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Measuring Earthquakes

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The vibrations produced by earthquakes are detected, recorded, and measured by instruments call seismographs. The zig-zag line made by a seismograph, called a "seismogram," reflects the changing intensity of the vibrations by responding to the motion of the ground surface beneath the instrument. From the data expressed in seismograms, scientists can determine the time, the epicenter, the focal depth, and the type of faulting of an earthquake and can estimate how much energy was released. The first indication of an earthquake is often a sharp thud, signaling the arrival of compressional waves. This is followed by the shear waves and then the "ground roll" caused by the surface waves. A geologist who was at Valdez, Alaska, during the 1964 earthquake described this sequence:  The first tremors were hard enough to stop a moving person, and shock waves were immediately noticeable on the surface of the ground. These shock waves continued with a rather long frequen...
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the Rock Cycle

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The Rock Cycle Igneous, metamorphic, and sedimentary rocks are related by the rock cycle, the circular process by which each is formed from the others. Rocks are weathered to form sediment, which is then buried. During deeper and deeper burial, the rocks undergo metamorphism and/or melting. Later, they are deformed and uplifted into mountain chains, only to be weathered again and recycled.  Rocks do not remain in equilibrium and are forced to change as they encounter new environments. The rock cycle is an illustration that explains how the three rock types are related to each other, and how processes change from one type to another over time. The rock cycle never stops.
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Temperature gradient and density

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Temperature gradient and density It is well known from deep mining operations that temperature increases downwards at an average rate of 300C per km. This rate is higher near a source of heat such as an active volcanic centre, and is also affected by the thermal conductivity of the rocks at a particular locality. Assuming for the moment that the temperature gradient continues at the average rate, calculation shows that at a depth of some 30 km the temperature would be such that most known rocks would begin to melt. The high pressure prevailing at that depth and the ability of crustal rocks to conduct heat away to the surface of the Earth result in the rock material there remaining in a relatively solid condition; but there will be a depth at which it becomes essentially a viscous fluid and this defines the base of the lithosphere (Greek: Iithos = stone). The mean mass density of the Earth, which is found from its size and motion around the Sun, is 5.527 gem"3. This is grea...
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