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If the Seafloor is Spreading, Then Why Doesn’t it Take Over the World?
Submitted by JR junior on Mon, 05/09/2011 - 15:08
In all the movies titled The Blob, the Blob continues to grow and spread, covering more and more area, like it might to cover the entire world, which sounds kind of like seafloor spreading. Unlike the Blob, though, you do not need to spray the seafloor with fire extinguishers and then drop it in the Arctic Circle to limit how far it spreads. Seafloors are continuously being dragged down into the mantle to become magma again, which means that seafloors spread without becoming wider (to learn more about how seafloor spreading occurs, read my last blog post).
Ocean crust is denser than continental crust, so in many places where the two crusts meet, the ocean plates sink below the continents (like a rock sinks in water that is less dense than it). In these places where the ocean plates bend down under the continental plates towards the mantle, the ocean floor forms trenches, the deepest places in the oceans. Some of these trenches are so deep that if you were somehow able to drive a car at 55 mph from the surface of the ocean to the bottom of the trench, it would take you over 7 minutes (you could also drop Mt Everest in them and not see the top above water, but that example is not original). In the map below, courtesy of NOAA, you can see the trenches surrounding almost all of the Pacific Ocean.
In other places on Earth, such as around almost the entire Atlantic Ocean, the ocean crust is actually pushing the continents around instead of sinking beneath them (the Atlantic only has a couple of small trenches around it). In the process, the Atlantic Ocean is also shrinking the Pacific Ocean. As the Atlantic Ocean floor expands, it pushes continents over the Pacific Plate, causing the Pacific to sink into the trenches that surround it.
The JOIDES Resolution is currently drilling on the Cocos Plate, which is one of the smaller plates found under the Pacific Ocean. The Cocos Plate has two seafloor spreading centers. The East Pacific Rise to the west adds rock to the Cocos Plate and so does the Cocos Ridge to the south of it (they are the red and green lines on the map below). At the same time it is spreading, the Cocos Plate is also sinking under the continental crust in the trench found just to the southwest of the Central American coast (the trench is the blue line with the triangles on it).
So if anyone ever makes a low budget horror movie about spreading seafloor attacking the Earth like it was the Blob, you now know the film’s scientific information may not be the best thing upon which to base your homework answers.
PS. Even though I am a blobfish, the Blob is no relation to me.
PPS. Because ocean crust keeps returning to the mantle, ocean crust rock is like a baby compared to senior citizen continental crust rock. The oldest continental rock found on Earth is over 4 billion years old. See if you can find ages of the oldest ocean crust rock in this diagram below.