|At this point we have reached the oldest rocks in the sequence within
the Marin headlands. These volcanic rocks, we believe, began the
sequence as new ocean floor was being formed. From photographic and
deep sea submersible observations we know that volcanic rocks which erupt
beneath the sea commonly form these pillow like shapes. We do not
see similar forms in lava that are forming on land.
These rocks were once basaltic in their chemistry like a majority
of the volcanoes that erupt to form the deep ocean floor, but they have
been altered by interaction with hot waters near their location of origin
or during their uplift from the ocean floor to a significantly different
chemical composition. The pillows remain, however, to give us important
clues to their origin.
information on pillow lava