National Geographic : 1976 Aug
This abnormally high activity continued for 15 days. Then, on February 19, a severe shock, registering 6 in magnitude, occurred very near us, off to the east where the fault zone runs between Cuba and Jamaica. This was the first time a major shock has been recorded directly on a subsea boundary. With this quake, stress in our area ap parently was relieved. The earth's inner tremblings returned to a lower level. Earth's Crust Moves at Tree-growth Pace Earthquakes had not brought us to the Cayman Trough, but they reminded us that the planet on which we exist is, in fact, a living, restless body.* It is as hard for most people to perceive the earth's crust moving as it is for a butterfly to sense the growth of a sequoia tree on which it alights. The butterfly has no way of compre hending the tree's slow growth, from a small seed to full grandeur. If asked whether the tree is alive, the butterfly might say, "Of course not. I've been here all my life, and it hasn't done a thing!" Likewise, a person liv ing on the earth for less than a hundred years has no direct way of sensing that his world's surface has been changing and moving for four and a half billion years. Once a molten body suspended in the cold reaches of space, the primordial earth cooled and eventually developed an outer crust. Water accumulated in its depressions to form the seas. To us this outer crust is solid ground, but actually it floats-thin and brittle-on a hot, plastic interior. Our ability to sense this crust is similar to that of an ant crawling across a water bed. The ant does not feel the bed moving. Like wise, we do not feel the earth give beneath our feet. If we were giants, however, hun dreds of kilometers high, the earth would respond to our load. As we walked across it, the hot plastic interior beneath the crust would flow away, reacting to our weight. The heat trapped deep inside the earth constantly seeks a way out. It has a slow, in exorable circulation system, which geophys icists think may involve convection currents slowly bringing semimolten material from the hot interior, called the mantle, up to the surface. Perhaps twenty crustal fragments, *See "This Changing Earth," by Samuel W. Matthews, in the January 1973 GEOGRAPHIC. RAYMONDM. WRIGHT,JAMAICANGEOLOGICALSURVEY AND(BELOW)WOODSHOLEOCEANOGRAPHIC INSTITUTION In the bizarre landscape of the deep, scientists aboard Alvin find startling proof of a constantly changing seafloor. Like a thermal spring at Yellowstone, a vent (above) once spewed hot water carrying dissolved minerals from deep within earth's crust. On contact with cold seawater, the minerals precipitated, spattering the sediment-covered seafloor. On a steep slope of the rift valley's west wall, Alvin noses in to pry out a sample of gabbro (left), the main rock of layer three, which the expedition sampled undersea for the first time. A ten-cm pink shrimp swims above the manganese-blackened outcrop. In an unexpected encounter, Alvin locates rock from layer four, the upper mantle, at 3,660 meters (below). It formed deep under ground and perhaps was forced up at this fault plane on the west wall. Only extensive analysis will give the answer.