The ABC's of Plate Tectonics

Sedimentation and Continental Growth

by Donald L. Blanchard

 

      Tectonic forces elevate the land, reducing the surface area of continental masses, while increasing their thickness. Erosion wears away the high places, transporting its sediments to the continents' edge and depositing them off the edge of the continental shelf onto the sea floor, thereby reducing the thickness of continents but increasing their surface area. The form that this "new land" takes depends on a number of factors: the age of the continental margin, the shape of the continental shelf, as determined during the rifting process, and the presence or absence of a subduction zone adjacent to the continental margin.


Rifts: The Making of a Continental Margin.
When a new spreading center forms beneath a pre-existing continent, a rift forms that will eventually, if allowed to proceed normally, divide the original continent into two, with new ocean floor being created to separate them. As the two newly formed plates begin to separate, molten material, mostly basalt, from the mantle beneath will flow upwards into the crack. The heat from this molten material is conducted to the continental material above, reducing its density and causing it to float higher in the mantle, producing a ridge of mountains above the spreading center. As spreading continues, blocks will break loose from the sides of the crack and subside into the void, creating the characteristic "rift valley," such as that in East Africa. As spreading continues, the rift valley will deepen, ultimately subsiding below sea level and allowing ocean water to fill the valley. The now water-covered subsidence blocks will later become the submerged continental shelves.

Depending on the rate of spreading and the amount of heat flowing into the rift, these continental margins may be broad or narrow. Also depending on heat flow, which to a large degree depends on the proximity of plumes, volcanoes may or may not form. Surface fissure flows of basaltic lava may also occur where heat flow is high. (Plumes generally appear to be associated with the formation of new spreading centers, a notion that will be discussed further in a later lesson.)

Continued spreading causes the complete separation of the two land masses, with new sea floor being created beneath the ever-widening ocean. As the continental margins move farther from the heat flow at the spreading, the mountains formed along the continental margins cool and slowly subside back into the mantle. Depending on how much material erosion has removed from their summits, they may sink below the waves and vanish forever.

Freshly rifted continental margins tend to have steep walls, the continental shelves plunging nearly vertically from a few hundred feet deep to the ocean floor, many thousands of feet below. As erosional sediments are washed off the land surface, they first cover the continental shelves, then wash over the precipice to fall on the ocean floor below. Very old continental margins, therefore, tend to have large accretionary wedges on the deep ocean floor piled up against the continental margin.

The rifting process is not always as 'clean' as the above description may sound. Sometimes a segment of spreading center may shift slightly while separation is occurring, causing some of the subsidence blocks to be separated from both daughter continents. These messy remnants may become submerged plateaus or 'banks' on the sea floor, coral atolls, or islands in their own right. New Zealand, for example, is a continental fragment left behind after a long forgotten rifting episode.


Passive and Active Margins:
The continental margins discussed in the preceding section are known as passive margins, where the continent and the adjoining ocean floor are part of the same plate. Passive margins occur on both sides of the Atlantic Ocean, in Europe, Africa, and North and South America. Margins where continent and sea floor are on separate plates, which usually implies a subduction zone adjacent to the continent, are known as active margins. Active margins occur around much of the Pacific Rim, in North and South America, the Alaska and Kamchatka Peninsulas, the Aleutian Islands, and Japan.

Barring a realignment of plate boundaries, passive continental margins can persist for very long spans of time, building accretionary wedges that can extend hundreds of miles out to sea. As the wedge of sediments thicken, the thickest portion can eventually rise above sea level, burying the original continental shelves and becoming 'new' dry land.

Sedimentary deposition along active margins is made somewhat more complicated by the presence of a subduction zone offshore. The rate of sedimentary deposition and the subduction rate, as well as the age of the continental margin, all influence the type of landforms created. The simplest case, which occurs with very young active margins, is where a range of volcanic mountains form on the leading edge of the continent as material from the subducting oceanic plate is heated and the lighter fractions melt their way up through the overlying continent. (The oceanic plate is always subducted under the continental plate.) As the mountains rise, some of the erosional sediments will be washed out to sea, eventually to tumble over the edge of the continental shelf and down into the offshore trench. A small percentage of these sediments may actually be carried beneath the continent, to contribute to the molten material feeding the volcanoes. The great majority of the sediments in the trench, however, will be scraped off of the subducting plate, to adhere to the edge of the continent, as will any extraneous material of lower density previously residing on top of the subducting plate, such as oceanic sediments, seamounts, or even small islands. As material is added to the continental margin, the added weight further depresses the subducting sea floor, effectively pushing the trench farther out to sea. The western coastline of South America remains in this state today, the Peru-Chile Trench being responsible for the formation of the Andes Mountains.


Island Arcs and Back-Arc Basins:
When the volume of sediments is great enough relative to the rate of subduction, the trench adjacent to an active continental margin can be overwhelmed by erosional deposits, and simply stop subducting. Continued convergence of the two plates will cause a new trench to form some distance offshore. Subduction at the new site will lead to the formation of a volcanic island arc, behind which remains a now passive section of sea floor, known as a back arc basin. The presence of an offshore subduction can also produce tension on the overriding continent, as subducting material from the oceanic plate entrains mantle material from beneath the continent. This can lead to rifting and crustal stretching within the back-arc basin.

Continued erosion from the mainland, plus added sediments eroded from the new volcanic islands, will eventually fill the basin up to sea level. Subsequent sediments from the mainland must then be washed over the now dry land, between the volcanic islands, and out into the trench.

Over a long enough span of time, this process of new island arcs and filled up back arc basins can be repeated many times, as a careful examination of the Pacific margin of the Asian continent will reveal. The Kamchatka Peninsula, the Kuril Islands, and the Japanese Archipelago are the most recent (and currently active) island arcs, while the Sea of Okhotsk and the Sea of Japan are back arc basins that have not yet filled with sediments. The Korean Peninsula is a former island arc, as are any number of arc-shaped mountain ranges on the Asian mainland. The Yellow Sea is very shallow (less than 600' or 180 m. deep) but not quite completely filled with sediments, while the Amur River and several other Manchurian rivers flow through former back arc basins that are now high and dry. Manchuria and much of far eastern Siberia, then, are composed of 'new' land created by erosion and island arc formation along a very old active margin. (An explanation of the distinction between 'new' land and 'old' land (or cratons) will be discussed in a subsequent lesson.)


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File created on May 3, 1998.
Last updated November 2004.