Last week I took a day trip to the coast. I have always been sort of curious about the history of Morro Rock, but since I have been in this class my curiosity has grown.. So I did some research...
At http://www.travellogs.us/Miscellaneous/Geology/Volcanic%20plugs/Volcanic%20Plugs.htm, I learned that Morro Rock is a volcanic plug. A volcanic plug is magma that cooled inside of the neck of a volcano. The intrusive rock is much harder and tougher than its surrounding rock so over time the rest of the sediment around it eroded away leaving Morro Rock behind. Morro Rock is a rhyolite plug that is from a volcano that was active about 24 million years ago.
According to Wikipedia.com, Morro Rock is part of what is called the Nine Sisters of San Luis Obispo County, which are a series of 9 volcanic plugs that are lined up along the Los Osos Valley.
http://academic.cuesta.edu/physci/Geology/morros.htm, states that "Potassium-argon dating yielded an age of 22.1 +/- 0.9 million years for the
crystallization of this rock."
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