4.11: Chapter Summary

The Earth and Plate Tectonics.

The Earth consists of a solid inner core, liquid outer core, plastic mantle, and solid overlying lithosphere. The mantle, especially the upper mantle or asthenosphere, is close to its melting point and can flow like a fluid, but very slowly. The thin lithosphere consists of continental or oceanic crust overlying a layer of solidified mantle material and is separated into plates that float on the asthenosphere. About 225 million years ago, all the continents were joined. Since then they have been separated by plate tectonic movements.

Plate Boundary Processes.

Lithospheric plates may pull away from (diverge), collide with (converge), or slide past each other. Oceanic crust is created at divergent plate boundaries and destroyed at convergent plate boundaries. Divergent plate boundaries are the oceanic ridges and areas where continents are being pulled apart. Oceanic ridges are undersea mountain chains with many active volcanoes, and they are offset at transform faults.

Convergent plate boundaries are downwelling zones where old oceanic crust is subducted. Subduction zones at the edges of continents are characterized by an offshore trench and coastal mountains formed by compression of the continental crust plate and accumulation of sediment scraped off the subducting oceanic crust plate. Subducted and heated crust melts and magma rises to form volcanoes on the continental crust plate. Subduction zones at which oceanic crust is at the edge of both plates are characterized by a trench and a magmatic arc (and sometimes a separate sedimentary arc) of islands on the nonsubducting plate. A high subduction rate may stretch the nonsubducting plate forming a back-arc basin. A collision where two continents meet at a convergent plate boundary is characterized by mountain chains created by compression of the continental crusts of the two colliding plates.

Hot Spots.

Hot spots cause persistent volcanic activity. Some are situated over zones where upwelling convection extends throughout the mantle. Lithospheric plates move independently of any movements of most hot spots. As the lithospheric plate, hot spot, or both move with respect to each other, hot-spot trails of islands and seamounts are formed.

Plate Interiors.

As new oceanic crust moves away from a divergent plate boundary, it cools, sinks isostatically, and is buried by sediment. Edges of continents that are not at plate boundaries are known as passive margins and are characterized by a flat coastal plain, shallow estuaries and swamps, and a wide, heavily sediment-covered continental shelf.

Sea-Level Change and Climate.

The Earth’s climate is naturally variable. When the average surface temperature changes, eustatic changes of sea level occur globally. When the Earth warms, sea level rises as ocean water expands thermally, and vice versa. At the Earth’s warmest temperatures, the oceans covered as much as 40% of the present land surface area. At its lowest temperatures, sea level was at least 100 m lower than it is today, and most of the continental shelves were exposed.

Isostatic leveling causes sea level to change in relation to the local coast. If continental crust is weighted by ice during a glacial period, or if its temperature falls (density increases), it sinks. If crust loses weight (as it does when ice melts during warm periods) or warms, it rises. However, isostatic leveling is very slow.

Present-Day Oceans.

The Pacific is the largest and oldest ocean. It is ringed by subduction zones and has many volcanic islands and atolls formed at hot spots and magmatic arcs. Because few rivers drain directly into it and sediments are trapped in subduction zones and marginal seas, its seafloor has a relatively thin sediment cover.

The Atlantic Ocean is widening as lithospheric plates move apart at the Mid-Atlantic Ridge. It has few islands and broad continental shelves. Compared with the Pacific Ocean, it has more rivers and thicker average sediment cover.

The Indian Ocean is the youngest ocean. It has a complex oceanic ridge system, few islands, and thick sediment cover, especially in the north, where major rivers empty from the new, easily erodible Himalaya Mountains created at the India–Eurasia continental collision.

There are four types of marginal seas: shallow seas where continental crust is submerged, long narrow seas where continents are breaking apart, seas between continents that are moving toward a future collision, and back-arc basins behind subduction zones.