National Geographic : 1985 Aug
N A CLEAR DAY from the top of Twin Peaks in San Francisco, you can sometimes see the Farallon Is lands emerging from the Pacific Ocean like passing ships of granite. In fact, those islands, some 40 kilometers offshore, are itinerants. Born far to the south, they have been working their way up the coast for millions of years. Their speed is impercepti ble. A runner moving at their pace would take 170,000 years to finish a ten-kilometer race. Yet in the time scale on which our 4.5 billion-year-old planet evolves, the Faral lons are dashing toward Alaska. Bits and pieces of ancient landforms, the San Franciscoareais ajigsaw of terranesin this computer-drawnmap. In a process still active, moving oceanplates have been plastering NorthAmerica's west coast with wayward crustalblocks for 200 millionyears. Today, west of the SanAndreas Faultand its subsidiaryHaywardFault,terranesare moving northabout six centimeters (two inches) a year. 154 Islands on the move. How most geologists would have scoffed at that idea just 20 years ago. But how little we knew then about the way our planet really works. No one under stood why volcanoes arise. Or how moun tains or even oceans are made. Nor did science realize that Earth's surface is broken into seven major and many minor plates: slabs of rigid rock averaging 100 kilometers (60 miles) thick that ride like icebergs in a more fluid layer of hot rock below. Twenty years ago few scientists dreamed that the continents rode those plates-growing, rift ing, and colliding as they drifted. I MARIN HEADLANDS, small blocks 90 to 180 millionyears oldon the California coast, are deep oceanic crust thrust up onto the land. SYOLLA BOLLY, 130-to-150-million-year-old chert and sandstone, was once buried in a deep subduction zone but has mysteriously resurfaced. SALCATRAZ, about 130 million years old, is a small remnant of a continentalmargin, most of which has disappeared.