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Our transcription: As mountain ranges are worn down, their roots are buoyed upward by the mantle. Because the mantle is far stiffer than the most fluid lava, the crust floats upward quite slowly sustaining a hilly topography in the landscape for hundreds of millions of years. As the crust rises, rocks from ever deeper levels inside the Earth are brought to the surface and worn away. The floating of Earth's crust atop the mantle is termed "isostasy". This is similar to what happens at sea, where large icebergs float with more ice extending beneath the surface than small ones do. In the same way, tall mountains usually have roots extending deeper into the Earth than low mountains made up of the same rock type. In both cases, far more mass lies hidden from view than can be seen at the surface.
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