المساهمات : 29
تاريخ التسجيل : 11/06/2011
العمر : 21
الموقع : ALRAMTHA
|موضوع: Volcanoes and Earthquakes الإثنين يونيو 13, 2011 5:45 pm|| |
On April 18, 1906, a disastrous earthquake struck San Francisco. Lasting less than a minute, it destroyed most of the city and killed 700 people. In 1976 the worst known earthquake killed over 650,000 people in Tientsin Province, China. Volcanoes and Earthquakes tells us about famous earthquakes and describes the natural forces that created them.
You probably know that earthquakes can be huge natural disasters. But why do they happen? The Earth's surface is formed of massive slabs of rock called plates. These plates, also called tectonic plates, are always moving. Sometimes they just slide past one another. At other times they actually collide with one another. Plate movement causes the buildup of tremendous quantities of energy in the rock. When the energy is released, it produces vibrations that travel through the rock, leading to earthquakes. During earthquakes, faults, or giant cracks, are produced by the pressure of the moving rock.
Earthquakes and volcanoes occur along the edges of the plates. Scientists have developed a theory that explains how these giant plates move, thereby creating, destroying, and re-forming continents and oceans over long periods of time. This theory is called the theory of plate tectonics.
According to the theory, there are two types of plates, oceanic and continental. Various types of movement occur along the different kinds of plate boundaries. Plate collisions create landforms such as coastal volcanoes, island arcs, and mountain chains. When plates move apart, they produce new ocean floor as magma from the mantle rises up through volcanoes and deposits new rock along the plate boundaries. In some areas plates slide alongside each other, neither creating nor destroying land.
When the plates move, they cause vibrations that produce earthquakes. Thousands of earthquakes take place every year, but only a few of them are destructive enough to be considered disasters. Many earthquakes are so mild that few people notice them. Scientists have developed an instrument called a seismograph, which measures and records the intensity of earthquakes. Because seismographs are very sensitive, they help scientists predict earthquakes, too. They can record even minor shocks and changes in the Earth's layers, and this helps to monitor the build-up of stresses that lead to earthquakes. By comparing the data from seismographs in different locations over the world, scientists can get a better picture of the forces at work underground.