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If rift valleys develop on both sides of a block that is left standing up, the block is known as a horst.
Many rift valleys have much the same width, often in the range of 30 to 45 kilometres. This may be the largest span that the crust can support before it collapses.
Rift valley landscapes
There are some spectacular examples of rifting, such
as the Basin and Range region of the southwestern United States. Here the crust has been gently doming
up and stretching for a long time, making the crust sixty kilometres wider in the last fifteen million years, and allowing blocks to sink and create rift valleys. They are the basins. The horsts that remain standing are the ranges.
One of the best-known rift valley basins in the world is Death Valley, whose floor is 83 metres below sea level. The nearby Panamint Range is nearly 4000 metres above sea level. Erosion has stripped away some of
the mountains, and this material has partly filled in the
(Above) When the land stretches, blocks of the crust fracture along parallel lines, and then some blocks sink, or founder, between others.
The foundering blocks make rift valleys, while the blocks that remain upstanding make block mountains or horsts.
(Left) This satellite picture of the Basin
and Range region of
the southwestern United States shows parallel dark-coloured horsts (ranges) separated by white-coloured basins. The white of the basins is produced by salt
lakes. The view looks southwestwards across Nevada to the snow- capped peaks of the Sierra Nevada mountains of California.
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