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Field Trip to the
California Coast

Field Trip Stop #1:

The Pillow Basalts of the Golden Gate Seashore

These are samples of pillow basalts typical of the central California coast. Basalt is the most common volcanic rock both of the sea floor and of continental volcanoes.

Geologic History:

1. Exuding lava:

Beneath the sea, basaltic magma exudes from midoceanic ridges where crustal plates spread in opposite directions. The seawater cools the surface of the basaltic magma quickly, forming a solid crust over still fluid, hot lava. These crusts crack and the molten lava oozes out, much like a strip of toothpaste from its tube. In turn, this strips cools, its surface cracks, and the process continues. The resulting rounded and elongate shapes of the basalt give it the name of pillow basalt.
2. Uplift:

At this stop, we are standing near a former spreading center or midoceanic ridge, under perhaps 10,000 feet of water. The pillows formed as lava erupted on the sea floor, and cooled quickly. As hot seawater circulating through it, a low-grade metamorphism took place and the minerals, such as chlorite, developed which give the basalt its dark green color, and its common name greenstone.

3. Faulting and erosion:
This basalt is about 1370 m thick and it is broken by many faults. Weathering (both biotic and abiotic) and erosion continue to act on these pillows revealing some of their interior structure.

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