Man's physical universe



River and the Snake River have cut through it. Some of these flows

were nearly a mile deep and leveled off the landscape. The Deccan

plateau in

India was once covered by lava flowing from fissures out

over nearly 250,000 square miles, to a thickness of 10,000 feet in places.

Similar plateaus have been found in South America and South Africa,

The Hawaiian Islands were formed by layer upon layer of lava flows,

extending seven miles above the ocean floors, tw^o miles of which are

above the sea.

Fig. 46. Wizard Island from the rim near Crater Lake Lodge, Crater Lake,

California. Wizard Island is a volcanic cone within the very large crater now

occupied by Crater Lake. (Courtesy of the U. S. Dept. of the Interior. Photograph

by George A. Grant, 193 L)

The lavas flowing from these fissures, such as occurred in Iceland and

the Hawaiian Islands, are dark in color, because they are rich in iron

but poor in silica. They flow freely for a long distance and do not cool

rapidly, and gases readily escape through the lava.

The lava that flows from the conelike volcanoes is much hotter but

cools very quickly to form masses through which gases cannot pass;

explosions generally occur, and steep cones are produced. The lightcolored

lavas are rich in silica but poor in iron. Tremendous pressure

may be built up in such volcanoes over long periods of time before they

More magazines by this user
Similar magazines