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How do rift valleys form between blocks of Earth’s crust?

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A car drives along the Trans-Canada Highway with Mount Sir Donald's sharp, snow-capped peak rising above forested slopes in the Selkirk Mountains, British Columbia. — mountain Mount Sir Donald in the Selkirk Mountains, British Columbia, looms above a segment of the Trans-Canada Highway.

Rift valleys form primarily through the extension of the Earth’s crust, which causes the crust to fracture and pull apart at divergent plate boundaries. As these large crustal blocks or lithospheric plates move away from each other, the brittle crust breaks along normal faults.123

This process creates a gap into which a central segment of the crust subsides or slides downward. The resulting down-dropped block, known as a graben, forms the relatively flat floor of the rift valley, while the surrounding blocks—often rising during this extension—form the steep, mountainous margins. These valleys are characterized by their elongated shape and are frequently associated with volcanism and seismic activity. While rift valleys are distinct from erosional valleys, they are often found in regions featuring horst and graben topography, where a series of raised and lowered crustal blocks create a landscape of alternating mountains and basins.1243