. Elementary physical geography . otect these fromdestruction, causing flat-topped hills and smalltable-lands. These lava-capped buttes or mesas(Fig. 218) are very com-mon in the regions be-tween the Rocky Mountains and the Pacific coast. Cause of Volcanoes. — The immediate cause of volcaniceruptions is the presence of steam; and in a measure theeruption may be compared to the bursting of a is steam present in a superheated condition, this tendsto find relief, and the eruption occurs. The origin of theheat which causes the melting of the rock cannot be has to do with the
. Elementary physical geography . otect these fromdestruction, causing flat-topped hills and smalltable-lands. These lava-capped buttes or mesas(Fig. 218) are very com-mon in the regions be-tween the Rocky Mountains and the Pacific coast. Cause of Volcanoes. — The immediate cause of volcaniceruptions is the presence of steam; and in a measure theeruption may be compared to the bursting of a is steam present in a superheated condition, this tendsto find relief, and the eruption occurs. The origin of theheat which causes the melting of the rock cannot be has to do with the heated condition of the earth, andsince we are not certain just what this condition is, we ofcourse are not able to state what causes the molten same cause that produces the folding of mountainsappears to operate in the formation of volcanoes; and thevolcanic action is in most cases, if not in all, an indicationthat the crust is folding. Earthquakes. — By far the greater number of earthquakes 384 PHYSICAL Fig. 243. The earthquake wave. E, epicen- trum. F, focus. j., • _•, occur either near volcanoes or among mountains, thoughsome have occurred at great distances from either of these. The earthquake is a jarringof the rocks, caused by someshock which is transmitted as aseries of spherical waves in alldirections through the point of origin of theshock is known as the focus(Fig. 243), and from this cen-ter the earth waves move in alldirections. If the rocks were of uniform texture, the earth-quake waves would have - a spherical form; but /:, ? Z ->--*. - .>- v~._W!. ^vsince the strata vary incharacter, the rate of mo-tion differs, and conse-quently the sphericalform is distorted (). The point on the earthssurface directly above thefocus is known as theepicentrum, and this isthe place where theshock first reaches thesurface. The waves comefrom the earth at equaldistances from this point,and on all sides of the rock texture
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