A glance at a map of the Earth reveals its two principal features; the ocean basins and the continents ▼. A significant difference between these two areas is their relative elevation, which results primarily from differences in their respective densities and thicknesses:

• Ocean basins. The average depth of the ocean floor is about 3.7 kilometers (2.3 miles) below sea level, or about 4.5 kilometers ( 2.8miles) lower than the average elevation of the continents. The basaltic rocks that comprise the oceanic crust average only 7 kilometers (4.3 miles) thick and have an average density of about 3.0 g/cm3.

• Continents. The continents are remarkably flat features that have the appearance of plateaus protruding above sea level. With an average elevation of about 0.8 kilometer (0.5 mile), continental blocks lie close to sea level, except for limited areas of mountainous terrain. Recall that the continents average about 35 kilometers (22 miles) thick and are composed of granitic rocks that have a density of about 2.7 g/cm3.

The thicker and less dense continental crust is more buoyant than the oceanic crust. As a result, continental crust floats on top of the deformable rocks of the mantle at a higher level than oceanic crust for the same reason that a large, empty (less dense) cargo ship rides higher than a small, loaded (denser) one.

Major Features of the Ocean Floor

If all water were drained from the ocean basins, a great variety of features would be visible, including chains of volcanoes, deep canyons, plateaus, and large expanses of monotonously flat plains. In fact, the scenery would be nearly as diverse as that on the continents ▲.

During the past 70 years, oceanographers have used modern depth-sounding equipment and satellite technology to map significant portions of the ocean floor. These studies have led them to identify three major regions: continental margins, deep-ocean basins, and oceanic (mid-ocean) ridges.

Continental Margin

The continental margin is the portion of the seafloor adjacent to major landmasses. It may include the continental shelf, the continental slope, and the continental rise.

Although land and sea meet at the shoreline, this is not the boundary between the continents and the ocean basins. Rather, along most coasts, a gently sloping platform, called the continental shelf, extends seaward from the shore. Because it is underlain by continental crust, it is clearly a flooded extension of the continents. A glance at the figure above ▲ shows that the width of the continental shelf is variable. For example, it is broad along the East and Gulf coasts of the United States but relatively narrow along the Pacific margin of the continent.

The boundary between the continents and the deep-ocean basins lies along the continental slope, which is a relatively steep drop-off that extends from the outer edge of the continental shelf, called the shelf break, to the floor of the deep ocean ▲. Using this as the dividing line, we find that about 60 percent of Earth’s surface is represented by ocean basins and the remaining 40 percent by continents.

In regions where trenches do not exist, the steep continental slope merges into a more gradual incline known as the continental rise. The continental rise consists of a thick wedge of sediment that moved downslope from the continental shelf and accumulated on the deep-ocean floor.

Deep-Ocean Basins

Between the continental margins and oceanic ridges are deep-ocean basins. Parts of these regions consist of incredibly flat features called abyssal plains. The ocean floor also contains extremely deep depressions that are occasionally more than 11,000 meters (36,000 feet) deep. Although these deep-ocean trenches are relatively narrow and represent only a small fraction of the ocean floor, they are nevertheless very significant features. Some trenches are located adjacent to young mountains that flank the continents. For example, in the figure above ▲ the Peru–Chile Trench off the west coast of South America parallels the Andes Mountains. Other trenches parallel island chains called volcanic island arcs.

Dotting the ocean floor are submerged volcanic structures called seamounts, which sometimes form long, narrow chains. Volcanic activity has also produced several large lava plateaus, such as the Ontong Java Plateau located northeast of New Guinea. In addition, some submerged plateaus are composed of continental-type crust. Examples include the Campbell Plateau southeast of New Zealand and the Seychelles Bank northeast of Madagascar.

Oceanic Ridges

The most prominent feature on the ocean floor is the oceanic ridge, or mid-ocean ridge. As shown in the figure above ▲, the Mid-Atlantic Ridge and the East Pacific Rise are parts of this system. This broad elevated feature forms a continuous belt that winds for more than 70,000 kilometers (43,000 miles) around the globe, in a manner similar to the seam of a baseball. Rather than consist of highly deformed rock, as do most of the mountains on the continents, the oceanic ridge system consists of layer upon layer of igneous rock that has been fractured and uplifted.

Being familiar with the topographic (or relief) features that comprise the face of Earth is essential to understanding the mechanisms that have shaped our planet. What is the significance of the enormous ridge system that extends through all the world’s oceans? What is the connection, if any, between young, active mountain belts and oceanic trenches? What forces crumple rocks to produce majestic mountain ranges? These are a few of the questions that will be addressed in the next chapter, as we begin to investigate the dynamic processes that shaped our planet in the geologic past and will continue to shape it in the future.

Major Features of the Continents

The major features of the continents can be grouped into two distinct categories: uplifted regions of deformed rocks that make up present-day mountain belts and extensive flat, stable areas that have eroded nearly to sea level. Notice in the figure below ▼ that the young mountain belts tend to be long, narrow features at the margins of continents and that the flat, stable areas are typically located in the interiors of the continents.

Mountains

The most prominent features of the continents are linear mountain belts. Although the distribution of mountains appears to be random, this is not the case. The youngest mountain belts (those less than 100 million years old) are located primarily in two major zones. The circum-Pacific belt (the region surrounding the Pacific Ocean) includes the mountains of the western Americas and continues into the western Pacific in the form of volcanic island arcs. Island arcs are active mountainous regions composed largely of volcanic rocks and deformed sedimentary rocks. Examples include the Aleutian Islands, Japan, the Philippines, and New Guinea.

The other major mountain belt extends eastward from the Alps through Iran and the Himalayas and then dips southward into Indonesia. Careful examination of mountainous terrains reveals that most are places where thick sequences of rocks have been squeezed and highly deformed, as if placed in a gigantic vise. Older mountains are also found on the continents. Examples include the Appalachians in the eastern United States and the Urals in Russia. Their once lofty peaks are now worn low, as a result of millions of years of weathering and erosion.

The Stable Interior

Unlike the young mountain belts, which have formed within the past 100 million years, the interiors of the continents, called cratons, have been relatively stable (undisturbed) for the past 600 million years or even longer. Typically, these regions were involved in mountain-building episodes much earlier in Earth’s history.

Within the stable interiors are areas known as shields, which are expansive, flat regions composed largely of deformed igneous and metamorphic rocks. Notice in the figure above ▲ that the Canadian Shield is exposed in much of the northeastern part of North America. Radiometric dating of various shields has revealed that they are truly ancient regions. All contain Precambrian-age rocks that are more than 1 billion years old, with some samples approaching 4 billion years in age. Even these oldest-known rocks exhibit evidence of enormous forces that have folded, faulted, and metamorphosed them. Thus, we conclude that these rocks were once part of an ancient mountain system that has since been eroded away to produce these expansive, flat regions.

Other flat areas of the craton exist, in which highly deformed rocks, like those found in the shields, are covered by a relatively thin veneer of sedimentary rocks. These areas are called stable platforms. The sedimentary rocks in stable platforms are nearly horizontal, except where they have been warped to form large basins or domes. In North America, a major portion of the stable platform is located between the Canadian Shield and the Rocky Mountains.

• Two principal divisions of Earth’s surface are the continents and ocean basins. A significant difference between them, their relative elevations, results primarily from differences in their respective densities and thicknesses.

• Continents have relatively flat, stable core areas called cratons. Where a craton is blanketed by a relatively thin layer of sediment or sedimentary rock, it is called a stable platform. Where a craton is exposed at the surface, it is known as a shield. Wrapping around the edges of some cratons are younger mountain belts, linear zones of intense deformation and metamorphism.

• The ocean basins are rimmed by shallow continental shelves, which are essentially flooded portions of the continents. The deep ocean includes vast abyssal plains and narrow, very deep ocean trenches. Seamounts and lava plateaus interrupt the abyssal plain in some places.

• abyssal plains: Vast level areas of the deep ocean floor, usually extending from the base of the continental rise and continuing to mid-ocean ridges.

• continental margin: The portion of the seafloor adjacent to the continents. It may include the continental shelf, continental slope, and continental rise.

• continental rise: The gently sloping surface at the base of the continental slope.

• continental shelf: The gently sloping submerged portion of the continental margin, extending from the shoreline to the continental slope.

• continental slope: The steep gradient that leads to the deep-ocean floor and marks the seaward edge of the continental shelf.

• cratons: The very old, stable parts of continental crust that have not been affected by significant tectonic activity during the Phanerozoic eon. Cratons consist of the shield and stable platform.

• deep-ocean basins: The portion of seafloor that lies between the continental margin and the oceanic ridge system. This region comprises almost 30 percent of Earth’s surface.

• deep-ocean trenches: Elongated depressions in the seafloor produced by bending of oceanic crust during subduction. Also simply called trenches.

• mountain belts: A geographic area of roughly parallel and geologically connected mountain ranges developed as a result of plate tectonics.

• oceanic ridge (mid-ocean ridge): A broad, linear ridge or rise on the ocean floor. The rift at the crest of the ridge represents a divergent plate boundary, where new oceanic crust is generated. Also called mid-ocean ridge or oceanic rise.

• seamounts: Isolated volcanic peaks that rise at least 1000 meters (3000 feet) above the deep-ocean floor.

• shields: Large, relatively flat expanses of ancient metamorphic rock within the stable continental interior.

• stable platforms: The part of a craton that is mantled by relatively undeformed sedimentary rocks and underlain by a basement complex of igneous and metamorphic rocks.