11  - Pillow lava
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.

Additional information on pillow lava

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