San Francisco Bay Region Geology and Geologic Hazards

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Why Make a Geologic Map?

An accurate geologic map is needed to understand the Earth’s resources and hazards.

A geologic map provides basic data for understanding both past and present-day processes affecting a region of the Earth. This kind of information is important for four main reasons:

  1. To provide geologic information that can help to reduce death and damage caused by geologic hazards such as earthquakes and landslides. Different types of geologic materials can amplify shaking or even liquify during earthquakes. Some also are more likely to produce landslides, or they may contain natural deposits of hazardous asbestos or mercury. A geologic map shows where these types of geologic materials are, as well as the location of faults that might generate earthquakes.

  2. To better find and protect or safely extract geologic resources. Concrete, sand, metals, petroleum, even groundwater, are all important geologic resources, but to benefit society, they first must be found. A geologic map shows the distribution of the rocks and sediments that are most likely to contain these resources. For example, if you needed sand and gravel to make concrete, would it be better to look in an area of solid granite (map unit Kgr) or in an area of loose river deposits (map unit Qha)?

  3. To improve our stewardship of the Earth through informed agriculture, construction, and environmental practices. A geologic map shows the distribution of the types of geologic materials that are likely to produce poor soils that are unsuitable for agriculture (for example, map unit Jsp). It also shows which rocks will provide the safest foundations for buildings and roads, as well as those which can help support important or endangered species.

  4. To help geologists unravel the geologic history of the region. The relations between the geologic materials and structures shown on the geologic map give clues about the sequence of events that happened in the area in the past. An improved understanding of the geologic history helps us to better understand the region's geologic resources and hazards.

Learn more about the USGS geologic mapping effort.

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Page Last Modified: 8/18/2006