IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3W_3C-P18

Date/Time: July 23 Poster

Vent migration and caldera collapse during the Minoan eruption of Santorini
Timothy H Druitt
Lab Magmas et Volcans, Blaise Pascal University, Clermont-Ferrand, France
E-mail: T.Druitt@opgc.univ-bpclermont.fr

The late 17th century BC Minoan eruption of Santorini discharged 30-60 km3 of rhyodacitic magma, andminor andesitic magma, and collapse deepened and widened a partially flooded 21 ka caldera alreadypresent in the northern half of the volcanic field. The resulting present-day caldera has distinct northern andsouthern basins, separated by a NE-SW line of tectonic weakness (Kameni Line, KL). Juvenile and accidental
component populations from the deposits exhibit systematic variations between the four eruptive phases, allowing
reconstruction of vent migration during eruption.

The plinian phase (phase 1) discharged rhyodacite and asuite of compositionally distinctive Ba-rich andesites from a subaerial vent situated on the KL. The vent thenmigrated northwards into the ancient caldera, causing eruption of syn-plinian base surges (phase 2) then cohesive,
low-temperature pyroclastic flows (phase 3). The andesitic magma ceased to be erupted as the vent migrated
away from the KL. However, the phase 3 tuffs contain abundant black, glassy andesitic lithics that are chemicallyvery similar to the juvenile Ba-rich andesite, and which are pieces of a partly subaqueous lava complex present in
the 21 ka caldera prior to the Minoan eruption. Blocks of lava compositionally identical to Minoan rhyodacite werealso discharged in abundance during phase 3. The lithic assemblage is uniform throughout phase 3, suggestinga single vent situated within the 21 ka caldera, consistent with Pfeiffers (2001) ballistic data. Phase 4 discharged
hot pyroclastic flows that poured into the sea forming three ignimbrite fans (NW, E and S). The first flows (phase4a) were erupted from one or more vents in the north and laid down lithic-block-rich ignimbrites and lag breccias
containing abundant clasts of Tertiary basement. These constitute much of the NW ignimbrite fan, and containgas escape pipes suggesting that some of the lithic debris was wet when incorporated into the flows. Subsequentflows (phase 4b) were much poorer in block-sized lithics and laid down the hot, fine-grained, tan-coloured ignimbrite
typical of phase 4. Package 4b, which accounts for most of the onland ignimbrite, contains only trace amounts ofbasement clasts, makes up the S and E ignimbrite fans, and overlies package 4a in the NW. The greater thicknessof the S and E fans compared to the NW fan suggests that, unlike phase 4a, the phase 4b flows were erupted
from the area of the present-day southern basin. Moreover, their hot, dry nature shows that that the erupting ventswere subaerial, being sited south of the flooded 21 ka caldera. In summary, the principal eruptive vents migrated
from the KL first northwards into the ancient flooded caldera (future northern basin), then southwards onto dry land(future southern basin). Caldera collapse in the north may have begun as early as phase 3, but the southern halfof the caldera was still subaerial when the eruption ceased.

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