7 Major Plates • African Plate, covering Africa - Continental plate • Antarctic Plate, covering Antarctica - Continental plate • Australian Plate, covering Australia (fused with Indian Plate between 50 and 55 million years ago) - Continental plate • Eurasian Plate covering Asia and Europe - Continental plate • North American Plate covering North America and north-east Siberia - Continental plate • South American Plate covering South America - Continental plate • Pacific Plate, covering the Pacific Ocean - Oceanic plate
What Drives Tectonics? • Definition : The cycle of movement in the asthenosphere that causes the plates of the lithosphere to move. Heated material in the asthenosphere becomes less dense and rises toward the solid lithosphere, through which it cannot rise further. It therefore begins to move horizontally, dragging the lithosphere along with it and pushing forward the cooler, denser material in its path. The cooler material eventually sinks down lower into the mantle, becoming heated there and rising up again, continuing the cycle. See also plate tectonics. Convection
What Kind of Boundaries are Between Plates • Main Motions • Convergent Boundaries • Divergent boundaries
Convergent Boundaries • 3 Types • Ocean to Ocean • Continental to Continental • Ocean to Continental
Ocean to Ocean • One plate is subducted, initiating andesitic ocean floor volcanism on the other. It eventually forms an island arc volcanic island chain with a deep ocean trench by its side. It is described by a progression from shallow to deep focus earthquakes from the trench toward the island arc. Also a back-arc basin may form if subduction rate is faster than forward motion of overriding plate
Ocean to Continental • Oceanic plate is dense and more heavey which causes it to subduct under the Lighter continental plate. It produces a deep ocean trench at the edge of the continent. About half the oceanic sediment descends with the subducting plate; the other half is piled up against the continent. The Subducting plate and sediments partially melt, producing andesitic or granitic magma. This produces volcanic mountain chains on continents called volcanic arcs and batholiths. Part of the oceanic plate can be broken off and thrust up onto the continent during subduction (obduction). Obduction can expose very deep rocks at the surface. It is characterized by shallow to intermediate focus earthquakes with rare deep focus earthquakes
Continental to Continental • Continental crust cannot subduct, so continental rocks are piled up, folded, and fractured into very high complex mountain systems. It is characterized by shallow-focus earthquakes, rare intermediate-focus earthquakes and practically no volcanism
Divergent Boundaries • Divergent bounders where the plates move away from one another, creating a environment that seems to stretch, characterized by shallow-focus earthquakes and volcanism. A release of pressure may causes partial melting of mantle and produces basaltic magma. This magma rises to surface and forms new oceanic crust. When it occurs with oceanic crust (oceanic ridges) and in continental crust (rift valleys).
Mid-Atlantic Ridge • The Mid-Atlantic Ridge forms part of the global mid-oceanic ridge system and, like all mid-oceanic ridges, is thought to result from a divergent boundary that separates tectonic plates: the North American Plate from the Eurasian Plate in the North Atlantic, and the South American Plate from the African Plate in the South Atlantic. These plates are still moving apart, so the Atlantic is growing at the ridge, at a rate of about 5–10 centimeters per year in East-West direction.
Divergent Boundaries • Continental to continental: Rift valleys are produced by tensional tectonic forces which occur at divergent plate boundaries. Rift valleys typically appear as a downdropped Graben between a pair of faults, or vertical Earth movements. Rift valleys are often associated with and flanked by Volcanoes. The margins of rifts are commonly uplifted, so that the downfaulting of the rift floor is associated with the uplift of both margins. Also inn continental rift valleys the true cross-sectional form is typically asymmetric, with the rift floors tilted toward the most elevated flank. Most of the subsidence is controlled by one border fault system, and most of the internal faults parallel the dip of the border faults
Con’t • Ocean to Ocean: Where 2 plates are moving away from each other, deep rifts are opened through the crust that allows magma from the upper mantle to rise to the surface and cool. Newly differentiated mantle material which include structures and sediments from and of basaltic flows, pillows, and breccias; basalt is a dark, fine-grain, high density igneous rock. All this up and down involves distorting the crust several things can happen like fractures, joints, slips, faults, and folds
The rocks of the oceanic crust are not older than 200 million years. The material of which the oceanic crust consists is for the greater part tholeiitic basalt (this is basalt without olivine). Basalt has a dark, fine and gritty volcanic structure. • The basalts the are from magma or lava when it reaches are made of the elements silicon, oxygen, aluminum, iron, magnesium, calcium, sodium, potassium, phosphorus, and titanium. Have layers of basaltic rock which are from the oceanic crust
Convergent Boundaries • Also deal with oceanic and Continental crust
Continental Crust • The continental crust is the layer of granitic, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. Granitic- A crystalline igneous rock that consists largely of alkali feldspar (typically perthitic microcline or orthclase, quartz, and plagioclase (commonly calcic albite or oligoclase). finer-grained rocks of this composition include rhyolite and aplite, and coarser-grained ones are granite pegmatite (Hart 1998).
Over View Subduction • Have oceanic sinking under the continental • Making basalt on the bottom with granite compositions on top • Main featured are mountains with volcanic activity • Magma rising from the mantle
Colliding plates • Massive pilling up • Granite formations • Mountains • Examples is the Himalayas
Cited Work and References • Websites • U.S. Geological Survey Special Publication: • wc.pima.edu/.../setting/geology_platetec.htm • http://wc.pima.edu/~bfiero/tucsonecology/setting/images/plates_ooc01.gif • gpc.edu/~pgore/Earth&Space/GPS/platetect.html: Pamela J.W. GoreGeorgia Perimeter College • http://www.cotf.edu/ete/modules/msese/earthsysflr/plates3.html • http://www.cotf.edu/ete/modules/msese/elevator.html • http://mediatheek.thinkquest.nl/~ll125/en/crust.htm
Con’t • Kious, W.J., and Tilling, R.I., 1996, THIS DYNAMIC EARTH--THE STORY OF PLATE TECTONICS: U.S. Geological Survey Special Publication, 77 p • Courtillot, Vincent and Vink, G.E., 1983, HOW CONTINENTS BREAK UP: Scientific American, v. 249, no. 1, pp. 42-49 • Bird, J.M., ed., 1980, PLATE TECTONICS (revised ed.): Washington, D.C., American Geophysical Union, 986 p.