QUIZ 23a. Plate tectonics - evidence for plate movement

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(a) At one time it was believed that the major features of the earth's surface were the result of the of the crust as the Earth down. Back in the 1920's 's theory of crustal movement, sometimes called , was not generally accepted until more than 50 years after it was proposed because he was not a geologist, he was (remember this was just a few years after the 1st World War) and there was no to support his theory - no suitable data was available.

(b) Tectonic may slide past each other and this is happening along the Californian coast giving rise to and they cause !

(c) Plates may move each other. As this happens, a thinner, oceanic plate can be driven down () beneath a granitic continental plate where it partially . The continental crust is compressed, causing and heated causing . Earthquakes are again produced and may rise through the continental crust to form . This is happening along the western side of South America (the Andes)

(d) Plates may move from each other. This causes which are filled by magma producing new, basaltic, oceanic crust. This is known as spreading and is happening along oceanic ridges, including the mid-Atlantic .

(e) The minerals in the magma record the direction of the Earth's at the time when the rising magma . Magnetic in oceanic crust occur in stripes parallel to ridges, matching the reversals of the Earth's magnetic field and so supporting the of sea-floor spreading.