Presentation on theme: "Plate Tectonics Underlies All Earth History"— Presentation transcript:
1Plate Tectonics Underlies All Earth History Chapter 7Plate Tectonics Underlies All Earth History
2EarthquakesEarthquake = vibration of the Earth produced by the rapid release of energy.
3Seismic WavesFocus = 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.
4Types of Seismic Waves1. Body wavesP-wavesS-waves2. Surface waves
5Types of Seismic Waves P-waves 1. Body wavesP-wavesPrimary, pressure, push-pull Fastest seismic wave(6 km/sec in crust; 8 km/sec in uppermost mantle) Travel through solids and liquidsS-waves2. Surface waves
6Types of Seismic Waves 1. Body waves P-waves S-waves Secondary, shaking, shear, side-to-side Slower (3.5 km/sec in crust; 5 km/sec in upper mantle km/sec) Travel through solids only2. Surface waves
7Types of Seismic Waves P-waves S-waves 2. Surface waves 1. Body wavesP-wavesS-waves2. Surface wavesL-waves or long waves Slowest Complex motion –Up-and-down and side-to-side Causes damage to structures during an earthquake
11Determining 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 discontinuitybetween mantle and core
12Determining 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.
13Determining 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).
14Determining 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.
15S-wave Shadow ZonePlace 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.
16P-wave Shadow ZonePlace where no P-waves are received by seismographs.Makes a ring around the globe. Smaller than the S-wave shadow zone.
19Continental Crust Granitic composition Averages about 35 km thick; 60 km in mountain rangesLess dense (about 2.7 g/cm3).
20Oceanic 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)
21LithosphereLithosphere = 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
22AsthenosphereAsthenosphere = low velocity zone at 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.
23Isostasy 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.
25MantleComposed 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)Eclogite2885 km thickAverage density = 4.5 g/cm3Not uniform. Several concentric layers with differing properties.
26Core Outer core Inner core Molten Fe (85%) with some Ni. May contain lighter elements such as Si, S, C, or O.2250 km thickLiquid. S-waves do not pass through outer core.Inner coreSolid Fe (85%) with some Ni1220 km radius (slightly larger than the Moon)Solid
27Core 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.
29FaultsA fault is a crack in the Earth's crust along which movement has occurred.Types of faults:Dip-slip faults - movement is verticalNormal faultsReverse faults and thrust faultsStrike-slip faults or lateral faults - movement is horizontal.
37Plate TectonicsPlate 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.
38The 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.
39Evidence in Support of the Theory of Plate Tectonics Shape of the coastlines - the jigsaw puzzle fit of Africa and South America.
40Evidence 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 tillitesGlacial striationsCoal depositsCarbonate depositsEvaporite deposits
41Evidence in Support of the Theory of Plate Tectonics Fossil evidence implies once-continuous land connections between now-separated areasImage from U.S. Geological Survey
42Evidence 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).
43Evidence in Support of the Theory of Plate Tectonics Geologic similarities between South America, Africa, and IndiaSame 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.
44Evidence in Support of the Theory of Plate Tectonics Geologic similarities between Appalachian Mountains and Caledonian Mountains in British Isles and Scandinavia.
45Evidence in Support of the Theory of Plate Tectonics Rift Valleys of East Africa indicate a continent breaking up.Image from U.S. Geological Survey
46Evidence in Support of the Theory of Plate Tectonics Evidence for subsidence in oceansGuyots - 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).
47Evidence 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.
48Evidence 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
49Paleomagnetism 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.
50Paleomagnetism 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.
51Paleomagnetism and Polar Wandering Curves The orientation of Earth's magnetic field is described by inclination and declination.
52InclinationInclination = the angle of the magnetic field with respect to the horizontal (or the dip of the magnetic field).Inclination = 90o at North magnetic poleInclination = 0o at the equatorInclination can be used to determine the latitude at which a lava body cooled, or at which sedimentary grains were deposited.
53DeclinationDeclination = the angle between where a compass needle points (magnetic north) and the true geographic north pole (axis of the Earth).
54Apparent Polar Wandering Paleomagnetic data confirm that the continents have moved continuously.When ancient magnetic pole positions are plotted on maps, we can see that they were in different places, relative to a continent, at different times in the past.This is called apparent polar wandering. The poles have not moved. The continents have moved.
55Apparent Polar Wandering Different polar wandering paths are seen in rocks of different continents.Put continents back together (like they were in the past) and the polar wandering curves match up.
56The lithosphere is divided into plates (about 7 large plates and 20 smaller ones).
57Lithosphere and Asthenosphere Lithosphere = rigid, brittle crust and uppermost mantle.Asthenosphere = partially molten part of upper mantle, below lithosphere.Rigid lithospheric plates "float" on flowing asthenosphere.Convection in asthenosphere moves tectonic plates.
58Two types of crust are present in the upper part of the lithosphere: Oceanic crust - thin, dense, basalticContinental crust - thick, low density, granitic
59Types of plate boundaries Divergent - The plates move apart from one another. New crust is generated between the diverging plates.Convergent - The plates move toward one another and collide. Crust is destroyed as one plate is pushed beneath another.Transform - The plates slide horizontally past each other. Crust is neither produced nor destroyed.
60Divergent Plate Boundaries Plates move apart from one anotherTensional stressRifting occursNormal faultsIgneous intrusions, commonly basalt, forming new crust
64Continental Collision Continental collisions form mountain belts with:Folded sedimentary rocksFaultingMetamorphismIgneous intrusionsSlabs of continental crust may override one anotherSuture zone = zone of convergence between two continental plates
65SubductionAn oceanic plate is pushed beneath another plate, forming a deep-sea trench.Rocks and sediments of downward-moving plate are subducted into the mantle and heated.Partial melting occurs. Molten rock rises to form:Volcanic island arcsIntrusive igneous rocks
66Ocean-to-Ocean Subduction An oceanic plate is subducted beneath another oceanic plate, forming a deep-sea trench, with an associated basaltic volcanic island arc.Image courtesy of U.S. Geological Survey
67Ocean-to-Continent Subduction An oceanic plate is subducted beneath a continental plate, forming a trench adjacent to a continent, and volcanic mountains along the edge of the continent.Image courtesy of US Geological Survey
68Ocean-to-Continent Subduction Zone Includes: Accretionary prism or accretionary wedge - Highly contorted and metamorphosed sediments that are scraped off the descending plate and accreted onto the continental margin.Mélange - A complexly folded jumble of deformed and transported rocks.
69Ocean-to-Continent Subduction Zone Includes: Ophiolite suite - Piece of descending oceanic plate that was scraped off and incorporated into the accretionary wedge. Contains:Deep-sea sedimentsSubmarine basalts (pillow lavas)Metamorphosed mantle rocks (serpentinized peridotite)Blueschists – metamorphic minerals (glaucophane and lawsonite) indicating high pressures but low temperatures.
70Transform Plate Boundaries Plates slide past one anotherShear stressTransform faults cut across and offset the mid-ocean ridgesA natural consequence of horizontal spreading of seafloor on a curved globeExample: San Andreas Fault
71Types of Transform Faults Because seafloor spreads outward from mid-ocean ridge, relative movement between offset ridge crests is opposite of that in ordinary strike-slip faults. Note arrows showing direction of movement.
72Plate Boundaries Red = Midoceanic ridges Blue = Deep-sea trenches Black = Transform faults
73Wilson CyclesPlate tectonic model for opening and closing of an ocean basin over time.Opening of new ocean basin at divergent plate boundarySeafloor spreading continues and subduction beginsFinal stage of continental collision
74Wilson CyclesOpening of a new ocean basin at a divergent plate boundary.Sedimentary deposits include:Quartz sandstonesShallow-water platform carbonatesDeeper water shales with chert
75Wilson Cycles2. Expansion of ocean basin as seafloor spreading continues and subduction begins. Sedimentary deposits include:GraywackeTurbiditesVolcanic rocksAlso mélange, thrust faults, and ophiolite sequences near the subduction zone.
76Wilson Cycles 3. Final stage of continental collision. Sedimentary deposits include:ConglomeratesRed sandstonesShalesDeposited in alluvial fans, rivers, and deltas as older seafloor sediments are uplifted to form mountains, and eroded.
77What Forces Drive Plate Tectonics? The tectonic plates are moving, but with varying rates and directions.What hypotheses have been proposed to explain the plate motion?Convection Cells in the MantleRidge-Push and Slab-Pull ModelThermal Plumes
78Convection Cells in the Mantle Large-scale thermal convection cells in the mantle may move tectonic plates.Convection cells transfer heat in a circular pattern. Hot material rises; cool material sinks.Mantle heat probably results from radioactive decay.Image courtesy of U.S. Geological Survey.
79Convection Cells in the Mantle Rising part of convection cell = rifting (mid-ocean ridge)Descending part of convection cell = subduction (deep sea trench)Image courtesy of U.S. Geological Survey.
80Ridge-Push and Slab-Pull Model Crust is heated and expands over a mid-ocean ridge spreading center. Crust tends to slide off the thermal bulge, pushing the rest of the oceanic plate ahead of it.This is called ridge-push.
81Ridge-Push and Slab-Pull Model Near subduction zones, oceanic crust is cold and dense, and tends to sink into the mantle, pulling the rest of the oceanic plate behind it.This is referred to as slab-pull.
82Thermal PlumesThermal plumes are concentrated areas of heat rising from near the core-mantle boundary. Hot spots are present on the Earth's surface above a thermal plume.The lithosphere expands and domes upward, above a thermal plume. The uplifted area splits into three radiating fractures, forming a triple junction. Rifting occurs, and the three plates move outward away from the hot spot.
83Thermal PlumesA triple junction over a thermal plume. Afar Triangle.
84Thermal Plumes Thermal plumes do not all produce triple junctions. Hot spots are present across the globe. If the lava from the thermal plume makes its way to the surface, volcanic activity may result.As a tectonic plate moves over a hot spot (at a rate as high as 10 cm per year), a chain of volcanoes is formed.
87Paleomagnetic Evidence Magnetic reversals have occurred relatively frequently through geologic time.Recently magnetized rocks show alignment of magnetic field consistent with Earth's current magnetic field.Magnetization in older rocks has different orientations (as determined by magnetometer towed by a ship).
88Paleomagnetic Evidence Magnetic stripes on the sea floor are symmetrical about the mid-ocean ridges (Vine and Matthews, 1963).
89Paleomagnetic Evidence Normal (+) and reversed (-) magnetization of the seafloor about the mid-ocean ridge. Note the symmetry on either side of the ridge.
90Magnetic Reversal Time Scale Reversals in sea floor basalts match the reversal time scale determined from rocks exposed on land. Continental basalts were dated radiometrically and correlated with the oceanic basalts. Using this method, magnetic reversals on the sea floor were dated.
91Calculating Rates of Seafloor Spreading Width of magnetic stripes on sea floor is related to time.Wide stripes = long timeNarrow stripes = short timeKnowing the age of individual magnetic stripes, it is possible to calculate rates of seafloor spreading and former positions of continents.
92Rates of Seafloor Spreading The velocity of plate movement varies around the world.Plates with large continents tend to move more slowly (up to 2 cm per year).Oceanic plates move more rapidly (averaging 6-9 cm per year).
93Youth of Ocean Basins and Sea Floor Only a thin layer of sediment covers the sea floor basalt.Sea floor rocks date to less than 200 million years (most less than 150 million years).No seafloor rocks are older than 200 million years.
94Measurement of Plate Tectonics from Space LasersMan-made satellites in orbit around Earth - Global Positioning SystemBy measuring distances between specific points on adjacent tectonic plates over time, rates of plate movement can be determined.
95Seismic Evidence for Plate Tectonics Inclined zones of earthquake foci dip at about a 45o angle, near a deep-sea trench. Benioff Zones, (or Wadati-Benioff Zones).The zone of earthquake foci marks the movement of the subducting plate as it slides into the mantle.The Benioff Zone provides evidence for subduction where one plate is sliding beneath another, causing earthquakes.
96Gravity EvidenceA gravity anomaly is the difference between the calculated theoretical value of gravity and the actual measured gravity at a location.Strong negative gravity anomalies occur where there is a large amount of low-density rock beneath the surface.Strong negative gravity anomalies associated with deep sea trenches indicate the location of less dense oceanic crust rocks being subducted into the denser mantle.
97Gravity EvidenceNegative gravity anomaly associated with a deep sea trench. Sediments and lower density rocks are subducted into an area that would otherwise be filled with denser rocks. As a result, the force of gravity over the subduction zone is weaker than normal.
98Thermal Plumes, Hot Spots and Hawaii Volcanoes develop over hot spots or thermal plumes.As the plate moves across the hot spot, a chain of volcanoes forms.The youngest volcano is over the hot spot.The volcanoes become older away from the site of volcanic activity.Chains of volcanic islands and underwater sea mounts extend for thousands of km in the Pacific Ocean.
99Thermal Plumes, Hot Spots and Hawaii A new volcano, Lo'ihi, is forming above the hot spot, SE of the island of Hawaii. The Hawaiian islands are youngest near the hot spot, and become older to the NW.
100Thermal Plumes, Hot Spots and Hawaii This chain of volcanoes extends NW past Midway Island, and then northward as the Emperor Seamount Chain. The volcanic trail of the Hawaiian hot spot is 6000 km long. A sharp bend in the chain indicates a change in the direction of plate motion about 43 million years ago.
101Exotic TerranesSmall pieces of continental crust surrounded by oceanic crust are called microcontinents.Examples: Greenland, Madagascar, the Seychelles Bank in the Indian Ocean, Crete, New Zealand, New Guinea.
102Exotic TerranesMicrocontinents are moved by seafloor spreading, and may eventually arrive at a subduction zone.They are too low in density and too buoyant to be subducted into the mantle, so they collide with (and become incorporated into the margin of) a larger continent as an exotic terrane.
103Exotic TerranesExotic terranes are present along the margins of every continent.They are fault-bounded areas with different structure, age, fossils, and rock type, compared with the surrounding rocks.
104Exotic TerranesGreen terranes probably originated as parts of other continents.Pink terranes may be displaced parts of North America.The terranes are composed of Paleozoic or older rocks accreted during the Mesozoic and Cenozoic Eras.