Rodeo Beach geology field trip

April 30, 2018 by Fred Allebach

The other day I had a chance to go to Rodeo Beach on the Marin Headlands, on a little household geology field trip. I was the driver and companion, and my partner was the geology student sent on assignment to identify chert, greywacke, and pillow basalt.

The above-noted rocks are part of the Franciscan Complex, which is an accreted Mesozoic terrane of former ocean bottom that was scraped off the Farallon Plate as it subducted under the North American Plate. The Marin Headlands and the California Coast Range are made up of this accreted Franciscan Complex rock, with the constituent rocks at different stages in the rock cycle. Rodeo Beach rocks represent all three rock cycle stages: igneous, metamorphic and sedimentary.

The subduction of the Farallon Plate, over the last 200 million years, is associated with various mountain building events. The creation of the Sierra Nevada granitic batholith being one prime example. The Farallon Plate has a remaining fragment that is still subducting off the California Coast. This is the Gorda Plate, on Cape Mendocino near Petrolia. Here, at the Mendocino Triple Junction, the San Andreas Fault, North American Plate and the Gorda Plate meet. The Gorda Plate is subducting to the southeast, driving volcanic activity farther inland at Mounts Lassen and Shasta. Sonoma’s thermal waters: Fetter’s Hot Springs, and the Geysers, represent the trailing edge of this ancient subductive volcanic phenomena.

Back on the beach, we found the assigned rocks, and began to notice that the beach itself was an aggregation of cool-looking sand and gravel made from the local rock formations. Apparently, this type of beach is unusual in California. Most beaches are made from sediments washed down by rivers.

On the north side of the beach were many surfers, and they were fun to watch. Since no materials are allowed to be taken from a National Recreation Area, we took pictures while strolling, observing and noticing features about the assigned rocks.

Greywacke is an unremarkable looking, texturally immature, lightly metamorphosed sandstone. What would you expect from a prematurely accreted ocean bottom? Chert is a fine-grained sedimentary rock, frequently with micro fossils. Chert was a favorite material to make stone tools. Why? Because of the concoidal fracturing properties made possible by chert’s microcrystalline silica content. Chert does not limit the tool maker by having to deal with pre-existing crystal cleavage planes, and this allowed for intentional shaping. Chert on the Marin Headlands can be seen in dramatic outcropppings of folded, twisted shapes known as clines and synclines. Pillow basalt is an igneous rock that gains its peculiar shape through lava being erupted under water, or better said, by subaqueous extrusion. This underwater lava extrusion can occur on tectonic spreading centers or near underwater volcanoes.

With all these cool geologic processes happening at Rodeo Beach, one might start wondering, how did this all get here and why? Of course, this was part of the assignments purpose, to pique thought and wonder, and as my scientific associate and life partner’s professor said, it’s so involved, “it makes your head hurt.”

Well, let’s not be shy to attempt an explanation. Heat energy is what makes earth a live planet. The earth’s molten core is what’s driving all this continental and oceanic tectonic crustal movement, evidence of which is seen at Rodeo Beach. The heat energy from the core drives convection currents through the mantle, that move the crustal plates on the earth’s surface around. The plates smash, grind, slide and subduct, and in the process, the crust deforms, making mountains and volcanos. That’s plate tectonic theory in a nutshell.

Out on Rodeo beach, the waves lap up like any other day. The Farallon Plate is gone, subducted, transmogrified into Sierra granites. Now at the continental margin is the San Andreas Fault, a strike-slip, or transform fault. Along the San Andreas Fault the Pacific Plate runs to the northwest, sliding along the North American Plate, building up pressure every 100 or so years that is released as earthquakes. That’s why we need to wire book cases with our geology books to the wall.

Who would have thought a geology field trip to Rodeo beach could have such far flung implications!?