Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 36 to 42. Earthquakes are the most lethal of all natural disasters. What causes them? Geologists explain them in terms of a theory known as plate tectonics. Continents are floating apart from each other; this is referred to as the continental drift. About sixty miles below the surface of the sea, there is a semi-molten bed of rock over which plates, or ...

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 36 to 42.

Earthquakes are the most lethal of all natural disasters. What causes them? Geologists explain them in terms of a theory known as plate tectonics. Continents are floating apart from each other; this is referred to as the continental drift. About sixty miles below the surface of the sea, there is a semi-molten bed of rock over which plates, or slabs, carry continents and sea floors at a rate of several inches a year. As the plates separate from each other, a new sea floor is formed by the molten matter that was formerly beneath. Volcanic islands and large mountain ranges are created by this type of movement. The collision of plates causes geological instability such as that in California called the San Andreas Fault, located between the Pacific and North American plates. The plates there are constantly pushing and pulling adjacent plates, thereby creating constant tremors and a potential for earthquakes in the area

Geologists would like to be able to predict earthquakes accurately. Using laser beams, seismographs, gravity-measuring devices, and radio telescopes, they are presently studying the San Andreas Fault to determine the rate of strain and the amount of ground slippage. Calculations indicate that sometime in the future, California will be struck by a major earthquake. In spite of the geologists' theory of plate tectonics, there are still gaps in man's understanding and knowledge of the causes of earthquakes. Powerful earthquakes have occurred in places where plate boundaries are hundreds of miles away. In the 1800s New Madrid, Missouri, and Charleston, South Carolina, were shaken by earthquakes that no one had foreseen.

Certain areas of the world are quake prone. Italy, Yugoslavia, and Algeria have experienced many quakes. In November 1980, Naples was struck by an especially devastating quake. China and Japan have also been hit by horrendous quakes. In 1923, Tokyo and Yokohama were reduced to rubble by gigantic tremors that were followed by fires, tornadoes, and finally a thirty-four-foot tsunami, or tidal wave, which was caused by the earth's drop into the waters of Tokyo Bay.

What effects have geologists' predictions of earthquakes had? The Chinese in Haicheng in 1974 were warned that an earthquake might occur within the next year or two. With the help of amateur seismologists' observations of animal behavior and the rise and fall of water in wells and measurements of quantities of radioactive gas in water, professional geologists were able, in January 1975, to predict an earthquake within the next six months. On February 4, Haicheng was destroyed, but because its residents had been evacuated, very few people were killed. In California, where earthquake is an ever-present menace, building codes now require quakeproof structures, and Civil Defense units have intensified their training in how to deal with disaster should it strike or, perhaps more accurately, when it strikes.

Continental drift is the concept that_____________.