VOLCAN-ECUADOR, GALAPAGOS-ISLANDS - EROSION AS A POSSIBLE MECHANISM FOR THE GENERATION OF STEEP-SIDED BASALTIC VOLCANOS

Volcan Ecuador (0-degree 02' S, 91-degrees 35' W) consists of two stro ngly contrasting components: the eroded and vegetated remnant of a onc e-circular main volcano with a probable caldera, and a prominent rift zone extending to the northeast that is neither strongly eroded nor we athered. There are about 20 young-looking flows and vents on this cald era floor but only one on the higher remnant of the main volcano. The southwest half of the main volcano is faulted into the ocean. The main part of Volcan Ecuador possesses steep erosional slopes (average 30-4 0-degrees) that cut into a sequence of flows that dip radially outward at < 10-degrees. In contrast, the northeast rift zone consists entire ly of young flows and vents. The upper 10 km of the rift zone forms a peninsula about 7.5 km wide that connects Volcan Ecuador to Volcan Wol f. The rift zone bends to the southeast and the lower 8 km is tangenti al to the coast of Volcan Wolf. The rift zone axis dips away from the northeast edge of the main volcano, and its flanks slope roughly north west and southeast at <4-degrees. The rift zone is the Galapagos struc ture that most closely resembles a Hawaiian rift zone because it is co nstructed of lavas from subparallel linear vents, shows evidence of a deep feeder conduit, and has changed its direction to avoid a direct i ntersection with neighboring Volcan Wolf. The steep erosional slopes e xtending around the perimeter of the main volcano (except to the south west where slumping occurred) were probably generated by marine erosio n during a prolonged period of eruptive inactivity (perhaps 20 000-30 000 years). Only a few post-erosional eruptions have taken place at th e main volcano in and near what was once the caldera. The entire rift zone postdates the period of prolonged erosion. Using the evidence for prolonged inactivity at Volcan Ecuador, we propose that erosion may h ave helped to produce steep slopes on the other western Galapagos volc anoes. On these more active volcanoes, however, numerous subsequent er uptions have completely mantled the erosional slopes with lava. The me chanism by which the volcanoes may shut off for long periods of time i s unknown, but the fact that the Galapagos hotspot is presently supply ing nine active volcanoes suggests that the magma supply at an individ ual volcano could vary greatly over periods of (tens of?) thousands of years.