Another Type of Pseudocrater?
Dennis Geist1, Karen Harpp1, Christopher Hamilton2, Mark Kurz3
Affiliations: 1Department of Earth and Environmental Science, Colgate University, Hamilton, NY USA; 2Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA; 3Woods Hole Oceanographic Institution, Woods Hole, MA USA
Presentation type: Poster
Presentation time: Tuesday 16:30 - 18:30, Room Poster Hall
Poster Board Number: 153
Programme No: 3.5.12
Abstract
Three ocean-entry crater fields in the Galapagos Archipelago are enigmatic: the craters are too numerous and randomly distributed to have erupted above dikes or conduits, yet unlike Iceland's pseudocraters they are in settings where there is little possibility of water or soil development in the near-surface substrate. 2.9 ± 0.9 ka and 10 ± 5 ka (Mahr et al., 2016) pāhoehoe fields on Cerro Azul and San Cristobal each host > 100 spatter and scoria cones. Some craters may align with sinuous lava tubes roughly perpendicular to the coast. A ~20-m topographic bulge occurs about halfway between the vents and the coast at both localities. On San Cristobal, 10 analyzed lava samples indicate large compositional heterogeneity. At both fields, many of the craters have 'a'ā breakouts. The third locality is Bartolome, an islet off the coast of Santiago, with 15 craters, although the crater distribution there may be due to intersecting fissures. One possible origin is that steam that forms at the lava's ocean entry travels up active lava tubes until it reaches a break-in-slope, where it is trapped, and a steam--lava mixture explodes through the crust. Alternatively, two subparallel fissures erupt simultaneously. The fissure at lower elevation forms a topographic barrier, and lavas from the two fissures mix within a pāhoehoe sheet flow. Enough of a piezometric head could develop that the upper crust fails, and lava erupts through the upper crust. These pseudocraters could thus be considered mega-hornitos.