CoachA tectonic plate (or simply a plate) is a large, rigid slab of rock that forms part of the Earth’s outer shell. A plate is made of the crust and of the uppermost, cold and rigid part of the mantle, together at least 80 kilometres thick on average; it moves slowly over the hotter and softer mantle beneath it.
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Example 18.8 (Divergent boundaries: Iceland and East Africa)
Down the middle of the Atlantic Ocean runs a chain of undersea mountains, the Mid-Atlantic Ridge, where the plates on either side move apart. Hot rock from the mantle rises into the gap, melts in part, and becomes new sea floor: a divergent boundary builds new plate. Iceland is a piece of this ridge that rises above the sea: there, North America moves westward and Eurasia eastward, and the island is slowly torn open. Between 1975 and 1984, the cracks near the Krafla volcano, in the north of the island, widened by about 7 metres in all. A divergent boundary can also cut through a continent: along the East African Rift, Africa is beginning to split, and the Red Sea is a young ocean that has already separated Arabia from Africa.
Example 18.9 (Convergent boundaries: the Andes and the Himalaya)
Off the west coast of South America, the Nazca plate, which carries ocean, moves towards the South American plate, which carries a continent. The ocean plate, heavier, bends and sinks beneath the continent, deep into the mantle. Along the line where it goes down, the sea floor plunges into a deep trench; above the sinking plate, the land is shaken by earthquakes and lined with volcanoes: the Andes. In the western Pacific, where one ocean plate sinks under another, the trenches are the deepest places on Earth: the Mariana Trench is nearly deep. When two continents meet, neither sinks: the crust crumples and thickens. India collided with Asia about 50 million years ago, and the two plates are still pushing; that is how the Himalaya and the Tibetan Plateau were raised.
Example 18.10 (A transform boundary: the San Andreas fault)
In California, the Pacific plate slides north-westwards past the North American plate along the San Andreas fault, about long. No plate is made or destroyed there; the two plates grind past each other, and the jolts of that grinding are earthquakes.