Axial, Brownbear and Cobb Seamounts: Examples of Growth and Demise of the Submarine Volcanic Edifice through Time.

Abstract:

Axial, Brownbear and Cobb Seamounts, located along and to the west of the Juan de Fuca Ridge (JdFR), are the most recent volcanic expressions of the Cobb Hotspot. Cobb Seamount is the oldest (3.3 Ma) and furthest from the hotspot source. Cobb exhibits a circular volcanic edifice that rises from ~2800 m at its base to within 34 m of the sea surface. Multibeam bathymetry of Cobb indicates there are wave-cut terraces at the summit, as well as landslide scars along the flanks of the volcano. Brownbear Seamount (0.5-1.5 Ma) has an elongated (N-S) shape, which indicates an interaction of the hotspot plume with the extensional-stress field of the JdF spreading center. Brownbear also has two (~4 km wide) summit calderas which likely formed due to collapse once plate motion moved the seamount west of the magma source. Axial Seamount, in contrast, is the volcanic edifice directly above hotspot plume and is therefore in a constructional phase. Axial also straddles the ridge and because of this exhibits an elongate structure with well defined rift zones forming its northern and southern flanks. Axial also has a single large summit caldera (~8 km wide) which has been the location of two eruptions in 1998 and 2011 which produced extensive lava flows within the caldera and down the south rift zone. We will present multibeam bathymetry of these three volcanoes, and include the results of previous surveys and observations, to infer the evolution of the volcanic construction and destruction processes through time along this seamount chain.