Chapter 7 Plate Tectonics Underlies All Earth History
Earthquakes Earthquake = vibration of the Earth produced by the rapid release of energy.
Seismic Waves • Focus = the place within the Earth where the rock breaks, producing an earthquake. • Epicenter = the point on the ground surface directly above the focus. • Energy moving outward from the focus of an earthquake travels in the form of seismic waves.
Types of Seismic Waves 1. Body waves • P-waves • S-waves 2. Surface waves
Types of Seismic Waves 1. Body waves • P-waves Primary, pressure, push-pullFastest seismic wave (6 km/sec in crust; 8 km/sec in uppermost mantle)Travel through solids and liquids • S-waves 2. Surface waves
Types of Seismic Waves 1.Body waves • P-waves • S-waves Secondary, shaking, shear, side-to-sideSlower (3.5 km/sec in crust; 5 km/sec in upper mantle km/sec)Travel through solids only 2. Surface waves
Types of Seismic Waves 1. Body waves • P-waves • S-waves 2. Surface waves L-waves or long wavesSlowestComplex motion – Up-and-down and side-to-sideCauses damage to structures during an earthquake
Seismographs • Earthquakes are recorded on an instrument called a seismograph. • The record of the earthquake produced by the seismograph is called a seismogram.
Determining the Earth's Internal Structure Earth has a layered structure. Boundaries between the layers are called discontinuities. • Mohorovicic discontinuity(Moho) between crust and mantle(Named for discoverer, Yugoslavian seismologist Andrija Mohorovicic) • Gutenberg discontinuity between mantle and core
Determining the Earth's Internal Structure The layered structure is determined from studies of how seismic waves behave as they pass through the Earth.P- and S-wave travel times depend on properties of rock materials through which they pass.Differences in travel times correspond to differences in rock properties.
Determining the Earth's Internal Structure • Seismic wave velocity depends on the density and elasticity of rock. • Seismic waves travel faster in denser rock. • Speed of seismic waves increases with depth (pressure and density increase downward).
Determining the Earth's Internal Structure Curved wave paths indicate gradual increases in density and seismic wave velocity with depth. Refraction (bending of waves) occurs at discontinuities between layers.
S-wave Shadow Zone Place where no S-waves are received by seismograph. Extends across the globe on side opposite from the epicenter. S-waves cannot travel through the molten (liquid) outer core.Larger than the P-wave shadow zone.
P-wave Shadow Zone Place where no P-waves are received by seismographs. Makes a ring around the globe.Smaller than the S-wave shadow zone.
The Earth's Internal Layered Structure • Crust • Mantle • Outer core • Inner core
Crust • Continental Crust (granitic) • Oceanic Crust (basaltic)
Continental Crust • Granitic composition • Averages about 35 km thick; 60 km in mountain ranges • Less dense (about 2.7 g/cm3).
Oceanic Crust • Basaltic composition • 5 - 12 km thick • More dense (about 3.0 g/cm3) • Has layered structure consisting of: • Thin layer of unconsolidated sediment covers basaltic igneous rock (about 200 m thick) • Pillow basalts - basalts that erupted under water (about 2 km thick) • Gabbro - coarse grained equivalent of basalt; cooled slowly (about 6 km thick)
Lithosphere Lithosphere = outermost 100 km of Earth. Consists of the crust plus the outermost part of the mantle. Divided into tectonic or lithospheric plates that cover surface of Earth
Asthenosphere • Asthenosphere = low velocity zone at 100-250 km depth in Earth (seismic wave velocity decreases). • Rocks are at or near melting point. • Magmas generated here. • Solid that flows (rheid); plastic behavior. • Convection in this layer moves tectonic plates.
Isostasy • Buoyancy and floating of the Earth's crust on the mantle. • Denser oceanic crust floats lower, forming ocean basins. • Less dense continental crust floats higher, forming continents. • As erosion removes part of the crust, it rises isostatically to a new level.
Mantle • Composed of oxygen and silicon, along with iron and magnesium (based on rock brought up by volcanoes, density calculations, and composition of stony meteorites). • Peridotite (Mg Fe silicates, olivine) • Kimberlite (contains diamonds) • Eclogite • 2885 km thick • Average density = 4.5 g/cm3 • Not uniform. Several concentric layers with differing properties.
Core • Outer core • Molten Fe (85%) with some Ni. May contain lighter elements such as Si, S, C, or O. • 2250 km thick • Liquid. S-waves do not pass through outer core. • Inner core • Solid Fe (85%) with some Ni • 1220 km radius (slightly larger than the Moon) • Solid
Core and Magnetic Field • Convection in liquid outer core plus spin of solid inner core generates Earth's magnetic field. • Magnetic field is also evidence for a dominantly iron core.
Faults • A fault is a crack in the Earth's crust along which movement has occurred. • Types of faults: • Dip-slip faults - movement is vertical • Normal faults • Reverse faults and thrust faults • Strike-slip faults or lateral faults - movement is horizontal.
Folds • During mountain building or compressional stress, rocks may deform plastically to produce folds. • Types of folds • Anticline • Syncline • Monocline • Dome • Basin
Folds • Anticline • Syncline • Monocline • Dome • Basin
Plate Tectonics Plate Tectonic theory was proposed in late 1960's and early 1970's. It is a unifying theory showing how a large number of diverse, seemingly-unrelated geologic facts are interrelated. A revolution in the Earth Sciences. An outgrowth of the old theory of "continental drift", supported by much data from many areas of geology.
The Data Behind Plate Tectonics Geophysical data collected after World War II provided foundation for scientific breakthrough: • Echo sounding for sea floor mapping discovered patterns of midocean ridges and deep sea trenches. • Magnetometers charted the Earth's magnetic field over large areas of the sea floor. • Global network of seismometers (established to monitor atomic explosions) provided information on worldwide earthquake patterns.
Evidence in Support of the Theory of Plate Tectonics • Shape of the coastlines - the jigsaw puzzle fit of Africa and South America.
Evidence in Support of the Theory of Plate Tectonics • Paleoclimatic evidence - Ancient climatic zones match up when continents are moved back to their past positions. • Glacial tillites • Glacial striations • Coal deposits • Carbonate deposits • Evaporite deposits
Evidence in Support of the Theory of Plate Tectonics • Fossil evidence implies once-continuous land connections between now-separated areas Image from U.S. Geological Survey
Evidence in Support of the Theory of Plate Tectonics • Distribution of present-day organisms indicates that they evolved in genetic isolation on separated continents (such as Australian marsupials).
Evidence in Support of the Theory of Plate Tectonics • Geologic similaritiesbetween South America, Africa, and India • Same stratigraphic sequence (same sequence of layered rocks of same ages in each place) • Mountain belts and geologic structures (trends of folded and faulted rocks line up) • Precambrian basement rocks are similar in Gabon (Africa) and Brazil.
Evidence in Support of the Theory of Plate Tectonics • Geologic similarities between Appalachian Mountains and Caledonian Mountains in British Isles and Scandinavia.
Evidence in Support of the Theory of Plate Tectonics • Rift Valleys of East Africa indicate a continent breaking up. Image from U.S. Geological Survey
Evidence in Support of the Theory of Plate Tectonics • Evidence for subsidence in oceans • Guyots - flat-topped sea mounts (erosion when at or above sea level). • Chains of volcanic islands that are older away from site of current volcanic activity -Hawaiian Islands and Emperor Sea Mounts(also subsiding as they go away from site of current volcanic activity).
Evidence in Support of the Theory of Plate Tectonics • Mid-ocean ridges are sites of sea floor spreading. They have the following characteristics: • High heat flow. • Seismic wave velocity decreases at the ridges, due to high temperatures. • A valley is present along the center of ridge. • Volcanoes are present along the ridge. • Earthquakes occur along the ridge.
Evidence in Support of the Theory of Plate Tectonics • Paleomagnetism and Polar Wandering Curves. The Earth's magnetic field behaves as if there were a bar magnet in the center of the Earth
Paleomagnetism and Polar Wandering Curves • As lava cools on the surface of the Earth, tiny crystals of magnetite form. • When the lava cools to a certain temperature, known as the Curie point, the crystals become magnetized and aligned with Earth's magnetic field. • The orientation of the magnetite crystals records the orientation of the Earth's magnetic field at that time.
Paleomagnetism and Polar Wandering Curves • As tiny magnetite grains are deposited as sediment, they become aligned with Earth's magnetic field. • The grains become locked into place when the sediment becomes cemented.