Comparison of the Submarine 1888 Ritter and the

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Comparison of the Submarine 1888 Ritter and the Subaerial 1980 Mount St Helens
Debris Avalanche Deposits

* Day, S (sday@pmc.ucsc.edu) , University of California, Earth Sciences Department
1156 High Street, Santa Cruz, CA 95064 United States
Silver, E (esilver@pmc.ucsc.edu) , University of California, Earth Sciences Department
1156 High Street, Santa Cruz, CA 95064 United States
Ward, S (sward@pmc.ucsc.edu) , University of California, Earth Sciences Department
1156 High Street, Santa Cruz, CA 95064 United States
Hoffmann, G (garyh@pmc.ucsc.edu) , University of California, Earth Sciences
Department 1156 High Street, Santa Cruz, CA 95064 United States
Lyons, A (infinitytree@hotmail.com) , University of California, Earth Sciences
Department 1156 High Street, Santa Cruz, CA 95064 United States
Llanes-Estrada, P (pllanes@geo.ucm.es) , University of California, Earth Sciences
Department 1156 High Street, Santa Cruz, CA 95064 United States

The lateral collapse of Ritter Island volcano, Papua New Guinea, on March 13th 1888
was nearly twice the volume of the lateral collapse of Mount St Helens on May 18th
1980 (4 to 5 km3 compared to 2.8 km3) and the resulting landslide traveled about twice
the distance (~75 km for Ritter and ~30 km for MSH). Both landslides descended
valleys producing topographically - controlled deposit distributions. Sonar mapping and
deep tow camera imaging indicate that the Ritter Island deposit is exceptionally well
exposed for a submarine debris avalanche deposit, most likely due to its very young age.
Comparing the Ritter and MSH deposits in terms of their geometry, areas of associated
substrate erosion, and the development of different morphological facies, provides
insights into the kinematic and mechanical similarities and differences between
subaerial and submarine debris avalanches resulting from volcano lateral collapses.
Although both deposits have block - rich and matrix - rich facies, extensive substrate
erosion in the distal part of the Ritter Island landslide resulted in the incorporation of
water - rich sediment and transformation of the debris avalanche into a debris flow rich
in sediment intraclasts.