The Urals, washed out by secular erosion, the highly uplifted mountain summits of Kamchatka, the Kurils, and Japan, and finally the tectonic line hidden beneath the waters of the Indian Ocean, represent three stages in the evolution of large mountainous structures on the Earth's crust. It is characteristic of deep faults in the Earth's crust that they form segments or systems of segments of small circles on the Earth's surface; from the geometrical point of view, these circles represent traces on a sphere along which the latter is intersected by surfaces. Thus, considered from outside, planetary deep faults appear to be regular geometric lines on the surface of the spherical Earth. This fact is evident from any careful study of a globe of the Earth.
During the present period, which is a period of jet aviation and space flights, planetary faults in the crust can be observed clearly in nature as well as on a globe of the Earth. The faults show up as regular geometric contours on the Earth's surface. When a jet aircraft at an altitude of 11 or 12 km passes the "roof of the world" (the Pamir highlands) and goes out into the aerial spaciousness over the Indian plain, it is clear to any geologist or geophysicist aboard that the Himalayas, the highest mountains in the world, also crown the boundary between two blocks of the Earth's crust: the high-mountain block of the Pamirs and Tibet, and the crystal massif of the Deccan. Like a wall, the Pamir-Tibetan highlands form a steep vertical barrier, and individual peaks, produced by intensive erosion of the high mountains, stand out upon the uplifting of the Earth's crust in this region. One of these peaks is the highest mountain on the Earth, Chomolungma [Mt. Everest].
Deep faults in the Earth's crust, which are an important characteristic of the crustal structure, are a consequence of the active internal life of our planet. Now let us see whether analogous faults exist on any of the other celestial bodies. We have only to consider carefully the surface of the Moon, in order to recognize upon it formations which are very reminiscent of the deep faults in the Earth's crust. The steep side of the
lunar Apennines which overhangs the flat, rather dark, surface of the Mare Imbrium reminds one immediately of the terrestrial Himalayas rising above the Indian plain. An intersection between the spherical surface of the Moon and a sharply incident surface lying along the arc of a small circle, exactly as in the case of the island arcs of the Earth, is observed along the line of the lunar Apennines and the other mountain chains of our satellite.
Consequently, deep planetary faults, together with their characteristic morphological features, apparently exist both on the Earth and on the Moon, and, in all probability, on the other terrestrial planets (Mercury, Venus, and Mars) as well. It is not impossible that on the strongly eroded surface of Mars the traces of deep planetary faults are observed by us in the form of the very puzzling "canals" of Mars, concerning the nature of which so many hypotheses have been made. Were we to observe from outer space the geometrically regular island arcs and mountain chains of our planet, or just the boundaries between two blocks of the Earth's crust, we might also be led to postulate an artificial origin for such quite natural planetary formations as deep faults.
It was noted at the beginning of this section that deep faults in the crust are regions where mineral deposits are formed. Thus the study of these faults is of great practical value as well as being of theoretical interest.
At present scientists are gradually exploring the possibility of investigating deep faults by comparative methods (by studying planetary cracks on the surfaces of other bodies in the solar system), in addition to penetrating into the Earth's interior by geophysical and geological means, which has always been (and still is) the chief method of studying the faults. Geologists, geophysicists, astrophysicists, and astronauts are thus all confronted by a common problem of greater complexity, the solution of which requires the combined efforts of representatives of all fields.