One of the outcrops I always look forward to revisiting when I get back to the Marin County area of California is the spectacular “Wall of Pillows” roadcut just downstream of Nicasio Reservoir along the Point Reyes-Petaluma Road and just uphill from Point Reyes Station. The roadcut is north-facing, so there’s really no good time of day to catch the sun on it. If you zoom in you can see my two field assistants, Edi and Berti, resting on some of the pillows.
More Pillows Than You Can Shake a Stick At
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The roadcut exposes a section of the upper oceanic crust scraped off the subducting Farallon Plate and incorporated into the Franciscan Subduction Complex. Pillow lavas such as these are diagnostic of fluid lavas (usually basaltic) that were extruded underwater, most commonly at the mid-ocean ridges. Although rarely witnessed, this type of volcanism is by far the most common form of extrusive igneous activity on Earth. The basaltic lava chills rapidly against the cold seawater and forms a glassy rind. The interior of each pillow subsequently inflates as more magma intrudes. This same type of texture can be observed in the photo below where 2.75 Ga pillows of the Wawa Greenstone belt are exposed near the Sir James open pit mine near Wawa, Ontario.
So how about it Geoblogosphere, have you seen any better pillows in your field areas? (I’m itching for a “pillow” fight!)