If the Seafloor is Spreading, Then Why Doesn’t it Take Over the World?

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.
The Blogfish
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. 


The Blob and the Blobfish

That was a ton of fun to read! And, like may of your blogs, it gives me questions.
Does the Blogfish like The Blob movie?
Where is the great rift valley in Africa?
Will Alaska and Russia collide?
What will happen to North America (besides a possible collision with asia)?
Do human activities effect seafloor spreading?
How does Antarctica factor into this?


Im a little confused!? How does the seafloor spread by it doesn't widen because it is melted by magma? Does it lengthen??? - CREATURE

The reason the seafloor does

The reason the seafloor does not continue to widen or lengthen is because as rock is added at one end of a seafloor plate, about the same amount of rock is being melted back into the mantle at another end. As the seafloor is pushed by convection currents into deep ocean trenches, the seafloor at the very deepest part of the trench is melting back into the mantle. So the seafloor loses rock there. As the convection current moves the seafloor, though, a hole opens in the middle of the seafloor that lava rises up into to form new rock. So every time rock is added at a seafloor spreading center and the seafloor spreads out from there, about the same amount of rock is being melted somewhere else in the seafloor, because the rock there is being pushed back into the mantle.

Spreading till nothing

In that case, would continents keep spreading till they were all small islands?

That's a good question.

That's a good question. Continents do occasionally break into mini-continents. This might be beginning to happen right now at the east Africa at the Great Rift Valley. The reason continents do not continue to break up in smaller and smaller pieces is plate tectonics will also push small continental plates back together to form larger plates. The Himalayas continue to grow right now because the Indian plate is being pushed into the Asian plate. Sometimes all the continents are pushed together to form a supercontinent, like Pangaea, but as convection currents in the mantle change, the supercontinents eventually break up into smaller continents again.