Man's physical universe

xanabras

DIASTROPHISM AND VULCANISM 159

is quite possible that the actual time of occurrence is determined by

such external factors as the change in pressure due to annual and daily

changes in atmospheric pressure. Earthquakes occur more frequently

during the night than the day and more frequently in winter than in

summer. Years of many sunspots show a greater number of earthquakes

than usual. Microseisms, or very small oscillations, are thought

to be produced by storms; they are much more frequent in winter than

in summer.

Mountain Ranges Have Been Formed by Slow Uplifts of the Earth's

Surface.

The forces which have caused earthquakes have been producing

very slow changes in the earth's surface. Some portions of .the earth's

surface are being elevated gradually. The Italian island of Palmarola

has risen more than 200 feet since 1822. The northern part of the

Scandinavian Peninsula has been rising for several thousand years.

Shore lines have been raised 1000 feet above the sea in northern

Sweden. An elevation is taking place in the region of the Great Lakes

along the Canadian border, where the present elevations rise 600 or 700

feet above former levels.

On the other hand, the eastern coast of the United States is sinking,

as is also the northern coast of France. The Bermuda Islands have

sunk to the extent that a former area of 576 square miles has been

reduced to 20 square miles.

Some mountains are wholly volcanic in origin, while others, like

the Catskills, were formed by erosion of high plateaus. The Black

Hills of South Dakota were formed by the erosion of a huge dome

upthrust 9000 feet by a lava intrusion.

The great mountain ranges of the world, like

the Alps, the Himalayas,

the Rockies, and the Sierra Nevadas, were formed by faulting

and folding. These mountain ranges represent rock layers produced by

sedimentation in shallow ocean beds to a depth of 25,000 to 30,000 feet,

which have been forced upward in the form of mighty folds. Inasmuch

as the continental shelves are not over 600 feet deep, a layer of sedimentary

rock 30,000 feet thick could be formed only by the gradual sinking

of these shallow basins as they became filled with sediment. At the

same time the neighboring land must have been rising gradually to

provide sufficient material for the tremendous amount of erosion necessary

to form such huge amounts of sediment. When the Appalachian

range was folded, the circumference of the earth was decreased about

100 miles or more, and it was decreased perhaps twice as much when

the Alps were formed.

More magazines by this user
Similar magazines