The Baikal rift is the most important of a system of rift valleys located in the USSR. A comparison of geological and geophysical data for the Baikal rift and other rift zones of the earth leads to the conclusion that their origin is determined by similar deep‐seated processes. Some mechanisms of rifting suggested earlier are considered. It is shown that rift‐valley formation cannot result from collapse of the central parts of large crustal arches. The concept of Vening‐Meinesz that considers rifting to result from isostatic subsidence of a wedge‐shaped crustal block also is problematic. A new mechanism for rifting is proposed that is based on modern geological and geophysical data. Rift valleys may be considered to result from crustal extension when neck‐shaped strains in the crust are developed. The lower part of the crust is plastically attenuated, and faults are formed only in the upper layer of the crust, which is highly viscous. It is suggested that large‐scale movements in the mantle created by gravity convection may be the cause of crustal extension. The mechanism of gravity convection has advantages over thermal convection.

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