The Handy Geography Answer Book. Paul A Tucci

THE PHYSICAL ENVIRONMENT

THE EARTH’S MATERIALS AND INTERNAL PROCESSES

      What are the three main layers of the interior of the Earth?

      Scientists do not know very much about what is below the surface of our planet. But we do know that it is divided into three layers: crust, mantle (upper and lower), and core (outer and inner).

      How thick is the Earth’s crust?

      The thickness of the Earth’s crust varies at different points around the planet. Under continents, the crust is approximately 27–31 miles (45–50 km) thick, but under the oceans, it is a mere 6.2 miles (10 km) thick, usually surrounded by an 18-mile- (30-km-) thick contour.

      What did Alfred Wegener discover?

      Although Wegener was not the first to discover that the continents seem to appear to fit together like pieces of a jigsaw puzzle, he was the first to create a theory that the continents seemed to have drifted apart, in 1912. His original paper was entitled “The Formation of the Major Features of the Earth’s Crust (Continents and Oceans).” It was not until long after his death in 1930 that modern technology and scientific analyses proved that he was correct.

      What is continental drift?

      The Earth is divided into massive pieces of crust that are called tectonic plates. These plates lie wedged together like a puzzle. The plates slowly move, crashing into each other to form mountain ranges, volcanoes, and earthquakes. The plates are like rafts floating on water; this is called continental drift. By 1968, the theory of continental drift was gradually replaced by the theory of plate tectonics.

      This contour map indicates the differing levels of thickness in the Earth’s crust as marked in ten-kilometer (six-mile) intervals.

      How do we know that the continents were once joined together?

      There is extensive evidence, dating back many millions, perhaps billions, of years, that the continents were joined together. Fossil evidence of plant and animal life that are similar have been found on different continents, as well as similar glacial sediment.

      How many tectonic plates are there?

      There are seven significant plates on the planet. Some of the largest include the Eurasian Plate, North American Plate, South American Plate, African Plate, Indo-Australian Plate, Pacific Plate, and Antarctic Plate. Some smaller plates are located between the major plates. The smaller plates include the Arabian Plate (containing the Arabian Peninsula), the Nazca Plate (located to the west of South America), the Philippine Sea Plate (located southeast of Japan, containing the northern Philippine islands), the Indian Plate (containing Southeast Asia, India, and South China to Eastern Indonesia), the Cocos Plate (located southwest of Central America), the Juan de Fuca Plate (just off the coast of Oregon, Washington, and Northern California), and the Caribbean Plate (beneath Central America and the Caribbean Sea).

      Pangea was a supercontinent that existed about 200 million years ago. It gradually separated into two large continents—Laurasia and Gondwana—about 120 million years ago, and then those continents divided further over time.

      What type of rocks are formed by lava?

      Igneous rocks are formed when liquid magma under the surface of the Earth, or lava on the surface of the Earth, cools and hardens into rock.

      What type of rocks are formed from particles?

      Sedimentary rocks are formed by the accumulation and squeezing together of layers of sediment (particles of rock or remains of plant and animal life) at the bottom of rivers, lakes, and oceans or even on land. The continual accumulation of more and more layers of sediment places a great amount of pressure on the lowest layers of sediment and, over time, compresses them into rock.

      What are recycled rocks?

      Metamorphic rocks are rocks that had a prior existence as sedimentary, igneous, or other metamorphic rocks. Underground heat and pressure metamorphose or change one type of rock into another, creating a metamorphic, or recycled, rock.

      What type of sand do we often see on beaches near volcanoes?

      Because the content of the sediment near volcanoes is comprised of dark black igneous rock, the resulting beach sand in such places as Hawaii and Indonesia is actually a dark brown or even black in color.

      Which is larger, clay or sand?

      A single grain of sand is 1,500 times larger than a grain of clay.

      What is a dike?

      A dike is magma that has risen up through a crack between layers of rock. When this magma solidifies, it becomes very solid rock. If the rock around it is eroded, a dike can form great rock monoliths above the ground.

      What are hot springs?

      Hot springs are created by underground water that is heated and percolates to the Earth’s surface. Aside from being natural baths, the steam from hot springs can be used to drive turbines, which create electricity. This type of energy production is called geothermal energy.

      Sinkholes have become a growing problem in the United States, destroying roads and sometimes even swallowing houses.

      Where are hot springs used by people around the world?

      Hot spring baths have been used in cultures throughout the world since ancient times. They are found in such places as Japan, Taiwan, Australia, the United States, Iceland, and Sicily.

      Why does the ground sink?

      In many places around the world, seemingly solid land lies over vast oil deposits or water aquifers. Without the liquid supporting it, the ground sinks into the space left behind. In some parts of California’s Imperial Valley, the land has dropped more than 25 feet (7.6 meters) due to underground water being removed from the area. Unless the pumping of underground water and oil is stopped, the land will continue to sink.

      Why do houses fall into sinkholes?

      Houses that sit upon limestone rock have the proclivity to fall into sinkholes. As underground water wears away the limestone rock, it creates underground caverns. If the water wears away too much limestone, the cavern may collapse, taking anything on the surface with it. A sinkhole is just one of the many reasons to have your home inspected by a geologist.

NATURAL RESOURCES

      What is a renewable resource?

      A renewable resource is one that can be replenished within a generation. Forests, as long as they are replanted, are a renewable resource. Materials such as oil, coal, and natural gas are known as nonrenewable resources because they require millions of years to be created. So, once the world’s supply of oil is gone, it’s gone for a long, long time.

      When will we run out of oil on Earth?

      Scientists estimate that there are approximately 1.3 trillion barrels of oil on reserve (not yet pumped) in the world’s oil fields, deep beneath the Earth. Debate continues whether or not we have reached the point of maximum production of this oil, which is called “peak oil.” Based upon today’s rate of demand and consumption, many experts believe that we have approximately 40 years before the world’s oil supplies will be nearing depletion. Many other scientists believe that new technologies and findings