California Geologic History Part 2

USA west coast - around 400 million years ago the ocean plate started going under the North American plate and volcanoes formed island arcs – like Japan today 400 – 200 mya This was already shown in part 1 The island arc stage was similar to current Japan and Indonesia

By 100 Million years ago the spreading centers off the Ca coast were getting closer to the subduction zone. The spreading centers formed a series of underwater mountains including the Davidson Seamount The Davidson seamount is part of the Marine sanctuary. Repeat of Part 1 slide

Step by step formation of the San Andreas fault – confusing but this is what caused the san Andeas fault to form. * Go to explanation on next slide then come back briefly. During this time the davidson seamount ceased to be volcanic because it was no longer above a spreading zone. Demo basalt sample similar to seamount

29-0 mya (Cenozoic) The oceanic plate that had been subducting beneath the western edge of North America became completely overridden and the North American and Pacific plates came into direct contact for the first time. The two plates began moving sideways The San Andreas Fault system was formed and continues to slide today. In far northern California and the Pacific Northwest, subduction continues today.

Evidence 1 Granite rocks from the southern Sierras are now at Point Lobos and other spots on N. Cal The rocks in question are not just the same general type but “exactly” the same composition. Show granite again.

Evidence 2 The movement of the San Andreas Fault has separated several older extinct volcanoes The Pinnacles is half of the Neenach volcano in Southern ca. Point out locations of Pinnacles and Neeach on opposite sides of the fault

Current Western USA In Ca the San Andreas Fault has moved inland. Subduction continues in the north forming the major volcanoes of the Cascades including Shasta and Mt St Helens “simple” view of current situation Point out the “ancient” stuff – far inland and the purple “island arc” stuff

Monterey Canyon Formation Not enough data to show whether the canyon will someday extend into the harbor The canyon mouth captures over 200,000 cubic meters of sand from the coastline. The lip of the canyon advances and retreats.

Monterey Canyon Formation Similar undersea canyons exist at the mouths of large rivers, but there is no big river at the head of Monterey Canyon today. The canyon may be the old outlet of a river draining the Central Valley, which was moved north by the action of the San Andreas Fault from the Santa Barbara area. The granite rocks around the bay are part of the evidence.

Monterey Canyon Formation-cont The huge flow of sand, along the bay, creates rapid currents which erode and enlarge the canyon There are numerous nearby faults which trigger underwater landslides deepening the canyon It’s a very unique, interesting feature still being actively investigated Students may become the scientists who learn much more

Many sources were used including http://montereybay.noaa.gov/sitechar/geol3.html. Wikipedia snobear.colorado.edu/.../CaliforniaMtns/California_geologic_history.ppt‎ Several old teacher text books http://serc.carleton.edu/details/images/18172.html USGS