Mixed deposits of simultaneous strombolian and phreatomagmatic volcanism: Rothenberg volcano, east Eifel volcanic field

Journal o f Volcanology and Geothermal Research, 30 (1986) 117--130 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

117

MIXED DEPOSITS OF SIMULTANEOUS STROMBOLIAN AND PHREATOMAGMATIC VOLCANISM: ROTHENBERG VOLCANO, EAST EIFEL VOLCANIC FIELD

B.F. HOUGHTON' and H.-U. SCHMINCKE ~ ' New Zealand Geological Survey, D.S.I.R., P.O. Box 499, Rotorua, New Zealand 2 Institut fiir Mineralogie, Ruhr- Universiffit Bochum, D4630 Bochum, F.R. Germany

(Revised and accepted December 3, 1985)

ABSTRACT Houghton, B.F. and Schmineke, H.-U., 1986. Mixed deposits o f Strombolian and phreatomagmatic volcanism: Rothenberg volcano, East Eifel'volcanic field. J. VolcanoL Geotherm. Res., 30: 117--130. The Pleistocene basanite-tephrite Rothenberg cone complex in the East Eifel was constructed by alternating dominantly Strombolian (S1--3) and dominantly phreatomagmatic (P1--3) phases of volcanism along a NNE--SSW linear vent system. Strombolian eruptions, from the central vent of the S1 scoria cone, and phreatomagmatic eruptions, from a vent on the southern margin of the cone, occurred simultaneously during the second phreatomagmatic phase (P2). The P2 deposits are a complex sequence in which Strombolian fallout ejecta is intimately admixed with phreatomagmatic fallout and pyroclastic surge material. Every bed contains at least trace amounts of ejecta from both sources but, at every site, an alternation of Strombolian-dominant and phreatomagmaticdominant units is recorded. Each bed also shows marked lateral changes with a progressive northward increase in the proportion of Strombolian material. The two eruptive styles produced morphologically distinct clast populations often with widely separated ( 5 - 7 ¢) grain size modes. The phreatomagmatic component of the P2 deposits is inferred to be the result of shallow interaction of external water and cool, partially degassed magma which reached the surface at a time when the magma column was retreating from the northern Strombolian central vent. The Rothenberg deposits illustrate the complexity and sensitivity of controls on Strombolian and associated phreatomagmatic volcanism, and the shallow depth of fragmentation during such eruptions. During such shallow eruptions minor, ephemeral and localised variations in the rate of rise and discharge of magma, and vent geometry and hydrology significantly influence the magma:water ratio and hence eruptive style.

INTRODUCTION Explosive volcanic eruptions are driven either by the explosive release of m a g m a t i c gas o r b y m a g m a - - w a t e r i n t e r a c t i o n ( S t e a r n s a n d M a c d o n a l d , 1946; Walker, 1973; Wilson, 1976; Sparks, 1978). Endmember styles defined for explosive basaltic volcanism (Walker and Croasdale, 1972; Fisher and

0377-0273/86/$03.50

© 1986 Elsevier Science Publishers B.V.

118 Schmincke, 1984) are Strombolian eruptions (magmatic gas) and Surtseyan or Vulcanian explosions (magma--water interaction). Many eruptions are, however, transitional in style, with both mechanisms operating in rapid succession or simultaneously. This is particularly true where magma and external water are in contact at very shallow levels, and localised variations in vent geometry and hydrology can influence the magma:water ratio. Under these circumstances very different conditions may pertain at two adjacent vents, or even two portions of a single vent, during one eruptive phase. Simultaneous discrete phreatomagmatic and Strombolian activity from a single vent or vent system have been recorded during several basaltic eruptions (Macdonald, 1962; Thorarinsson et al., 1964; Kienle et al., 1980). However, there are no detailed descriptions of deposits in which the products o f the t w o discrete eruptive styles are admixed on the scale of a single bed. This paper describes a well exposed complex tephra unit at Rothenberg volcano which is interpreted as the product of thorough mixture of coeval Strombolian and phreatomagmatic fallout and surge material, from adjacent portions of a single vent system. The unit is described in detail here, because such deposits are previously undocumented. This paper forms part of a series on Strombolian and associated phreatomagmatic volcanism (Houghton and Hackett, 1984; Houghton and Schmincke, in prep.). ROTHENBERG VOLCANO Rothenberg is a large Pleistocene cone complex, adjacent to the phonolitic Laacher See Volcano, in the East Eifel volcanic field (Fig. 1). The complex consists of a t u f f ring, t w o younger coalesced scoria cones and a single lava flow (Schmincke, 1977; Duda and Schminke, 1978). The older, northern tephritic scoria cone is a complex feature affected b y synvolcanic faulting and intruded by dykes coeval with the younger southern basanitic cone (Karukuzu, 1982). The outer wall of the complex (Fig. 2) consists of a composite sequence of fallout and surge-emplaced units divided into 3 Strombolian (S1, $2, $3) and 3 phreatomagmatic {P1, P2, P3) phases (Schmincke, 1977). A cross-section through the deposits, close to the change in slope between the inner and outer walls of the cone complex is exposed in a major quarry at grid reference 2 5 8 6 9 5 5 8 6 0 (Fig. 3). The Strombolian deposits consist of alternating meter-thick coarse-grained (Md ~ 3 0 mm) and fine-grained (Md < 1 5 mm) beds. The coarse-grained beds are lithic-poor, moderately sorted and contain numerous dm-sized b o m b s which have broken on impact. The fine-grained beds are less well sorted but are also lithic free. Both types o f bed contain rare outsized (m-scale) slabby blocks of degassed lava. The deposits of phreatomagmatic phases consist of alternations of lithic-rich fallo u t and ash-rich pyroclastic surge deposits.

119 D E P O S I T S O F P H A S E P2

Deposits o f the second phreatomagmatic phase (P2) vary from 6 to 12 m in thickness and show rapid lithological changes along the northern and eastern walls of the quarry {Fig. 3b). A striking feature is the presence of two discrete populations of essential clasts within the deposit: (1) ragged, highly vesicular scoria; and (2) angular to subrounded poorly vesicular to nonvesicular clasts. Despite this contrast in morphology, the two populations are petrographically identical. Four stratigraphic sections through P2 are shown in Fig. 3. Four subunits have been defined {Table 1), some rich in clast type (1) and some in (2), but each shows a progressive northward increase in the abundance o f {1) relative to (2). P2a consists o f an alternation of black, scoria lapilli- and bomb-beds identical to the underlying S1 Strombolian tephra, with cm-scale massive fine ash beds. The proportions of ash-sized and lithic material increases southwards {Fig. 3). P2b is made up o f laminated fine and coarse ash beds containing low-angle cross-lamination, dune bedforms and internal thickness variations. I

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120

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r Fig. 2. a. Sketch of eastern face of Rothenberg quarry showing alternation of phreatomagmatic (P, dotted) and Strombolian (S) phases, after Karukuzu (1982). b. Composite stratigraphy for Rothenberg cone complex. Plots of (c) percentage of tephra finer than 1 mm (F') against stratigraphic height; and (d) proportion of highly vesicular clasts (V'). V' differentiates between S beds (V' = 98--100) and P beds (V' = 5--35), and F' between the coarse-grained, well sorted S beds (F'<5) and the fine-grained poorly sorted S beds (F' = 5--20) (see Fig. 7). P2b t h i c k e n s m a r k e d l y s o u t h w a r d ( b e y o n d section 4) into a valley, f o r m e d in part b y the o u t e r slope o f the $1 scoria cone, where it contains black, p o o r l y vesiculated b o m b s which have f o r m e d thin t h e r m a l haloes in the adjacent fine ash. P2c is described in detail below. P2d resembles P2a in consisting o f alternating b r o w n , p o o r l y sorted lithic-rich ash beds and black m o d e r a t e l y well-sorted scoriaceous lapilli bands. P2c

P2c is a distinctive, m a n t l e - b e d d e d fallout lapilli h o r i z o n . P2c shows t h e m o s t striking evidence f o r a d m i x t u r e o f S t r o m b o l i a n and p h r e a t o m a g m a t i c ejecta during d e p o s i t i o n . Typical coarse-grained, matrix-free, well-sorted, lithic-poor S t r o m b o l i a n fallout beds d o m i n a t e in section 1. At section 2 P2c contains t w o diffuse b u t distinct c o n c e n t r a t i o n s o f b o t h accidental lithics and angular to r o u n d e d , p o o r l y vesicular to nonvesicular, juvenile clasts (Figs. 4 and 5). These t w o lithic c o n c e n t r a t i o n s t h i c k e n and f o r m discrete d m - t h i c k scoria-poor, ash- and lithic-rich beds b y section 3, and scattered lithics n o w o c c u r above and b e l o w these beds. The c o n t r a s t b e t w e e n sections

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Fig. 3. Stratigraphic sections through subunit P2, R o t h e n b e r g volcano. Localities for the sections are shown on (a) a sketch of the cone and (b) sketches of the n o r t h e r n and eastern quarry faces through the volcano.

2 and 3 is striking as they are only 70 m apart. Quarrying has removed subunits c and d at section 4. GRANULOMETRIC

AND COMPONENT ANALYSES

One hundred and thirty eight samples from Rothenberg (52 from P2) were sieved for grain size analyses. Sieve fractions coarser than 500/~m were split into 4 components (1) highly vesicular, ragged scoria, (2) weakly vesicular angular to rounded juvenile clasts, (3) nonvesicular juvenile and accessory lithics, and (4) crystals (Fig. 6).

Fallout deposits excluding P2 The characteristics o f $1--$3, P1 and P3 are discussed elsewhere (Houghton and Schmincke, in prep.) and are merely summarised here (Table 1, Fig. 6), as a basis for interpreting the data from P2. The combination of grain size and c o m p o n e n t data gives distinctive signatures for Strombolian (Fig. 6a)

122

and phreatomagmatic (Fig. 6b) fallout tephra. All Strombolian samples are lithic-free and have a single mode of highly vesicular scoria. The coarsegrained Strombolian deposits (Md = 30--90 mm) are better-sorted (o¢ = 0.6--1.1) than all other samples and show positive skewness (Fig. 6a) largely TABLE

1

Characteristics and inferred origin of subunits of P-2 deposits, Rothenburg volcano Subunit

Description

Bedding characteristics

1 l i t h i c - r i c h , poorly sorted ash beds and intercalated black scoria-rich lapillibeds and better sorted lithic-bearing coarse ash units

m a n t l e b e d d i n g in lapilli a n d c o a r s e a s h bands, s o m e fine ash bands s h o w l o w angle cross bedding and truncation of underlying units

Interpretation

In section 2

thickness (m) 0.5

1.8

1.5

28

black lapiUi bed consisting almost entirely of ragged highly vesicu l a r s c o r i a in n o r t h e r n sections. Passing southward a progessively increasing amount of finer dense essential and access o r y Lithic c l a s t s recorded.

Mantle- and showerbedded throughout

laminated fine-grained and poorly sorted ash beds with balhsticand flow-emplaced large bombs and blocks showing zigzag cracking. Bombs form cmscale pink thermal halos in surrounding fine ash.

low angle cross bedding, variations in bed thickness along strike, dune forms in f i n e beds. I m p a c t craters associated with some large blocks. Bedding in f i n e u n i t s e x t e n d s 'through' large clasts without mantling.

thickness (m)

mantle bedding and

thickness (m)

*pred. = predominantly

--

% stromboLian scoria *pred.53

black, lithic-p oor, framework-supported lapilLi- a n d b o m b - b e d s with minor intercalated lithic-rich ash bands containing ' f l o a t i n g ' lapilli. L a t t e r increase in abundance southward and hecome dominant by s e c t i o n 4.

4

3

?

thickness (m) 4.3

0.6

0.6

% strombolian 110

0.4

97

1.0

75

1.1

--

scoria ?

alternation of strombolian tephra fall f r o m a m o r e northern source and phreatomagmatic fallout; surge and surge cloud deposition from a more southerly source exclusively fallout deposit but produced by intimate mixture of clasts from the two sources. Northern strombolian source dominant t h r o u g h o u t b u t increase involvement of phreatomagmatic material passing southward

dominantly surge deposits from the 4.0+ southern source chann e l l e d in a v a l l e y south of section 4 w i t h a d m i x e d ballistic clasts from southern (accessory and e s s e n t i a l Lithie b l o c k s ) and northern (bombs) sources

impact s a g s a s s o c i a t e d w i t h lapilIi- a n d b o m b - 2 . 0 1 . 9 2.0 3.0 beds. Ash bands mass i v e in s e c t i o n s 1 - - 3 , but with low-angle % stromboIian scoria c r o s s b e d d i n g etc. i n southern exposures. >90 84 62 11

accumulation of strombolian tephra fall i n n o r t h e r n a n d w e s t e r n a r e a s o n l y infrequently influenced by initial phreatomagmatic eruptions from south source producing pyroclastic surge deposits mostly confined to SE quadrant and localised alrfall and surge cloud material

123 VI

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Fig. 5. Photograph of P2 deposits at locality 2. Lithic concentration within P2c is arrow. ed. Scale (lower right-hand corner) is 2 m long.

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125 TABLE 2

Characteristics o f p y r o c l a s t i c d e p o s i t s o f R o t h e n b e r g scoria c o n e . Asterisks indicate that fine grain size o f the samples p r e c l u d e d d e t e r m i n a t i o n o f data. D a t a is f r o m all u n i t s e x c e p t P2 m i x e d d e p o s i t s Deposit

Coarse s t r o m bolian fine-grained strombolian phreatomagmatic fallout phreatomagmatie surge

Median diameter (Md) m m

30--90

% finer Inman than 1 mm sorting a~

Mean specific gravity of essential clasts

Standard deviation o f specific gravity

% wallrock % highly lithics vesicular scoria

1--5

0.6--1.1

1.2--1.9

0.15--0.36

0

6--16

5--20

1.5--2.5

1.2--1.9

0.15--0.35

0

1--8

5--55

1.0--2.0

1.5--2.4

0.2--0.7

40--88

2.3--3.5

0.1--1.5

12--53

100 99--100 5--35

due to breakup of large clasts on impact. Fine-grained Strombolian deposits are often very poorly sorted (a# = 1.5--2.5) and o@ increases steadily with decreasing grain-size. The phreatomagmatic deposits are rich in accessory or wall-rock lithic clasts and nonvesicular to weakly vesicular essential clasts. Phreatomagmatic fallout samples are finer-grained (Md = 1--8 mm) but often better sorted (a¢ = 1.0--2.0) than the fine-grained Strombolian samples (Fig. 6b).

P2 samples Data from representative P2 samples are shown in Fig. 6. Most samples from P2b and P2d show the typical fine-grained, lithic-rich and unskewed pattern of P1 and P3 phreatomagmatic airfall deposits. P2a and P2c however show two types of unusual bimodality. Samples 7, 26, 28, and 30 show a subordinate lithic grain-size mode superimposed on the 'normal' Rothenberg Strombolian pattern. The two modes are separated by up to 7 ¢ units, making it extremely unlikely that the 2 clast populations are the products of a single explosive event. The remaining samples, especially samples 8--14, contain variable proportions of two populations of similar grain size, one of highly vesicular scoria and the other of lithics and weakly vesicular juvenile clasts. The abundance and grain size of the two populations vary independently, but often combine to produce a single broadened grain size peak for entire samples. The random variations within the 2 populations suggest these samples too are not produced by a single explosive mechanism. The fields of Rothenberg Strombolian, fine-grained Strombolian and phreatomagmatic fallout deposits, based on samples from S1--3, P1 and P3, show considerable overlap on the Md-a¢ diagram (Fig. 7a) of Walker and Croasdale (1972). This diagram is therefore of limited use in interpreting the P2 deposits. A more useful plot (Fig. 7) is F' (weight% of clasts finer than 1 mm) against V' (weight% of highly vesicular clasts). On this plot the 4 fields

126

of Strombolian, fine-grained Strombolian and phreatomagmatic fallout and phreatomagmatic surge deposits, as inferred from field studies, are clearly distinguished (Fig. 7). Except for samples from the P2 mixed deposit, there is an absence of samples with V' values between 25 and 95, distinguishing those samples for which disruption was principally by release and expansion of magmatic gas, from those where magma--water interaction drove the eruption. P2 samples plot in three fields on the F'-V' diagram (Fig. 7). Most P2b and P2d samples plot in the phreatomagmatic fallout field. Those samples with pronounced grain size bimodality plot along a trend between the Strombolian and surge fields. The remaining samples, mostly from P2c, plot in the gap between the Strombolian and phreatomagmatic fallout fields (Fig. 7). (a) fine grained st rombolian

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Fig. 7. Plot of proportion of highly vesicular clasts (V') against percentage of tephra finer than 1 mm (F') for all P2 samples. Crosses are samples from P2a inferred to be an admixture of Strombolian airfall and pyroclastic surge and/or surge cloud depositions. Closed circles are P2b and P2d phreatomagmatic airfall deposits. Open circles are mixed Strombolian and phreatomagmatic airfall deposits. Fields of S1 to $3 Strombolian and fine-grained Strombolian airfall and P1 and P3 phreatomagmatic airfall and pyroclastic surge deposits shown for comparison. (inset). Plots median diameter (rod) versus Inman sorting coefficient (a¢) for the P2 samples (dots). Overlapping fields for coarse- and fine-grained S1 to $3 Strombolian and P1 and P3 phreatomagmatic fallout deposits are outlined, and,demonstrate the problems in using this plot to interpret the P2 deposits.

127 DENSITY MEASUREMENTS Ten to thirty juvenile clasts, in the 16--64 mm size range, were split from each P2 sample for d.ensity determinations. Clast volume was measured by immersion in water, after coating by a measured amount of latex. Specific gravity means and ranges, for samples from P2c, are shown in Fig. 4. These data are typical of all samples from P2 and indicate that there are discrete ranges o f density for the t w o morphological classes of juvenile clasts. The ragged, highly vesicular scoria clasts show a limited total range of specific gravity values, from 0.98--1.69, with average values of 1.13--1.41. Fracture-bounded poorly- or nonvesicular grains have average specific gravities of 2.12 to 2.40 (20--29% vesicles). Individual samples from P2 contain either only scoriaceous clasts or clasts of both types. In the latter case, the density ranges of the t w o populations do not overlap and appear to vary independently. INTERPRETATION AND MODEL While alternation of dominantly Strombolian and dominantly phreatomagmatic eruptions characterises the entire history of Rothenberg volcano, and even subunit P2 alone, the complex nature of P2 cannot be explained by a simple temporal alternation of eruptive styles at a single vent. The two clast populations present in P2 (1) highly vesicular scoria, and (2) weakly vesicular clasts and juvenile and accessory lithics, although n o w admixed in variable proportions, are therefore interpreted as the products of discrete Strombolian and phreatomagmatic explosions. P2 itself is interpreted as a sequence o f complex proximal deposits produced during establishment, brief dominance and then cessation of activity from a parasitic phreatomagmatic vent, on the margin of a near continuously erupting Strombolian cone. The Strombolian vent was the northern vent responsible for the S1 and $2 phases of the eruption. Assymmetrical b o m b sags and bed-forms in the surge deposits, suggest the second, phreatomagmatic source lay south and west of section 4, b u t its exact location has been obscured by the later growth of the southern cone during $3 volcanism. The 2 vents were probably separated by no more than 500 m and perhaps by as little as 200 m along a NNE--SSW fissure system. The most clearcut evidence for admixture of ejecta from 2 sources is the rapid n o r t h - ~ o u t h change in clast morphology in P2c. However, all samples, from any part of P2, show at least minor admixture, even where one clast population, and by inference eruptive style, was overwhelmingly dominant. Evidently both vents were active throughout P2 time and at any time the proportions of clast t y p e and degree of admL~.ture were determined by the relative vigour of the t w o vents and the transport processes operating (i.e., exclusively fallout, or fallout and simultaneous pyroclastic surge). The model for the eruption is as follows: Initial P1 vent-clearing phreatomagmatic eruptions at Rothenberg were

128

succeeded by open vent $1 Strombolian volcanism (Fig. 8a). The magma rose near isothermally and was discharged rapidly. Magma:water ratios remained high and cont act with external water was limited. $1 volcanism led to a progressive degassing of magma remaining in the vent and discharge rate decreased. The partially degassed magma intruded to shallow levels under o t her portions of the cone, although a mixture of newly arrived actively vesiculating magma and relatively degassed 'long-residence time' magma was still being discharged at the central vent (Fig. 8b). Magma may even have begun to retreat from the central vent when the phreatomagmatic volcanism c o m m e n c e d from the southern vent (Fig. 8c).

(a)

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northern vent

(c)

(b)

southern vent

(d)

Fig. 8. S c h e m a t i c e r u p t i o n sequence, a. E r u p t i o n o f $1 f r o m t h e n o r t h e r n S t r o m b o l i a n

vent. b. Ponding of lava in the northern vent and magma intrusion beneath the site of the southern vent. c. Simultaneous eruptions from the northern and southern vents, d. Resumption of purely Strombolian eruption from the northern vent depositing $2. The level o f activity from the two vents varied independently, with activity from the northern Strombolian vent occasionally reduced to infrequent weak explosions ejecting isolated bombs. Phreatomagmatic volcanism dom i na t e d t h r o u g h o u t deposition o f P2b and most o f P2d, and Strombolian eruption for P2a and P2c. Strombolian eruption from the nor t he r n vent was almost continuous during P2a deposition. Phreatomagmatic explosions were limited in size and infrequent. When these occurred during breaks in Strombolian activity discrete ash-rich beds are intercalated in the Strombolian sequence, and when Strombolian activity was continuous the phreatomagmatic material form ed a sparse matrix in the interstices of framework-supported Strombolian b o m b and lapilli beds (Fig. 6).

129

P2b is interpreted as reflecting alternating surge and phreatomagmatic fallo u t deposits during relative quiescence of the northern, Strombolian vent. Strombolian influence is limited to isolated b o m b s which c o m m o n l y produced pink mm-thick thermal haloes in the underlying fine brown ash. P2c is the product of admixture of fallout ejecta from the two vents. Strombolian volcanism predominated, with less voluminous phreatomagmatic explosions from the southern source. Phreatomagmatic activity was more important during deposition of the overlying P2d before ceasing completely (Fig. 8d), prior to the eruption of $2. $2 was then erupted from a source coincident with, or very close to, the northern or S1 vent. It is significant that, while both the $1 and $3 Strombolian sequences commence with fine-grained blocky beds containing poorly vesicular juvenile clasts, inferred to be the products of vent-clearing prior to establishment of true 'open-vent' conditions, the first $2 deposits are typical coarse-grained, framework-supported b o m b beds, suggesting that open vent conditions prevailed at the northern vent throughout P2 into $2 time. DISCUSSION AND CONCLUSIONS The P2 deposits are a graphic illustration of the complex and sensitive controls of explosive basaltic volcanism and the very shallow depth of fragmentation during such eruptions. While magma was actively vesiculating and being discharged from the central northern vent at Rothenberg, the role of external water was minor, and confined to occasionally increasing the degree of fragmentation and explosivity during essentially 'dry' eruptions. Phreatomagmatic volcanism was only possibly from a vent on the margin of the cone where (1) vesiculation was not fragmentating the rising magma, and (2) there was access of external water in a shallow aquifer. This contrast in behaviour at two vents which are clearly connected at shallow depth (Fig. 8) also emphasizes the very shallow depth of fragmentation in some explosive basaltic eruptions. The entire history of Rothenberg cone complex is an alternation of Strombolian and phreatomagmatic activity; P2 is a special case of interaction between Strombolian and phreatomagmatic vents during a single eruptive phase. The changing nature of the deposits reflects the relative vigour of activity at these vents. This in turn appears to have been controlled principally by: (1) the rate of magma rise; (2) the rate of discharge; (3) the vent geometry, particularly the extent to which they would permit lava ponding and/or access of external water; and (4) the condition o f the magma, whether prior to degassing, actively vesiculated, or degassed. ACKNOWLEDGEMENTS This research was completed while one of the authors held an Alexander yon H u m b o l d t research fellowship. Drafts of the manuscript were generously

130 r e v i e w e d b y B.P. K o k e l a a r , I.A. N a i r n , G . P . L . W a l k e r , C . J . N . W i l s o n , K . H . Wohletz, J.A. Wolff, a n d J.V. Wright. The c o m p l e t e d m a n u s c r i p t received generous reviews f r o m two a n o n y m o u s reviewers.

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