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Transcript
Continental Drift Chapter 10 Wegener’s Hypothesis Once a single supercontinent Started breaking up about 200 mya Continents drifted to current location Evidence, but no mechanism Pangaea Pangaea Wegener’s Evidence 1. Fossils – Similar ones found in areas thought to have been connected 2. Rock formations – – Rock ages same on African & S. American coast Continuation of mountain ranges across continents 3. Climatic evidence – – Glaciers in Africa and S. America Tropical fossils in colder climates Rock Evidence Fossil Evidence Climate evidence Glacial evidence in south Africa, Australia, South America, and India. Occurred at same time. Pangaea Breakup Breakup of Pangaea (cont) Breakup of Pangaea (cont) Breakup of Pangaea (cont) Mid – Ocean Ridges Under sea mountain ranges Sediment thicker farther away from ridges Sediment closer to ridges is younger Ocean floor is young compared to Earth Mid - ocean Ridges Mid – Ocean Ridges Sea Floor Spreading Suggested by Harry Hess Ocean ridge is a crack (Rift) in Earth’s crust Magma fills the crack As ocean floor spreads, continents also move Sea Floor Spreading Sea Floor Spreading Paleomagnetism Study of magnetic properties of rocks Iron rich minerals in magma align with Earth’s magnetic field As magma solidifies, magnetic orientation becomes permanent Residual magnetism is paleomagnetism Earth as a Magnet Polar Wandering Makes more sense for the continents to move than for the North Pole to move Magnetic Properties from Pangaea Paths coincide if continents started as Pangaea Magnetic Reversals Earth’s magnetic field does not always point north Magnetic reversals – orientation is opposite of normal “Normal” rocks and “Reversal” rocks line up by time period Alternating normal and reversed polarity over time Geomagnetic time scale Magnetic Reversals Magnetic Reversals Plate Tectonics Theory that explains how large pieces of lithosphere, called plates, move and change shape Study of formation of features in Earth’s crust Lithosphere Solid outer layer of Earth Consists of the crust and rigid upper part of the mantle Broken into blocks called plates Plates float on the asthenosphere Asthenosphere Solid plastic layer of mantle below the lithosphere Made of mantle rock that flows very slowly Allows tectonic plates to move Lithosphere & Asthenosphere Crust types 1.Oceanic crust – Dense – Rich in magnesium and iron 2.Continental crust – Low density – Rich in silica (SiO2) Continental & Oceanic Crust Tectonic plates Can be either or both oceanic and continental crusts 15 major plates Earthquakes – sudden shifts along plate boundaries Volcanoes – plate motions generate magma that erupts Tectonic Plates Types of Plate Boundaries 1. Divergent boundaries – – – – Plates move away from each other Most on the ocean floor Form ocean ridges (rifts) Plates separate at rift valley Divergent Boundary Divergent Boundary (on land) Types of Plate Boundaries 2. Convergent boundaries – Plates move toward each other – Plates eventually collide – 3 types of collisions Convergent Boundary Continental – Oceanic Crust Collisions – Subduction zone – oceanic sinks under continental – Magma rises to surface and forms volcanic mountains Continental – Oceanic Collision Convergent Boundary Oceanic – Oceanic Crust Collision – Forms ocean trench (subduction) – Magma forms island arc (Example: Japan) Convergent Boundary Continental – Continental Collision – Forms mountains – Edges crumple and thicken – Creates uplift Continent Collisions The Himalayas Types of Boundaries 3. Transform boundaries – Plates slide past each other – Sudden motion produces Earthquakes – No magma involved Transform boundary San Andreas Fault Earthquakes – 1 Year North America – Earthquakes 10 Year Period Causes of Plate Motion 1. Mantle convection – Convection currents drag plates 2. Ridge Push – Newly formed rock slides downhill at mid ocean ridge 3. Slab pull – Subducting plate pulls more crust with it Plate Motion Rifting Breaking apart of Earth’s crust Terrane Piece of lithosphere with a unique geologic history incorporated into another piece of lithosphere Western U.S. Terrane Alaska Accretions Continental Growth Effects of Continental Change Climate changes Life – As populations separate, new species develop