Pillow Basalt at Palos Verdes
Pillow basalt is located at the base of a large
north-south trending scarp within the Portuguese Bend Landslide. This picture was taken facing
east.
Movement within the Portuguese Bend landslide area has exposed rocks formed
by seafloor volcanism: pillow basalt.
Pillow basalt is an igneous rock formed when magma came into direct contact with water, such as
during seafloor volcanic eruptions. Basalt is a
common volcanic (or igneous extrusive) rock that often has a dark color and a very fine graininess.
It is called a
volcanic or extrusive rock because it forms from magma that erupted, or was extruded,
from a volcano at the Earth's surface.
This differs from igneous intrusive rocks. This type of rocks forms from magma that
cools much more slowly, insulated deep in the Earth's warm crust.
A common type of igneous intrusive rock is granite. It has a coarse-grained texture due to the
very slow cooling of the magma.
When molten rock is allowed to cool slowly, say, over thousands of years, crystals can grow into
large sizes. These crystals of igneous intrusive rocks are visible to the naked eye, unlike their extrusive
counterpart which cools in a geologically brief time: within a few days to months.
The rapid cooling of lava prevents most crystals from growing to a visible size.
The bulbous-looking rocks are
pillow basalt.
Before Palos Verdes Peninsula began rising out of the sea over a million years ago,
the shallow seafloor experienced underwater volcanism which released lava directly into the ocean.
When the molten rock came in contact with water, it hardened instantly on the outside, forming a crust of
uncrystallized rock. The interior cooled more slowly, forming small crystals, but visible only by
microscope. The crust, or rind, of the pillow basalt can be easily distinguished from its
more crystalline interior in the following close-up picture.
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