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World Earthquakes Earthquakes outline plates • Looking at the pattern of major worldwide earthquakes over the past century shows a pattern. • Earthquakes are concentrated into areas called “earthquake zones” and most of these zones form bands or lines. • Scientists came to realize that these bands represent divisions in the lithosphere and separate it into pieces (plates). Plate boundaries • These divisions within the lithosphere are called “plate boundaries”. It is here that plates interact with each other, resulting in major geologic change, features, and events. • Plates may interact in 1 of 3 ways: – Collide , divide (separate), or slide (horizontally past each other). Plate movement Types of Plate Boundaries DIVERGENT CONVERGENT Convergent (compressional) boundaries • Plates collide along convergent boundaries. • The results of this interaction depends on the types of crust collidiing. • Collision zones form where continents collide with other continents. • Subduction zones form where continents collide with ocean crust or ocean crust collides with other ocean crust. Collision zones • The worlds largest mountain ranges are formed from colliding landmasses. • Continental crust cannot sink into the mantle, so the only place to go is up. Here, the crust buckles, fractures, and thickens to form huge mountains. • The fracturing of the crust creates frequent and sometimes violent earthquakes. plate animations site: collision zone Convergent (cont-cont) {violent earthquakes; fold mountains} The Himalayas Subduction zones • Formed by the collision of oceanic crust with either continental or oceanic crust. • Ocean crust is dense enough and thin enough to be “dunked” back into the mantle where it collides with less dense crust. • This process of ocean crust being absorbed back into the mantle is called subduction. Trenches • A trench is a deep, V-shaped crevice (canyon) on the sea floor. • They are created during subduction where the edges of the plates bend downward. • They mark the location of a convergent boundary. Convergent Boundaries Convergent (oceanic-continental) {violent earthquakes & volcanoes} Peru-Chile Trench Aleutian Trench Puerto-Rico Trench Volcanoes • Some of the world’s most violent volcanoes form in subduction zones, usually as a part of an extended range of mountains and volcanoes called a volcanic arc. • These are sparked off by the melting of rock as a plate subducts, heats up, and partially melts. Convergent (oceanic-continental) {violent earthquakes & volcanoes} Convergent (oceanic-oceanic) {violent earthquakes & volcanoes} Divergent Boundary • Plates separate (divide) along a divergent boundary. • Most divergent boundaries are on the sea floor in the form of mid-ocean ridges. • Some form on land in the form of wide rift valleys (Iceland, Eastern Africa). Over time the land splits and a new sea forms. • Volcanic eruptions and earthquakes are common along these boundaries. Divergent boundary {rift valley, mild earthquakes/ volcanoes} A Rift Valley Transform Boundary • Plates slide past each other without much of a collision. • A large crack (called a fault) come to the surface, known as a transform (or strikeslip) fault. • Motion along the fault generates frequent and sometimes severe earthquakes. • The San Andreas fault in California is an example. Transform boundary {strike-slip faults, earthquakes} On the Web • Plate Boundaries RAP • Musical overview