Man's physical universe

xanabras

DIASTROPHISM AND VULCANISM 161

filled in a shallow basin extending as far west as Iowa with debris to a

depth of 5000 to 20,000 feet. As this basin sank and the neighboring

highland rose, lateral pressures were created in the sinking area because

the tougher areas surrounding this sedimentary rock would not yield.

Something had to happen to relieve the strain, and it is only natural

that the weaker sedimentary rocks

of the sinking basin would be the

ones to buckle up. These folds

were then elevated by other isostatic

readjustments.

We have explained the isostatic

movement of the earth's surface in

words that would indicate that we

are sure of our explanation. Actually

this theory is the only even

partially satisfactory one that has

been advanced, and the

geologist

feels it represents merely the first

approach to a complete theory.

Fig. 45. A fold in a rock stratum.

(Courtesy of the U. S. Geol. Survey.)

Volcanic Activities Bring About Other Important Changes in the Earth's

Surface.

Volcanic eruptions occur on the land and in the oceans. Sometimes

new islands are thus formed. For example, the island called "Old

Bogoslof " in the Bering Sea was raised to an elevation of nearly half

a mile by volcanic activity during the period 1796-1823. In 1883 a

second island, "New Bogoslof," appeared, while a third island was

raised in 1906.

Only about 430 volcanoes have been active within the time of

recorded history. Many of the very numerous prehistoric volcanic

activities were not the explosive types with their typical spouting of

rocks and ash, belching of gases, flames, and smoke from cones, as

found in modern volcanoes ; but they more closely resembled the great

lava flows of Iceland.

In 1783, a series of earthquakes in Iceland preceded

two great rivers of molten rock that poured out of fissures, 24

miles one way and 40 miles in the opposite direction, covering 200

square miles of land.

Parts of Ireland, the Hebrides, and large sections of Greenland were

formed in this way, while 200,000 square miles of the northwestern

states of the United States were covered with lava flowing from cracks.

This flow is very evident to anyone who visits the Columbia River

plateau. This lava may be seen especially well where the Columbia

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