PILLOW LAVA
Pillow Lava:
When flowing basaltic lava meets ocean water it quickly
chills on the surface to become stone. However, because rock
is an excellent insulator, the molten rock below the chilled
crust will continue moving under the weight of the molten rock
streaming behind it, forming
tubes of lava.
These lava tubes
frequently burst, bifurcating into spreading fingers of lava
across the seabed. In places the newly formed tubes swell into
pod-like masses before freezing to rock. In cross section, an
accumulation of these frozen fingers of lava look like stacked
pillows.
Pillow Lava - 2:
When basalts erupt underwater, they
commonly form pillow lavas, which are
mounds of elongate lava "pillows" formed by
repeated oozing and quenching of the hot
basalt. First, a flexible glassy crust forms
around the newly extruded lava, forming an
expanded pillow. Next, pressure builds until
the crust breaks and new basalt extrudes like
toothpaste, forming another pillow. This
sequence continues until a thick sequence
may be deposited. When geologists find
pillow basalts in ancient rock sequences, they
may conclude that the area was once under
water.
San Francisco Bay Region:
Ancient pillow basalt occurs in outcropping throughout
the San Francisco Bay region (such as these at Point Bonita in
the Marin Headlands). At Uvas Reservoir (near Morgan Hill)
limestone occurs between basalt pillows. At the time this pillow
basalt formed, the water was shallow and warm enough for
lime mud to accumulate on the seabed; such conditions might
have occurred on top of a submarine volcano). Today, lime
mud accumulates only in tropical to subtropical marine settings.
-- Excerpts from:
Philip Stoffer, 2002, Rocks and Geology in the San Francisco Bay Region, USGS Bulletin 2195, and
USGS Volcano Hazards Program Photoglossary, 2003
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