The President Jackson Seamounts are a series of seamounts ( underwater volcanoes) located on the Pacific Plate, off of

California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...

. It consists of 8 seamounts, 4 independent and 4 morpohologically fused, just west of the northern

Gorda Ridge The Gorda Ridge (41°36'19.6"N 127°22'03.1"W), aka ''Gorda Ridges'' tectonic spreading center, is located roughly off the northern coast of California and southern Oregon. Running NE – SW it is roughly in length.Carey, Stein, & Rona. (1990). ...

. They are generally very small, and arranged linearly. The assembly consists of 8 seamounts arranged in a chain. The easternmost of the seamounts is located about east of the axial ridge that spawned the volcanoes,

. The chain was mapped initially with SeaBeam equipment in 1985, however its incomplete coverage limited its usefulness. A GLORIA sidescan survey conducted in 1986 further established the rough size and locations of the seamounts. A further sampling cruise by the

United States Geological Survey The United States Geological Survey (USGS), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, ...

(USGS) recovered

pillow lava Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava underwater, or ''subaqueous extrusion''. Pillow lavas in volcanic rock are characterized by thick sequences of discont ...

and

hyaloclastite Hyaloclastite is a volcanoclastic accumulation or breccia consisting of glass (from the Greek ''hyalus'') fragments (clasts) formed by quench fragmentation of lava flow surfaces during submarine or subglacial extrusion. It occurs as thin margin ...

from 4 of the 8 volcanoes. The samples seemed to be of more primitive origin then those erupted elsewhere on the ridge. In 1998, the

Monterey Bay Aquarium Research Institute The Monterey Bay Aquarium Research Institute (MBARI) is a private, non-profit oceanographic research center in Moss Landing, California. MBARI was founded in 1987 by David Packard, and is primarily funded by the David and Lucile Packard Foundation ...

conducted two dives on a cone cluster located southeast of the main structure. The chain consists of eight volcanoes. Of these, four are self-dependent, whereas the other four form twin fused, morphologically complex, flat-topped structures. The volcanoes form a linear chain, and are relatively small, measuring less than in diameter, and are nearly circular in shape. They average ± in volume, but range from to . In addition to the eight main seamounts, there is a scattered disk of minor volcanic cones and flows about southeast of the chain. The summits of the seamounts, with the exception of the second one from the northwest, are all flat. Several have nested or cross-layered

caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...

s and

pit crater A pit crater (also called a subsidence crater or collapse crater) is a depression formed by a sinking or collapse of the surface lying above a void or empty chamber, rather than by the eruption of a volcano or lava vent. Pit craters are found ...

s (29 in total), many of which push into the flank of the volcanoes. In several cases, newer volcanic structures have all but erased older ones. In addition to all this the seamounts are pocketed by multiple small cones, and are littered by debris ranging far and wide from the range. There is evidence of erosional activity throughout, with erosional valleys and landslide debris visible.

References

{{reflist Seamounts of the Pacific Ocean Ridge volcanoes Seamount chains

See also

References