by Jane Nielson, The Community Voice
May 5, 2011
Like all landscapes, the Laguna de Santa Rosa is the sum of many natural events. Long ago, deep-seated forces of sub-duction laid the foundation of coastal California. Tectonic forces shoved the eastern Pacific tectonic plate, formed at a volcanic ridge in the early Pacific Ocean, beneath continental rocks of the overriding North American plate. (Recall that sub-duction has caused the world’s largest recorded earthquakes, including the recent Sendai, Japan quake.)
Some 30 million years ago, the East Pacific plate’s mid-Pacific ridge began to disappear beneath North America. Starting from what is now northern Mexico and progressing northward, the San Andreas “transform fault” gradually replaced the sub-duction zone.
Now 810 miles long, the San Andreas Fault currently ends abruptly at Cape Mendocino. To the north, a small remnant of the East Pacific plate still dives into a sub-duction zone beneath North America, creating Mt. Lassen, Mt. Shasta, and the Cascades volcanoes of Oregon and Washington. Eventually, that plate and its ridge will disappear beneath North America, extending the San Andreas Fault to the Aleutian Islands.
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