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Transcript
Seismic records have led geologist to believe that the Earth has a layered structure, and can be thought of as being a bit like a cracked egg. The solid crust is broken up into smaller bits, called plates, which float on a dense mantle. Parts of the mantle are molten liquid and movements in this liquid cause the plates to drift into one another. It is the meeting of the plates that causes the earthquakes. The density of the material forming the Earth increases with depth. Nuclear reactions within the solid inner core maintain the high temperatures of the body of the Earth. The outer core is liquid and large convection currents within this layer give rise to the Earth’s magnetic field. P-Waves The speed of both types of waves increase as they travel through the Earth’s interior. However, the speed rapidly changes when they cross between layers of material. S-Waves The speed of the p waves decrease considerably when encountering the outer core, but S waves are stopped altogether. A B Direct P-wave shadow zone The diagram above shows the paths taken by P and S waves as they travel through the interior of the Earth. Point A - Both P and S waves are gradually refracted as they travel and therefore follow curved paths as they spread out from the epicentre. This indicates that the density of the material within a layer gradually increases with depth. P and S waves are detected up to angles of 1030 from the epicentre. Point B - At certain points the refraction becomes more pronounced as the waves meet a definite boundary between layers. No S waves can travel through angles greater than 1030 hence the boundary at B must separate solid from liquid. This is the evidence that the outer core is made of a dense liquid. P waves entering the liquid outer core slow down and are focused (like waves travelling through a lens) into the region at the bottom of the diagram (angles greater than 1400). A shadow zones for direct travelling P waves exists between 1030 and 1400, however, weak P waves are detected in this region due to the refraction of the inner core. 1 Show P + S waves curving through the mantle. (Not interacting with the outer core) 2. Show S-waves forming a shadow zone. 3. Show P-waves refracting through core.