TITLE     INTRODUCTION      DETRITAL ROCKS       CHEMICAL ROCKS      BIOCHEMICAL ROCKS    ENVIRONMENTAL CLUES      CREDITS

CHERT

biochemical limestone        diatomite        coal

The sedimentary rock chert can form in several ways.   From the biochemical aspect, it forms from deposition and compaction of silica-rich skeletons of diatoms, radiolarians (a common ocean planktonic animal), and tiny sponge fragments (called spicules) on the ocean floor.   Chemically derived chert can form as siliceous skeletons or silca-rich rocks dissolve, releasing silicon into the ocean water.   Silicon and oxygen will then bond together, precipitating onto the ocean floor.   Since chert is composed of silca, it is very hard and durable.   Ancient people often relied on chert as the raw material for the formation of tools such as hide scrapers, drills, and spear points.

Image 1 shows a collection of tiny radiolarian skeletons, common constituents in biochemical chert (image from Dr. Bert Conrey).   Images 2 provides a view of a typical chert hand sample.   Image 3 shows geology student Scott next to an outcrop of Monterey Formation shale (light) and chert (dark) along the California coast (image by Dr. Rick Behl).   Image 4 shows Bruce next to a chert outcrop in north-central Nevada..

           image 1                          image 2                         image 3                        image 4                  

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