Anatomy of Earth Help (page 2)
Anatomy of Earth
The Earth has a slightly larger diameter at the equator than at the poles. This oblateness , or flattening, is caused by the rotation of the planet. The effect is too small to be visually apparent as the planet is seen from space; the outer planets, especially Jupiter and Saturn, are much more oblate than is the Earth and actually look that way. The Earth’s diameter is 12,756 km (7,926 mi) in the plane of the equator and 12,714 km (7,900 mi) as measured along the rotational axis from pole to pole.
The Power Of Time
Time is one of the most powerful forces in the Cosmos. The Earth-dwelling two-legged creatures who call themselves Homo sapiens have not developed a mature concept of this power and how it can be harnessed. If they had a better understanding of time and how things happen in the long term—millions upon millions of their years—many of the mysteries that befuddle them would become clear and simple in their minds.
Suppose it were possible to look at time so that a year seemed to pass in a fraction of a second? How would Earth look when beheld from such a perspective? The precession of the axis would be apparent; Earth would look like a furiously spinning top. The continents would drift around like ice floes on an Arctic lake during the springtime thaw. In some places large chunks of land would break away from continents. In other places islands would bump into continents and join up with them. Crumpling of the crust, caused by the drifting of land masses, would create mountain ranges. The Hawaiian Islands would drift toward the northwest, eroding down into the ocean at the northwestern end and being born anew in continuous volcanic eruptions at the southeastern end. The sea level would rise and fall periodically; glaciers would advance and retreat. The Earth, which seems like a stable place on a day-to-day scale, would be revealed as dynamic, fluid orb. One might be tempted to suppose that Earth has a life of its own, that it is a gigantic biological cell. An idea of this sort has been posed by some respected Earth scientists. It is called the Gaia hypothesis . However, this notion has not been proven true, and many academics have dismissed it as unscientific.
Given sufficient time, rivers cut canyons hundreds of meters deep. One of the best-known examples is the Grand Canyon in the southwestern United States. It is hard to imagine, on an hour-by-hour or day-by-day scale, how the little Colorado River could have gouged out such a ravine, but time is patient beyond all human understanding. Time has unlimited endurance. It works day and night; it never rests. It carves and chips and grinds, builds new structures atom by atom or cell by cell, and keeps on doing its work for human lifetime after human lifetime, generation after generation, age after age. Time has been at work on Earth for more than 4 billion human-defined years and will continue to mold and change the planet for at least that many years yet to come.
The Earth can be considered to have four distinct layers. The central portion, called the inner core , is believed to be solid. It is extremely hot and consists mainly of iron and nickel. These metals are ferromagnetic , meaning that they can be magnetized. Surrounding the inner core is a liquid iron and nickel layer called the outer core . This liquid flows in huge eddies that are thought to be responsible for the magnetization of the core and hence for the existence of the geomagnetic field .
Above the outer core lies the mantle , consisting of rock similar to granite (called basaltic rock ). The consistency of the mantle would appear solid if you could take a piece of it and hold it in your hand. However, on a long time scale and considered in its entirety, it is a fluid mass. As the eons pass, the mantle flows much like hot tar or molasses, rising up from the center of the planet toward the surface in some places and descending in other zones. One theory holds that this is a mechanism for the transfer of heat from the hot core regions to the surface, where the heat energy ultimately is transferred to the atmosphere and radiated into space. The up-and-down currents result in lateral movement near the upper reaches of the mantle.
The outermost layer, called the crust , floats on top of the mantle and, as the ages pass, moves around on it. The lateral movements of the mantle carry chunks of crust along. The crust is not a uniform, continuous mass but instead has regions where it is deep (about 30 km, or 20 mi) and other regions where it is shallow (perhaps as thin as 10 km, or 6 mi). The thickest parts of the crust form the continents and larger islands. The thin regions lie beneath the seas and oceans. Figure 8-3 is a simplified cross-sectional diagram of the Earth as it would appear if it were sliced in half at the equator.
Practice problems of this concept can be found at: The Planet Earth Practice Problems
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