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Transcript
Earth’s Many Layers Lecture #4 POP QUIZ! • If we could cut open the Earth with a gigantic knife, the interior would most resemble . . . ♣A peanut M&M ♣An apple ♣An onion ♣A Twinkie Layers of the Earth • No holes or pockets • 3 main layers ♣Like a peanut M&M • Several subdivisions ♣More like an onion The Crust • Outer layer (thinnest) • 2 types ♣ Continental crust = rock of continents o Thickness varies (average = 35 km) o Composition varies (average = granite) The Crust • 2 types ♣Continental crust ♣ Oceanic crust = rock under oceans o Thickness ~7 km o Composed of basalt The Mantle • Middle layer • Crust/mantle boundary = the Moho ♣Thickness = Moho to 2900 km ♣ Composed of peridotite The Core • Inner layer ♣ From 2900 km to center of Earth • Composed of ironnickel alloy * ♣ Inner part is solid ♣ Outer is liquid The Core (cont.) • How can inner be solid?? • Solid vs. liquid depends on: ♣ Temperature ♣ And pressure • Example: water ♣ Altitude = sea level 5000 ft 10,000 ft ♣ Boiling T = 212 oF 203 oF 194 oF The Core (cont.) solid • Liquid metal flows. liquid ♣ Metals conduct electricity • Circulation of electrical conductor => magnetic field Crust Mantle Layers of the Earth Outer core Inner core Relative Thickness of Earth’s Layers • Best model . . . ♣ An egg! How Did Layers Form? • Initially, Earth = homogeneous mixture ♣ Rock + metal • Heated by: ♣ Energy of impacts ♣ Radioactive decay How Did Layers Form? (cont.) • By differentiation ♣Def: separation of homogeneous material into parts with different composition Formation of Core • Early Earth began to melt ♣ Heavy iron sank a C n Iro e h p o r t s a t ♣ Lighter rock rose • Separation of core from mantle Schematic View Formation of Crust • Continental crust began to form ~ 4 Ga ♣ By partial melting ♣ Has grown gradually since then • Oceanic crust (oldest ~ 200 Ma): ♣ Continuously formed (mid-ocean ridge) ♣ Then destroyed (sinks into mantle) Crustal Extremes Old zircon crystal in continental crust Young pillow basalt in oceanic crust How Do We Know? • Observe structures in deep mines? • Explore interior of faults? • Drill holes from crust to core? • Study how earthquake waves travel? How Do We Know? (cont.) • Direct sampling ♣ Crust o Easy! o Many samples ♣ Mantle o Hard! o Some samples ♣ Core o Impossible! o No samples World’s Deepest Hole • Northern Russia • 1970-1994 • 7.5 miles deep • Drill core samples from 3.6 miles down • Tried to drill to Moho No go! How Do We Know? (cont.) • Earthquake waves ♣ Travel down ♣ Reflect or bend at layers ♣ Bounce back to surface ♣ Speed increases with increasing density Earthquake Waves How Do We Know? (cont.) • Lab experiments ♣High T & P ~ interior conditions ♣ Measure: o Density o Speed of EQ waves ♣ Results confirm interpretation - layers How Do We Know? (cont.) • Study of meteorites ♣ 2 kinds fall: o Stony meteorites ~ mantle o Iron meteorites ~ core A Second Kind of Layering • Based on strength of rocks • Rocks near surface ♣ Cool, strong, stiff ♣ Lithosphere = crust + upper mantle o ~ 100 km thick A Second Kind of Layering (cont.) • Rocks at depth ♣ Hotter, weaker, softer ♣ Asthenosphere = layer that flows ♣ Within mantle (under lithosphere) • Plastic vs. brittle deformation Comparison Compositional Strength Layering Layering Crust Lith Mantle Asthen