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Earth's Layered Structure: Seismic Waves, Composition & Discovery

Learn about Earth’s layered structure, seismic wave paths, composition, and exploration of its layers through seismic data and drilling. Discover the physical properties and composition of the lithosphere, asthenosphere, mantle, and core.

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Earth's Layered Structure: Seismic Waves, Composition & Discovery

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  1. Warm Up 12/4 • When does liquefaction occur? • When loosely consolidated soils saturated with water are shaken by earthquake waves • What factors determine how much damage is inflicted on a building? • Intensity and duration of vibrations, nature of material on which the structure is built, and design of the structure • Why is it incorrect to refer to tsunamis as tidal waves? • Tidal waves are caused by the gravitational pull of the sun and moon, while tsunamis are caused by earthquakes

  2. Go Bruins!

  3. Earth’s Layered Structure

  4. Layers Defined by Composition • If Earth were made up of the same materials throughout its interior, seismic waves would travel in straight lines at constant speeds • Seismic waves reaching seismographs located farther from an earthquake travel at faster average speeds, due to increased pressure at depth • Earth’s interior consists of three major zones defined by its chemical composition – the crust, mantle, and core

  5. Paths of Seismic Rays

  6. The Crust • Crust – thin, rocky, outer layer of Earth; divided into oceanic and continental crust • The oceanic crust is roughly 7 kilometers thick and composed of the igneous rocks basalt and gabbro • The continental crust is 8-75 kilometers thick (avg. 40 km) and consists mostly of the granitic rock granodiorite • The rocks of the oceanic crust are younger then the rocks of the continental crust

  7. Mantle and Core • Mantle – a solid, rocky shell that extends to a depth of 2890 km • Over 82% of Earth’s mass is contained in the mantle • The boundary between the crust and mantle represents a chemical composition change • Core – a sphere composed of an iron-nickel alloy

  8. Layers Defined by Physical Properties • Earth’s interior has a gradual increase in temperature, pressure, and density with depth • Depending on the physical environment (temperature and pressure), a material may behave like a brittle solid, a putty, or a liquid • Earth can be divided into layers based on physical properties – the lithosphere, asthenosphere, outer core, and inner core.

  9. Lithosphere and Asthenosphere • Lithosphere – relatively cool, rigid shell consisting of the crust and upper most mantle • Averages about 100 kilometers in thickness • Asthenosphere – soft, comparatively weak layer beneath the lithosphere • The rocks within the asthenosphere are close enough to their melting points that they are easily deformed

  10. Lower Mantle • From a depth of 660 km down to near the base of the mantle • More rigid layer that is still prone to gradual flow • At the bottom of the mantle, rocks behave more like those in the asthenosphere

  11. Inner and Outer Core • The core, which is composed of an iron-nickel alloy, is divided into two regions with different physical properties • Outer Core – liquid layer 2260 km thick, flow produces Earth’s magnetic field • Inner Core – sphere with a radius of 1220 km, compressed into a solid state by immense pressure

  12. Earth’s Layered Structure

  13. Discovering Earth’s Layers • Moho – boundary separating the crust from the mantle where the velocity of seismic waves abruptly increases • Seismic waves from even small earthquakes can travel around the world • The outer core causes P waves that travel through it to arrive several minutes after expected and are bent by 100o away from the earthquake (shadow zone) • It was further shown that S waves do not travel through the outer core, making geologists conclude that the outer core is composed of a liquid

  14. Discovering Earth’s Composition • Early seismic data and drilling technology indicate that the continental crust is mostly made of lighter, granitic rocks • The crust of the ocean floor has a baslatic composition • The composition of the mantle and core is known from more indirect data • Scientists use lava and meteorites to infer the composition of the mantle and core • Earth’s core is thought to be mainly dense iron and nickel, similar to metallic meteorites. The surrounding mantle is believed to be composed of rocks similar to stony meteorites

  15. Earth’s Interior Showing P and S Wave Paths

  16. Assignment • Read Chapter 8, Section 4 (pg. 233-237) • Do 8.4 Assessment #1-6 (pg. 237)

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