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How does crustal extension cause faults to develop?

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tectonic basins and rift valleys Escarpments of the Great Rift Valley rising above the plain north of Samburu Game Preserve, central Kenya. Beisa oryxes graze in the foreground.

Crustal extension causes faults to develop because the brittle Earth’s crust fractures when subjected to stretching forces. As large crustal blocks or lithospheric plates diverge, the extension creates a gap, causing a block of rock—known as the hanging wall—to slide downward relative to the adjacent block, or footwall.12

These fractures are known as normal faults, which typically feature steep dip angles between 45° and 90°. When these faults occur in groups, they can create horst and graben topography, where blocks of crust are either raised (horsts) or lowered (grabens). The down-dropping of a block between surrounding fault blocks creates a rift valley or tectonic basin, a process often compared to a keystone falling as the walls of an arch move apart.231