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Why are Lake Baikal and Lake Tanganyika so deep?

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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.

Lake Baikal and Lake Tanganyika are the two deepest lakes in the world because they occupy basins formed by complexes of grabens—downdropped blocks of the Earth's crust.1

These basins are created through the extension of the Earth’s crust, which causes it to fracture. As large crustal blocks or lithospheric plates diverge, a smaller block slides down into the resulting gap, creating a deep tectonic depression or rift valley.2

Specifically, Lake Baikal lies in a deep structural hollow surrounded by mountains, with sedimentary strata on its floor that may be as much as 20,000 feet thick. Similarly, Lake Tanganyika occupies the southern end of the Western Rift Valley, where the land rises steeply from the shores, characteristic of these geologically young, fault-formed landscapes. Because these lakes are situated within these deep, fault-formed troughs, they have achieved their extreme depths.3451