Transcript PPT
Plate Tectonics I. Continental Drift • Suggested by Alfred Wegener • Hypothesis-- Continents had once been one large land mass that had separated and moved apart. • Called this supercontinent Pangea, meaning “all Earth” A. Evidence for Continental Drift •Shape- continent shapes fit together like a puzzle. •Fossils- same species on separate continents. Rock Evidence - rock types on edges of continents match. Glacial Features Evidence - best explained by joined continents Glacial Features Evidence Climate Evidence Tropical salt bed deposits found in pole regions, glacial features in tropics, etc. The fossilized coral reef in Isle Lamotte, Vermont, shows that the area was once in the tropics. Evidence for Continental Drift • • • • • Shape- continent shapes fit together like a puzzle Fossils- same species on separate continents Rocks- rock types on edges of continents match Glacial Features- best explained by joined continents Climate- tropical salt bed deposits found in pole regions, glacial features in tropics Trouble With Continental Drift Holmes’ Solution B. New Evidence from Ocean Floor • Wegener’s theory rejected by geologists • Revived by evidence from study of magnetic anomalies on ocean floor on either side of Mid-Atlantic Ridge • Modern theory explains movement of the continents and changes in the Earth’s crust Map of Ocean Ridges Magnetic Record on Ocean Floor II. Modern Theory of Plate Tectonics 1. Plates- rigid blocks of Earth’s crust and upper mantle (lithosphere) move over the surface spreading apart and bumping into each other 2. Powered by convection currents in nearly molten layer of mantle (asthenosphere) Major Tectonic Plates III. Plate Boundaries A. Diverging Boundaries • where 2 plates are pulling apart by tension forces • mid ocean ridge has central crack called a rift zone • at times the ridge opens to release basaltic magma forming new oceanic crust • moves 2.5 cm per year • quiet volcanic activity Ocean Ridges and Rifts are associated with diverging boundaries The Sinai Peninsula is a rift zone B. Transforming Boundaries 1. boundaries at which plates move past one another in opposite directions or in same direction but at different rates 2. high area for earthquakes but low volcanic activity 3. San Andreas Fault is example Transformation-- plates moving beside each other Earthquakes are common along transform boundaries Earthquakes are common along transform boundaries. C. Converging Boundaries • Plates move toward each other from compressive forces • subduction is one plate diving beneath another • collision of two continental plates can produce mountains (Himalayas) Convergence -- two plates moving towards each other Collisions- Mountains can be formed when continental plates converge Subduction denser plate dives under another descending plate melts deep sea trenches on the ocean floor melting magma is less dense, rises forming a chain of volcanoes Volcanoes and earthquakes are associated with subduction. Deformation of the Crust a. tension- stretching or pulling apart b. compression- forces that push against a body from opposite sides and squeezes it into folds c. shearing- forces that push against a body from different sides, producing twisting and tearing Forces that cause rocks to break Folds • bends in rock layers caused by compression • form alternating ridges (anticlines) or troughs (synclines) Anticline • Ante “up” • Folds Upward Syncline • Syn = Sink • Bends downward Monocline Monoclines • Simple fold with only one bend Folded Mountains • Formed from compression forces. • Appalachian Mountains (Green Mts. are part of these) • Alps • breaks in rocks • called faults when movement occurs along break. • energy released as seismic waves. Fractures Normal Fault • Normal Fault – hanging wall is below the foot wall. • Caused by tension. Reverse Fault • Reverse Fault – Hanging wall is above the foot wall. • Caused by compression. Thrust Fault • Type of reverse fault • Almost horizontal (normal reverse faults are more vertical) • Mixes Up the order of the layer – older rocks above younger rocks Lateral Faults • Shearing causes side to side movement • Example is the San Andreas Fault • A series of normal faults in one area causes a series of mountains and valleys known as fault block mountains. Fault Block Mountains Plutonic Mountains • Plutonic mountain are formed from vertical forces pushing magma upward against overlying rock. • Plutons – molten rock built up under the surface that causes the surface to bulge Batholiths • Batholiths are plutons exposed by erosion. • Cadillac Mountain, Acadia National Park, ME • Half Dome, Yosemite National Park, CA Floating Crust • Isotasy – balance of two forces • Top Force= mass of crust and gravity pushing crust into the mantle • Bottom force= crust is less dense than mantle and buoyancy pushes crust up How does isostasy affect Mountain Ranges? • Mountains contain more mass • Result is they sink deeper into the mantle • Erosion reduces the mass of the mountains • Buoyancy takes over and pushes mountain back up Volcanoes Types of Volcanoes Shield Volcanoes •Low, dome-shaped •mostly basalt •quiet flowing •Hawaiian Islands, Iceland are examples Shield Volcano: Haleakala, Hawaii •Violent eruptions blow out bits of lava that harden in the air. Cinder Cones •Accumulated cinders form cones. •Gas-rich basaltic lava. •Small, short-lived Puu Lilinue on Mauna Kea in Hawaii. Strato- Composite or Composite Cones.Volcanoes up of alternating layers of lava • Made Cone-shaped andofash. structures made alternating layers of lava and ash. Associated with subduction zones. Mt. Hood, Oregon Composite Cone: Mount Shasta, California Mt. St. Helens, Oregon Calderas Large depression in volcano Formed from a) violent eruption of composite volcano or b) collapse of shield volcano after magma chamber empties Domes •small with steep sides and dome-shaped mass within crater •formed from slower moving, more viscous lava Island-Arc Volcanoes • trenches with new rocks being formed from quiet flowing basaltic lava • island arcs formed at subduction zones Hot spots • areas in mantle where radioactivity causes more heat that melts rocks as plate moves over • cause volcanic island chains such as Hawaii • also cause of Yellowstone thermal activity The Hawaiian Islands were formed over a hot spot Factors Affecting Eruptions Lava Viscosity Dissolved Gases •resistance to flow •High temp. = more viscous •Increases with silica content. •Rhyolitic – more silica, less fluid, more violent flow. •Basaltic – less silica, more fluid, quieter flow. •Mostly water vapor & CO2 •Held in magma by pressure, released as pressure decreases •Provides force to eject material, sometimes violently. •More gas = more violent eruption. Environmental Hazards of Volcanoes •Pollution •Lava Flows •Falling Ejecta •Ash Falls (Building Collapse, Crop Destruction) •Mudflows •Direct Damage •Floods •Blast •Pyroclastic Flow •Toxic Gas Pyroclastic Flow or Nuee-Ardente (French: Fiery Cloud) http://www.youtube.com/watch?v=XLmbnnh5OLs •Associated w/ Composite Cones •Hot gases, glowing ash, rock fragments •Race downhill @ 200 km/hr Earthquakes and Volcanoes Earthquakes Epicenter and Focus 1. Focus• actual point on the fault where movement occurs and vibrations begin • usually deep beneath the surface 2. Epicenter • where the effects of the seismic activity are first felt on the surface directly above the focus • point of greatest damage Measuring Earthquakes • Intensity- the damage an earthquake causes at the surface -can be either physical or geological- measured with Mercalli scale • Magnitude- the strength of an earthquake from the seismograms- Richter scale • Seismographs- instruments that record tremors traveling through the Earth Seismograph Richter Magnitudes Effects < 3.5 Generally not felt, but recorded. 3.5-5.4 Often felt, but rarely causes damage. <6.0 At most slight damage to well-designed buildings 6.1-6.9 Can be destructive in areas up to about 100 kilometers across where people live. 7.0-7.9 over Major earthquake. Can cause serious damage larger areas. >8 Great earthquake. Can cause serious damage in areas several hundred kilometers across. Earthquake Waves P-waves- primary or "pressure” wavestravel forward- the 1st vibrations feltcompression waves S-waves- secondary or "shaking” waves- move forward but vibrate at right angles to the direction of movement- shear waves Locating an Epicenter • Distance from seismic stations is calculated by difference in arrival times of P-waves and S-waves • Epicenter can be located by triangulating data from at least three stations Earthquakes and Plate Tectonics • Most earthquakes (and volcanoes) occur along plate boundaries A Week of Earthquakes How Do We Know There’s a Core? When waves strike the interface between two substance of different densities, they can be reflected or refracted. The End