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Mid-Ocean Ridge: Definition, Facts, Formation & Example

Mid-Ocean Ridge: Definition, Facts, Formation & Example
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Lesson Transcript
Instructor: Terry Dunn

Terry has a master's degree in environmental communications and has taught in a variety of settings.

Mid-ocean ridges...on the surface, it sounds obvious what these might be, but there are complicated forces that create them. Here, you will learn what mid-ocean ridges are, where they are, how they are created, and what makes them fascinating geological formations.

Definition of the Mid-Ocean Ridge

People have always wondered what's lurking below the water. Movies have played on our fears and lack of knowledge. Creature From the Black Lagoon, Piranha, Jaws, you name it...there's a particular creepiness that comes with not knowing what's under there. But in real life, there is something going on under the ocean that is enormous, that is sometimes insanely hot, and that houses strange creatures that live in total darkness. They are mid-ocean ridges, which form where two tectonic plates are spreading apart, which is called seafloor spreading. Tectonic plates are large pieces of the earth's crust, and they interact in a number of ways. Where they pull apart is called a divergent plate boundary.

Formation of Mid-Ocean Ridges

Because mid-ocean ridges are places where tectonic plates are pulling apart, you might expect to find an incredibly deep canyon, but that's not exactly the case. As the tectonic plates pull apart, molten rock, or magma, comes up from below to fill in the gaps. In fact, it sometimes pushes from below first, forming a raised area and forcing the more solid rock above to crack apart. The new rock that forms from the molten rock is usually basalt, the most common type of rock on Earth. Where there are weaker areas in the crust along the mid-ocean ridges, the ridge can divide into segments, called fracture zones. To add to the drama, there are often small earthquakes at mid-ocean ridges.

Mid-ocean ridges can spread slowly or quickly. The slow-spreading ridges often crack and crumble, and leave behind ridges and valleys. Fast-spreading mid-ocean ridges, seem to spread with more fluidity and have a smoother look. Slow-spreading and fast-spreading are relative terms. Mid-ocean ridges typically spread less than an inch up to four inches a year. 

Location of Mid-Ocean Ridges

It turns out, global mid-ocean ridges are largest mountain chain on Earth. They wind through more then 40,000 miles of ocean floor making the earth look like it has a series of zippers. About 90% of mid-ocean ridges are underwater, but there is a length through Iceland that is above sea level. Mid-ocean ridges are typically half a mile to six miles wide and over a mile underwater. 

Two of the most well-known mid-ocean ridges are the Mid-Atlantic Ridge and the East Pacific Rise. As you might guess, most of the Mid-Atlantic Ridge is in the Atlantic, and most of the East Pacific Rise is in the Pacific. The Mid-Atlantic Ridge is spreading one to two inches a year, and the East Pacific Rise is spreading at two to six inches per year. The mid-Atlantic Ridge begins in the Arctic Ocean extends across Iceland all the way to the bottom of the Atlantic near Africa before dividing into two. Our world is not unlike a ceramic vase that has been cracked in multiple places!

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