MadSci Network: Earth Sciences |
Amy, What a clever and thoughtful question you asked, one that gets right to the heart of plate tectonics - what's pushing, what's pulling, and what's going where? You are correct to notice that the Eurasian Plate (which includes the crust under the eastern half of the northern Atlantic Ocean, not just the continents of Europe and Asia) must also be moving away from the mid-Atlantic ridge, in the opposite direction to the North American plate. Both of those plates meet the Pacific plate around the edges of the Pacific Ocean. As the Atlantic Ocean Basin expands, the Pacific Ocean Basin is indeed getting smaller. In terms of geography, this is happening relatively equally on both sides. The west coast of North America and the east coast of Asia are both moving toward the International Date Line. With the Global Positioning System (GPS - using the same GPS satellites that people use to find their way in their car these days, but with much more accurate receivers), we can measure plate motions. Here is a map showing the motions of some spots on different plates: http://www.geodesy.miami.edu/plate_motion.html Continental spots are in red, oceanic spots in white. You can see that the arrows form curves within a plate. That's because moving on a sphere is different from moving on a flat surface and so the plates get around by rotating around on the surface. The length of the arrow tells you how fast the plate is going. You can see that Eurasia is going a little faster than North America, but there's not a huge difference. So, both Eurasia and North America are rotating away from the center of the Atlantic and toward the center of the Pacific. However, the Pacific plate isn't just sitting around. It is also moving, to the northwest, away from western North America and toward Alaska and Japan. As the Pacific ocean gets smaller, the Pacific plate underneath it has to go somewhere. That extra plate is subducted, pulled back into the mantle and recycled. Most of that subduction is happening along the northwest side of the Pacific ocean, causing big earthquakes and volcanoes in western Alaska, the Kamchatka Peninsula (part of Russia), Japan, and the Phillipines as the Pacific plate slips down underneath those areas. In fact, the fact that the Pacific plate moves rapidly and moves toward the big subduction zones tells us something very important about plate tectonics. It tells us that the most important thing driving plate motions is the pull of the sinking crust in a subduction zone. This pull is much larger that the small push generated at ridges and also larger than the drag on the plate from the moving mantle underneath. Hope that helps answer your question, Dave Smith, Da Vinci Science Center
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