Man's physical universe

xanabras

UNIT III

CONTINUOUS CHANGES IN THE EARTHS SUR-

FACE HAVE BROUGHT ABOUT CONDITIONS

WHICH MAKE POSSIBLE THE LIFE OF MOD-

ERN MAN

INTRODUCTION TO UNIT III

In speaking of the earth, geologists refer to the outer solid portion

as the lithosphere, the oceans, etc. as the hydrosphere, and the gaseous

envelope which surrounds the earth as the atmosphere. Inasmuch as

the earth rotates daily in the path of the sun's radiant energy, it is

heated unequally from day to night.

This unequal heating of the earth

produces changes in the densities of the hydrosphere and atmosphere,

which, under the influence of gravity, result in local circulations in these

spheres. A more important cause of circulation in the atmosphere and

hydrosphere is the difference between the radiation received at the

poles and that received at the equator. The rotation of the earth

affects the direction of the above circulation. The radiant energy of

the sun also brings about the evaporation of water. The water vapor so

formed is transported by air currents to colder places, where it condenses

as rain. Thus the energy of the sun brings about weather

changes, winds, and rains, which constitute the physical bases for the

changes in the earth's surface known as gradation.

Gradation is the geological activity which tends to level the surface

of the lithosphere; it consists of two processes: degradation, or wearingdown

of the higher places, and aggradation, or building-up of the lower

places.

It seems probable that the earth underwent readjustments in

volume, due to rearrangements of the rock materials under the stress

of pressure, and that the surface therefore was thrown into wrinkles,

thus forming mountain ranges. These mountain ranges were in turn

torn down by various agents of erosion with a redistribution of pressure

on the surface that resulted in upheavals of new mountain ranges.

Some people have attributed this wrinkling process to the gradual

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