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Ch – 15 Plate Tectonics Earth’s layers by physical properties Crust and upper mantle: – Lithosphere – rigid solid which make up the tectonic plates, includes both crust and upper mantle – Asthenosphere – partially molten “weak” layer Lower mantle (mesosphere) mostly solid Core – outer core (molten) – inner core (solid) Ocean and Continental Crust • Oceanic Crust – primarily basalt – 4-7 km thickness (thin relative to continental crust) – denser (heavier) than continental crust • Continental Crust – primarily granite – 20-70 km thickness – less dense (will not undergo subduction) Age of Sea Floor Rocks (red-young, blue-old) Fig. 6.10, p.139 Plate Tectonics: the new paradigm From left to right: • Transform boundary (conservative) • Convergent boundary (destructive) • Divergent boundary (constructive) What happens at a divergent plate boundary Sea Floor Spreading • Two plates move apart • Mantle material upwells to create new seafloor • Mid-Oceanic ridges (underwater mountain range) develop along welldeveloped divergent boundaries • Mid-Atlantic Ridge • East Pacific Rise Geologic features found at divergent boundaries • volcanic activity (often underwater) • mid-ocean ridge (underwater mountain chain) • very young volcanic rock • “linear” seas (e.g. Red Sea, Sea of Cortez) • rift valley – long narrow valley, such as found in East Africa Figure 15.10 Sea Floor Spreading on Land • Sea floor spreading adds thin, lowelevation ocean crust to landmass. Eventually water fills in • Arabian peninsula split from African continent • Process continues in East Africa rift valleys (note lakes filling in low lying ocean crust) • Somali Plate? Geologic features found at divergent boundaries • volcanic activity (often underwater) • mid-ocean ridge (underwater mountain chain) • very young volcanic rock • “linear” seas (e.g. Red Sea, Sea of Cortez) • rift valley – long narrow valley, such as found in East Africa • shallow focus earthquakes What happens at a convergent plate boundary 1. Oceanic-continental subduction – Denser oceanic lithosphere sinks into the asthenosphere under more buoyant continental lithosphere – Pockets of magma develop and rise – Continental volcanic arcs – chain of volcanoes a short distance from plate boundary (e.g. Andes, Cascades) – Deep focus earthquakes Figure 15.14a What happens at a convergent plate boundary 2. Oceanic-oceanic subduction • Two oceanic plates converge and the older, denser one descends beneath the younger, more buoyant one. • Pockets of magma develop and rise • Volcanic Island Arcs forms as volcanoes emerge from the sea • Examples include Japan, Philippines, and the Aleutian Island, • Deep focus earthquakes What happens at a convergent plate boundary • Subduction (Cont’d) • Oceanic-oceanic convergence • Two oceanic slabs converge and the older, denser one descends beneath the younger, more buoyant one. • Forms volcanoes on the ocean floor • Volcanic Island Arcs forms as volcanoes emerge from the sea • Examples include the Aleutian, Mariana, and Tonga islands Figure 15.14b What happens at a convergent boundary Continental Collision (no subduction) • Continental-continental convergence • When subducting plates contain continental material, two continents collide • Can produce non-volcanic mountain ranges such as the Himalayas Figure 15.14c What happens at Transform Fault Boundaries Conservative boundary (no loss or gain of lithosphere) Plates slide past one another • Most transform faults join two segments of sea-floor spreading • Significant non-oceanic tranform fault boundaries include • San Andreas Fault, • Alpine Fault • Anatolian Fault (Turkey) Figure 15.16 Figure 15.17 Modern discoveries supporting Plate Tectonic Theory • Mid-ocean ridges – underwater mountain chains that circle the globe and often mimic the shape of the coastline • Distribution and depths of earthquakes and volcanoes • Relatively young age of the oceanic crust (less than 180 million years) • Lack of deep-ocean sediment Testing the plate tectonics model Evidence for the plate tectonics model • Paleomagnetism • Probably the most persuasive evidence for sea floor spreading • Ancient magnetism preserved in rocks • Paleomagnetic records show Earth's magnetic field reversals recorded in rocks as they form at oceanic ridges Figure 15.19 Paleomagnetic reversals recorded by basalt flows at mid-ocean ridges Figure 15.24 Testing the plate tectonics model Evidence from ocean drilling • Some of the most convincing evidence confirming seafloor spreading has come from drilling directly into oceanfloor sediment • Age of deepest sediments • Thickness of ocean-floor sediments verifies seafloor spreading Testing the plate tectonics model Hot spots and mantle plumes • Caused by rising plumes of mantle material • Volcanoes can form over them (Hawaiian Island chain) • Originate at great depth, perhaps at the mantle-core boundary Figure 15.18 Testing the Plate Tectonics Model • Earthquake depths • Definite patterns exist – Shallow focus occur along the oceanic ridge system – Almost all deep-focus earthquakes occur in the circum-Pacific belt, particularly in regions situated landward of deep-ocean trenches What drives plate motion Driving mechanism of plate tectonics • No one model explains all facets of plate tectonics • Earth's heat is the driving force • Several models have been proposed Fig. 6-13, p.136 What drives plate motion Slab-pull and slab-push model • Descending oceanic crust pulls the plate • Elevated ridge system pushes the plate • Plate-mantle convection • Mantle plumes extend from mantle-core boundary and cause convection within the mantle Several mechanisms contribute to plate motion Figure 15.26 Whole-mantle convection Figure 15.27 B