Basalt

Time
Miocene Epoch

In Our Region
The segment of East Pacific Rise located at the mouth of the Gulf of California, east of the southern tip of the Baja Peninsula.

Description
This block of pillow basalt originally erupted from a volcanic vent on the sea floor. When the molten lava contacted cold sea water, a jet black glassy rind instantly formed. The resulting rounded shape is referred to by geologists as a "pillow" structure. The on-going deep sea volcanic eruption of lava formed new sea floor hundreds of feet thick.

An Inside Look This particular sea floor basalt was collected in 1998 at 23°32' North latitude, 108°25' West longitude, from 2450 meters water depth on the crest of the Alarcon Rise.

Origins
During the Miocene Epoch the crust east of the Baja California region began to thin as western North America experienced extreme tectonic extension. A proto-Gulf formed in this area and extended north into the area of present day Palm Springs. Large-scale faulting west of the Baja Peninsula eventually shifted to the east and Peninsular California was transferred from the North American lithospheric plate to the Pacific Plate. Eventually, the eastern zone of faulting became associated with the major spreading plate boundary known as the East Pacific Rise. Crustal movement along this complex plate boundary is responsible for the opening of the Gulf of California and the on-going northwest movement of the Baja Peninsula.

Text: Tom Deméré
Photograph: François Gohier

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