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Transcript
Heat readily escapes from small rocks but is retained in larger bodies. (Courtesy NASA.) Light and Atoms Back The Age of the Earth ! Radioactive decay used to determine the Earth’s age " " Radioactive atoms decay into daughter atoms The more daughter atoms there are relative to the original radioactive atoms, the older the rock is ! Radioactive potassium has a half-life of 1.28 billion years and decays into argon which is a gas that is trapped in the rock unless it melts " " " " Assume rock has no argon when originally formed Measuring the ratio of argon atoms to potassium atoms gives the age of the rock This method gives a minimum age of the Earth as 4 billion years Other considerations put the age at 4.5 billion years Motions in the Earth’s Interior ! Introduction " Heat generated by radioactive decay in the Earth creates movement of rock " This movement of material is called convection " Convection occurs because hotter material will be less dense than its cooler surroundings and consequently will rise while cooler material sinks ! Convection in the Earth’s Interior " The crust and mantle are solid rock, although when heated, rock may develop convective motions " These convective motions are slow, but are the cause of: earthquakes, volcanoes, the Earth’s magnetic field, and perhaps the atmosphere itself 1 Convection currents are created when a heat source raises the temperature at the bottom of a liquid or gas, as in a pot of boiling water. Convection currents in the Earth’s liquid magma just underneath the crust are responsible for the movement of the continental plates. Examples of convection: (A) In our atmosphere, puffy cumulus clouds form when the Sun heats the ground and warms the air so that it rises. (B) You can see rising and sinking motions in a pan of heated soup. (C) An artist's view of convection in the Earth's interior. Light and Atoms Back Motions in the Earth’s Interior ! Plate Tectonics " Rifting " " " " Subduction " " " Hot,molten material rises from deep in the Earth’s interior in great, slow plumes that work their way to the surface Near the surface, these plumes spread and drag the surface layers from below The crust stretches, spreads, and breaks the surface in a phenomenon called rifting Where cool material sinks, it may drag crustal pieces together buckling them upward into mountains If one piece of crust slip under the other, the process is called subduction Rifting and subduction are the dominant forces that sculpt the landscape – they may also trigger earthquakes and volcanoes 2 (A) Rifting may occur where rising material reaches a planet's surface. (B) Subduction builds mountains where material sinks back toward the interior of the Earth. Back Light and Atoms Continental Drift The continents shift and move slowly over time. At one point, the continents are believed to have been together, in the form of a supercontinent called Pangaea. This supercontinent then separated into smaller continental plates. Some plates separated further, creating mid-oceanic ridges, while others collided, creating large mountain ranges. Eventually the familiar continents of today emerged. The boundaries of Earth’s tectonic plates show different motions Colliding Separating Sliding past one another The Earth’s Major Tectonic Plates 3 Motions in the Earth’s Interior ! Plate Tectonics (continued) " The shifting of large blocks of the Earth’s surface is called plate tectonics Early researchers noted that South America and Africa appeared to fit together and that the two continents shared similar fossils " It was later proposed (1912) that all the continents were once a single supercontinent called Pangea " The Earth’s surface is continually building up and breaking down over time scales of millions of years " Map of the Earth, showing its plates. Smaller plates include the Cocos (Co), Caribbean (Ca), Juan de Fuca (Jf), Arabia (Ar), Philippines (Ph), and Scotia (Sc). Light and Atoms Back Breakup of Pangea and the Earth today. Notice the close match of the African and South American coastlines. Light and Atoms Back 4