DIKES and SILLS


Dikes:
A sheetlike body of igneous rock that cuts across layering or contacts in the rock into which it intrudes. Dikes form when magma rises into an existing fracture, or creates a new crack by forcing its way through existing rock, and then solidifies. Hundreds of dikes can invade the cone and inner core of a volcano, sometimes preferentially along zones of structural weakness.

Sills:
A tabular body of intrusive igneous rock, parallel to the layering of the rocks into which it intrudes.

Remnants of the Past:
Stocks, sills, dikes, laccoliths and other intrusions are remnants of past igneous activity and are exposed at the surface long after erosion has stripped away any ancient volcanoes and other overlying rocks and sediments that may have existed in an area.


See: Plutons



-- Excerpts from:
Volcano World Website, 2003, USGS Volcano Hazards Programs Photoglossary, 2003, and Philip Stoffer, 2002, Rocks and Geology in the San Francisco Bay Region, USGS Bulletin 2195

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