Research Technical Report

Hyaloclastite from Miocene Seamounts Offshore Central California: Compositions, Eruption Styles, and Depositional Processes

Davis, A.S., and D.A. Clague (2003)

In: Explosive Subaqueous Volcanism, J.D.L. White, J.L. Smellie, and D.A. Clague (eds), Geophysical Monograph 140, American Geophysical Union, 129-142.

ABSTRACT

Hyaloclastite deposits are abundant on mid-Miocene volcanic seamounts offshore central California. The glass compositions are predominantly evolved hawaiite and mugearite, although minor amounts of tholeiitic to alkalic basalt are also present. Textural features give evidence for different eruption styles. For the evolved, alkalic compositions, fragmentation occurred primarily in response to exsolution of magmatic gases as the magma approached and erupted on the seafloor. Textural features of pyroclasts suggest formation of lava fountains of limited size and height, depending on water depth. Monomict, clast-supported hyaloclastite of highly vesicular pyroclasts suggest limited dispersal and deposition near vent sites. Matrix-supported, polymiet breccias are reworked and displaced by currents into deeper water. The narrow range of glass compositions extending over multiple layers of volcanic sandstone suggests deposition from a slurry of tephra and water directly related to an eruption. Basaltic hyaloclastite of vesicle-free, angular glass fragments from the deepest site apparently formed from quench granulation. Sulfur contents suggest eruption depths ranging from near sea level to over 2000 m for samples collected from 1300 to over 3400 m depth, implying large amounts of subsidence for these seamounts.