National Geographic : 1952 Jan
Our Home-town Planet, Earth National Geographic Photographer Willard R. Culver Tremendous Pressures Bring About Fantastic Changes in Earth's Deep Rocks and Metals Metals flow like wax, and "incompressible" water is compressed to half its normal volume under loads up to 6,250,000 pounds to the square inch (pages 127 and 128). This hydraulic press at Harvard University can duplicate pressure 500 miles down and give clues to structural changes. Prof. Percy W. Bridgman attaches wires to measure the electrical conductivity of compressed copper. earthquake belts. Here faults reaching down inside the earth tap reservoirs of molten magma (pages 116, 118, 119). Gases are trapped inside this upwelling magma; releasing the pressure inside a vol cano is much like uncapping a bottle of ginger ale. In both cases bubbles of gas froth out. The hot lava that flows forth in a volcanic eruption is really magma froth, a mixture of molten rock and gas bubbles. When Volcanoes Explode When lava boils up inside a tight-plugged volcano, the gases trying to escape may build up enough pressure to blow up the entire mountain. This happened when Mount Kat mai exploded in Alaska in 1912, creating the spectacular Valley of Ten Thousand Smokes.* It happened again when Mount Krakatau (or Krakatoa) in the Dutch East Indies exploded in 1883, destroying 36,000 lives.t Rising there today is a new volcano, called by the natives Anak Krakatau, "Child of Krakatau." Lava spewed out by volcanoes becomes highly fertile soil through weathering. The volcanic island of Java, only the size of New York State, supports 40 million people and is one of the most densely populated areas of the globe. Ice Once Covered Third of Land Another slow but mighty influence on man's earthly destiny is the gradual march of the Ice Ages. Fed by accumulating snow, enormous masses of ice, in places perhaps as much as two miles thick, have crept outward periodi cally from Arctic and Antarctic centers and down from high mountains and tablelands. At least once these glaciers covered nearly a third of the earth's land area. * The National Geographic Society's discovery and investigation of this remarkable volcanic region was described in the NATIONAL GEOGRAPHIC MAGAZINE by Dr. Robert F. Griggs, leader of The Society's expedi tions, in "Valley of Ten Thousand Smokes," January, 1917, and February, 1918; and "Our Greatest National Monument," September, 1921. t See "Eruption of Krakatoa," by Sir Robert Ball, NATIONAL GEOGRAPHIC MAGAZINE, June, 1902.