A relief map of the world reveals the structure of global mountain systems: the great backbone of both North and South America from the Rockies to the Andes, where the Pacific has pushed underneath spouting volcanoes; the high t peaks of the Alps and Himalayas where continental land masses have collided; and the ridges and wrinkles that mark ancient oceans long since squeezed out of existence. With the oceans drained, other even larger features become visible. The ocean ridge system, where new crust is formed, consists of long mountain ranges. Isolated groups of volcanoes such as the Hawaiian chain stand out as great underwater mountains composed of millions of cubic kilometers of basalt. The ocean trenches, where crust is swallowed, plunge up to 14000 m beneath the sea, and are flanked by volcanic atolls. Thought the ancient wrinkles reveal a long history, continuing activity shows that the Earth is still a dynamic planet.
A normal fault is where one block slides down the fault face compared with the other block. A strike slip fault is where one plate grates alongside another. The movement in this case is not vertical but horizontal. Features called horsts and grabens result from blocks moving between two faults.
The most famous strike slop fault in the world runs from San Francisco in the north to the hills behind Los Angeles. It is the San Andreas Fault and, with its branches and tributaries, has been on the move almost continuously for thousands of years. Hardly a day goes b without a few tremors along its length. Quakes brought San Francisco to halt in 1989 and Los Angles in 1994 but the last big ones were in 1906 and 1857 respectively.