DYNAMICAL GEOLOGY. The surficial forces of dynamical be discussed under the process of grada tion, the internal forces under the processes of diastrophism and volcanism.
The external' forces are con tinually at work tearing down earth structure at one point, building up at another. This tearing down and rebuilding is known as grada tion. Degradation, erosion and denudation are practically synonyms for the tearing down , process; aggradation and deposition for the ,' building up. The first is the reducing .of rock material to fine particles, ready for re moval and is known as weathering. The re moval of the material is known as transporta tion, and both steps are subprocesses of ero sion. The main agents by which these pro. cesses are carried on are the atmosphere ground water, running water, oceans,,lakes and organisms.
The Work of the Atmosphere is both me chanical and chemical. Mechanically, it works by means of wind, frost, sudden temperature changes and rain impact; chemically, by oxida tion, carbonation, hydration and solution.
Wind work is everywhere operative but is much more effective in arid than in humid re gions. Deserts are especially exposed to its activity, due to lack of vegetation, which in humid regions forms a protective cover. Wind laden with drifting sand. is an important agent of erosion. Rocks of uniform hardness are worn smooth, rocks that are less homogeneous are etched into relief by the cutting away of softer parts, cliffs are undercut and desert re gions are thus rendered famous for their fantas tic forms. Desert sands are in many places strewn with highly worn and polished pebbles which represent the harder particles of rock left behind after the softer rock is ground fine and blown away. Wind also has a notable transport ing power. At no time is the air free from fine dust. Falls of dust and volcanic ash on ships at sea and films of dust on unbroken snow fields of high altitudes testify to the carrying power of even moderate breezes. The great sand and dust storms of arid regions are well known. On the margins of deserts thick and extensive deposits of wind-blown soil called loess occur. Famous loess beds occur in China and in the United States, in the latter particu larly along the lower courses of Missouri and Mississippi rivers. Wind-drifted sand accumu lates in hills whose gentle slopes are toward the wind. Because sand drifts up the incline and slides down the steeper lee slope, such sand dunes continually migrate in the direction of wind movement and not infrequently encroach upon arable land causing destruction of prop erty. As a result of such damage much has been done in the study of the prevention of dune migration. Wind-blown material of this sort may become buried, ultimately consolidated and finally form sedimentary rocks. Eolian de posits are called terrestrial or continental, as opposed to marine deposits laid down in the sea. Wind also influences climate and rainfall,
controls waves and currents and acts in many ways as an indirect geologic agent. See DESERT; DUNE; EOLIAN DEPOSITS; LOESS; METEOROLOGY; WIND; etc.
Freezing water exerts an expansive force of about 2,000 pounds to the square inch. Water in minute pores of the rocks, or in larger open ings such as joints or bedding is an efficient agent of disintegration in those lati tudes and at those altitudes subject to sudden and repeated freezing and thawing. It crowds and wedges off large angular blocks from every exposed ledge. The great talus slopes of moun tain regions afford well known illustrations. See FROST; ICE; TALUS; etc.
If a rock be heated in a campfire and drop ped into cold water, the sudden contraction of a rapidly cooling exterior over a still heated interior causes the disruption of the mass. A similar phenomenon takes place in desert re gions where, due to lack of moisture and pro tecting vegetation, the daily range of tempera ture is extreme. In the Sahara the daily range is said to be over 100° F., and explorers state that at night there may frequently be heard the report of breaking rocks. A rock like a granite made up of several different minerals is more subject to such disintegration than a sand stone which is homogeneous in composition. This is because the different minerals expand at different rates upon heating and tend to tear asunder. Change of temperature usually results in the breaking off of curved concentric masses of rock, hence the term exfoliation is sometimes applied to it. See Dona; Ex romAnoN ; etc.
In soft rocks, the impact of falling rain drops, entirely aside from any erosive effect of running water, has a disintegrating action. This is usually effective only in regions of light vegetation, and is therefore most strikingly ex hibited in our so-called obad lands." (See BAD LANDS). The above forces are wholly me chanical in their activity. They are for the most part agents of weathering, and only in the case of the wind are they important in trans porting or depositing soils.
The chemical work of the atmosphere will be described briefly under the topics of oxida tion, carbonation, hydration and solution. Oxi dation is the union of some rock element with oxygen. The affinity of ferrous iron and sul phur for oxygen is a familiar fact and the group of sulphur bearing minerals (the sul phides), particularly pyrite and chalcopynte, as well as many minerals containing ferrous iron, especially the so-called ferro-magnesian min erals of which the pyroxenes and amphiboles are examples, are subject to oxidation. The iron ultimately forms hematite or limonite and the sulphur becomes sulphuric acid. This process of oxidation is of great economic im portance in the secondary enrichment of sul phide ore bodies. See CHALCOPYRITE; ECONOM