Plate tectonics is associated with subsidence of many types and scales, particularly on or near plate boundaries. Plate tectonics is associated with the large-scale vertical motions that uplift entire mountain ranges, drop basins to lower elevations, and form elongate depressions in the Earth's surface known as rifts that can be thousands of feet (km) deep. Plate tectonics also causes the broad flat coastal plains and passive margins to slowly subside relative to sea level, causing the sea to encroach slowly onto the continents. More local scale folding and faulting can cause areas of the land surface to rise or sink, although at rates that rarely exceed half an inch (1 cm) per year.
Extensional or divergent plate boundaries are naturally associated with subsidence, since these boundaries are places where the crust is being pulled apart, thinning, and sinking relative to sea level. Places where the continental crust has ruptured and is extending are known as continental rifts. In the united states, the Rio Grande rift in New mexico represents a place where the crust has begun to rupture, and it is subsiding relative to surrounding mountain ranges. In this area, the actual subsidence does not present much of a hazard, since the land is not near the sea, and a large region is subsiding. The net effect is that the valley floor is slightly lower in elevation every year than it was the year before. The rifting and subsidence is sometimes associated with faulting when the basin floor suddenly drops, and the earthquakes are associated with their own sets of hazards. Rifting in the Rio Grande is also associated with the rise of a large body of magma beneath socorro, and if this magma body has an eruption it is likely to be catastrophic.
The world's most extensive continental rift province is found in East Africa. An elongate subsiding rift depression extends from Ethiopia and somalia in the north, south through Kenya, uganda, Rwanda, Burundi, and Tanzania, then swings back toward the coast through malawi and mozambique. The East African rift system contains the oldest hominid fossils, and is also host to areas of rapid land surface subsidence. Earthquakes are common, as are volcanic eruptions such as the catastrophic eruption of Nyiragongo in Congo in January of 2002. Lava flows from Nyiragongo covered large parts of the town of Goma, forcing residents to flee to neighboring Rwanda.
subsidence in the East African rift system has formed a series of very deep elongate lakes, including Lakes Edward, Albert, Kivu, malawi, and Tanganyika. These lakes sit on narrow basin floors, bounded on their east and west sides by steep rift escarpments. The shoulders of the rifts slope away from the center of the rift, so sediments carried by streams do not enter the rift, but are carried away from it. This allows the rift lakes to become very deep without being filled by sediments. It also means that additional subsidence can cause parts of the rift floor to subside well below sea level, such as Lake Abe in the Awash depression in the Afar rift. This lake and several other areas near Djibouti rest hundreds of feet below sea level. These lakes, by virtue of being so deep, become stratified with respect to dissolved oxygen, methane, and other gases. methane is locally extracted from these lakes for fuel, although periodic overturning of the lake's water can lead to hazardous release of gases.
When continental rifts continue to extend and subside, they eventually extend far enough that a young narrow ocean forms in the middle of the rift. An example of where a rift has evolved into such a young ocean is the Red sea in the middle East. The borders of the Red sea are marked by large faults that down drop blocks of crust toward the center of the sea, and the blocks rotate and subside dramatically in this process. most areas on the margin of the Red sea are not heavily developed, but some areas, such as sharm Al sheikh on the southern tip of the sinai Peninsula, have large resorts along the coast. These areas are prone to rapid subsidence by faulting and pose significant risks to the development in this and similar areas.
Transform plate boundaries, where one plate slides past another, can also be sites of hazardous subsidence. The strike-slip faults that comprise transform plate boundaries are rarely perfectly straight. Places where the faults bend may be sites of uplift of mountains, or rapid subsidence of narrow elongate basins. The orientation of the bend in the fault system determines whether the bend is associated with contraction and the formation of mountains or extension, subsidence, and the formation of the elongate basins known as pull-apart basins. Pull-apart basins typically subside quickly, have steep escarpments marked by active faults on at least two sides, and may have volcanic activity. some of the topographically lowest places on Earth are in pull-apart basins, including the salton sea in California and the Dead sea along the border between Israel and Jordan. The hazards in pull-apart basins are very much like those in continental rifts. An example of a trans form boundary with coastal subsidence and uplift problems is found in southern California along the San Andreas fault. Many areas south of Los Angeles are characterized by faults that down-drop the coast because the faults have an extensional component along them. Further north, near Ventura and Santa Barbara, the San Andreas fault bends so that there is compression across the fault, and many areas along this segment of the coast are experiencing tectonic uplift instead of subsidence.
Convergent plate boundaries are known for tectonic uplift, although they may also be associated with regional subsidence. When a mountain range is pushed along a fault on top of a plate boundary, the underlying plate may subside rapidly. In most situations erosion of the overriding mountain range sheds enormous amounts of loose sediment onto the under-riding plate, so the land surface does not actually subside, although any particular marker surface will be buried and subside rapidly.
On March 27, 1964, southern Alaska was hit by a massive magnitude 9.2 earthquake that serves as an example of the vertical motions of coastal areas associated with a convergent margin. Ground displacements above the area that slipped were remarkable—much of the Prince William Sound and Kenai Peninsula area moved horizontally almost 65 feet (20 m), and moved upward by more than 35 feet (11.5 m). Other areas more landward of the uplifted zone were down dropped by several to 10 feet. Overall, almost 125,000 square miles (200,000 km2) of land saw significant movements upward, downward, and laterally during this huge earthquake.
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