North American Plate
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The North American Plate is a tectonic plate covering most of North America, Greenland and part of Siberia. It extends eastward to the Mid-Atlantic Ridge and westward to the Chersky Range in eastern Siberia. The plate includes both continental and oceanic crust. The interior of the main continental landmass includes an extensive granitic core called a craton. Along most of the edges of this craton are fragments of crustal material called terranes, accreted to the craton by tectonic actions over the long span of geologic time. It is believed that much of North America west of the Rockies is composed of such terranes.
The easterly side of the North American Plate is a divergent boundary with the Eurasian Plate to the north and the African Plate to the south forming the northern part of the Mid-Atlantic Ridge. The southerly boundary with the Cocos Plate to the west and the Caribbean Plate to the east is a transform fault, represented by the Cayman Trench under the Caribbean Sea and the Motagua Fault through Guatemala. The rest of the southerly margin which extends east to the Mid Atlantic Ridge and marks the boundary between the North American Plate and the South American Plate remains poorly understood and undefined. The westerly boundary is the Queen Charlotte Fault running offshore along the coast of Alaska and the Cascadia subduction zone to the north, the San Andreas Fault through California, the East Pacific Rise in the Gulf of California, and the Middle America Trench to the south. On the northerly boundary is a continuation of the Mid-Atlantic ridge called the Gakkel Ridge. The rest of the boundary in the far northwestern part of the plate extends into Siberia. This boundary continues from the end of the Gakkel Ridge as the Laptev Sea Rift, on to a transitional deformation zone in the Chersky Range, then the Ulakhan Fault, and finally the Aleutian Trench to the end of the Queen Charlotte Fault system.
On its western edge the Farallon Plate has been subducting under the North American Plate since the Jurassic period. The Farallon Plate has almost completely subducted beneath the western portion of the North American Plate leaving that part of the North American Plate in contact with the Pacific Plate as the San Andreas Fault. The Juan de Fuca, Cocos, and Nazca Plates are remnants of the Farallon Plate.
The boundary along the Gulf of California has not yet been clearly described and research is ongoing. The Gulf is underlain by the northern end of the East Pacific Rise. West of the Rise is the Pacific Plate. East of the Rise, most tectonic maps show the North American Plate. It is generally accepted that a piece of the North American Plate was broken off and transported north as the East Pacific Rise propagated northward, creating the Gulf of California. The block which broke off is known geologically as the Salinian Block. However, it is as yet unclear whether the oceanic crust east of the Rise and west of the mainland coast of Mexico is actually a new plate beginning to converge with the North American Plate, consistent with the standard model of rift zone spreading centers generally.
A few hotspots are thought to exist below the North American Plate. The most notable hotspots are the Yellowstone, Raton and Anahim hotspots. These are thought to be caused by a narrow stream of hot mantle convecting up the Earth's core-mantle boundary called a mantle plume,[1] although some geologists prefer upper-mantle convection as a cause.[2][3] The Yellowstone and Anahim hotspots are thought to have first arrived during the Miocene period and are still geologically active, creating earthquakes and volcanoes. The Yellowstone hotspot is most notable for the Yellowstone Caldera and the many calderas that lie in the Snake River Plain while the Anahim hotspot is most notable for the Anahim Volcanic Belt, currently found in the Nazko Cone area.
[edit] Plate motion
For the most part, the North American Plate moves in roughly a southwest direction away from the Mid-Atlantic Ridge. The motion of the plate cannot be driven by subduction as no part of the North American Plate is being subducted (except for a very small section comprising part of the Puerto Rico Trench), thus other mechanisms continue to be investigated. One recent study suggests that a mantle convective current is propelling the plate. [4]
[edit] See also
[edit] References
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