Part II: Solar System The Earth The Earth A. The Interior B. The Surface C. Atmosphere 2 Updated: July 14, 2007 A. Interior of Earth 1. Differentiated Structure 2. Seismography 3. Composition of layers 3 1a. Interior The earth is differentiated Inner Core: solid iron Outer Core: liquid iron Mantle: heavy rock (olivine) Crust: light rock (granite) 4 1b. How did it get this way? Differentiation! Heat came from radioactive decay (e.g. Aluminum 26) and/or intense meteor impacts, heated up interior, heavy stuff sank to center. This releases even more gravitational potential energy! 5 1c. More heat is released as liquid core freezes Melting point increases with pressure, so the melting point temperature is higher deeper inside the earth. The very center is hence solid. 6 At current rate, it will take several billion years for entire core to become solid. 1
2. How do we know about inside the earth? We haven t drilled very far into the earth 1961 Project Moho went 183 meters into the oceanic crust (which is 5000 meters thick!) 1970 Russians went 12.3 km, but this was on land where the continental crust is 35 km thick! They did not get to the Basalt layer. 2007 Japan s Chikyu Hakken mission will attempt to go 7000 meters, the first to drill deep enough to get sample from the upper mantle! 7 2a Seismographs 8 100 AD: Chinese (Zhang Heng) have primitive device 1880 John Milne invents modern seismograph (shear waves) 2b Two Basic Wave Types (Andrija Mohorovičić 1909) (pressure waves) 9 2c Liquid Core 10 Richard Dixon Oldham (July 31, 1858 July 15, 1936) was a British geologist who, in 1906, argued that the Earth must have a molten interior as S waves were not able to travel through liquids nor through the Earth's interior. S Shadow Zone (shear waves) S waves can only travel in solids! P can travel in solid, liquid and gas! 2d Speed of Waves 11 2e. Velocities of P and S waves in different layers of the Earth 12 Beno Gutenberg 1914 (worked with Richter) determines travel times for waves. Measures Temp and pressure inside of earth Measures size of core from P Wave Shadow Zone. P wave velocities: in granite: 6 km/sec in basalt: 7 km/sec in peridotite: 8 km/sec 2
2f Reflection & Refraction 13 Seismic waves are reflected or bent at the boundaries of different layers inside the earth 2g. Solid Inner Core (1936) The pressure at the center of the Earth is 3.6 million bars Solid inner core discovered 1936 by Inge Lehmann from reflection/refraction of waves (for a time called the Lehmann discontinuity). Size not determined until 1960s from shockwaves from underground nuclear tests (echoes bouncing off inner core). 14 3a. Composition: The crust 1909 Andrija Mohorovicic realized that the velocity of seismic waves is related to the density of the material that they are traveling through. He determined the abrupt change in density from the crust to the mantle (The Mohorovicic discontinuity, or the Moho Upper Crust is Granite Lower Crust is Basalt 15 3a. Composition: The crust Earth s interior is layered on a very large scale CRUST: basalts and granites. 0.4% of Earth s mass is in the crust. The crust is ~5 km thick under the oceans and ~40 km thick under continents The rigid crust is the uppermost part of the lithosphere (which is ~100 km thick) The asthenosphere is a plastic or partially molten layer below the crust, and is ~100-200 km thick 16 Need diagram! 3b. The Mantel 17 Inner Core 18 MANTLE: made of silicate rocks (dunite, peridotite). 67% of the mass of Earth is in the mantle Increasing pressure at greater depth in the mantle causes mineral transitions (atoms are compressed into more dense arrangements). This results in changes in the seismic wave velocities at various depths Pressure at the top of the mantle is ~36,000 bars Pressure at the bottom of the mantle is ~1.3 million bars In 1996, Xiadong Song and Paul Richards confirmed a prediction that the inner core rotates slightly faster than the rest of the Earth. The magnetic forces of the geodynamo seem to be responsible. Over geologic time, the inner core grows as the whole Earth cools. Iron crystals freeze out at the top of the outer core and rain onto the inner core. At the base of the outer core, the iron freezes under pressure taking much of the nickel with it. The remaining liquid iron is lighter and rises. These rising and falling motions, interacting with geomagnetic forces, stir the whole outer core at a speed of 20 kilometers a year or so. 2002 evidence of an inner inner core 300 km radius. 3
3d. What is the Earth made of? 19 B. Surface of Earth 20 Crust and Mantle: Silicate rocks Silicon (Si) is an abundant element Si + Oxygen + metals (Fe, Mg, Al, etc.) = silicate rocks Interior (Core and Inner Core) Atmosphere: Gases (oxygen, nitrogen, etc.) Hydrosphere: Water Magnetosphere: Charged atomic particles from the Sun B. Earth s Surface 1. Plate Tectonics 2. Continental Drift 3. Volcanoes 4. Rocks 21 Forces that Shape the Face of the Earth Tectonism The world-wide pattern of earthquakes shows that they occur along lines in the Earth s crust These lines define the margins of segments of the Earth s crust These are called tectonic plates 22 The distribution of earthquakes defines several plates into which the Earth s crust is divided 23 24 Fig 7-6, p.157 4
The San-Andreas Fault running through California. 25 Forces that Shape the Face of the Earth Tectonism -- continued 26 This represents motion along the margin of two sub-plates, and is the source of many dangerous earthquakes. Plates move a few cm per year Earthquakes Sea-floor spreading (few cm per year). 2 km 2 of new sea floor per year. Enough to resurface ocean floors every 100 million years Subsidence of crust at continent margins Vertical uplift of mountains (few mm per year) Fig 7-8, p.160 This shows the subduction of an oceanic crustal plate under a continental mass. Friction of the subducting plate with the thicker land mass causes rock melting; the melted rock (magma) rises along cracks and fissures, and erupts as volcanoes. 27 Subduction and volcanism on the NW coast of North America 28 Fig 7-7b, p.159 29 30 The ocean floor spreads as new lavas come up from the mantle Volcanoes rise where ocean floor crust is subducted below the continents Convective motion in the mantle Tectonism is driven by the heat in the Earth s deep interior 5
What would happen if the Earth cooled? 31 Sea-floor spreading at the Mid-Atlantic Ridge 32 Plate tectonic motions would stop The magnetic field would collapse, and Earth would be unprotected from the solar wind Life would cease to exist, except perhaps deep in the surface rocks The atmosphere and oceans would evaporate into space What other consequences can you imagine? The Mid-Atlantic Ridge is a fracture zone that runs through the middle of the Atlantic Ocean, from a point near Antarctica to a point north of Iceland. This Ridge is the site of ocean-floor spreading, a process that makes new ocean floor and moves the two plates (North American Plate and Eurasian Plate) laterally, west and east. The Mid-Atlantic Ocean Ridge 33 Lava rising along the long fracture that constitutes the Mid-Atlantic Ridge forces the two plates on the left and right to move laterally to make room for the newly forming crust. 34 This continuing replacement of the Atlantic Ocean floor ensures that the oceanic crust is very young. Fig 7-7a, p.159 Crustal plates close to home: the Juan de Fuca Plate is moving east under the North American Plate, causing the Cascade volcanoes from N. California to British Columbia 35 2. Continental Drift 36 Earthquakes and eruptions of Mt. St. Helens are the most recent consequences of this motion 6
37 38 39 Rates of plate tectonic motion 40 How can we measure this? 1. Date ocean sediments (determine age of sediments and measure distance from ridge) 2. Magnetic reversals (date magnetic reversals and measure distance from ridge - wider stripes indicate faster plate motion) 3. Satellite laser ranging techniques (bounce laser beams off a satellite and to a station on another plate - time to reach station is proportional to distance traveled) 4. Hot spots (provide a fixed reference point to allow absolute motion to be determined) What do we find? Rates of plate tectonic motion 41 42 7
Rates of plate tectonic motion 43 Rates of plate tectonic motion 44 What do we find? What do we find? 45 46 47 48 8
3. Volcanoes 3 50 Forces that Shape the Face of the Earth Volcanism 10-30 volcanoes erupt every year New land is created Lava flows cover hundreds of km 2 Gases (H 2 O, sulfur dioxide, CO 2 ) enter the atmosphere 51 Heimaey, Iceland, 1973 Tolbachik, Kamchatka, 1975 52 53 54 Lava flow in Hawaii Fig 7-5, p.157 Fig 7-CO, p.152 9
55 The Hawaiian Island Chain has been building on a Pacific Basin Hot Spot for 50 Million Years Mid-Ocean Volcanoes (e.g., Hawaii) 56 A pocket of liquid rock (magma) remains stationary in the upper mantle, while the Pacific plate slowly moves across it. Continued eruptions from the hot spot build volcanic mountains (eventually islands) in a chain, as the Pacific plate moves along. 4. Rocks 57 58 59 60 10
The History of the Earth 61 62 Earth is a complex system of interacting elements Lithosphere Atmosphere Hydrosphere Biosphere Magnetosphere Notes Clean Up Part B Need overview of california and its faults, local geography of hayward etc. 1906 earthquake stuff? 63 11