Morro Rock is one in a series of volcanoes from the Miocene age, during the Miocene Rotation of what are now the Santa Ynez and Santa Monica Mountains broke away from the North American Mainland, and pivoted in a clockwise direction, with the Santa Monica end held against the continent while the other end, now Point Conception, rotated like the hand of a clock to end up pointing just north of Westward. The sea floor thinned both north and south of the rotating terrane, and volcanoes came up. Those south of the terrane are now embedded in the Channel Islands. Those north of the terrane are now exposed as large roundish rocks or volcanic necks, running from Morro Rock to San Luis Obispo. Morro is the most familiar of those. The terrane between was mostly underwater during the rotation. Once locked into roughly its current position, pressure of the Pacific Plate against the North American Plate pushed up the mountains, which are a layer-cake of Tertiary rocks, the most recent of which is the Monterey Shale visible along the beaches earlier in this series. Here's another view.