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Transcript
The Interior of the Earth Chapter 30 The Interior of the Earth Quick Quiz • What are the parts of a continent? – shield, stable platform, fold mountains Yesterday: The main features of the earth’s surface are continents and ocean basins. Today: What are the main features of the earth’s interior? How did we find out what these features are? Merry Christmas • How do you figure out what’s in the box? – – • Ocean? – anything with the word abyssal, ocean or sea in it. How do we know what’s inside the Earth? 1. 2. 3. 4. 5. Drill a hole. . . a very deep hole! “Weigh” the earth. “Listen” to earthquakes. (Best) Examine meteorites. Study magnetic properties. – – – Peek Weigh Shake Look for clues around the house (shopping bags, price tags) Borrow a metal detector…(they measure magnetic fields generated in metals) Drilling a Hole Ocean drilling program Deepest hole penetrated 2,111 m (1.31 mi) Leg 148, E Pacific Ocean Shallowest water depth: 37.5 m (123 ft) Leg 143, NW Pacific Ocean Greatest water depth: 5,980 m (3.72 mi) Leg 129, W Pacific Ocean “Weighing the Earth” (The Cavendish Experiment) Direct Observations (Sir Henry Cavendish (1798)) gold Inclusions in Volcanic Eruptions Pieces of rock from deep in the Earth are brought up in magma as it rises to the surface 348 lb balls of lead F = GmM , so if you know G, m, d and F you can d2 figure out the mass of the Earth Evidences from Earth’s Mass & Density Density of overall Earth is 5.5 g/cm3 Granite has a density of ~2.7 g/cm3 Continental Crust Basalt has a density of ~3.0 g/cm3 Oceanic Crust Peridotite has a density of ~3.3 g/cm3 This represents the rocks from the Upper Mantle What can we conclude about the interior of the earth compared to the crust? A. The interior is more dense B. The interior is less dense C. The interior has the same density So, what does this tell us about the Earth’s Interior? From density alone can we tell which of the models below is correct? Gradually increasing density model Layered density model Inferences from Meteorites Stony Chondrite Iron Stony Achondrite Evidence from Meteorites Meteorite - chunk of rock from space that lands on earth. (“falling star”) “Stony” meteorites a. Silicate material b. “Less” dense “Metallic” meteorites a. Iron, nickel b. “More” dense Planetesimal Asteroids, Meteors Kaboom Lots of little pieces Silicates (80%) Iron, nickel (20%) Earth as a magnet • Must be magnetic material – Iron, nickel or both • Permanent magnet loses magnetism at high temperature, so must be due to current. • Molten iron/nickel has electric current. Conclude, at least some portion of the earth’s core is molten iron and nickel. The two ways to make a magnet 1. Iron and nickel atoms in certain arrangements N S 2. electric currents current flowing through a wire generates a magnetic field “Listening” to Earthquakes In the early 1950’s sensitive devices were developed to monitor nuclear bomb testing and stations were set up around the world to detect explosions. The devices also monitor earthquakes and have given us a great deal of information about the earth’s interior. Earth Types of Waves Which damage is from a P wave? Surface waves cause damage and destruction Volume waves compression waves primary (P) waves, arrive first travel through solids and liquids shear waves secondary (S) waves, arrive second will not travel through liquids Seismograph What has been learned from seismic waves (earthquakes) about the earth’s interior? 1. How does the speed of the waves change with depth? Does speed change gradually, or are their abrupt changes in medium? 2. What are the layers like that influence the waves? Do the waves stop entirely, speed up or slow down? What does this tell us. 3. What are the shadow zones and why do they occur? Speed of Seismic Waves • P faster than S • Speed generally increases with depth, except – – – Mohorovicic discontinuity Asthenosphere boundary Mantle / core boundary Earthquake P waves shadow S waves • S waves cannot travel through liquid • Resulting shadow zone is bowl-shaped • Implies core is liquid • S waves stop here! • Speed constant in inner core Shadow zones, the movie • Waves change speed abruptly when they transition from solid to liquid • This causes bending (refraction) • Refraction makes it impossible for waves to arrive in the shadow zone • Resulting shadow zone is doughnut-shaped shadow A Real Shadow Zone Shadow Zones Areas where no waves can be detected So what do we conclude? Earth is Layered (Differentiated) Compositional Layers Core – made of Iron Mantle – made of Peridotite Crust – made of granite & basalt Mechanical Layers Inner Core – Solid Outer Core – Liquid Mesosphere – Solid, but plastic Asthenosphere – Mostly solid, plastic, 1-6% liquid Lithosphere – Solid, brittle The layers of the earth P Shadow zone between the dark ovals Two Depth Characterizations Composition crust Physical Characteristics lithosphere mantle asthenosphere Mechanical Layers Question A strong earthquake causes “S” (shear waves) and “P” waves to travel out through the earth. Which statement(s) is (are) true about the shadow zone(s) on the opposite side of the earth? a. neither “S” nor “P” waves are seen beyond about 103o b. “S” waves are never seen beyond 103o. “P” waves are not seen just beyond 103o but appear again beyond about 143o. Therefore the shadow zone for “S” waves is circular but the shadow zone for “P” waves is doughnut shaped. c. The shadow zone for “S” waves is evidence for a liquid core in the earth. d. The shadow zone is the result of a very dense igneous rock in the upper mantle that prevents both “S” and “P” waves from getting through. e. both b and c above.