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Tham khảo tài liệu 'societ english1', ngoại ngữ, anh ngữ phổ thông phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | EARTH AND SPACE SCIENCE waves. Tsunamis or tidal waves are different. They result from underwater earthquakes volcanic eruptions or landslides not wind. Energy from the Core Another source of Earth s energy comes from Earth s core. We distinguish four main layers of Earth the inner core the outer core the rocky mantle and the crust. The inner core is a solid mass of iron with a temperature of about 7 000 F. Most likely the high temperature is caused by radioactive decay of uranium and other radioactive elements. The inner core is approximately 1 500 miles in diameter. The outer core is a mass of molten iron that surrounds the solid inner core. Electrical currents generated from this area produce the earth s magnetic field. The rocky mantle is composed of silicon oxygen magnesium iron aluminum and calcium and is about 1 750 miles thick. This mantle accounts for most of the Earth s mass. When parts of this layer become hot enough they turn to slow moving molten rock or magma. The Earth s crust is a layer from four to 25 miles thick consisting of sand and rock. The upper mantle is rigid and is part of the lithosphere together with the crust . The lower mantle flows slowly at a rate of a few centimeters per year. The crust is divided into plates that drift slowly only a few centimeters each year on the less rigid mantle. Oceanic crust is thinner than continental crust. This motion of the plates is caused by convection heat currents which carry heat from the hot inner mantle to the cooler outer mantle. The motion results in earthquakes and volcanic eruptions. This process is called plate tectonics. Tectonics Evidence suggests that about 200 million years ago all continents were a part of one landmass named Pangaea. Over the years the continents slowly separated through the movement of plates in a process called continental drift. The movement of the plates is attributed to convection currents in the mantle. The theory of plate tectonics says that there are now twelve .
