Pebbles on Rodeo Beach

December 16, 2005

Pebbles_rodeo_beach copy

Provided and copyright by: David Lynch
Summary authors & editors: David Lynch

Like a sea of jewels, the pebbles at Rodeo Beach in the Golden Gate National Recreation Area sparkle with brilliant colors. They're mostly chert - dark red jasper and jade-green agate, with occasional yellow carnelians and the odd surf-polished shell fragment. There’s no conventional beach sand here, it’s all tiny rounded pebbles, mostly smaller than a penny and about the size of a pepper corn. The Cretaceous chert weathers out of the surrounding sea cliffs and gets pounded and polished by the breaking surf. Each storm brings a fresh crop - some larger than others, and the colors change with the season and currents.

This chert is formed in the Pacific Ocean. As the Farralon Plate marched eastward and then under the North American Plate, it carried sediments from the bottom of the ocean. The skeletons of tiny radiolarians are made of silica SiO2. Over millions of years, they rained down on the igneous seafloor, forming a layer of ooze. Over time and pressure, the blanket of skeletons lithified into chert. Most of the chert was subducted down into the mantle but some of it got scraped off and plastered against the western North American Plate. Thanks to erosion, this chert that found its way to Rodeo Beach. See also the Earth Science Picture of the Day for October 3, 2005.

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