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Rift Valley: Definition & Facts

Instructor: Terry Dunn

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

Rift valleys are created slowly, but they are literally earth-shattering. In this lesson you will learn about the powerful forces that create them, their special features, and aspects about the world's largest rift valleys.

What is a Rift Valley?

You've heard of rifts between siblings and valleys of depression, so a rift valley sounds like a pretty dire, contentious place. Rift valleys are, in fact, places of turmoil and change, but it's slow motion change and we owe our continents to them. As you might guess, a 'rift' is a split or break and a 'valley' is a low point. The rift forms when the earth's crust divides. The valley that forms in the divide is usually flat, narrow, and has steep sides.

The image that comes to mind might be the Grand Canyon or the steep-sided valleys in the Alps, but those valleys were formed by the movement of water, either flowing or frozen (glaciers). A rift valley is not related to valleys formed by those types of erosion. They are formed as tectonic plates move. Tectonic plates are kind of like puzzle pieces of the earth's crust and they shift very slowly. Yes, the ground beneath your feet is not as still as you might believe! Where two pieces of the puzzle lie next to each other they either push up against one another, slip under one another, or pull apart. It's in the places where they pull apart that a rift valley forms.

How Do Rift Valleys Form?

Rift valley formation may be slow, but it can be dramatic. It usually starts with a hotspot below the crust where the pressure builds, the crust rises, and then starts to crumble apart. Earthquakes, lava flows, and volcanoes are spawned and the tectonic plates are spread slightly further apart. As the plates spread, a valley is formed. The area where the plates are pulling apart is called a divergent boundary. The valley floor is called a graben (sometimes used interchangeably with the term rift valley) and the valley sides are called horsts. Over time, the grabens fill up with water to create a lake or they fill up with sediments that flatten out the valley floor. The process happens under the ocean and also on land. When it happens in the ocean it's called seafloor spreading.

Diagram of a divergent boundary
A divergent boundary diagram

Examples of Rift Valleys

There are some great examples of rift valleys both under the ocean and on land. The Mid-Atlantic Ridge under the Atlantic Ocean is where the North American tectonic plate and the Eurasian tectonic plate meet. The rift is about 9 miles across and is increasing by about an inch per year. Where it is splitting, new molten lava is filling in the gap. The East Pacific Rise is another large area of seafloor spreading.

The Great Rift Valley in Tanzania
Great rift valley, Tanzania

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