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Studies on the age of honeycombs and tafoni features
CATENA
Vol. 7, 317-325
Braunschweig 1980
STUDIES ON THE AGE OF HONEYCOMBS AND TAFONI FEATURES
D. Kelletat, Hannover SUMMARY In this short paper some examples oftafoni and similar features of cavernous weathering are described from Western Scotland and Southern Greece, including Crete and the Santorini group. All these forms belong to the younger Holocene. This age is indicated by young tectonic emergence from the sea, historical data, or the well known volcanic history ofSantorini. PreHolocene tafoni are neither more widespread nor better developed than those of the last 2000 or 3000 years. Very different rocks are weathered into tafoni features: old and young sandstones and calcareous sandstones, pliocene conglomerates, eolianites, or lavas and holocene tephra. The tafoni are nearly in all cases strongly oriented towards the present sea front, and therefore salt weathering should be the most effective process. ZUSAMMENFASSUNG Es werden Tafoni und Wabenbildungen yon Westschottland und Griechenland (einschl. Kreta und Santorin) vorgestellt. Die Beispiele zeigen, dab for die Ausbildung sehr kr/iftiger Waben- und Tafonierscheinungen ein Zeitraum yon wenigen Jahrtausenden, fallweise auch etlichen Jahrhunderten gentigt. Die GelS.ndesituation-j ung aus dem Meer ausgetauchte Partien oder erst in historischer Zeit geschaffene vulkanische Oberfl/ichen - schliegtj edes h6here Alter mit Sicherheit aus. Ebenfalls vorhandene Tafoni, for die aufgrund ihrer Lage auch ein pleistoz/ines Alter in Frage k/ime, sind weder nach der Verbreitung noch nach der Intensit/it der Ausbildung entwickelter als diejungholoz/inen Erscheinungen. Die von tier Tafonierung betroffenen Gesteine sind sehr vielf~iltig: Pal/iozoische und mesozoische Sandsteine und Kalksandsteine, neogene Konglomerate, quart/ire marine Sedimente, wtirmzeitliche Glacisfanglomerate oder Aolianite, verschiedene jungpleistoz/ine und holoz/ine Vulkanite wie Laven, Schlacken, Aschen, Tuffe und Bimse. Im Mittelmeergebiet ist eine Zunahme von H/iufigkeit und Qualit/it der Formausbildung bei denjungen Tafoni in Richtung zu st/irker ariden Verh/iltnissen durchaus zu erkennen. Insbesondere werden dabei immer mehr sonst weniger geeignete Gesteine davon erfagt. Nahezu alle Vorkommen weisen zudem eine starke Orientierung zur Ktiste hin auf, und zwar sowohl nach der H6henlage als auch nach der Exposition. Das ist an vielen Einzelfelsen zu studieren, deren dem Meer abgewandte Flanken keine entspechenden Verwitterungsspuren zeigen. Hartrindentafoni kommen vor, doch diirfte der wesentliche Prozeg der der Salzsprengung bzw. der Hydratationsverwitterung sein.
1.
INTRODUCTION
For more than 100 years tafoni and honeycomb features have been reported as phenomena of macro- and/or microclimatic aridity. Case hardening and especially salt weathering are considered to be the most effective causes of cavernous weathering (see FRENZEL 1965, WILHELMY 1972, KLAER 1973, HOLLERMANN 1975). The youthful-
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ness of these weathering features is demonstrated by the honeycombs found on older sandstone buildings. The strong increase in these phenomena in recent times seems to point to a relationship with increased air pollution. About the age and time of formation of the large tafoni found at places not affected by pollution, however, only sporadic information exists. This is demonstrated for example by HOLLERMANN's (1975) research on Tenerife (Canary Islands). He pointed out that although active and recent features occur, the origin of most cavernous surfaces dates back to early Quaternary times. As a contribution to the problem of the age and rate of formation oftafoni and honeycomb features, this paper is restricted to Holocene phenomena.
2.
THE OBSERVATIONS
2.1. HONEYCOMBS ON HOLOCENE ABRAISONAL PLATFORMS AND CLIFFS IN WESTERN SCOTLAND Fine examples of very young and intensive honeycomb weathering are even found in cool-temperate and humid latitudes. For example large portions of cavernous features of honeycomb type are found on rocky surfaces in paleozoic sandstones ("Old Red") on the east coast of the Isle of Arran (Western Scotland), only a little above high-water level (Photo 1). Precipitation here is more than 1000 mm/year. The honeycombs decorate part of an abrasional platform which has emerged from the sea due to isostatic uplift during the Holocene. This platform is still reached by heavy storm waves. For this reason the honeycombs show signs of polishing. The formation of honeycombs, however, must be more rapid than the rate of abrasion by waves. Considering the position and the rate of the isostatic uplift of the Isle of Arran, these honeycombs must date back less than 1000 years. The honeycombs at a cliff near Elgol (SW-Isle of Skye, Western Scotland), built in cretaceous calcareous sandstones, should have nearly the same age. Here, the precipitation, at the foot of the more than 1000 m a.s.l, high Cuillin Hills, is over 1200 ram/year. As seen in Photo 2, the honeycombs very nearly span a huge notch in the cliff. Because of the continuing isostatic uplift of this island, only the lowest parts of the notch are still polished by beachpebbles, and there the honeycombs have been partly destroyed. Only slightly higher the honeycombs are filled with sand, mixed with fine salt cristalls, and they are very sharply developed (Photo 3). The overhang of the notch gives shelter from direct precipitation ( = microclimatic aridity), while saltwater spray can easily reach the hollow from the sea.
2.2.
HONEYCOMBS AND TAFONI IN COASTAL AREAS OF GREECE
In the warm and partly arid coastal regions of southern and south-eastern Greece honeycomb and tafoni weathering forms have a wide distribution. The precipitation per year ranges from 600 m m down to 200 mm. As the precipitations becomes less and the number of arid months increases (up to 8 in eastern Crete), so the tafoni-phenomena become more distinct. Tafoni-galleries with large cavities border the cliffcoasts of eastern Crete, which are built up of pliocene conglomerates and fanglomerates of the younger Pleistocene. The sea-facing slopes, up to 200 m a.s.l, also show these features (Photo 4). On the higher slopes as well as on active cliffs, the dimensions and the intensity oftafoni-patterns are very similar. The forms are
Photo 1: Honeycomb weathering on the upper part of an abrasive platform of Old Red Sandstone on the east coast of the Isle of Arran (Western Scotland).
Photo 2: Honeycombs in cretaceous calcareous sandstone at an holocene notch near Elgol, Isle of Skye, Western Scotland.
Photo 3:
Section of Photo 2: Part of the notch near Elgol, no reached by surf.
Photo 4:
Tafoni galleries in pliocene conglomerates on the steep coast W of Ierapetra, Crete.
Photo 5: Honeycombs competing with sand-polishing in tertiary glauconitic sandstoneat a place which emerged from the sea less than 2000 years BP. West of Palgochora, Crete.
Photo 6: Tafoni, adjoining fissures and layers of consolidated ashes of the Kokkino Vouno, Santorini.
Photo 7: Tafoni with canopies in leaked cinders at the caldera wall near Athinios, Santorini.
Photo 8: lapilli.
Structure of the cliffat Cape Mavro near Akrotiri, Santorini. Changing layers of cinders and
Photo 9: Tafoni in phonolithic rocks at the caldera wall near Manolas, Isle ofTherasia, Santorini.
Photo 10: Tafoni-like features in pumice of the Santorini-volcano from 1450 BC at the cliff near Akrotiri, Santorini.
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KELLETAT
also found in flysch and cristalline schists. Along the steep coasts of Peleponnesos and Crete, which are built up ofclastic sediments, another observation can be made: the single fragments of rocks are weathered back between the calcareous-hardened cement, mostly to an eggshell-like rest of them. Limestones, sandstones, flysch, cristalline schist, and ophiolites are amongst others all affected by this process. This phenomenon, however, is restricted to areas of severe seawater spraying, and ends about 10 m above sea level or 50 m inland. The occurence of cavernous features on active cliffs gives a time range for their maximum age, which is about equal to that of the younger Holocene. This time span can be shortened in some places, since honeycombs (Photo 5) and huge tafoni, for example in Western Crete (see KELLETAT 1979, p. 64, photo 22), are found at places which tectonically emerged from the sea less than 2000 years ago. Within the Mediterranean area it is also possible to date tafoni by archeological and historical evidence. As an example the large cavities on the walls of a quarry dating from roman times near Phalasarna in Western Crete may be given (KELLETAT 1979, p. 56, photo 18). However, the material in this quarry - an eolianite from the Wtirm-phase - like most uniformgrained rocks, must be regarded as being very succeptable to this type of weathering.
2.3.
TAFON1 IN VOLCANIC ROCKS OF THE SANTORINI-GROUP
The Santorini-archipelago in the southern Aegean Sea can be regarded as an excellent natural testing site to determine the age and rate oftafoni formation. Here tafoni features can be seen in all kinds of volcanic rocks from hard lavas to allmost unconsolidated pumice. Their maximum age is given by the great eruption of the Santorini volcanoe in the 15. century BC, which could be dated radiometrically and archeologically. By this eruption, all surfaces of the Santorin islands were covered with pumice, ignimbrites or lahars, and immediately after the eruption the large caldera was formed. The maximum age of all Santorini surfaces, of about 3500 years, may well be significantly younger in some places, especially on the steep coasts or valley margins. A number of photographs are described below which provide information about specific features of the weathering and the rock types involved. The steep walls of the Kokkino Vouno northeast of Phira, which are formed by postvolcanic faults in weak consolidated cinders and tuffes, show a complete pattern of galleries and pits (Photo 6). As has already been pointed out by HIJLLERMANN (1975), concerning the tafoni of the volcanic rocks of Tenerife, there is a marked adaption to the strata and joints. The hollowing near the surface of slopes - as well as that at the huge boulders - shows the phenomenon of basis-tafoni. In the more consolidated pyroclastics, nearly all steep slopes on Santorini contain tafoni which are dapted to the strata, but which have an irregular shape within more compact layers. There are especially canopies connected with case-hardening such as in the leaked cinders at the calderawall near Athinios (Photo 7), or on the perpendicular 50 m high cliffs on the east coast of Thira near Akrotiri (Photo 8). The hardest rock, having tafonies on the Santorini-archipelago are lavas of different chemical composition, but mostly acid. Photo 9 from the"Scala" below the village of Manolas on Therasia Island shows them in fine-grained phonolites. The situation at the caldera wall excludes an age of more than 3500 years. Finally tafoni and tafoni-like features of various forms have developed in loose material which has obtained a certain stability by drying out near the surface. Photo 10 shows examples
HONEYCOMBS AND TAFONI FEATURES
325
from gullied "upper pumice" near Akrotiri on the east coast of Thira. This pumice was ejected by the minoan eruption, it then out washed and was later gullied from the active cliff side. The canopies and cavities in this material can only be very short-lived features.
REFERENCES FRENZEL, G. (1965): Studien an mediterranen Tafoni. Neues Jahrbuch for Geologie und Pal~iontologie, Abhandlungen 122, 3, 313-323. HOLLERMANN, P. (1975): Formen kavern6ser Verwitterung ("Tafoni") auf Teneriffa. CATENA, Vol. 2, 385-410. KELLETAT, D. (1979): Geomorphologische Studien an den Ktisten Kretas. Abhandlungen der Akademie der Wissenschaften in G6ttingen, Mathematisch-physikalische Klasse, 3. Folge, Nr. 32. KLAER, W. (1973): Untersuchungen zur klimagenetischen Geomorphologie im Granit auf Korsika. Erdkundliches Wissen, Beiheft Geographische Zeitschrift 33 (Plewe-Festschrift), 247-260. WILHELMY, H. (1958): Klimamorphologie der Massengesteine. Braunschweig. WILHELMY, H. (1972): Geomorphologie in Stichworten. II. Exogene Morphodynamik: Verwitterung, Tal- und F15.chenbildung. Hirts Stichwortbiicher. Anschrifl des Autors: Prof. Dr. Dieter Kelletat, Geographisches Institut der Universi~t Hannover, Schneiderberg 50, D-3000 Hannover 1
Vol. 7, 317-325
Braunschweig 1980
STUDIES ON THE AGE OF HONEYCOMBS AND TAFONI FEATURES
D. Kelletat, Hannover SUMMARY In this short paper some examples oftafoni and similar features of cavernous weathering are described from Western Scotland and Southern Greece, including Crete and the Santorini group. All these forms belong to the younger Holocene. This age is indicated by young tectonic emergence from the sea, historical data, or the well known volcanic history ofSantorini. PreHolocene tafoni are neither more widespread nor better developed than those of the last 2000 or 3000 years. Very different rocks are weathered into tafoni features: old and young sandstones and calcareous sandstones, pliocene conglomerates, eolianites, or lavas and holocene tephra. The tafoni are nearly in all cases strongly oriented towards the present sea front, and therefore salt weathering should be the most effective process. ZUSAMMENFASSUNG Es werden Tafoni und Wabenbildungen yon Westschottland und Griechenland (einschl. Kreta und Santorin) vorgestellt. Die Beispiele zeigen, dab for die Ausbildung sehr kr/iftiger Waben- und Tafonierscheinungen ein Zeitraum yon wenigen Jahrtausenden, fallweise auch etlichen Jahrhunderten gentigt. Die GelS.ndesituation-j ung aus dem Meer ausgetauchte Partien oder erst in historischer Zeit geschaffene vulkanische Oberfl/ichen - schliegtj edes h6here Alter mit Sicherheit aus. Ebenfalls vorhandene Tafoni, for die aufgrund ihrer Lage auch ein pleistoz/ines Alter in Frage k/ime, sind weder nach der Verbreitung noch nach der Intensit/it der Ausbildung entwickelter als diejungholoz/inen Erscheinungen. Die von tier Tafonierung betroffenen Gesteine sind sehr vielf~iltig: Pal/iozoische und mesozoische Sandsteine und Kalksandsteine, neogene Konglomerate, quart/ire marine Sedimente, wtirmzeitliche Glacisfanglomerate oder Aolianite, verschiedene jungpleistoz/ine und holoz/ine Vulkanite wie Laven, Schlacken, Aschen, Tuffe und Bimse. Im Mittelmeergebiet ist eine Zunahme von H/iufigkeit und Qualit/it der Formausbildung bei denjungen Tafoni in Richtung zu st/irker ariden Verh/iltnissen durchaus zu erkennen. Insbesondere werden dabei immer mehr sonst weniger geeignete Gesteine davon erfagt. Nahezu alle Vorkommen weisen zudem eine starke Orientierung zur Ktiste hin auf, und zwar sowohl nach der H6henlage als auch nach der Exposition. Das ist an vielen Einzelfelsen zu studieren, deren dem Meer abgewandte Flanken keine entspechenden Verwitterungsspuren zeigen. Hartrindentafoni kommen vor, doch diirfte der wesentliche Prozeg der der Salzsprengung bzw. der Hydratationsverwitterung sein.
1.
INTRODUCTION
For more than 100 years tafoni and honeycomb features have been reported as phenomena of macro- and/or microclimatic aridity. Case hardening and especially salt weathering are considered to be the most effective causes of cavernous weathering (see FRENZEL 1965, WILHELMY 1972, KLAER 1973, HOLLERMANN 1975). The youthful-
318
KELLETAT
ness of these weathering features is demonstrated by the honeycombs found on older sandstone buildings. The strong increase in these phenomena in recent times seems to point to a relationship with increased air pollution. About the age and time of formation of the large tafoni found at places not affected by pollution, however, only sporadic information exists. This is demonstrated for example by HOLLERMANN's (1975) research on Tenerife (Canary Islands). He pointed out that although active and recent features occur, the origin of most cavernous surfaces dates back to early Quaternary times. As a contribution to the problem of the age and rate of formation oftafoni and honeycomb features, this paper is restricted to Holocene phenomena.
2.
THE OBSERVATIONS
2.1. HONEYCOMBS ON HOLOCENE ABRAISONAL PLATFORMS AND CLIFFS IN WESTERN SCOTLAND Fine examples of very young and intensive honeycomb weathering are even found in cool-temperate and humid latitudes. For example large portions of cavernous features of honeycomb type are found on rocky surfaces in paleozoic sandstones ("Old Red") on the east coast of the Isle of Arran (Western Scotland), only a little above high-water level (Photo 1). Precipitation here is more than 1000 mm/year. The honeycombs decorate part of an abrasional platform which has emerged from the sea due to isostatic uplift during the Holocene. This platform is still reached by heavy storm waves. For this reason the honeycombs show signs of polishing. The formation of honeycombs, however, must be more rapid than the rate of abrasion by waves. Considering the position and the rate of the isostatic uplift of the Isle of Arran, these honeycombs must date back less than 1000 years. The honeycombs at a cliff near Elgol (SW-Isle of Skye, Western Scotland), built in cretaceous calcareous sandstones, should have nearly the same age. Here, the precipitation, at the foot of the more than 1000 m a.s.l, high Cuillin Hills, is over 1200 ram/year. As seen in Photo 2, the honeycombs very nearly span a huge notch in the cliff. Because of the continuing isostatic uplift of this island, only the lowest parts of the notch are still polished by beachpebbles, and there the honeycombs have been partly destroyed. Only slightly higher the honeycombs are filled with sand, mixed with fine salt cristalls, and they are very sharply developed (Photo 3). The overhang of the notch gives shelter from direct precipitation ( = microclimatic aridity), while saltwater spray can easily reach the hollow from the sea.
2.2.
HONEYCOMBS AND TAFONI IN COASTAL AREAS OF GREECE
In the warm and partly arid coastal regions of southern and south-eastern Greece honeycomb and tafoni weathering forms have a wide distribution. The precipitation per year ranges from 600 m m down to 200 mm. As the precipitations becomes less and the number of arid months increases (up to 8 in eastern Crete), so the tafoni-phenomena become more distinct. Tafoni-galleries with large cavities border the cliffcoasts of eastern Crete, which are built up of pliocene conglomerates and fanglomerates of the younger Pleistocene. The sea-facing slopes, up to 200 m a.s.l, also show these features (Photo 4). On the higher slopes as well as on active cliffs, the dimensions and the intensity oftafoni-patterns are very similar. The forms are
Photo 1: Honeycomb weathering on the upper part of an abrasive platform of Old Red Sandstone on the east coast of the Isle of Arran (Western Scotland).
Photo 2: Honeycombs in cretaceous calcareous sandstone at an holocene notch near Elgol, Isle of Skye, Western Scotland.
Photo 3:
Section of Photo 2: Part of the notch near Elgol, no reached by surf.
Photo 4:
Tafoni galleries in pliocene conglomerates on the steep coast W of Ierapetra, Crete.
Photo 5: Honeycombs competing with sand-polishing in tertiary glauconitic sandstoneat a place which emerged from the sea less than 2000 years BP. West of Palgochora, Crete.
Photo 6: Tafoni, adjoining fissures and layers of consolidated ashes of the Kokkino Vouno, Santorini.
Photo 7: Tafoni with canopies in leaked cinders at the caldera wall near Athinios, Santorini.
Photo 8: lapilli.
Structure of the cliffat Cape Mavro near Akrotiri, Santorini. Changing layers of cinders and
Photo 9: Tafoni in phonolithic rocks at the caldera wall near Manolas, Isle ofTherasia, Santorini.
Photo 10: Tafoni-like features in pumice of the Santorini-volcano from 1450 BC at the cliff near Akrotiri, Santorini.
324
KELLETAT
also found in flysch and cristalline schists. Along the steep coasts of Peleponnesos and Crete, which are built up ofclastic sediments, another observation can be made: the single fragments of rocks are weathered back between the calcareous-hardened cement, mostly to an eggshell-like rest of them. Limestones, sandstones, flysch, cristalline schist, and ophiolites are amongst others all affected by this process. This phenomenon, however, is restricted to areas of severe seawater spraying, and ends about 10 m above sea level or 50 m inland. The occurence of cavernous features on active cliffs gives a time range for their maximum age, which is about equal to that of the younger Holocene. This time span can be shortened in some places, since honeycombs (Photo 5) and huge tafoni, for example in Western Crete (see KELLETAT 1979, p. 64, photo 22), are found at places which tectonically emerged from the sea less than 2000 years ago. Within the Mediterranean area it is also possible to date tafoni by archeological and historical evidence. As an example the large cavities on the walls of a quarry dating from roman times near Phalasarna in Western Crete may be given (KELLETAT 1979, p. 56, photo 18). However, the material in this quarry - an eolianite from the Wtirm-phase - like most uniformgrained rocks, must be regarded as being very succeptable to this type of weathering.
2.3.
TAFON1 IN VOLCANIC ROCKS OF THE SANTORINI-GROUP
The Santorini-archipelago in the southern Aegean Sea can be regarded as an excellent natural testing site to determine the age and rate oftafoni formation. Here tafoni features can be seen in all kinds of volcanic rocks from hard lavas to allmost unconsolidated pumice. Their maximum age is given by the great eruption of the Santorini volcanoe in the 15. century BC, which could be dated radiometrically and archeologically. By this eruption, all surfaces of the Santorin islands were covered with pumice, ignimbrites or lahars, and immediately after the eruption the large caldera was formed. The maximum age of all Santorini surfaces, of about 3500 years, may well be significantly younger in some places, especially on the steep coasts or valley margins. A number of photographs are described below which provide information about specific features of the weathering and the rock types involved. The steep walls of the Kokkino Vouno northeast of Phira, which are formed by postvolcanic faults in weak consolidated cinders and tuffes, show a complete pattern of galleries and pits (Photo 6). As has already been pointed out by HIJLLERMANN (1975), concerning the tafoni of the volcanic rocks of Tenerife, there is a marked adaption to the strata and joints. The hollowing near the surface of slopes - as well as that at the huge boulders - shows the phenomenon of basis-tafoni. In the more consolidated pyroclastics, nearly all steep slopes on Santorini contain tafoni which are dapted to the strata, but which have an irregular shape within more compact layers. There are especially canopies connected with case-hardening such as in the leaked cinders at the calderawall near Athinios (Photo 7), or on the perpendicular 50 m high cliffs on the east coast of Thira near Akrotiri (Photo 8). The hardest rock, having tafonies on the Santorini-archipelago are lavas of different chemical composition, but mostly acid. Photo 9 from the"Scala" below the village of Manolas on Therasia Island shows them in fine-grained phonolites. The situation at the caldera wall excludes an age of more than 3500 years. Finally tafoni and tafoni-like features of various forms have developed in loose material which has obtained a certain stability by drying out near the surface. Photo 10 shows examples
HONEYCOMBS AND TAFONI FEATURES
325
from gullied "upper pumice" near Akrotiri on the east coast of Thira. This pumice was ejected by the minoan eruption, it then out washed and was later gullied from the active cliff side. The canopies and cavities in this material can only be very short-lived features.
REFERENCES FRENZEL, G. (1965): Studien an mediterranen Tafoni. Neues Jahrbuch for Geologie und Pal~iontologie, Abhandlungen 122, 3, 313-323. HOLLERMANN, P. (1975): Formen kavern6ser Verwitterung ("Tafoni") auf Teneriffa. CATENA, Vol. 2, 385-410. KELLETAT, D. (1979): Geomorphologische Studien an den Ktisten Kretas. Abhandlungen der Akademie der Wissenschaften in G6ttingen, Mathematisch-physikalische Klasse, 3. Folge, Nr. 32. KLAER, W. (1973): Untersuchungen zur klimagenetischen Geomorphologie im Granit auf Korsika. Erdkundliches Wissen, Beiheft Geographische Zeitschrift 33 (Plewe-Festschrift), 247-260. WILHELMY, H. (1958): Klimamorphologie der Massengesteine. Braunschweig. WILHELMY, H. (1972): Geomorphologie in Stichworten. II. Exogene Morphodynamik: Verwitterung, Tal- und F15.chenbildung. Hirts Stichwortbiicher. Anschrifl des Autors: Prof. Dr. Dieter Kelletat, Geographisches Institut der Universi~t Hannover, Schneiderberg 50, D-3000 Hannover 1