How the Mariana Trench Became Earth's Deepest Point

James Cameron's dive to the deepest part of the Mariana Trench took him seven miles below sea level. Why is this abyss so deep? There are at least three contributing reasons that come together.
One reason the Mariana Trench is so deep, he added, is because the western Pacific is home to some of the oldest seafloor in the world—about 180 million years old.

Seafloor is formed as lava at mid-ocean ridges. When it's fresh, lava is comparatively warm and buoyant, riding high on the underlying mantle.

But as lava ages and spreads away from its source, it slowly cools and becomes increasingly dense, causing it to settle ever lower—as is the case with the Mariana Trench.

Read the rest at National Geographic News. Link

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Just for perspective, this trench is 7 miles; the Earth's diameter is 8000 miles. If the Earth were shrunk to the size of a billiard ball this trench would be about 2/1000 inch deep. It is one of the only earth features that could be felt by a human at that scale. It would be a very faint scratch that you _might_ be able to detect it with your fingernail. Overall, the Earth's surface is smoother than most billiard balls.
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