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Why does stretching of Earth’s crust cause blocks of crust to drop down?

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Snow-capped, rugged Atlas Mountains encircle a green valley with scattered trees, terraced fields, and a few small buildings in Morocco. — valley The rugged Atlas Mountains surround a valley in Morocco.

When Earth’s crust undergoes extension, or stretching, the brittle rock is subjected to tensional forces that cause it to fracture. As large crustal blocks or tectonic plates move apart, the space between them increases.123

This process leads to the formation of normal faults, where the block of rock positioned above the fault plane—known as the hanging wall—slides downward relative to the block below, the footwall. As the hanging wall drops into the gap created by the diverging crust, it forms a depression known as a graben, or rift valley. In this geological context, the down-dropped block effectively becomes the floor of the valley, while the surrounding blocks may remain elevated or rise, creating the characteristic mountainous margins of these tectonic landforms.132