n be determined from the pattern of magnetic anomalies either side of a spreading ridge. Either side of a spreading center, weak magnetic anomalies 5-50 km wide and hundreds of kilometers long can be identified. Molten rock cools between diverging plates the magnetic minerals present align themselves with the orientation of the Earth's magnetic field at that time. The polarity of the Earth has changed at regular intervals throughout geological time. Magnetic north has alternated between the Arctic (normal polarity) and the Antarctic (reversed polarity). These long linear strips of magnetic anomalies form a symmetrical pattern either side of a spreading center. A record of the changes in the Earth's magnetic polarity has been established and dated for the Cenozoic and is the basis for magnetostratigraphy. This record, in conjunction with the magnetic stripes found either side of a spreading ridge, allows the rate and pattern of sea floor spreading to be examined. At a convergent boundary two plates are in relative motion towards each other. One of the two plates slides down below the other at an angle of around 45 degrees and is incorporated into the Earth's mantle along a subduction zone. The path of this descending plate can be found from analysis of deep earthquakes and the initial point of descent is marked on the surface by a deep ocean trench . Plate area is reduced along the subduction zone. When two plates of oceanic crust collide a volcanic island arc may form. As one of the plates is subducted beneath the other it begins to melt at a depth of between 90 and 150 km and the resulting magma rises to the surface above the subduction zone to form a chain or arc of volcanoes. The edge of the plate, which is not descending, is therefore marked by a chain of volcanic islands. A transform boundary occurs where two plates slide against each other. But rather than sliding smoothly, the plates build up tension, then release the tension wit...
