San Francisco Bay Region Geology and Geologic Hazards

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Geologic Materials

Geologic materials are the rocks and sediments that make up the land where we live.

The characteristics of geologic materials reflect the processes that form them and the environments in which they form. Geologists divide these materials into three basic rock types. Igneous rocks originate as extremely hot melted rock below the Earth's surface. If the melted rock cools slowly under the surface, it forms plutonic rock (named after Pluto, the Roman god of the underworld), such as granite. If, instead, the melted rock stays hot and rises to the surface, it can either ooze out or explode to form volcanic rock (named after Vulcan, the Roman god of fire), such as basalt and obsidian. When rocks get buried or are pushed deep into the Earth, the pressure and heat changes them into metamorphic rocks, such as marble and slate. Serpentinite, the California state rock, is another example of metamorphic rock. Sediments are mostly bits and pieces of older rocks that have been transported by wind and water to accumulate on beaches and in sand dunes, on lake and river bottoms, and on ocean floors. Given enough time, sediments may be buried under subsequent accumulations and then squeezed or cemented together to form sedimentary rocks, such as sandstone and shale. The remains of plants and animals get caught up in these accumulations to form fossils, which are found only in sediments and sedimentary rocks. Although fossils usually are sparse, a few sedimentary rocks are made almost entirely of fossils; for example, chert is made from millions of tiny plankton fossils. This map shows where the different rock types are found in the San Francisco Bay region. The map also shows accumulations of young sediments that have not yet been converted to rocks, such as sand dunes, bay mud, stream deposits (alluvium), and deposits on marine terraces (flat surfaces cut into coastal rocks by waves and then lifted above sea level by the same forces that drive the San Andreas Fault).

image collage - description below
Examples of geologic materials in the San Francisco Bay region (their basic rock types and map units are given in parentheses). (A) Granite (plutonic, Kgr); inset shows a close-up of the large mineral crystals that make up granite. (B) Obsidian (volcanic, Tpmv). (C) Slate (metamorphic, Kfm). (D) Serpentinite (metamorphic, Jsp). (E) Thin layers of sandstone and shale (sedimentary, KJs). (F) Fossil-bearing sandstone (sedimentary, Tos). (G) Chert (sedimentary, KJfc); inset shows a microscopic view of one of the millions of plankton fossils (order Radiolaria) that make up chert. (H) Bay mud deposits (sediments, Qhym), which over geologic time may be buried by other deposits, compacted, and transformed into shale (sedimentary). (I) Sand dunes (sediments, Qs).


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Page Last Modified: 8/18/2006