Modeling the formation of axisymmetric terraces in submarine explosive eruption deposits with analog experiments

Abstract:

Recent high-resolution three-dimensional seismic imaging studies reveal striking quasi-axisymmetric terraces expressed in the bathymetry and internal structure of deposits surrounding submarine calderas of Santorini, Greece and Sumisu, Izu-Bonin Arc (Figure \ref{fig}a \& b), which are composed primarily of unconsolidated tephra related to the most recent eruptions of these volcanoes.

Appropriately scaled analog experiments on multiphase jets simulating submarine explosive eruptions demonstrate that eruption column collapse may occur intermittently or periodically through the growth and descent of sedimentation waves from the top of the column, which, in turn, emplace axisymmetric terraces in the proximal deposit.

The dynamics of these sedimentation waves and the geometry and architecture of the resulting deposits are sensitive to the depth of the water layer and interactions between the jet and the ocean-air interface (free surface).

Where jets rise through the free surface, descending sedimentation waves spread at the free surface before accelerating to the base of the water layer to produce relatively broader terraces (Figure \ref{fig}c \& d).

On the basis of our results, a comparison of the bathymetries for Santorini and Sumisu suggests that whereas both events occurred under comparable eruptive conditions at the volcanic vents, Santorini may have erupted through a comparatively shallow water layer.

Figure Caption

a. Bathymetry of proximal deposit around Santorini caldera, Greece exhibiting quasi-axisymmetric terraces surrounding an annular raised plateau.

b. Bathymetry of Sumisu caldera, Izu-Bonin arc and surrounding proximal deposit from the 30-60 ka eruption.

c. Proximal deposit with a raised annular plateau surrounded by axisymmetric terraces emplaced by a multiphase jet that rose above the free surface of a relatively shallow water layer.

c. Terraced proximal deposit emplaced by sediment waves descending around a multiphase jet that did not penetrate the free surface.