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Seismic waves.

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Introduction Seismic waves are generated in the earth by the movements of earthquakes or explosions. Depending on the material travelled through, surface and body waves move at unpredictable speeds. Layers of the earth, including the core, mantle, and crust, have been distinguished using seismic wave profiles. Seismic waves from explosions have been used to understand the subsurface structure of the crust and upper mantle and in the exploration for oil and gas deposits. There are several different kinds of seismic waves, and they all move in different ways. The two main types of waves are body waves and surface waves. Body waves can travel through the earth's inner layers, but surface waves can only move along the surface of the planet like ripples on water. Earthquakes radiate seismic energy as both body and surface waves. Types of Wave Body Waves P Waves The first kind of body wave is the P wave or primary wave. This is the fastest kind of seismic wave. The P wave can move through solid rock and fluids, like water or the liquid layers of the earth. It pushes and pulls the rock it moves through just like sound waves push and pull the air. ...read more.


We also know that the density of the rock that is on the surface is 2800kg/m3. Therefore, there must be denser material further towards the centre of the Earth. We can find out more about the Earth's structure by looking at records of P-waves and S-waves. These conclusions can be made: * Seismic waves cannot be travelling through the earth in straight lines * No S-waves travel to the opposite side of the earth from their origins...something must block them. * P-waves do arrive on the other side of the Earth but not in a ring-shaped shadow zone. Scientists have explained these facts by saying: * Density increases as you go further into the Earth. This changes the speed of the waves and means that they get refracted. * The Earth has a central core and the outer must be liquid because S-waves are blocked and cannot travel through liquid. * The core has a density of 11,000 kg/m3. Patterns in P-wave refraction have shown this. The size of the core was measured in 1914 by Bene Gutenburg using seismic waves tracing. He found its radius to be 3470 km. Inge Lehmann in 1936 studied how seismic waves travelled through the core. ...read more.


Oceanic crust makes up the other 60 percent of the earth's solid surface. Oceanic crust is, in general, thin and dense. It is constantly being made at the bottom of the oceans in places called mid-ocean ridges. Mid-ocean ridges are undersea volcanic mountain chains formed at plate boundaries where there is a build-up of ocean crust. This production of crust does not increase the physical size of the earth, so the material produced at mid-ocean ridges must be recycled, or consumed, somewhere else. Geologists believe it is recycled back into the earth in areas called subduction zones, where one plate sinks underneath another and the crust of the sinking plate melts back down into the earth. Oceanic crust is continually recycled so that its age is generally not greater than 200 million years. Oceanic crust averages between 5 and 10 km thick. It is composed of a top layer of sediment, a middle layer of rock called basalt, and a bottom layer of rock called gabbro. Both basalt and gabbro are dark-coloured volcanic rocks. Conclusion Earthquakes happen when tectonic plates go under, into or alongside each other. P and S waves originate from the earthquake and travel through the Earth. S waves do not travel through liquid and this has helped scientists find out about the Earth's structure. ...read more.

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