Geology posts are the least popular topics here at Wickersham’s Conscience. Ironically, geology has seen a complete revolution in WC’s lifetime, as extensive, dramatic and important as the revolutions in electronics and biology. So despite strong evidence the subject bores his readers to tears, WC will revisit the matter of plate tectonics.
Many years ago, WC’s neighbor, Dr. Dave Stone, had that bumper sticker on his giant Ford SUV. It was Stone’s puckish sense of humor: he’s a highly regarded physicist, specializing in the paleomagnetic evidence that supports plate tectonics, a/k/a continental drift. The aphorism has generalized to mean attempting to stop the unstoppable.
Because plate tectonics has become unstoppable. The science has advanced dramatically since its inception in the early 1960s. Initial skeptics, including those documented by John McPhee in his superb geology series, have pretty much been overwhelmed by the massive evidence supporting the theory. Through massive oversimplification, borrowed illustrations and short sentences, WC will undertake to explain the rudiments of plate tectonics in a single blog post.
The surface of the earth, the lithosphere, floats like pudding scum on the semi-liquid mantle that makes up the majority of the planet. The “pudding scum,” the lithosphere, is broken into several dozen pieces, called plates. Driven by currents in the mantle and gravity, the plates move. Some slide by each other, creating great earthquake faults like the San Andreas Fault, in California. Other pieces slide under one another, with colder, denser ocean plate on the underside melting and dribbling up through the ligher, warmer continental crust to the surface, creating chains of volcanoes like the Cascades in Oregon and Washington, and the Andes, along the western side of South America. And sometimes pieces of the continental crust collide, driving up immense mountain ranges like the Alps in Europe, where the African Plate has collided with the Eurasian Plate, and the Himalayas, where the India Plate has crashed into the underbelly of the Eurasian Plate. And, helping drive this very slow motion hurdy-gurdy, plates are pushed apart at spreading centers, like the Mid-Atlantic Ridge.
A represents an upwelling current in the upper mantle. B is the movement force on the plate by the spreading center. C shows the subduction of the colder, denser oceanic crust beneath the warmer, less dense continental crust. The downward pull of gravity on the subducting plate also contributes to plate movement. D shows an oceanic trench, created at the margin of the continental crust. E shows the oceanic ridge, where the ocean crust has been pushed up and, because it is warmer and less dense, it has risen above the abyssal plain. F shows a plume of hotter, semi-liquid mantle rising from deeper in the earth, as a result of the convection at A and the chemistry of high-pressure, high temperature rocks. And G shows a volcano created by the melting of the oceanic crust, as the subduction process drags the crust down to levels where it can melt.
The plates don’t move quickly; the fastest move at about the same speed your fingernails grow. But over geologic time, over tens of millions of years, looking backwards in time. that’s ample speed to have Africa and South America snuggled against each other, as their shapes suggest. And, sure enough, you can trace geologic formations from Africa to South America, if the arriviste Atlantic Ocean didn’t intervene.
Remember all this movement, this crust-creation at spreading centers and plate subduction at boundaries takes place on the surface of a sphere. So the geometries of movement can be complex. To accommodate straight lines on a sphere, the spreading centers aren’t continuous, but are offset by faults. Subduction zones aren’t continuous, either. Think about the complexities of the west coast of the United States, with the San Andreas fault, a transform fault, along the southern and central California coast, and a subduction zone and attendant volcanoes in Oregon, Washington and southern British Columbia. it’s all a result of the collision of the Pacific Plate and the North American Plate, beginning about 30 million years ago.
From left to right, you can see a simplified version of the Pacific Plate very slowly – this is geology, after all – obliterating the late, great Farallon Plate and nearly demolishing the Juan de Fuca Plate as it collides with the North American Plate.
At the letter M in the Present, there’s a triple junction, where the Pacific Plate, the North American Plate and the Juan de Fuca Plate all touch. Triple junctions are where the geologic action is at, in this case where a transform fault meets a subduction zone. “Geologic action” also means earthquakes, tsunamis and other kinds of real-time geology. It’s not a good place to have a city. But WC digresses.
So the basic motions of plates are expansion, at spreading centers; subduction, where ocean floor slides under contents; faults, where plates slide by each other, and collisions, where continental plates meet. That’s pretty much it. Sure, there are layers upon layers of complications coming out of those four basic kinds of plate motion. And plate tectonics has been going on for an extraordinarily long time, perhaps as long as 4 billion years, so deciphering what has happened in the past adds whole new layers of complexities.
But for cocktail party conversation – or for the line at the coffee shop – you only need to remember spreading centers, subduction, faults and collisions. The rest is details.
Hey, let’s watch Gondwanaland break up…