The best explanation for Pocket Basin and related features is that the ground above the sites of the explosions was weighted down by the water of small lakes which had formed in melted-out pockets of glacial ice. Such localized melting of the glaciers would occur where the ice was in direct contact with underlying thermal features. A rapid draining of the lake waters would then produce a sudden release of pressure over the hot area, resulting in an unusually violent thermal eruption. [Illustration: MUD VOLCANO near Pocket Basin in the Lower Geyser Basin. The mud is formed by chemical decomposition of the rocks chiefly by the action of carbon dioxide and sulfuric acid. The splatter, 5-6 feet high, is caused by the escaping gases. (Fig. 53)] Faulting and its control of thermal activity Most of the major thermal areas of Yellowstone are related to the ring fracture zones of the Yellowstone caldera (fig. 22). Many deep-seated faults and fractures in these zones are presumably situated above the main source of heat of the thermal system. Thus, they provide convenient avenues of travel for underground waters to circulate to great depths, there to become heated and then rise to the earth’s surface (fig. 45). A few areas like Mammoth Hot Springs and Norris Geyser Basin, on the other hand, are not within the ring fracture zones of the caldera. In these areas, the thermal activity is commonly related to other prominent zones of faulting which also afford readymade channelways for the circulation of hot water and steam. Earthquakes Earthquakes occur frequently in areas of active faulting and volcanism; they are caused by sudden movements between adjacent blocks of the earth’s crust as the crust adjusts to new conditions and pressures. Because of its volcanic history and the fact that very recent fault movements have occurred there, it is not surprising that Yellowstone is an especially active earthquake area. Sensitive instruments (_seismographs_) record an average of about five earth tremors daily in and around the Park, and on rare occasions they may record 100 tremors or more in a single day. Nearly all these tremors are so slight that they cannot be felt by man, but at times, perhaps only once in a human lifetime, one is triggered with high enough intensity to sharply draw our attention to the very real earthquake potential that exists constantly in this geologically active area. Such a high-intensity quake occurred in the Yellowstone region near midnight on August 17, 1959.