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Transcript
ASTRONOMY 161 Introduction to Solar System Astronomy Class 15 The Earth Monday, February 12 The Earth: Physical characteristics Mass = 5.97×1024 kg Mean radius = 6,373 km Polar radius = 6,357 km Density = 5,515 kg/m³ Sidereal rotation period = 0.9973 d (23.934 h) Axial tilt = 23.4° The Earth: Key Concepts (1) The study of seismic waves tells us about the Earth’s interior. (2) The Earth is layered into crust, mantle, inner core, and outer core. (3) The Earth is layered because it underwent differentiation when molten. (4) The lithosphere is broken into plates that move relative to each other. (5) The motion of liquid metal in the outer core produces a magnetic field. (1) The study of seismic waves tells us about the Earth’s interior. How can we study the deep interior of the Earth? Average density of the Earth = 5500 kg/m3 Earth is too dense to be solid rock: Basalt = 3300 kg/m3 Granite = 2700 kg/m3 Earthquakes produce two types of seismic waves: P waves [Primary, Pressure]: Sound waves travel through solids and liquids. S waves [Secondary, Shear]: Transverse (side-to-side) waves that do NOT travel through liquids. Seismic waves radiating through the Earth after an earthquake: Note: S waves do not travel through the outer core! (2) The earth is layered into a crust, mantle, inner core, and outer core. From the outside in: 1) Crust: solid rock 5 km thick basalt (ocean floor) 35 km thick granite (continents) 2) Mantle: partly solid rock, partly semisolid (plastic) rock 2900 km thick 3) Outer (liquid) core: molten iron and nickel 2200 km thick 4) Inner (solid) core: solid iron and nickel 1300 km in radius The center of the Earth is as hot as the surface of the Sun. Why doesn’t the inner core melt?? The high pressure in the inner core keeps it solid. (3) The Earth is layered because it underwent differentiation when molten. When young, the Earth was heated by the impact of planetesimals. For a while, the Earth was molten. In a liquid, dense stuff sinks, lowdensity stuff floats. When differentiation was complete, Earth cooled and (partly) solidified. Crust of the ocean floor floats on the mantle “like skin on cocoa”; lower-density continents are the “marshmallows”. (4) The lithosphere is broken into plates that move relative to each other. The crust plus the upper mantle form the lithosphere (solid but brittle). Beneath the lithosphere is the asthenosphere (plastic). The asthenosphere, heated from below, undergoes convection. Convection Convection currents in the asthenosphere have broken the lithosphere into sections called plates. There are 15 large plates. Continuing convection in the asthenosphere causes plates to move relative to each other. The study of plate motion is called plate tectonics. The motion of continents was first suspected by Sir Francis Bacon (17th cent). Best known for leading the scientific revolution with his new 'observation and experimentation' theory. America and Europe are moving apart by 3 centimeters per year (= 30 km per million years). This “continental drift” is measured using GPS (global positioning systems). 200 million years ago, the Americas, Europe, and Africa formed a single supercontinent, PANGAEA. The boundaries between plates are geologically active, with many volcanoes and earthquakes. Example: the Ring of Fire around the Pacific Ocean. Types of plate boundaries 1) Oceanic rift: plates moving apart, new rock forming. 2) Subduction zone: plates moving together, ocean crust shoved under continent. 3) Mountain-building zone: plates moving together, continents collide & buckle upward. 4) Transverse fault: plates slipping past each other, with lots of friction. The summit of Mount Everest is made of marine limestone. It has been lifted 9 kilometers high! The San Andreas transverse fault: Motion jumps during a quake: (5) The motion of liquid metal in the outer core produces a magnetic field. A compass points to the Earth’s North Magnetic Pole (not the true North Pole!) Why is the Earth a giant magnet? Remember: Moving charged particles create a magnetic field. Inside the Earth, convection currents exist within the liquid outer core. These currents carry electrons around, creating a magnetic field. The Earth’s magnetic field stretches far beyond the surface. Our magnetic field deflects the solar wind (electrons and protons streaming away from the Sun).