Earth Features and Their Meaning / An Introduction to Geology for the Student and the General Reader

by William Herbert Hobbs

CHAPTER IV

THE ROCKS OF THE EARTH’S SURFACE SHELL

The processes by which rocks are formed.—Rocks may be formed in any one of several ways. When a portion of the molten lithosphere, so-called magma, cools and consolidates, the product is igneous rock. Either igneous or other rock may become disintegrated at the earth’s surface, and after more or less extended travel, either in the air, in water, or in ice, be laid down as a sediment. Such sediments, whether cemented into a coherent mass or not, are described as sedimentary or clastic rocks. If the fluid from which they were deposited was the atmosphere, they are known as subaërial or eolian sediments; but if water, they are known as subaqueous deposits. Still another class are ice-deposited and are known as glacial deposits.

Fig. 16.—Laminated structure of sedimentary rock, Western Kansas (after a photograph by E. S. Tucker).

But, as we have learned, rocks may undergo transformations through mineral alteration, in which case they are known as metamorphic rocks. When these changes consist chiefly in the production of more soluble minerals at the surface, accompanied by thorough disintegration, due to the direct attack of the atmosphere, the resulting rocks are called residual rocks.

The marks of origin.—Each of the three great classes of rocks, the igneous, sedimentary, and metamorphic, is characterized by both coarser and finer structures, in the examination of which they may be identified. The igneous rocks having been produced from magmas, which are essentially homogeneous, are usually without definite directional structures due to an arrangement of their constituents, and are said to have a massive structure. Sedimentary rocks, upon the other hand, have been formed by an assorting process, the larger and heavier fragments having been laid down when there was high velocity of either wind or water current, and the smaller and lighter fragments during intermediate periods. They are therefore more or less banded, and are said to have a bedded or laminated structure ([Fig. 16]).

Again, igneous rocks, being due to a process of crystallization, are composed of mineral individuals which are bounded either by crystal planes or by irregular surfaces along which neighboring crystals have interfered with each other; but in either case the grains possess sharply angular boundaries. Quite different has been the result of the attrition between grains in the transportation and deposition of sediments, for it is characteristic of the sedimentary rocks that their constituent grains are well rounded. Eolian sediments have usually more perfectly rounded grains than subaqueous deposits.

Glacial deposits, if laid down directly by the ice, are unstratified, relatively coarse, and contain pebbles which are both faceted and striated. Such deposits are described as till or tillite. If glacier-derived material is taken up by the streams of thaw water and is by them redeposited, the sediments are assorted or stratified, and they are described as fluvio-glacial deposits.