OUTER LAYERS OF THE EARTH
Earth's crust and the uppermost part of the mantle are known as the lithosphere. This is a fairly rigid zone that extends about 100 kilometers below the surface of the earth. The crust extends some 60 kilometers or so under the continental surface, but only about 10 kilometers below the ocean floor. The continental crust has a lower density than the oceanic crust does. It is primarily a light granitic rock rich in the silicates of aluminum, iron, and magnesium. In a simplified view the continental crust can be thought of as layered: On top of a layer of igneous rock (molten rock that has hardened) lies a thin layer of sedimentary rocks (rocks formed by sediment and fragments that water deposited); there is also a soil layer deposited during past ages in the parts of continents that have had no recent volcanic activity or mountain building.
Sandwiched between the lithosphere and the lower mantle is the partially molten material of the asthenosphere, about 150 kilometers thick. It consists primarily of iron and magnesium silicates that readily deform and flow under pressure.
In efforts to date various regions of the continents, geochemists have shown with radioactive-dating techniques that the oldest rock formations on the continents are between 3.8 and 3.5 billion years old. For North America the oldest part, called a continental shield, is a crescent-shaped region bordering the west and south sides of Hudson's Bay. A younger crescent lying roughly to the west and south surrounds this oldest region, and the westernmost and southernmost parts of the continent are even younger. A somewhat similar pattern exists for the other continents.
Antarctic Plate
The inference is that the continents are not original with the formation of the earth 4.6 billion years ago but are a secondary aspect that continues to grow. We know that the continental margins, particularly the western edge of North America, are new additions to the continents. The coastal regions are building due to the deposition of sediments washed down by rivers from the interior of the continent. In striking contrast the oldest known parts of the oceanic crust are about 200 million years old or almost twenty times younger than the oldest parts of the continents.
PLATE STRUCTURE OF THE LITHOSPHERE:
CONTINENTAL DRIFT
The idea that the continents drift relative to each other was one that few hurried to accept when the German geologist Alfred Wegener (1880-1930) proposed it in 1912. Yet recent research has revealed a variety of evidence showing that the lithosphere is indeed segmented into about a dozen or so major plates of different sizes. Floating on the earth's mantle, they move slowly, carrying the continents with them at an average rate of several centimeters each year. This motion is known as plate tectonics, or more popularly as continental drift.
Exploration of the ocean floor has shown the existence of a number of midocean ridges that rise several
kilometers above the ocean floor and are thousands of kilometers long. They mark one type of plate boundary. For example, the Mid-Atlantic Ridge separates the North and South American plates from the Eu rasian and African plates, while the East Pacific Ridge separates the Pacific plate from the North American, Cocas, and Nazca plates. It appears that lava, forced upward from the asthenosphere into a midocean ridge, pushes out laterally from the ridge to form new plate material, which gradually cools, thickens, and solidifies at the trailing edge of the older plate material Rock samples from as far down as 8 kilometers below sea level verify that earth's youngest volcanic rocks are those found near these midocean ridges.
We have fu rther confi rmation that the plates move from the shape, geological structure, and fossil record of the continents. Still more evidence comes from rocks. Igneous rocks with similar magnetic fields, which were frozen at the time the rocks solidified, have been found at continental margins that are now widely separated.
Another line of evidence derives from the heat flow out of the earth's interior. Compared to the energy falling on the earth from the sun, the interior flow is scarcely a trickle: The heat conducted through an area the size of a football field is roughly equivalent to the energy given off by three 100-watt light bulbs. Yet over the 4.6-billion-year history of the earth this trickle of energy has contributed to the work of making continents drift, opening and closing ocean basins, building mountains, and causing earthquakes. The geographic variation in the heat flow from the interior is not great, but the global variation shows that the major oceanic ridges are high-heat-flow zones while the older conti nental shields and sedimentary regions are low-heat-flow zones.
How are the plates transported across the mantle?
It appears that they are driven by the horizontal flow of convective currents within the mantle, circulating in the upper, softer portion of the mantle. Often the leading edge of one plate is pushed downward and forced into the mantle, to create a deep trench. This process can form a coastal mountain belt, like the Andes, on the overriding plate. As the other plate descends over millions of years, it heats up and becomes part of the general circulation in the asthenosphere. Plates separate along midocean ridges. Most of the great geologic processes-volcanic activity, mountain building, formation of ocean trenches, earthquakes-are concentrated on or near plate boundaries.
A CHANGING FACE FOR THE EARTH
About 200 million years ago the last mass movement of the continents began. Earth then had only one consolidated land mass, today called Pangaea. It is believed that this supercontinent accumulated from migrations produced by previous drifting. Some 20 million years later sea-floor spreading had separated the supercontinent into two segments: Laurasia in the north and Gondwana in the south. About 45 million years later the North Atlantic and I ndian Oceans had widened and South America had begun to separate from Africa, while India drifted northward. During the next 70 million years the South Atlantic Ocean widened into a major ocean, the Mediterranean Sea began to open up, and North America just began to separate from Eurasia.
A computer-generated projection for the next 50 million years suggests that the Atlantic and Indian oceans will enlarge, and the Pacific will contract. Australia will continue drifting northward toward a possible collision with Eurasia. Africa's northward movement will doom the Mediterranean. In 10 million years Los Angeles, which is part of the Pacific plate, will have come abreast of San Francisco, which is sitting on the North American plate, and from there will eventually slide into the Aleutian Trench.
Average plate motions are on the order of 5 to 6 centimeters per year so that the reshaping of the earth's face is quite dramatic when one considers the age of the earth. In about 2 billion years the gradual cooling of the earth from heat loss will mean that the asthenosphere will flow less readily and that the platemotion phase of the earth's evolution will probably come to an end. Thus the earth will enter a new phase, in which the plate motions of the earth's lithosphere are not responsible for most of the large-scale terrain features. Large mountain ranges, like the Himalayas, will no longer be uplifted, and they will erode away over millions of years.
GEOGRAPHY OF THE SURFACE
As far as surface geography is concerned, there appear to have been two major terrain-shaping mechanisms at work on the earth (and for that matter presumably on the moon, Mars, Venus, and Mercury, the terrestrial planets). These are impact cratering by meteoric bombardment and thermal-tectonic activity due to an outflow of thermal energy from the deep interior of the earth. Erosion by wind, water, and life and tectonic activity (deformations and motions of the crust), with its accompanying crustal strains and slippages, are the dominant mechanisms now (only on the earth). They have all but erased the results of the impact-cratering phase in the earth's history, except that remnants of the last of that phase remain in nearly a hundred ancient impact structures, some of which re as large as the largest visible ones on the moon. It is estimated that on the earth tectonic activity with its accompanying volcanic activity dominates better than 90 percent of the present terrain, with not more than 10 percent of the cratered terrain remaining. Present evidence suggests that surface evolution on the other terrestrial planets, as revealed by various space missions, has not been so heavily influenced by thermal-tectonic activity as that for the earth.
