LAND FORMS, THEIR DEFORMATION AND FORMATION 



61 



active volcanoes, an insignificant figure as compared 

 with the number of inactive ones. Of the active pre- 

 sumed 430, 80 are submarine and 350 are on land; 

 336 are associated with the Pacific Ocean and 94 with 

 the Indio-Atlantic region; 275 are in the Northern 

 Hemisphere, and 155 are in the Southern. 



Volcanoes are distributed fairly evenly in a so- 

 called "circle of fire" around the Pacific Ocean. In 

 western North America, the only volcanoes classified 

 as active are in Alaska, California (Mount Lassen) 

 and Mexico (Paricutin). The most common pattern 

 of volcanic distribution is straight to arcing or curving 

 because they are found along lines of weakness 

 (faults) in the earth's crust. For example, the Cas- 

 cade Range of California, Oregon, Washington, and 

 British Columbia constitutes a single set of volcanoes 

 on a fault line. Also, this range shows that the west- 

 ern North American exception to the "circle of fire" 

 condition is mostly due to inactivity of existing vol- 

 canic peaks. Actually, the Cascades contain many 

 volcanoes that were active within the last few thou- 

 sand years. 



LIFE HISTORY OF VULCANISM 



Any possible history of vulcanism must consider 

 what happens to young composite cones, lava domes, 

 cinder cones, lava beds, laccoliths, dikes, sills, stocks, 

 and batholiths. Fortunately, vulcanism products and 

 their destructional cycles are generally so distinctive 

 that only the barest mention of their erosion is 

 needed. This is especially true when the characteris- 

 tic rocks of each land form are known. 



At maturity, composite cones have walls with gul- 

 lies and tops without craters. Also, lava domes are 

 dissected and cinder cones degraded. Lava beds are 

 indicated by a plain and plateau (capped by the old 

 lava bed) topography; laccoliths are identified by the 

 stage of a mature dome mountain. In addition, dikes 

 and sills may be exposed on the surface. Some dikes 

 have the general appearance of volcanic plugs; how- 

 ever, dike base outlines tend to be somewhat oval, 

 whereas plug base outlines are much more rounded. 

 Finally, stocks and batholiths are uncovered and sup- 

 ply mountainous terrain. 



The peneplain of old age may have remnants of 

 the composite cone in the form of necks and dikes 

 surrounding a plug; remnants of both cinder cones 

 and lava domes in the form of degraded plugs; per- 

 haps some elevation indicating past lava fields; per- 



haps the much eroded remnants of laccoliths, or only 

 a semblance of radial drainage; and some indication 

 of dikes, stocks, and batholiths. 



DIASTROPHISM 



Deformations in the earth's crust are of two types: 

 those in which mountains are formed, and those in 

 which continental masses are raised or lowered with 

 little folding of the crust. Both types of diastrophism 

 tend to occur very slowly, usually about 2 feet per 

 hundred years; however, individual earthquakes 

 might cause 50-foot vertical or horizontal displace- 

 ment of rock layers. 



Diastrophism can be thought of as processes that 

 warp, tilt, uplift, and depress the crust. Moreover, 

 when these processes work together, they cause local 

 tension, compression, torsion, or shear. For this 

 reason, diastrophism may be considered as many dis- 

 tinct phenomena — uplifting, depressing, warping, 

 folding, jointing, and faulting. 



PROCESSES 



In studying the features produced by diastrophism, 

 the attitude, or position, of rock layers in reference to 

 compass directions and to the horizontal are im- 

 portant. To appreciate these relationships, the terms 

 strike and dip must be understood (Figure 4.23). Strike 

 is applied to any continuous layer of rock seen on the 

 surface and is no more than the compass direction of 

 this rock layer. Dip is the number of degrees that a 

 rock layer angles downward into the crust. Dip might 

 best be visualized in terms of sediment deposition and 

 later tilting. When a layer is deposited it is hori- 

 zontal to the surface and has zero degrees dip; as the 

 layer is tilted from the horizontal, its angle of dip 

 increases. 



Uplifting and Depressing. These two already men- 

 tioned processes appear as simple vertical movements 

 of fairly large expanses of land, with no displace- 

 ment of strike or dip. All large-scale movements of 

 continents are of this type. 



Frequently, these vertical displacements involve 

 shorelines. For example, shallow ocean floors may 

 be uplifted into a shoreline of emergence, or shores 

 depressed into ones of submergence. 



Warping. Warping provides a gently tilted, rolling 

 countryside in which neither elevations nor depres- 

 sions are obvious. The primary elevations are indi- 

 vidual domes and the main depressions are basins. 



