Chapter 30 The Interior of the Earth The Interior of the Earth 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? Quick Quiz What are the parts of a continent? shield, stable platform, fold mountains Ocean? anything with the word abyssal, ocean or sea in it. Merry Christmas How do you figure out what s in the box? Peek Weigh Shake Look for clues around the house (shopping bags, price tags) Borrow a metal detector (they measure magnetic fields generated in metals) How do we know what s inside the Earth? 1. Drill a hole... a very deep hole! 2. Weigh the earth. 3. Listen to earthquakes. (Best) 4. Examine meteorites. 5. Study magnetic properties. 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
Direct Observations Inclusions in Volcanic Eruptions Pieces of rock from deep in the Earth are brought up in magma as it rises to the surface Weighing the Earth (The Cavendish Experiment) (Sir Henry Cavendish (1798)) gold 348 lb balls of lead F = GmM, so if you know G, m, d and F you can d 2 figure out the mass of the Earth Evidences from Earth s Mass & Density Density of overall Earth is 5.5 g/cm 3 Granite has a density of ~2.7 g/cm 3 Continental Crust Basalt has a density of ~3.0 g/cm 3 Oceanic Crust Peridotite has a density of ~3.3 g/cm 3 This represents the rocks from the Upper Mantle So, what does this tell us about the Earth s Interior? 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 From density alone can we tell which of the models below is correct? Inferences from Meteorites Stony Chondrite Gradually increasing density model Layered density model 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 Silicates (80%) Iron, nickel (20%) Asteroids, Meteors Kaboom Lots of little pieces 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 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. 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 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 Which damage is from a P wave? 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 S waves stop here! Speed constant in inner core Earthquake P waves shadow Shadow zones, the movie S waves shadow 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 S waves cannot travel through liquid Resulting shadow zone is bowl-shaped Implies core is liquid
Shadow Zones Areas where no waves can be detected A Real Shadow Zone P Shadow zone between the dark ovals 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 Two Depth Characterizations Composition crust mantle Physical Characteristics lithosphere asthenosphere The layers of the earth 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 103 o b. S waves are never seen beyond 103 o. P waves are not seen just beyond 103 o but appear again beyond about 143 o. 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.