Crazy Theory 12,407

[Urbane Guerrilla]

Not just millions, but billions. The actual site of the lubrication is as deep as the Moho, if not deeper. The movement is in the lower mantle, which creeps along at just about the pace that a piano string slackens with time from its last tuning. The plates all move, conveyor-belting along gradually, taking about 150 million years for a plate surface uncomplicated by a continental landmass from emergence at a spreading center (those submarine mountain ranges) to its resubmergence into a subduction zone at the edge of a plate.

When there's a continent in the mix, its generally less dense rock causes it to float atop the mantle. Great continents can accrue from island arcs and largeish islands being drifted into landmasses. Also, great continents split up from being drifted apart at spreading centers. On land, these are known as rift valleys and there is one running up much of eastern Africa right now, related to the terrestrial splitting known as the Red Sea.

The geological conditions that make petroleum are more stable sorts of places: sedimentary areas (even sedimentary zones near volcanic hot spots; there is oil within Yellowstone Park's bounds that wells up in some ponds, and nobody knows how much or little is there) that remain, over millions of years, at about the temperature of a fresh cup of coffee. It's believed to take about a million years of this slow cooking to turn algal organic matter into petroleum, and then one needs a petrological structure that concentrates it, as oil never stays where it's made but squeezes along to a catch-pocket.

For a wideranging and highly readable introduction to tectonic motions and their incidental effects, you can't do better than John McPhee's quintilogy Annals of the Former World, now available under one cover in softback.