The most prominent features of the Earth, based primarily on size, are the continents and ocean basins. Plains, plateaus, and mountain ranges are the second most prominent features on the solid parts of the Earth. The continental masses with their major surface features appear to have a high degree of permanence and to be the products of forces that act over extremely long periods of time.

Although the term continent generally refers to large and extensive landmasses or mainlands, such as continental Europe, it has distinctive meanings in the Earth sciences. Geographically, "continent" refers to the surface of large continuous landmasses that make up about 29.2 percent of the Earth's surface. These large land areas are: Asia (29.9 percent of the land area), Africa (20.6 percent), North America (14.8 percent), South America (12 percent), Antarctica (10.5 percent), Europe (7 percent), and Australia (5.2 percent). Collectively, these continental areas occupy about 57,100,000 square miles (148,000,000 square kilometers) with a mean height above sea level of about 2,760 feet (840 meters). Distinctive parts of continents bounded by high mountains or water, such as Greenland or the India-Pakistan area, are called subcontinents. Because the Australian landmass is surrounded by water and isolated from the other continents, it is sometimes referred to as an island continent. Small continents such as Madagascar or the Seychelles Islands, are called microcontinents because the crust of which they are composed is similar to continental lithosphere, or the thick rocky layer of continental crust, and hence they are quite different from true oceanic islands.

The continents are distributed unevenly over the Earth's surface. More than 65 percent of all the land area, for example, lies in the Northern Hemisphere, which is sometimes referred to as the "land hemisphere." The Southern Hemisphere is truly an oceanic realm. Only about 11 percent of the Southern Hemisphere is above water.

The definition of continent includes the underwater portions of continental margins, that is, the continental shelf, the continental slope, and the continental rise. Continental margins are enormous, stretching for a total distance of 217,500 miles (350,000 kilometers). Approximately 15 percent of the area of the world's oceans lies over the submerged margins of continental crust. Some continental margins extend underwater to depths approaching 6,560 feet (2,000 meters). The width of the continental shelf ranges from zero to 930 miles (1,500 kilometers) but averages about 48 miles (78 kilometers). The continental slopes, which extend from the edge of the shelf to about 2,000 meters deep, are deeply furrowed by submarine canyons that serve as routes for sediment transport away from the continental shelves. The continental rise occurs on the seaward side of slopes where deep-sea trenches are absent.

Analysis of the forces of compression and tension in the Earth's crust, obtained from monitoring earthquakes indicates that the typical continental structure consists of a series of layers . The type of crust that underlies the continents and continental shelves has distinctive rocky and chemical properties. Although the character of the continental crust is still not perfectly understood, studies in different parts of the world suggest that there is considerable variety in the makeup of the crust. The crust is generally considered to be everything above a zone called the Mohorovicic Discontinuity, or Moho. This transitional zone is located where earthquake shock-wave transmission suddenly changes from about 4.3 miles (7 kilometers) per second to about 4.8 miles (7.8 kilometers) per second. It is believed that such a transition can be caused by a change in uniform material to a denser state. This so-called M-discontinuity varies in depth from 3 miles (5 kilometers) under the deep oceans to 40 miles (70 kilometers) under the Andes Mountains, with an average depth of 19 miles (30 kilometers). There is also another zone of relatively low seismic velocity and low rigidity, possibly due to partial melting, that lies below the Moho within the upper mantle, between 37 and 155 miles (60 and 250 kilometers) deep. Everything above this low-velocity zone is termed lithosphere, meaning rocky sphere, of which the crust and Moho are a part (see Earth).

Chemically, continental crust is termed sial, meaning that it is rich in silica-alumina rock. In contrast with continental crust, the basalt-type of ocean crust is termed sima because it is rich in silica-magnesium rock. The density of the upper layer of continental crust is about 1.56 ounces per cubic inch (2.7 grams per cubic centimeter), and the velocities of seismic waves through it are normally less than about 4.3 miles (7.0 kilometers) per second.

The crust of the continents is believed to have originated by a chemical change in which lighter materials were produced from a heavier, denser interior; that is, from the volcanic basalt of the mantle. Because the entire crust totals only about 0.01 percent of the volume of the present mantle, a great deal more crust formation is still possible. Scientists speculate that as the mantle chemically split up into different components, relatively light elements rose to the surface to form the crust, atmosphere, and sea water. Large portions of the crust solidified a long time ago. Dating of continental rocks suggests that the continental crust did not begin to stabilize until about 3.9 billion years ago, nearly 1 billion years after the core and the mantle separated.

Studies of heat flow, seismology, and gravity surveys combine to suggest several different regional types of crust. Shield and plains-type crust, also referred to as cratonic crust, averages between 29 and 31 miles (47 and 50 kilometers) in thickness. Mountain-type crust, or "orogenic" crust, at about 21-24 miles (33-38 kilometers) thick is considerably thinner than the shield type. Block-faulted intermontane plateaus and subsidence belts are types of crust characterized by the presence of geologic faults. They are tectonically active zones of crustal stretching or twisting.

Each and every continent consists of a specific group of components that includes shields, platforms, intracratonic and marginal basins, mountain belts, block-faulted belts, volcanic plateaus, and volcanic belts. Differences between continents are basically reduced to considerations of relative sizes and ratios of these components. Africa's shield areas, for example, are the largest while Asia has more fold mountains and volcanic belts. Climatic belts have profound effects on each continent producing differing histories of weathering, erosion and deposition, landforms, soils, vegetation, and human occupation.

No two mountain chains are alike, but many characteristic features occur repeatedly in different mountain chains. The younger orogenic belts, such as the Alps, have high elevations and rough mountain scenery. They are frequently paralleled at lower elevation by belts of relatively simple folds as occur in the Appalachian Valley. The older mountain belts tend to be largely eroded down to plains or rolling uplands such as the Appalachian Piedmont. The complex patterns of the older mountain belts are probably the result of perpetual rupture, continental collision, and other processes. Still other belts may be revived by the process of block faulting and uplift to form such features as France's Massif Central, Germany's Black Forest, or the Rocky Mountains of Wyoming and Colorado.

Ocean crust is created at oceanic ridges and destroyed in subduction zones where one plate travels under another. The ocean basins are thus relatively young features, most of them not older than the Jurassic Period, which began about 190 million years ago. The continents are, in many parts, very old--some 20 times as old as ocean basins.

According to a principle called isostasy, there is a tendency for the larger, lighter units of the Earth's crust to rise. According to their total mass, thinner and more dense units such as ocean basins stand lower. Thicker and less dense continental landmasses are buoyed up. In mountainous areas, for example, erosion tends to thin the crust from above. Because the mass is reduced, the crust rises to maintain isostatic equilibrium.

Eventually, erosion and isostatic uplift reach equilibrium, and, barring new structural disturbance, there will be no further change and the mountainous terrain will have been reduced to a plain. Many of the great cratons that are exposed today are the planed-down roots of very old mountain ranges. Because high mountains form a relatively small part of the land surface, the mean elevation of the subaerial crust is only 2,756 feet (840 meters). The highest mountains commonly occur near the continental coasts, and large areas of low-lying land--the cratons--tend to be located in the central parts of the continents. A rise in sea level would, for example, produce a significant shift in the shoreline position, an increase in the area of continental shelf, and a decrease in area of subaerial continent. The general height of the continental nuclei above sea level seems, however, to remain fairly constant.

The idea that the continents move about the surface of the planet is an old one. It was formulated initially to explain the striking parallels of the Atlantic coasts of Africa with North and South America. The close geometric fit of the continental margins was not universally accepted as evidence that the Atlantic Ocean was once closed.

The German scientist Alfred L. Wegener and others proposed a hypothesis of continental displacement where large plates of continental crust moved freely across a bed of oceanic crust. The main lines of evidence supporting the idea that the continents were once joined together and then later moved apart are based on the study of ancient climates and fossils, the geometrical fit of the continents, and the matching of geological features across oceans. Opponents of the drift theory proposed counterarguments for each line of evidence and were partially successful because initially the hypothesis lacked quantitative data. Developments during the 1960s provided the quantitative data needed to support the continental drift theory. This data suggested a spreading of the sea floor from the oceanic rift systems. According to the concept of seafloor spreading, the upwelling of mantle material and its spreading from the oceanic ridges caused the ocean floor and coupled continental blocks to move away from the ridges.

The concept of plate tectonics combines the satisfactory parts of the hypothesis of continental drift and seafloor spreading. In this theory of global tectonics the lithosphere is divided into a number of plates with horizontal movement described according to the laws of torsionally rigid bodies. Each plate consists of both continental and oceanic crust, so it must not be assumed that plate boundaries coincide with coastlines along continental margins.

Wegener suggested that before the Mesozoic Era, which began about 225 million years ago, the continents were grouped together into a single block or two. Wegener proposed the name Pangaea for the single continent that he believed once existed. The leading proponent of the theory of two primordial super continents, the South African geologist A.L. du Toit, maintained that from the middle of the Paleozoic Era, or about 400 million years ago, to the beginning of the Tertiary about 65 million years ago, the Northern Hemisphere continent of Laurasia was separated from the continent of Gondwanaland in the Southern Hemisphere by the Tethys Sea. The Tethys was a huge ocean from which the Alps and Himalayan mountain chains eventually emerged. This growth of the continents is consistent with the major structural features of the Earth as they exist today.