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Earth’s Layered Structure Earth’s Layered Structure Earth’s interior consists of three major zones defined by their chemical composition—the crust, mantle, and core. Density estimates for the layers of the earth (in grams per cubic centimeter): Continental Crust: 2.7 to 3.0 Oceanic Crust: 3.0 to 3.3 Mantle (silicates): 3.3 to 5.7 (increasing with depth?) Outer Core (liquid): 9.9 to 12.2 Inner Core (solid): 12.6 to 13.0 Earth’s Layered Structure The earth’s mantle is sub-divided into two layers: 1. The upper (brittle layer) 2. The lower (liquid) layer The earth’s upper mantle and crust form a relatively cool ridged shell called the lithosphere. Beneath the lithosphere is the asthenosphere: A ductile layer where rock is at or near its melting temperature. Earth’s Layered Structure The core is divided into Two different zones: 1. Outer core = 3500 ºC 2. Inner core = 5000 ºC The outer core is a liquid because the temperatures exceeds the melt point the iron-nickel alloy that makes up the outer core. The inner core is much hotter but remains a solid under pressure. The inner core is also comprised of a iron-nickel alloy. The earth’s crust is composed of 2 types of crust: Oceanic Crust: Rock type: Basaltic (mafic composition) Thickness: 3-7 km 1. Oceanic Crust 2. Continental Crust Continental Crust Rock type: Granitic (felsic) Thickness: 8-50 km Plate Tectonics Plate Boundaries and Motion Plate Tectonics Plate tectonic theory is founded on observation of crustal plate movement And the internal structure of the earth. 3 Types of Plate Boundaries • Divergent Boundary – moving _____ apart • Convergent Boundary – moving ________ together • Transform Fault Boundary – moving ________________________ sideways past each other The thick continental plate, pulls-apart, thinning by extensional forces, and fractured into a rift-shaped structure. The continental plate is arched upwards from the convection current's pushing the crust upward Divergent Plate Boundary: Continental Divergent boundary Divergent boundary of two continental plates pull apart. East African Rift rift valley Creating a __________. Example: _____________ Divergent Plate Boundary - Oceanic: Convergent boundary Subduction Convergent boundary of an oceanic plate and a continental plate. Forms a _______ volcanic mountain range trench Examples: Cascades Andes Mts and a ______. _______ or _______ Oceanic Plates Convergent Boundary Convergent boundary of two oceanic plates. island arc and Creates an ________ trench Example: _____ Japan a _____. Convergent Boundary Continental Plates. Convergent boundary of two continental plates. folded mountain Forms a ______ range. Appalachians Examples: ___________, Himalayas Alps, ______________ Transform Plate Boundary Transform-fault boundary where the North American and Pacific plates are moving ____ past each other. San Andreas Fault in California Example: ________________ Transform boundary Transform boundary Plate Boundary Summary Type of Boundary Process Involved Characteristic features Examples Divergent Sea-floor spreading Mid-ocean ridge Rift valleys, Earthquakes, Volcanic Activity Mid-Atlantic Ridge East Pacific Rise Convergent Ocean-ocean subduction Deep sea trenches Islands of Indonesia, Mariana Islands Ocean-continent subduction Volcano along coast of continent Continent-continent High continental collision Transform Plates sliding past each other West coast of S. America mountain chains Himalayas Earthquake activity San Andreas Fault, North Anatolian Fault, Fracture zones along mid-ocean ridge Plate Boundaries Review • Places where plates move apart are called divergent _____________ boundaries. rift valley • When continental plates diverge a ___________ is formed. • When two oceanic plates converge what is created? _________________ an island arc and a trench • The Appalachians formed mainly from continental folded plate collisions and therefore are a __________ mountain range. Convection currents • The force moving the plates is ____________ . What drive Plate Movement? Plates probably move as a result of a combination of the driving mechanisms. Plate separation may be initiated by mantle convection or a rising mantle plume; Plate Motions are classified into two categories; A. Plate-generated movement. B. Mantle-generated movement. A. Plate-generated movement. 1. Slab pull. Hypothesis that the weight of the cold slab descending into the asthenosphere pulls the whole plate along. Normal faulting in the ocean crust provides evidence for these plates experiencing tensional stresses. 2. Ridge push. Hypothesis that topography of the elevated mid-ocean spreading ridges drives the plates by gravity away from the ridge. Calculations suggest that plates could move down the gentle slope formed by the thermal profile of the lithosphere-asthenosphere boundary. However, during initial rifting, plates begin to move without the help of an elevated ridge. B. Mantle-generated movement. 1. Mantle convection cells. Rising warm rock from the mantle spreads laterally upon encountering the lithosphere, applying shear stress and causing it to move. a. Shallow convection model: only the asthenosphere convects, down to a depth of 700 km. b. Deep convection model: the whole mantle down to the core convects. 2. Thermal plumes. Vertical columns of of upwelling mantle 100-250 km in diameter lift the overlying lithosphere and spread laterally, which applies drag to the lithosphere. Many thermal plumes lie at divergent plate margins. Iceland is an example.