4.8: Plate Interiors

Oceanic crust is formed and destroyed by the tectonic processes that occur at divergent and convergent plate boundaries and hot spots. Its topography is modified by various processes over the tens or hundreds of millions of years between its creation and destruction. The edges of the continents, which are formed at divergent plate boundaries, are modified by a variety of processes between the time they are formed and the time they enter convergent plate boundaries.

Oceanic Crust

As it moves away from an oceanic ridge, the lithosphere cools, becomes denser, and sinks steadily deeper into the asthenosphere. Therefore, the seafloor becomes lower with increasing distance from the oceanic ridge. As the cooling crust sinks, it is progressively buried by a continuous slow “rain” of solid particles through the water column that accumulate as sediments on the seafloor (Figs. 4-20, 4-27). Because sediments tend to accumulate faster in topographic lows, the original topography of the rugged oceanic crust that is formed at oceanic ridges is progressively buried and smoothed as the crust moves away from the oceanic ridge. The effect is very similar to that of a snowfall. If left undisturbed, a few centimeters of snow obscures features, such as street curbs and potholes, and softens larger features by mounding around them. As more snow accumulates, larger features are buried, and even cars in a parking lot may be difficult to find. Similarly, the lower topography of the oceanic crust is completely obscured after it has traveled a few hundred kilometers away from the ridge, though the oceanic crust takes millions of years to move such distances. The higher topography of the oceanic crust survives the sedimentation as rounded hills or mountains rising above the surrounding flatter areas. 

Figure 4-27. The history of an ocean from its creation at an oceanic ridge to its disappearance at a continental collision.

The largest topographic features of oceanic hot spots and convergent boundaries where both plates have oceanic crust (e.g. magmatic arcs) are commonly cone-shaped volcanoes. Their conical form is preserved and even enhanced by sediment accumulation. Therefore, much of the deep-sea floor is characterized by generally cone-shaped abyssal hills and mountains (called seamounts), even in regions far from the oceanic ridges or hot spots (Fig. 3-4).

Some seamounts have flat tops and are called “tablemounts” or guyots. Volcanic mountain cones that have sufficient elevation when first formed at the oceanic ridge or at hot spots emerge above sea level as islands. The island tops are eroded by wind, water, and waves much faster than the volcano can cool and sink isostatically. Thus, before such volcanoes sink isostatically below the surface, their tops are totally eroded away. The eroded flat tops are preserved once the volcanoes are completely submerged because erosion is extremely slow under the ocean surface, far away from winds and surface waves. The Hawaiian Island–Emperor Seamount chain exemplifies the various stages of this process (Fig. 4-25).

Some seamounts form the submerged base of nearly circular coral reefs called atolls (Fig. 4-28). Reef-building corals grow only in shallow water (less than several hundred meters), and most species inhabit the warm tropical oceans (Chap. 15). In such regions, a coral reef may become established around islands that are formed when the tops of oceanic ridge or hot-spot volcanoes extend to or above sea level (Fig. 4-29). The coral reef continues to build upward from the flanks of the volcano as the volcano sinks isostatically. If the upward growth rate of the reef is fast enough to match the sinking rate of the volcano, the top of the live coral remains in sufficiently shallow water to continue growing.

Figure 4-28. Satellite image of a coral atoll with its shallow interior lagoon (Hull Island, Kiribati).

Figure 4-29. Coral reefs form in shallow water in the tropics and subtropics. Reefs are well developed around volcanic islands created at hot spots because there is little runoff of freshwater and sediment, both of which inhibit healthy growth of coral reefs. Coral reefs evolve as volcanic islands are formed and then sink isostatically as they move off the hot spot and cool. (a) Fringing reefs are formed around the perimeter of rising or static volcanic islands. (b) When the island sinks, a barrier reef is formed as the fringing reef grows upward. (c) Eventually, the island sinks completely beneath the surface, leaving an atoll where the barrier reef continues to grow upward.

The coral first forms a fringing reef adjacent to the island (Fig. 4-29a). As the volcano continues to sink, this reef continues to grow upward and becomes a barrier reef separated from the island by a lagoon (Figs. 4-29b, 4-30). Eventually, the volcano completely sinks and leaves only an atoll (Figs. 4-28, 4-29c). On the historic Beagle voyage of 1831 to 1836, Charles Darwin visited Keeling Atoll and several reef-fringed islands of the South Pacific. Even though Darwin had no knowledge of plate tectonics, he proposed an explanation for the formation of atolls that is very similar to our understanding of that process today.

Figure 4-30. This volcanic island, Bora Bora in the Leeward group of the Society Islands of French Polynesia, is surrounded by a lagoon and a well-developed barrier reef.

Continental Edges

We have seen how the processes at convergent, divergent, and transform fault plate boundaries form and shape the edges of continents. However, fewer than half of the coasts (or margins) of today’s continents lie at plate edges. Most are located in the middle of lithospheric plates. Most of these coastlines were formed initially at the rift zones created as Pangaea broke apart. Because there are few earthquakes or volcanoes at such continental margins, they are called passive margins. The Atlantic coasts of North America and western Europe are examples.

Passive Margins

Development of a passive margin begins as the edge of a new continent is formed at a mature continental rift valley (Fig. 4-31). The new edge is isostatically elevated because of heating in the rift zone. Consequently, rivers drain away from the edge toward the interior of the continent. Shallow seas form in the rift as blocks of the continental crust edge slide down into the rift zone. Because few rivers carry sediment into the seas, the turbidity of the water in these seas is low. Consequently, the light needed for photosynthesis (CC14) penetrates deep into the clear waters, and primary productivity (growth of marine organisms that create organic matter from carbon dioxide and an energy source; Chap. 12) is high. High productivity leads to large quantities of organic matter that may accumulate in the sediment of the new marginal seas. Because the seas are shallow and the rift valley is narrow, they may be periodically isolated from exchange with the large ocean basins. Under these conditions, thick salt deposits may be formed as seawater evaporates and its dissolved salts precipitate (Chap. 6).

Figure 4-31. A passive margin develops after the initial stages of the creation of a new ocean. (a) A rift forms at a continental margin. Layers of volcanic ash, desert sands, and evaporites form as lakes develop and dry up in the rift valley. (b) As the new ocean forms, reefs and high-productivity lagoons form along the ocean edge because rivers flow inland and turbidity is low in such young oceans. (c) As the margin moves away from the new oceanic ridge, it cools and subsides. Rivers start to flow toward the new ocean, depositing lithogenous sediments in the previously productive lagoons and reefs. Productivity in the coastal waters, especially coral reef growth, is suppressed by high turbidity caused by the river inputs of suspended sediment. (d) Sediment layers build up on the passive margin to form a broad sediment-covered continental shelf. The buried lagoon and reef sediments often form oil reservoirs.

As the passive margin moves away from the divergent plate boundary, which by that time has developed an oceanic ridge, both continental crust and oceanic crust cool and subside isostatically. Eventually, the edge of the continent subsides sufficiently that the direction of the slope of the landmass is reversed. Then the rivers that flowed away from the margin during its early history instead flow into the ocean at the margin. This runoff brings large quantities of sediment from the land, which increases turbidity and siltation in the coastal ocean, thus reducing light penetration and primary productivity. Both the marginal seas and their deposits of organically rich sediment eventually are buried deeply as they subside farther below sea level. When organic sediments are sufficiently deep to be heated and subjected to high pressure, the organic matter may be converted into hydrocarbon compounds, such as those found in oil and gas. If the overlying rocks are permeable, the oil and gas migrate toward the surface. Where the overlying rocks are not permeable, they trap the oil and gas, forming reservoirs. Reservoirs formed at passive margins provide most of the world’s oil and gas.

The characteristic feature of a passive margin is a coastal plain, which may have ancient, highly eroded hills or mountain chains inland from it. In addition, the coastal region is generally characterized by salt marshes and many shallow estuaries. Offshore, the continental shelf is wide and covered by thick layers of sediment. These features have been modified in many areas by changes both in sea level and in the distribution of glaciers that, in turn, were caused by climate changes.

The Fate of Passive Margins

The depth and width of the continental shelf at passive margins can be influenced by isostatic changes caused by the heating of relatively immobile continental blocks (CC2). For example, the continental shelf of much of the west (Atlantic) coast of Africa is narrower than those of many other passive margins. The reason is that the entire continent has been lifted by isostatic leveling in response to the heat accumulation that is causing the East African Rift to form. Africa’s Atlantic coast may eventually become a new subduction zone if the Atlantic Ocean crust near Africa cools sufficiently and if the East African Rift continues to expand. Another possibility is that the rifting in East Africa will simply stop. Many factors will influence the outcome, including how much heat is available in the mantle to drive the East African rifting process and to continue spreading at the Mid-Atlantic Ridge, how cold and dense the Atlantic Ocean crust is near West Africa, and the driving forces on other plates.

Whatever happens in Africa, the probable ultimate fate of some passive margins is that they will become subduction zones. This will be the outcome if the oceanic crust at the passive margin cools sufficiently to subduct into the mantle, causing the plate movements to change and the ocean to begin to close. Eventually, the ocean could be destroyed as two continents meet at a collision margin.