Man's physical universe

xanabras

DIASTROPHISM AND VULCANISM 165

These volcanic activities observed on the surface are merely slight

indications of the inner turmoil which causes them. All movements of

lava and the phenomena connected with these movements are grouped

under the name "vulcanism." Many lava flows which did not reach

the surface filled in fissures between rocks, forming dikes of hard rock

that protrude above the softer rocks around them when they are

exposed to weathering conditions. The Palisades of the Hudson were

formed by intrusions of lava between beds of rock, forming thick sills.

In several areas, these intrusions

cause the upper layers of rock to

bulge upward into dome-shaped hills

or mountains, called laccoliths. The

Black Hills are typical laccoliths.

Lava-filled tubes of volcanoes are

called necks. When lava cools so

quickly that it does not have time

to crystallize, volcanic glass, or obsidian,

is the result. Under the high

pressures in the earth's interior the

materials, although solid, have the

capacity to fiow slowly and thus

bring about equalization of pressure

differences on the surface, flowing

from the regions of high pressure

under regions of lower pressure and

thus elevating them.

It is quite possible that this semifluidity

of the interior solid matter

of the earth would account for the

accumulation of the denser materials

at the center and thus make

the theory of an original molten

earth unnecessary.

Under these conditions of high pressure there must also be molecular

rearrangements which would tend to decrease the pressure. Certain

changes in the nature of rocks can thus be explained.

Probably molten rock is produced in localized sections of the earth

which are under great strain.

Great cracks in the earth's surface afford the outlet for the hot materials

below, and volcanoes occur in chains along these cracks. Inasmuch

as earth movements also occur along these cracks, volcanic portions

of the globe are likewise subject to earthquakes.

Fig. 49. Columnar jointing, Devs

Tower, Wyoming. (Black Hills

Studios.)

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