear some errors because the lack of updating of the information in some of the consulted references and the subjectivity in the interpretations depending on each author. However, these last restrictions may be partially settled with an active updating and upgrading of the information included in the CCDB. Apart from the errors due to the quality of the included data, the observations we have made in the previous section are strongly

Fig. 11. (A) Results for the type of pre-caldera volcanic activity associated with the calderas included in the CCDB. NANA: Total number of calderas with information valid for the type of volcanic activity analysis. (B) Results for the timing of collapse caldera onset analysis. NANA: Total number of calderas with information valid for the timing of collapse caldera onset analysis. (C) Results for the type of post-caldera volcanism analysis. NANA: Total number of calderas with information valid for the type of postcaldera volcanism analysis.

the relation between the dimensions of the collapse and the subsidence type/geometry, we observe that large calderas are exclusively plate/piston or trap-door (Fig. 12C). If we combine the different results obtained when analysing the information included in the class “Geodynamical setting” with the caldera size, we observe that large calderas (area N500 km2) are all located in continental crust, whereas those located on areas of transitional and oceanic thin or thick crusts have all an area smaller than 500 km2 (Fig. 13A). Also interesting is that an important percentage (46.6%) of the calderas with an area smaller than 500 km2 are associated with C-type subduction zones. By contrast, those structures larger than 500 km2 are located principally in areas of continental rifting (Fig. 13B). 4. Discussion What we have presented here are only a few examples of the whole range of possibilities offered by the CCDB. A full description of

Fig. 12. Results obtained when analysing the magma composition (A), the type of tectonic faulting (B) and the subsidence type/geometry (C) of those calderas with an area exceeding the 500 km2.

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Fig. 13. Results obtained when combining the available information concerning the area of the calderas with the results obtained for the crustal type (A) and the tectonic setting (B) analyses.

dependent on the amount of available information. Obviously, general tendencies supported by a considerably high number of calderas will be more reliable than those supported by only a small number of samples. Occasionally, due to the low number of calderas with available information, quantification becomes meaningless and the reliability of the results obtained is doubtful. Not only is a huge amount of information necessary, but also a uniform distribution of the information is essential for performing a reliable analysis. As an example, we have commented that the number of calderas in North America is considerably higher than in the other studied regions (Fig. 8A). It is reasonable to think that this high number of calderas owes the fact that this area has been more intensively studied and thus more calderas have been detected and/or more information is available. This unusual high number of calderas could also be due to better preservation conditions and observation possibilities of the area or because this area is especially prone to the occurrence of caldera-forming eruptions. Hence, it is worth analysing the tectonic and magmatic evolution of North America in this context for future works. Independently of the abovementioned potential reasons, the direct consequence of having such amount of calderas in a specific area may misrepresent some of the results obtained. This is the case, for example, of the age distribution (Fig. 8B). Recent structures are normally more susceptible to detection and observation since they are typically better preserved than older ones. Therefore, with increasing age we expect a gradual decrease in the number of calderas. Instead, we observe a considerable high number of calderas with an age comprised between 25 and 50 Ma. Notice that in Fig. 5, due to the type of graph, flat parts of the line indicate that there are several calderas with approximately the same age. These calderas with an age of 25– 50 Ma, are principally located in the region of North America. Due to the high number of calderas in this area, the existence of a feature

common to almost all or at least a great majority of the calderas distorts the whole CCDB data analysis. Although being only a characteristic of the calderas located in North America, due to the high number of samples in that region, this feature appears as the most widespread throughout the calderas of the CCDB. Concerning the dimensions of the collapse calderas included in the CCDB, we observe that almost all samples have an area between 20 and 500 km2 (Fig. 8C). Possibly, smaller structures (caldera area b20 km2) are badly preserved and larger ones (caldera area N500 km2) have a lower probability of occurrence. Alternatively, an area between 20 and 500 km2 is a compromise between probability of occurrence and capacity of preservation. The study of the dimensions of collapse calderas leads us to consider a possible maximum concerning the size of a collapse caldera. As with other geological structures (e.g. plutons) we assume that there must be a dimensional threshold for collapse calderas. The largest caldera included in the CCDB is Blacktail (USA) with an area of 4712.4 km2 (100 × 60 km) and 1500 km3 of caldera-forming related deposits. Thinking about the factors controlling this threshold we expect to find a certain influence of the maximum volume of storable magma in the crust and the horizontal extension of the associated magma chamber. Crust thickness, regional and local stress fields, composition and time of residence of magma, are likely some of the main influential factors. A more exhaustive analysis of this point would be very interesting but it is out of the scope of this study. Additionally, the fact that almost all large calderas (caldera area area N500 km2) are associated with extensional structures suggests that the existence of these structures is, as some authors propose (e.g. Gudmundsson 1998, 2007), a requisite or a favourable factor for the formation of large calderas. Since the rest of large calderas are associated with transtensional structures but never with a compressional component we can say that, apparently, tectonic compression structures work against the formation of large collapse calderas. Regarding the type of collapse, the most abundant are plate/piston and secondly trap-door and funnel, probably, because these collapse types may be the most suitable due to mechanical reasons as shown by analogue modelling (e.g. Martí et al., 1994; Roche et al., 2000; Martí et al., in press-a) or these morphologies are the most recognizable or preserved in the field. The main difference between piecemeal and plate/piston collapses is the coherence of the caldera floor during subsidence (Lipman, 1997). However, the caldera floor is not always exposed and observable. In many cases, post-collapse volcanic activity products such as ignimbrites or lava flows cover it (e.g. Las Cañadas, Tenerife; Ablay and Martí, 2000; Kilauea, Hawaii; Walker, 1988). Additionally, it is evident that piecemeal floors are not easily recognized. In some cases, the distribution of the post-caldera activity reveals the fractures in disrupted floor (e.g. Menengai, Kenya; Leat, 1984). Regardless the specific collapse type, we can say that the most common observable features in the field are ring fractures delimiting a coherent or disrupted subsiding piston (i.e. plate/piston or piecemeal collapses, respectively) or asymmetrical collapses (i.e. trap-door collapses). Some authors hold (e.g. Sawada, 1984; Yoshida, 1984) that the type of collapse calderas in Japan is directly related to the stress field at the time of the caldera-forming eruption. Calderas related to periods of extension are commonly plate/piston like (e.g. Ishizuchi, Japan; Yoshida, 1984), whereas structures occurred during more recent arc-normal compression episodes tend to be funnel type (e.g. Hakone, Japan; Kuno et al., 1970; Aramaki, 1984). If we compare this statement with our results we can observe that effectively, those areas associated with compressional structures are related to funnel type collapses. Apparently, there is a strong connection between the collapse type and the type of regional/local tectonic faulting. However, we should not forget that in this example, the number of calderas without information is quite high. Several authors (e.g. Clemens and Vielzeuf, 1987; Wickham, 1987; Vigneresse et al., 1999) have insisted in the idea that the type of crust

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(i.e. its rheology, thickness, composition) may influence the composition of magma and its storage condition. In our analysis, we have observed that almost all calderas of the CCDB are located in areas of continental crust and on transitional thick crust (Fig. 10B). Notice that all calderas in the North America region and in Japan and the Marians Island region are uniquely associated with continental and transitional thick crust, respectively. Consequently, the abovementioned distortion of the results due to the high number of caldera samples in both areas can be a possible explanation for the distribution observable in Fig. 10B. However, it is also possible that the thickness and composition of these crust types may favour the formation of collapse calderas. By contrast, the low number of caldera samples in areas of oceanic crust may indicate that the mechanical properties and composition of oceanic crust are not suitable for collapse caldera formation. It is possible that oceanic crust is too thin to host or favour collapse calderas. This could be the explanation for the lack of calderaforming eruptions involving huge volumes of magma in oceanic crust environments. Of course, this last observation can be a question of preservation because most of the latter calderas are located on islands and a considerable percentage of extruded material may be deposited into the sea, thus preventing a correct volume estimate. It is also possible to think that collapse calderas associated with oceanic crust may lay under the sea level and are undetectable. Areas of oceanic crust may coincide with poor studied areas and consequently, the number of identified and described caldera samples is lower. Also interesting are the results concerning the tectonic setting of collapse calderas. Almost all calderas included in the CCDB are located in areas of C-type subduction zone or continental rifting. If we take a look at the spatial distribution of the calderas associated with C-type subduction zones, we detect that a high percentage (31.5%, 34 calderas) is located in Japan. It is possible that the subduction process itself, which leads to certain structural and magmatic conditions, favours the occurrence of caldera-forming eruptions. It is important to remind that large calderas (caldera area N500 km2) are associated with areas of continental rifting. We suspect that from a tectonic and/or compositional point of view, areas of continental rifting are favourable for the formation of larger calderas. On the one hand, the extensional stress field implicit in these areas may favour the generation of extensional faults, i.e. collapse structures (e.g. Olsen et al., 1987; Tandon et al., 1999; Henry and Aranda-Gomez, 2000; Acocella et al., 2002). The number of calderas located at hotspots near an oceanic ridge is higher than those located in isolated hotspots. Apparently, the interaction of the continuous magma supply of hotspots and the extensional structures of the oceanic ridge favour the occurrence of caldera-forming eruptions, i.e. the generation of magma chambers and stress configuration susceptible for calderaforming eruptions. 5. The CCDB online The final and better use of this database is to convert it into an open and accessible tool for all researchers working on collapse calderas. Therefore, we are working on the development of the CCDB website, structured in three main areas (Fig. 14): (1) CCDB, (2) Register and (3) CCDB Community. The CCDB area will be open to everybody and consists of a short introduction, explanation and user's manual of the CCDB. In this section, we offer an explanation of the objectives of the CCDB, the different information classes and the fields included in the CCDB. In the Register area, persons interested in the database should register in order to obtain a user name and a password. These will be necessary to receive the CCDB and to enter in the third area: CCDB Community. Registration is only for security reasons to avoid any misuse of the database and non-controlled modifications. Users will be asked to introduce their name and surname, as well as the information of their institute or organization. In the CCDB Community area the web page

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visitors will be able to suggest updates and corrections of the database, to send news about events or publications related to caldera collapse studies or to enter in the CCDB Forum. The principal objectives of this section are twofold. First, to maintain the collapse caldera database always updated with the incorporation of new information and data. Second, to give the CCDB users the possibility of asking or consulting aspects related to caldera collapse processes to other CCDB members. The CCDB webmaster, the person in charge of modifying, updating and correcting the database and of keeping the CCDB in good working order. Each CCDB member will be able to connect with the Webmaster to inform him/her about collapse caldera news, updates or corrections. Only published or accepted works should support each correction or update. Once updated or corrected the webmaster will inform all CCDB users about the new database version. 6. Summary and conclusions The CCDB is a new database compiling existing information on collapse calderas. We suggest that all information included in the CCDB should be available for each recorded caldera. In order to have a precise description of a collapse caldera, we consider convenient to have information on the collapse depression, the caldera-forming deposits, the associated magmatic system, the geodynamic setting where the caldera is located, the type of pre-caldera volcanism, the caldera-forming eruption sequence and the post-caldera evolution. In this paper, we present the analysis of information included in the latest version of the CCDB. The scope has been to find and show general trends among different parameters and their effect on the resulting collapse caldera structures. From the first set of results we can already make some significant observations on collapse calderas: (1) among all recorded caldera samples the most common geometry is the plate/piston like (35%), followed by funnel and trap-door collapse calderas (20.2% each); (2) felsic, calc-alkaline magmas are among the most abundant compositions; (3) CCDB calderas are located preferentially in C-type subduction zones (40%) and a in areas of continental rifting (25.5%); (4) calderas are mainly (86%) located in areas of continental standard-thin or thick and transitional thick crust; (5) frequently (53.3%), pre-caldera volcanic activity involves the development of long-lived stratovolcanoes and stratocones, being less common pre-caldera volcanic activity only related to lava flows and domes or the absence of a previous volcanic edifice; (6) large calderas (caldera area N500 km2) are mostly located in areas of extensional tectonic faulting, their compositions are uniquely felsic cal-alkaline (mostly rhyolitic), and are exclusively plate/piston or trap-door; and, (7) large calderas (area N500 km2) are all located in continental crust and principally in areas of continental rifting, whereas those calderas with an area smaller than 500 km2 are associated with transitional and oceanic thin or thick crusts C-type subduction zones. These observations are strongly dependent on the amount and quality of available information. Despite its possible restrictions, information contained in the CCDB may be useful to infer general tendencies among collapse calderas worldwide and to speculate about factors controlling caldera collapse processes. We have shown that there are areas that display a particular tectonic and magmatic evolution that favours caldera formation (North America and Northern Mexico). We expect this database to be a useful tool for studying and understanding these volcanic structures, as it allows a statistical evaluation of several depending parameters that control caldera formation. The generation, consultation and evaluation of this Collapse Caldera DataBase (CCDB) is not only restricted to this work. The objective is to publish the database in the form of an interactive web page permitting registered users to access to the database and share the recorded information. The database is coded to implement further extension and upgrading of recorded data.

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Fig. 14. Screenshots of the CCDB web page prototype. Three different sections compose the web page. The first one, called CCDB, will be open to nay user and consists of a short introduction, explanation and user's manual of the calderas database. A second section, Register, will include a registration form. Users interested in the database will have to register in order to obtain a user name and a password. These will be necessary to receive the CCDB and to enter in the third section: CCDB Community. Registration is only for security reasons in order to avoid any misuse of the database and non-controlled modifications. In the CCDB Community users will be able to suggest updates and corrections of the database, to send news about event or publications related to caldera collapse studies or to enter in the CCDB Forum.

However, we cannot ignore limitations associated with the available field data and their effect on the interpretation of the compiled information. It is therefore imperative to accept that both field data and interpretations are dynamic entities, i.e. they are and have to be subjected to both continuous revision and updating. This is why the CCDB needs to be open and made it available to all scientists interested on caldera studies, as only the contribution of all them will allow the CCDB to grow up and to certainly become the useful tool we have envisaged. Acknowledgements This research has been partially funded by the EC EXPLORIS project (EVR1-2001-00047). J. Martí is grateful for a MEC grant (PR-2006-

0499). A. Geyer is grateful for a MEC post-graduate fellowship (AP2002-1010). We thank A. Ordoñez who helped us with the database development and R. Scandone and G. Valentine for their constructive reviews. Appendix A. Database codes In this Appendix we also provide the code for the different possibilities of the field. (A) List of World regions and their corresponding code in the database: 1, Europe/Mediterranean; 2, Africa; 3, Indian Ocean; 4, New Zealand, Tonga and Kermadec Island; 5, Melanesia (Papua New Guinea and Vanuatu); 6, Indonesia; 7, the Philippines; 8,

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(B)

(C)

(D)

(E)

Japan and the Marianas Islands; 9, the Kurile Islands; 10, Kamtchatka and mainland Asia; 11, Alaska; 12, North America; 13, Hawaii; 14, Central America; 15, South America; 16, the Caribbean Sea; 17, Iceland; 18, Atlantic islands (Azores, Canary, and Cape Verde Islands); 19, Antartica; 9999999, Unknown. List of World subregions and their corresponding code in the database: 101, Italy; 102, Greece; 103, Turkey; 104, Iran; 105, USSR-W; 106, UK; 107, Israel; 108, Germany; 201, Red Sea and Ethiopia; 202, Africa-E; 203, Africa-C; 204, Africa-W; 205, Africa-N; 301, Arabia-W; 302, Arabia-S; 303, Indian O-W; 304, Indian O-S; 401, New Zealand; 402, Kermadec Islands; 403, Tonga; 404, Samoa; 405, Fiji; 500, Papua/New Guinea–New Zealand; 501, Papua/New Guinea–New Zealand; 502, Papua/ New Guinea–New Zealand; 503, Papua/New Guinea–New Zealand; 504, Papua/New Guinea–New Zealand; 505, Solomon Islands; 506; 507, New Hebrides; 508; 509, Australia; 601, Sumatra; 602; 603, Java; 604, Lesser Sunda Island; 605,Banda Sea; 606; 607; 608; 701; 702; 703; 704; 705, SE Asia; 801, Taiwan; 802,Ryuku Islands and Kyushu; 803, Honshu-Japan; 804, Izu-Mariana Island; 805, Hokkaido-Japan; 806, Southwest Japan; 900, Kurile Islands; 1000, Kamtchatka; 1001, USSR; 1002; 1003, Manchuria; 1004, Tibet; 1005, USSR-SE; 1006, Mongolia; 1007; 1008, Korea; 1101, Aleutian Islands; 1102, Alaska Pennynsula; 1103, Alaska-SW; 1104, Alaska-W; 1105, Alaska-E and SE; 1200, Canada; 1201,US-Washington; 1202, US-Oregon; 1203, US-California; 1204, US-Idaho; 1205, USWyoming; 1206, US-Utah; 1207, US-Nevada; 1208, US-Colorado; 1209, US-Arizona; 1210, US-New Mexico; 1211, Texas; 1302, Hawaiian Islands; 1303, Pacific-C; 1401, Mexico; 1402, Guatemala; 1403, El Salvador; 1404, Nicaragua; 1405, Costa Rica; 1501, Colombia; 1502, Ecuador; 1503, Galapagos Islands; 1504, Peru; 1505, Bolivia and Chile-N; 1506, Chile Island; 1507, Chile-C and Argentina; 1508, Chile-S; 1600, West Indies; 1700, Iceland-W; 1701, Iceland-SW; 1702, Iceland-S; 1703, Iceland-N; 1704, Jan Mayen; 1801, Atlantic-N; 1802, Azores; 1803, Canary Islands; 1804, Cape Verde Islands; 1805, Atlantic-C; 1806, Atlantic-S; 1900, Antartica; 2001, Artic Ocean; 9999911 or ?, Unknown subregion of Alaska; 9999998 or ?, Unknown subregion of Japan; 9999999 or ?, Unknown word region and subregion. List of the different subsidence types/geometries according to the classification of Lipman (1997) and their corresponding code in the database: ?, Unkown; C, Composite; CA, Chaotic; F, Funnel; F/P, Funnel/plate; F?, Funnel? ; F_P, Funnel-plate; F_T, Funneltrapdoor; KT, Krakatau type; LC, Laccocaldera; P, Plate; P?, Plate?; P2p, Plate 2 phases; PaPC, Plate and piecemeal; PC, Piecemeal; PC?, Piecemeal?; PC2p, Piecemeal 2 phases; P-D, Plate-downsag?; PI, Plate (inward); PI_D?, Plate (inward)downsag?; PO, Plate (outward); T, Trapdoor; T_D, Funneldownsag; T_PC, Trapdoor-piecemeal. List of the different magma compositions and their corresponding code in the database: ?, Unknown; A, Andesite; A_D, Andesite–dacite; AaD, Andesite and dacite; B, Basalt; B_R, Basalt–rhyolite; BA, Basaltic andesites; D, Dacite F, Felsic; I, Intermediate; L, Latite; L_T, Latite–trachyte; M, Mafic; P_T, Pantellerite–trachyte; QL, Quartz latite; QL_R, Quartz latite– rhyolite; QLaR, Quartz latite and rhyolite; R, Rhyolite; R_A, Rhyolite–andesite; R_D, Rhyolite–dacite; RaA, Rhyolite and andesite; RaD, Rhyolite and dacite; RaT, Rhyolite and trachyte; RD, Rhyodacite; S, Silicic; SH, Shoshonitic; T, Trachyte; TaB, Trachyte and basalt; TaP, Trachyte and phonolite; TaTP, Trachyte and trachyphonolite; TB, Tholeiite basalt; TEP, Tephriphonolite; TP, Trachyphonolite. List of the different rock suites and their corresponding code in the database: ?, Unknown; ALKAf, Alkaline felsic; ALKAi, Alkaline intermediate; ALKAif, Alkaline intermediate–felsic;

(F)

(G)

(H)

(I)

(J)

(K)

(L)

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ALKAm, Alkaline mafic; ALCAmf, Alkaline mafic–felsic; CALC– ALKA, Calc-alkaline–alkaline; CALCf, Calc-alkaline felsic; CALCi, Calc-alkaline intermediate; CALCi_ALKAf, Calc-alkaline intermediate–Alkaline felsic; CALC-if, Calc-alkaline intermediate– felsic; CALCm, Calc-alkaline mafic; CALCmf, Calc-alkaline mafic–felsic; THOLE, Tholeiite. List of the different plate tectonic settings and their corresponding code in the database: ?, Unknown; BAR, Back-arc rifting; CR, Continental rift; H, Hotspot; HOC, Hotspot near or over an ocean ridge; IAC, Island arc collision; OC, Ocean ridge; SC, Chilean-type subduction SM, Mariana-type subduction; T, Transform boundary; TJ, Triple plate junction. List of the different crustal types and their corresponding code in the database: ?, Unknown; C, Continental silicic thick crust; Cd, Continental silicic standard-thin crust; O, Oceanic basaltic thick crust; Od, Oceanic basaltic standard-thin crust; T, Transitional thick crust; Td, Transitional standard-thin crust. List of the different types of local tectonic faulting and their corresponding code in the database: ?, Unknown; COMPR, Compressional structures; EXT, Extensional structures; IF, Intersection of faults; Scomp, Transpressional structures; SHEAR, Shear structures; Stens, Transtensional structures. Types of pre-caldera volcanism and their corresponding codes in the database: ?, Unknown; AV, Andesite volcano; BV, Basalt volcano; C, Cone; CA, Caldera; CA?, Caldera; CC, Caldera cluster; L, Lavas; LD, Laccolithic dome; LFD, Lava flows and domes; LLSTR, Low, lava-dominated stratovolcano; LPSTR, Low, pyroclastic-dominated stratovolcano; M, Maars; NPE, No previous edifice; NPE?, No previous edifice?; PP, Pyroclastic plateau; RCC, Rift/Caldera cluster; STR, Stratovolcano; STRC, Stratocone; SUB Submarine; SV Shield volcano; VC, Volcano cluster; VE, Volcanic edifice; VE?, Volcanic edifice?. Timing of caldera onset and its corresponding codes in the database: ?, Unknown; A, Caldera collapse onset at the beginning of the caldera-forming eruption sequence; B, Caldera collapse onset preceded by a non-caldera eruptive episode. List of the different types of post-caldera volcanic activity according to Walker's (1984) proposal and their corresponding code in the database: ?, Unknown; C, Central or near-central vent; CaR, Central vent and multiple vents located at the caldera margins; CC, Caldera collapse; L, Straight line or linear zone distributed vents; M, Single vent at o near the caldera margins; Ms, Multiple vents at o near the caldera margins; R, Vents located along an arcuate line parallel to the caldera margin; RS, Vents controlled by regional structures; S, Vents scattered within the caldera. Caldera preservation (i.e. erosion level) and their corresponding code in the CCDB: WP, Well-preserved; PD, Partially eroded or destroyed; CD, completely destroyed.

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