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Transcript
The Structure of the Earth Composed of Core Mantle Crust The Earth’s Interior The Core The center of the earth 3500 km in radius Divided into • inner core – solid • outer core – liquid Composed of iron with some nickel 3000 to 5000°C The Earth’s Interior The Mantle 2900 km thick Composed of mafic minerals similar to olivine and perhaps ultramafic minerals such as peridotite 2800 to 1800°C The Earth’s Interior The Crust 8 to 40 km thick Mostly igneous rock, with significant proportions of metamorphic rock and a thin layer of sedimentary rock. The boundary between the mantle and the crust is sharply defined and is the Mohorovicic discontinuity or Moho The Crust Continental crust is composed of two zones Lower, continuous, mafic layer (more dense) Upper, continuous, felsic layer (less dense) • Composed mainly of granitic rock The ocean basins are composed of only one layer Almost entirely mafic rocks - gabbro and basalt The crust is much thicker beneath the continents (35km) than the ocean basins (7km) Lithosphere & Asthenosphere Lithosphere The outer shell of rigid, brittle rock which is broken into lithospheric plates Includes the earth’s crust and the upper part of the mantle 60 to 150 km thick Thickest under the continents and thinnest under ocean basins Asthenosphere a weak plastic layer located beneath the lithosphere The lithosphere moves on the asthenosphere Temperature reaches 1400°C Outer Layers of the Earth Crust continental ocean Lithosphere Asthenosphere Continental Drift Continental Drift Theory proposed by Alfred Wegener, in the early 1900’s Evidence included: Distribution of fossils and plant species Glaciations Geologic Structures and Deposits Continental Drift Fossil of a Glossopteris leaf Fossils of this land plant have been found in South America, Africa, India, Australia, and Antarctica and provide evidence that all of these landmasses were joined as one continent in the Permian Continental Drift Wegener’s 1915 map of the fit of the continents bordering the Atlantic Ocean His explanation of the way the continents moved was weak and highly criticized Pangaea Plate Tectonics http://pubs.usgs.gov/publications/text/dynamic.html#anchor19309449 Major Concepts Continental Drift (early 1900’s) supported by a variety of geologic evidence; however, this theory did not adequately explain the forces that caused the plates to move Major breakthrough in the development of a complete theory of the earth’s dynamics occurred in the early 1960’s, when the topography of the ocean floors was mapped and magnetic and seismic characteristics were determined Evidence Symmetrical magnetic banding found on both sides of the MidAtlantic ridge General paleomagnetic stratigraphy for the past 160 million years Major Concepts The lithosphere is divided into plates, bounded by oceanic ridges, trenches, mountain ranges, and transformed faults Plates separate where convection currents in the mantle rise and magma makes its way to the surface where it spreads at oceanic ridges Plates descend into the mantle beneath trenches and are consumed Plate Tectonics Refers to all forms of breaking and bending of the lithosphere (this includes the crust) Tectonic Processes include Extension → pulling apart → faulting Compression → crushing, squeezing together → folding and faulting Cross Sections Plate Motions & Interactions Two types of lithospheric plates Ocean lithosphere and continental lithosphere Lithosphere floats on the asthenosphere Ocean and continental lithosphere ‘float’ at different levels Plates are simultaneously undergoing both accretion and consumption Rates of each determine the size of the plate Plate Motions – Spreading Boundaries Plate spreading allows magma to rise to the surface to form new crust Basaltic magma congeals in the rift At depth under the rift, magma solidifies into gabbro Basalt and gabbro form new oceanic crust Plate Motions - Subduction Thin ocean plate is subducted under the thicker less dense continental plate Leading edge of the plate is cooler and denser and sinks under its own weight once subduction is initiated Eventually the plate melts, denser mantle rocks become part of the mantle Less dense oceanic and continental sediments become magma and rise to the surface forming chains of volcanoes Plate Motions – Transform Boundaries Plates not converging or diverging but sliding past each other along a transform fault Associated with mid-oceanic ridges Earthquakes Plate Boundaries Transform boundary Spreading boundary Converging boundary Plate Boundaries Summarized Spreading boundaries New lithosphere is created Converging boundaries Sea-floor spreading along the axial rift Lithosphere is being consumed – subduction Along active continental margins Transform boundaries Plates are moving horizontally past each other along a transform fault Driving Force for Plate Tectonics Heat produced by radioactive decay of unstable isotopes in the crust and mantle Continents of the Past During the 1960s, seismologists showed lithospheric plates are in motion Evidence based on magnetic data from the rocks on either side of plate boundaries Continents are moving today and have for at least the past 2 billion years Tectonic Features of the World