GEORGE POST / SCIENCE PHOTO LIBRARY GEORGE POST / SCIENCE PHOTO LIBRARY
Basalt outcrop. This dark rock formation is formed of volcanic (igneous) rock. It is a plug, a solidified piece of magma that once blocked the opening of a volcano. Over time, the rest of the volcano was eroded, leaving just the plug remaining. The plug itself has been eroded, and rocks and dust from the plug have formed a deposit at its base. Photographed near the Ute Mountain Ute Indian Reservation, Colorado, USA.
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