Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

THE ORIGIN AND DESCENT OF ROCKS.

It has been the current opinion that the earth was once in a molten state, and thence cooled to a solid condition, and hence that all the primitive rocks were igneous. Even those who think that the earth may never have passed through a molten state agree that the oldest known rocks are either true igneous rocks, or rocks of very similar nature. A molten magma may, therefore, be taken as the mother state of the rocks. Starting with this conception, the natural order of events suggests the inquiries (1) how rocks are formed from molten magmas, (2) what natures they assume, (3) how other rocks are derived from them, (4) how still other rocks are derived from these derivatives, and so on. To answer these inquiries is to trace out the generations of rocks and learn the general history of rock-formation.

(1) The process by which igneous rocks are formed from lavas is actually taking place in existing volcanoes. As these are widely scattered over the face of the earth, the material poured out by them represents different parts of the interior and varies in nature accordingly. This affords the means of studying the differences that arise from differences of material. This is a radical consideration, for variations in composition give rise to the most fundamental distinctions between rocks, though by no means the only ones. Rocks which have the same composition often differ greatly in texture or structure, owing to the varying conditions under which they were formed. In the solidification of rocks from the molten state, the rate of cooling causes many differences. A means of studying this is afforded by the various lava flows that are now being poured out on the surface under different conditions; but a more important means is afforded by extinct volcanoes, especially by those which have been deeply cut open by erosion. In certain very ancient volcanoes, not only have the solidified lava streams of the surface been cut across by erosion, but the lava that remained in the crater, or in the neck that led up from below, is laid bare for inspection. Exposures of even more profound nature have been made by the great disruptions which the outer part of the crust has suffered. In certain tracts there have been profound fractures, and the formations on one side of these have settled down and on the other side have been pushed up (faulted), so as to expose parts that were once much below the surface. Sometimes also the crust has been folded and crumpled, and the wrinkles thus formed have afterwards been worn away or cut open by deep valleys, and rocks that were once deeply buried have been laid bare. By the revelations made in these and other ways, it has been learned that at various times in the history of the earth molten matter has been thrust into fissures or intruded between layers of the crust and cooled there, without coming to the surface. Sometimes the lava appears to have forced its way into the rocks, and sometimes to have lifted the upper beds and formed great subterranean layers or tumor-like aggregates, called bathyliths and laccoliths ([Fig. 334]). Such intruded bodies of molten rock, solidifying under the varying conditions of such subterranean situations, are a fruitful source of instruction respecting the influence of varying rates and modes of cooling, as well as of other attendant conditions.

Fig. 334.—Diagram of a laccolith. (Gilbert.)

It will thus be readily seen that the rate of cooling of the various molten rocks must have differed very greatly. In the portions poured out upon the surface there were sometimes narrow streams and thin sheets, giving large exposure in proportion to the mass ([Fig. 335]), and sometimes thick flows and deep pondings in basins and choked valleys, giving massive bodies with relatively small surface exposure. There were explosions of the lava into minute particles with almost instantaneous cooling, and there were eruptions beneath the sea the peculiar effects of which are rather matters of inference than of positive knowledge. In the portions underground there were insinuations into thin fissures, on the one hand, and in-thrustings of thick bodies, on the other. Some intrusions entered the upper part of the crust where the rocks were cold and wet, and some were thrust into the deeper portions where the rocks were warmer and less penetrated by water. Sometimes the lava rose rapidly and was little cooled in passage, sometimes slowly with more cooling en route, and sometimes there were long halts between eruptions, with much opportunity to cool. An almost infinite variety of conditions is thus presented, and with it a rich field for the study of the modes of solidification.

Fig. 335.—Fresh lava flow, with large surface exposure. Holemaumau, Hawaii. (Libbey.)

In the underground intrusions the additional factor of high pressure was also present, and this is the third important condition in determining the nature of igneous rocks.

The three factors, composition, rate of cooling, and degree of pressure, require special consideration.