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Inside the Earth Earth’s Structure Earth’s Layered Structure • Earth’s interior consists of three major zones defined by their chemical composition— • the crust, • mantle, • and core. Review of Earth’s Interior • Chemical Classification • Crust - Thin, rocky outer layer – Continental Crust: • Mostly granite • Average thickness 30km – Oceanic Crust: • Mostly basalt • Average thickness 5 – 8 km • Denser than continental crust Divisions in the Earth's Interior Inside the Earth’s Interior • Mantle – Approximately 2900 km thick – 870o to 2200o – Composed of silicon, oxygen, iron and magnesium – Upper layer contains the asthenosphere http://teach.fcps.net/trt20/projects/EKU/layers/wwr69.gif Inside Earth’s Interior • Core – Approximately 3550 km thick – 2200o to 5000o – Composed of iron and nickel – Tremendous pressure http://teach.fcps.net/trt20/projects/EKU/Default.htm Earth’s Layered Structure http://phschool.com/atschool/earth_science/powerpoints/PHESCh01.ppt#16 Inside Earth’s Interior • Physical Classification: 5 layers based on physical traits • Lithosphere: – Crust and uppermost part of mantle – Rigid layer of rock – “Litho” means stone – Approximately 100 km thick Lithosphere • The Earth’s crust and upper mantle are broken into sections called plates • Plates move around on top of the mantle like rafts 2 Types of Plates • Ocean plates - plates below the oceans made up of mostly basalt • Continental plates - plates below the continents made up of mostly granite Inside Earth’s Interior • Asthenosphere: – Upper mantle – higher temperature and pressure – Soft rock that the lithosphere floats on – “Asthenes” means weak – Convection currents in the asthenosphere cause plates to move • Mesosphere – Rest of mantle which is solid material due to high pressure Earth’s Outer Core • At the center of Earth is the core. • The core is made mostly of iron and nickel and possibly smaller amounts of lighter elements, including sulfur and oxygen. • The core is about 4,400 miles (7,100 kilometers) in diameter, slightly larger than half the diameter of Earth and about the size of Mars. • The outermost 1,400 miles (2,250 kilometers) of the core are liquid. • Currents flowing in the core are thought to generate Earth's magnetic field. Earth’s Inner Core • Geologists believe the innermost part of the core, about 1,600 miles (2,600 kilometers) in diameter, is made of a similar material as the outer core, but it is solid. • The inner core is about four-fifths as big as Earth's moon. • Earth gets hotter toward the center. Geologists believe the temperature of Earth's outer core is about 6700 to 7800 degrees F (3700 to 4300 degrees C). The inner core may be as hot as 12,600 degrees F (7000 degrees C)--hotter than the surface of the sun. But, because it is under great pressures, the rock in the center of Earth remains solid. How deep is deep? • The deepest mine in the world, a gold mine in South Africa, reaches a depth of 3.8 km. • You would have to travel 1600 times that to reach the Earth’s center. • 6,000 km Exploring the Earth • Geologist use two main types of evidence to learn about the Earth’s interior. Direct evidence from rock samples. Indirect evidence from seismic waves. Rock Samples • Rocks from inside Earth give geologists clues about Earth’s structure. Rock Samples • Geologists have drilled holes as deep as 12 km into Earth. • The drills bring up samples of rocks to study. • Geologists can make inferences. • In addition, forces inside the Earth blast rock to the surface from depths of more than 100 km. • These rocks provide information about the interior. Finding Indirect Evidence • Seismic Waves • Primary Waves (P waves) – Back and Forth motion – First to arrive after earthquake – Can travel through liquid and solids • Secondary Waves (S waves) – Up and Down motion – Cannot travel through liquid Seismic Waves Paths Through the Earth S Waves P Waves http://phschool.com/atschool/earth_science/powerpoints/PHESCh01.ppt#16 Finding Indirect Evidence • The denser the rock the more quickly the wave will travel • As P waves travel from one type of material to another the waves bend or refract • S waves stop at the outer core indicating that it must be liquid Finding Indirect Evidence • The Moho – The name of the boundary between the mantle and the crust. Named for Andrija Mohorovicic – An area where seismic waves suddenly speed up (due to rock density) Shadow Zone • Absence of P waves from about 105 degrees to 140 degrees around the globe from an earthquake • Can be explained if Earth contains a core composed of materials unlike the overlying mantle Earth’s Interior Showing P and S Wave Paths