Mound of Gouge
July 06, 2006
Provided and copyright by: David Lynch
Summary authors & editors: David Lynch
Gouge is a powdery, chalky and sometimes putty-like or sticky material found at a fault plane. It's basically pulverized rock. During an earthquake, frictional heating between the grinding plates creates extremely high temperatures, occasionally melting the rock in very small regions and producing unusual chemistry. Being finely divided, gouge undergoes rapid chemical change as a result of absorbing water and accelerated weathering. The width of the gouge zone grows with each earthquake, starting out narrow - few mm to cm in young faults - and broadening to many feet or more in older faults. The steep, deeply eroded faces are hallmarks of gouge.
For millions of years, the San Andreas Fault has been grinding the North American Plate against the Pacific Plate, i.e. it has been grinding salt and peppery diorite against blue-green Pelona Schist, In this view taken looking north across the fault just west of the Big Pines Ranger Station near Wrightwood California, we see a mountain of gray gouge that's diorite based. Yet directly behind me across the fault on the Pacific Plate, the gouge is green because it started out as Pelona Schist. This area shows clear examples of both gouge and the different rocks on either side of the San Andreas Fault. The road in the middle of the picture is LA County Road N4, the Big Pines Highway.
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