Ribbon Chert

October 03, 2005

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

Along Conzelman Road in the Golden Gate National Recreation Area in California, are some of the finest exposures of ribbon chert in the world. Here geologist Michael Sawlan examines a road cut of reddish chert, the color being due to oxidized iron (rust). Chert is a type of quartz formed when single-celled radiolarians living in the ocean die and sink to the bottom of the ocean. Their skeletons are made of silica (SiO2) and they accumulate in beds hundreds of feet thick. The sediments initially form as “ooze”, a thick porridge blanketing the underlying the igneous basalt of the sea floor. As the plates march toward the continents and away from spreading centers, pressure from overlying sediments cause the ooze to harden into layers of chert with thin layers of shale between them. When the chert beds and igneous sea floor reach the continent, they are either subducted under the continent, or mashed against it in distorted slabs called accreted terranes. The Marin Headland cherts above are part of accreted terranes than include Franciscan rocks like serpentine (metamorphosed basalt) and greywacke sandstone (lithified continental sediments). Like limestone and coal, chert is a rock with a biological origin and the radiolarian skeletons can be seen in microscopic examinations. Owing to its smooth sharp edges, chert has often been used to make arrow heads.

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