CALDERAS ON VENUS AND EARTH (I). PLANET EARTH: OVERVIEW OF CALDERAS. A. S. Krassil-nikov1, 2 and J. W. Head2, 1Vernadsky Institute, 119991, Moscow, Russia, kras@geokhi.ru, 2Department of Geological Science, Brown University, Providence, RI, 02912, USA.

Introduction. Calderas are defined as large, more or less

circular, volcanic collapse depressions with diameters consid-erably larger than any included vent [1,2] resulting from col-lapse into partially drained near-surface magma chamber [2]. They have been defined as having diameters >1 km [3], with smaller depressions being called craters. Usage of term caldera, includes features described as “cauldrons”, which represent a variably deeper erosional level of the same fundamental struc-tures [4, 5]. Our goal here is to overview types of calderas on Earth, mechanisms and models of their formation for following comparison with venusian calderas [6]. Methods. We summa-rized aspects of calderas formation using models of caldera formation [5,7], maps and description of typical calderas [8-10], and overviews of calderas formation and classification [2,5,7].

Caldera geometry, structural elements. Structural and morphologic elements of a simplified caldera model include [5,7] (Fig. 1-3): topographic rim, inner topographic wall, bound-ing faults (if present), structural caldera floor, intracaldera fill (ash-flow tuff, lava flows, and landslide debris from caldera walls), and the underlying magma chamber or solidified pluton.

Classification of calderas. There are few classifications of calderas (genetical, chemical, tectonic). We took one of them by [2] with some modification and simplification according to Vic Camp volcanology website [11]. Three morphological classes are important on the Earth: basaltic shield volcano calderas, Crater-lake type of calderas, and ash flow calderas.

Basaltic shield volcano calderas. The summit regions of many active shield volcanoes are marked by calderas (Fig. 1). It is generally believed that shield caldera formation is due princi-pally to drainage of magma rather than explosive removal of it from a magma chamber. Instead, they subside in increments to produce a nested structure of pits and terraces. Basaltic calderas like these are gradually enlarged by episodic collapse, due to the extraction of lava from shallow-level magma chambers underly-ing the summit areas. Shield calderas form in basaltic volcanoes, with tholeiites being typical rock types for both large and small shields, sometimes in aluminious basalts and basaltic andesites [2,12].

Fig. 1. Perspective view of Mokuaweoweo caldera, Maona Loa volcano, Havaii, view toward NNE, “Landsat-7” image is overlapped on topo data, vertical exaggeration x2. Diameter of caldera is ~5.5x3 km. Crater-lake type of calderas (Fig. 2) is generated after the main phase of a Plinian eruption, during collapse of a stratovolcano (Mt. Mazama in Crater-lake caldera) into the void of the under-lying, depleted magma chamber. Although the waning phase of a Plinian eruption is often associated with the generation of pyroclastic flows, piston-like collapse of the volcanic edifice can generate the additional eruption of voluminous (0.1-100 km3), pumice-dominated sheet flows along ring fractures sur-rounding the collapsing mass. These sheet flows form thick deposits of ignimbrite, the hallmark of both Crater-lake type and ash flow calderas. Caldera formation has occurred two or more times in many stratovolcanoes; many stratovolcanoes have a caldera as their final evolutionary stage. Stratocone calderas usually form in volcanoes made of basaltic andesite or andesite, with occasional basaltic, trachytic, and phonolitic structures [13]. Ejecta associated with stratocone caldera formation is typi-cally dacitic to rhyolitic.

Fig. 2. Geologic map of Crater-lake caldera (Oregon) overlain on shaded relief map. On base of [8,9] with correction.

Ash flow calderas result from collapse following the eruption of extremely large (100-1000 km3) volumes of dacitic to rhyolitic ash flows (Fig. 3). These calderas are the largest, and most were not formed on existing massive volcanoes. Ash flow calderas also dominantly erupt dacite to rhyolitic ignimbrite or alcalic rocks. Many calderas larger than about 20 km diameter have

resurgent centers, apparently updomed during or by the refilling of the underlying magma chamber [2,14] (Fig. 3). With diame-ters ranging from 15 to 100 km, resurgent calderas dwarf those of the Crater-Lake type. They are similar to Crater-Lake type calderas in that they are also generated by crustal collapse above shallow magma chambers. Resurgent calderas, however, are too

Microsymposium 38, MS051, 2003(A4 format)

CALDERAS ON VENUS AND EARTH (I): A. S. Krassilnikov and J. W. Head

large to have been associated with a Crater-Lake type central volcano. Apart from their large size, the definitive feature of resurgent calderas is a broad topographic depression with a cen-tral elevated mass resulting from post-collapse upheaval of the

caldera floor. The caldera floor is typically filled with rhyolitic lavas, obsidian flows, and domes; the uplifted centers often contain elongate rifts (graben) along their crests (Fig. 3).

Fig. 3. Geological map of Yellowstone caldera (Idaho, Wyoming, Montana) by [10] overlain on shaded relief map. Resurgent domes are located in NE and SW parts of caldera.

Fig. 4. Models of subsi-dence geometry in rela-tion to depth and roof geometry of underlying magma chamber. From [5] with modification.

Caldera diameters [2,13,15]. Shield cal-deras comprise about 65 % of calderas with diameters D<2 km, but frequency declines fairly rapidly to 0 % at D=20 km. Stratovol-cano calderas are the dominant class of terrestrial caldera in the range D=20 km. Ash flow calderas first occur at D=5 km, rise to dominance at D=13 km, and account for all tabulated terrestrial calderas with D>20 km.

Caldera subsidence processes. Many calderas have such varied transitional geometries and structures that subclassifica-tion into discrete types seems less useful than relating subsi-dence geometry and resulting structures to a few geometrically simplified end-members [5,7] (Fig. 4). Small calderas (<3-5 km diameter) commonly have funnel geometry (e) because of dominant enlargement by slumping into an areally restricted vent. Many larger calderas dominantly involve plate (piston) collapse (a) of a coherent floor, bounded by steeply dipping ring faults; they are inferred to reflect voluminous eruptions from large shallow magma chambers. Trap-door subsidence (c), bounded by an incomplete ring fault and by a hinged segment, reflect early downsagging and incipient plate collapse related to

smaller eruptions, an asymmetrical magma chamber, or regional tectonic influences. Deep magma chambers or small eruptive volumes may favor downsag subsidence (d) without large bounding faults, although the appearance of downsag can be generated by younger volcanic units draped over a pre-existing caldera or structural basin. Pervasively brecciated chaotic dis-ruption (b) of subsiding caldera floor is uncommon, at least at calderas more than a few kilometers across. Interpreting caldera structures in terms of a continuum of subsidence styles, rather than as end-member types, can clarify relations between erup-tive and structural processes in comparison with size of the eruption and geometry of the cogenetic magma chamber. The size of magma chambers and volume of ash-flow eruptions strongly influence both caldera-subsidence processes and the lithologies of exposed intracaldera volcanic fill.

Evolution. Five principal stages can be recognized [4]: 1) precursor; 2) caldera collapse; 3) early post-collapse volcanism; 4) major ring-fracture volcanism; and 5) hydrothermal activity. Development can be terminated at any stage; stages can also be repeated. Resurgent doming can occur during stage 3 or later; doming can also be enhanced by reactivation of cauldron struc-tures by postvolcanic basin and range faulting.

Acknowledgements. The authors thank A.T. Basilevsky for help

and RFBR grant # 02-05-65068 for support. References. [1] Williams H (1941) Univ. Calif. Berkley Pub. Geol.

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