The Pillow Lavas of Marin

One of many interesting rocks in Marin County, pillow lavas clearly formed underwater as basalts. They were likely formed in very deepwater and perhaps, even south of the present Equator of the Earth. Estimates of their origins at around 18 degrees South Latitude have been made using paleomagnetic evidence (Marin is at ~38 degrees North Latitude) so they are widely traveled rocks.

Although geologists had long suspected that the pillow structure of these rocks formed underwater, it was not confirmed until the 1970's when divers observed and filmed pillows forming in shallow waters off the coast of Hawaii. A good place to see the pillow structure is in the Marin headlands just North of the Golden Gate. The small rock connected by an arch at Point Bonita  lighthouse contains a good cross-section of the interlocked and now, tilted pillows. They also have been extensively metamorphosed by hot seawater and converted into what are called greenstones. Evidence found in pillowed exposures in the Marin Headlands supports the idea that hot water bathing these rocks was important in the early history of the pillows. Secondary minerals in veins perhaps at one time occupied by hot water are common as seen in this photo.

An example of this hotwater "in action" in a modern spreading center can be seen in this "black smoker" photo from the USGS. In this picture, superheated water (up to 380 degrees Celsius) is surging from a "chimney" made of mineral deposits. A black smoker is a type of hydrothermal vent or spring.

LavaVideo Productions of Hawaii has given us permission to use clips from their excellent videos entitled "Pele Meets the Sea" and "Pillow Lava Formation" which show examples of submarine vulcanism (note: the Hawaiian Islands evolved from submarine volcanoes).

The relative age relationship of the pillows in Marin to the rest of the rock layers is shown in this geologic column.

Other Pillow Photos on the web:

Pillows created by lava flow entering a river/lake and subglacial eruption (Volcano World)

Pillow from summit area of Loihi Seamount off Hawaii
(School of Ocean and Earth Science and Technology at the University of Hawaii)

Description of "pillow tongue" formation (Volcano World)

Description of "pillow sack" formation (Volcano World)

Other Volcano Info:

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