Summary: Through the year, we’ve talked about events that broke apart and combined the various tectonic plates on the earth, but today, as we’ve almost reached the present, I wanted to just summarize the way things are today. First, I know I talked repeatedly about oceanic crust and continental crust. They are quite different from each other, in density, thickness, and mechanical behavior, and those differences drive subduction and plate tectonics. But the two types of crust also move together, a lot. The North American Plate includes all of the North American continent – except the bit of California west of the San Andreas Fault – but it also includes the oceanic crust beneath the North Atlantic Ocean, all the way out to the Mid-Atlantic Ridge. Iceland straddles the mid-ocean ridge, a pile of volcanic material erupted because of a hotspot at depth, but the west half of Iceland is part of the North American Plate, and the east half is on the Eurasian Plate. Depending on exactly how you want to define “major,” there are 9 to 16 major plates. Africa has two sub-plates – Arabia, which is tectonically separated from Africa, but only by the width of the young Red Sea, and the Somalia Plate, breaking away along the East African Rift. The Somalia Plate is certainly separating from Africa, but in many ways and in many places, they’re still attached to each other too. Then there’s the North American and South American Plates, both of which include the continent and the western half of the Atlantic Ocean. There is only a vague boundary between the North and South American Plates, because they are to a large extent moving together at a similar speed and direction. There is an extensional rift between North America and Greenland – but it failed, and Greenland is now completely attached to North America, and is moving with it. Australia and India have their own plates and include more oceanic crust than continental, but like the Americas, they are almost locked together now that India has collided with Eurasia and slowed down. Antarctica also has its own plate, and except in some small sections, Antarctica is entirely surrounded by oceanic rifts. Everything else is pulling away. How can that be possible? You can’t pull away everywhere. Well, you can, for a while – the excess is taken up by collisions elsewhere on the globe, but eventually, even if the active rift between West and East Antarctica opens up, part of the Antarctica Plate will start colliding somewhere. The Pacific Plate is entirely oceanic – no continental material except the bit of California west of the San Andreas Fault, which is traveling with the Pacific Plate. And the Pacific Ocean is really underlain by two large plates – the larger Pacific, and east of the East Pacific Rise spreading center, the Nazca Plate which is subducting beneath South America to lift up the Andes. The eastern half of the oceanic plate in the North Pacific was called the Farallon Plate, but it has been almost entirely subducted beneath North America. That subduction made for the various mountain-building events that created the Rockies, the Sierra Nevada, and the Coast Ranges. The other big plate is Eurasia – Europe, and Asia except for Arabia and India and the far eastern tip of Siberia, plus the east half of the North Atlantic Ocean. The smaller plates make for some interesting geography and tectonic activity. Arabia, breaking away from Africa, makes the Red Sea, and colliding with Asia makes the mountains of Turkey, Iraq, and Iran, and the Caucasus. The Philippine Plate is a small oceanic plate in the western Pacific Ocean. The volcanoes of the Philippines, Taiwan, and southern Japan are the result of the Philippine Plate subducting beneath the complex eastern margin of Asia, and the Mariana Trench – the deepest point on the ocean floor – is on the opposite side of the Philippine Plate, where the Pacific Plate is subducting beneath it. When two oceanic plates collide as they are doing
