2Types of Plate Boundaries DivergentConvergentTransform
3Plates move apart, resulting in upwelling of material from the Mantle to create new sea floor. Plates move together, causing one of the slabs of lithosphere to be consumed into the Mantle as it decends beneath the overriding plate.Plates slide past each other, without creating or destroying lithosphere.DivergentConvergentTransform Fault
4W. W. Norton. Modified from Cox and Hardt, 1986. Fig.4.28W. W. Norton. Modified from Cox and Hardt, 1986.Each plate is bounded by a combination of these types of boundaries.
5Zones of Plate Convergence New lithosphere is continually being created at spreading centers:Since the total surface area of the Earth remains constant, lithosphere must also be destroyed somewhere else.Zones of Plate Convergence
6When two plates collide, the leading edge of one is bent downward, allowing it to descend beneath the other plate.
7Upon entering the hot asthenosphere, the plunging plate (which is relatively cold) begins to warm and loses its rigidity.
8The descending lithosphere reaches a depth of up to 700 km (depending on its angle of descent) before its leading edge becomes assimilated into the material of the upper mantle.W. W. Norton
9Three types of convergent plate boundaries: The nature of convergent boundaries is influenced by the type of crustal material involved.
11When the leading edge of a plate capped with continental crust converges with oceanic crust: The less dense continental material (granitic) remains “floating”, while the more dense oceanic slab (basaltic) sinks into the asthenosphere.
12A region where an oceanic plate descends into the asthenosphere is called a subduction zone. As the oceanic plate slides beneath the overriding plate, the oceanic plate bends, producing a deep-ocean trench.
13Oceanic-Continental Convergence The oceanic crust is bent (at an angle of about 45º), permitting it to descend into the lithosphere.
14Oceanic-Continental Convergence Upon entering the hot asthenosphere, the downward moving plate and the water soaked sediments carried upon it begin to melt.
15This newly formed magma is less dense than the surrounding mantle rocks, which causes the magma to rise to the surface.
16Fig.4.14aMost of this magma will be emplaced in the continental crust and form intrusive igneous rocks.
28Oceanic - Oceanic Convergence When two oceanic plates converge, one descends beneath the other initiating volcanic activity (similar to the oceanic - continental case), but the volcanoes form on the ocean floor rather than on continents.
29Volcanic Island ArcDry land emerges from the ocean depths, forming a chain of volcanic islands called a volcanic island arc.
30Examples of an island arcs are the island chains of the western Pacific Ocean, such as Japan. In the case of Japan, volcanic islands are being created by the collision of the Pacific plate with the Eurasian plate. The Pacific plate being subducted beneath the Eurasian plate.
31Japan Subduction Zone and Island Arc The Pacific plate subducting beneath the Eurasian plate.
32The January 17, 1995 Kobe, Japan Earthquake Magnitude 6.9
33W. W. Norton. Modified from Cox and Hardt, 1986. Fig.4.28W. W. Norton. Modified from Cox and Hardt, 1986.
34Example of an island arc off the coast of Alaska Aleutian IslandsExample of an island arc off the coast of Alaska
35Great Alaska Earthquake The second largest earthquake ever recorded occurred along an oceanic - continental plate boundary in Alaska.Great Alaska EarthquakeMarch 28, 1964Magnitude 9.2Elementary school in Anchorage, Alaska destroyed by the 1964 Alaska earthquake.Dallas, Texas Seismogram