Mid-ocean Ridge Melting - The Ballon Analogy

     The mid-ocean ridges are mountain chains that curve around the Earth like baseball seams. Sadly, these exist almost entirely at the bottom of the oceans (but you may have guessed that from the name!). From these mountains, lava (liquid rock) comes slowly pouring out, from somewhere within the Earth. Exactly where, is an important, and not well understood question, but we'll get to that later. The lava becomes solid rock and creates new "crust." The "crust" is a specific portion of the Earth's anatomy. It is a region of the Earth that is more rigid (like a pie crust), than the inside (which is still rock, but since it is hotter, can 'flow' in a similar manner to a bending spoon). The inside of the Earth is actually split into a few different parts, but for the sake of simplicity, I will leave that to your interests to figure out elsewhere (try wikipedia). Anyway, the part of the inside of the Earth that I study is called the "mantle." It moves around slowly, similar to a boiling pot of water, heat from the bottom causes the mantle/water to move up, as it moves up (away from the heat source) it cools down, and when it reaches the top of the Earth/pot, it returns downward. This is called a convection cell, and there are many of them inside of the Earth's mantle. 

     The mid-ocean ridges are where two of these convection cells are moving upwards together, and then move away (see this image for a better understanding of all this stuff).

Artists conception of the interior of the Earth: The motion of the Mantle (the middle, orange region), can really be seen nicely here. The Core (the red part, at the center) is the source of heat that causes the mantle to move up in this image. The Mid-ocean ridge is the thin yellow line on the surface of the Earth. The melting region is the triangle formed just below the ridge.              Image Source: National Geographic

     Now, there is a lot of discussion about what causes these movements, but the basics are that the heat inside of the Earth is escaping, and that the ridges are an area of the Earth where things are moving away from each other. Just below the surface, there is a region of the mantle which is just hot enough to cause some of the material to melt. This melted material is lighter than the rock, and so it moves upwards. Now as you can imagine, the region containing melt is quite large, because a lot of the mantle is hot here. One of the main problems when this is realized, is that if everything is moving up in this large region, why do we only see a thin ridge, where all the lava comes out. One idea that explains this, that I use, is that there is a layer in the mantle that funnels the lava to the ridges. Think of it as a roof, and picture yourself in an attic. In this attic you have a bunch of balloons, if you let go of them, you would see them travel up, hit the bottom-side of the roof, and then get funneled to the center. This is the way that we think melt from a large area, arrives at the much smaller mid-ocean ridge. In this analogy, the center of the roof would be just below the mid-ocean ridge. 

The 'Balloon Analogy": The crust is represented by the green panels, and the arrows show that the crust is moving away from the mid-ocean ridge. The Grey is the roof that focuses the balloons (melt) to the ridge. The red balloons turn into green crust. The red arrows show the direction of the convection cells in the mantle. 

     This "roof" is a somewhat complicated structure, and the way we understand and explain it actually allows us to predict the way and the locations that it will cause the lava to come out. Now you'll have to imagine a roof with more than two sides, maybe a house with a corner in the middle. This corner, will get more of those balloons than other parts of the ridge. And extending the analogy to the Earth, a larger amount of lava, and hence a larger amount of rock will appear here. 

     The beauty of this relatively simple idea, is that we can now start to add more complicated things to different parts of the attic, and see where, and how the roof structure will create changes that we can then see at the surface of the Earth (except this part of the surface is at the bottom of the ocean, but that just means we get to go down in submarines and explore!). And when we find the things that our complications predict, we then add this small piece of knowledge to the growing body of knowledge about the Universe. Which is really what it's all about.

The 'Balloon Analogy 2: Son of Balloon Analogy": The crust is represented by the green panels, and the arrows show that the crust is moving away from the mid-ocean ridge. The Grey is the roof that focuses the balloons (melt) to the ridge. Only in this cartoon we have a corner. There is more crust (the blippy lines at the top of the image) at this corner. And we've also added a different kind of rock (yellow balloons) to the idea. The red balloons turn into green crust, the yellow balloons turn into yellow-green crust. You can begin to imagine what adding more colors, in different regions can do!