The hypotheses regarding the origin of the rift valleys are generally grouped in two categories: 1. Tensional Hypothesis 2. Compressional Hypothesis.
Category # 1. Tensional Hypothesis:
The earlier hypothesis of the origin of the rift valleys was based on the basic concept of the ‘dropped keystone of the arch’ of a building. According to this concept the rift valleys were related to the hollow space created by the dropping of the keys stone of an arch of a building downward.
In other words, an open space is formed at the middle portion of an arch of a building when the keystone or key brick falls downward due to cracks developed in the arch. Similarly, when two parallel cracks develop in the crustal surface due to tensional forces and when the side blocks on either side of the two cracks or fractures are pulled apart due to tensional forces, the middle portion between two parallel normal faults moves downward and thus an open space is formed. This open space becomes a rift valley.
This ‘key stone hypothesis’ was severely criticized because it was based on erroneous concepts and beliefs. For example, there is wide open space below the arch of a building and hence the keystone or key brick, after the arch develops cracks, can easily fall down but there is no open space beneath the crustal rocks and thus there would be difficulty for the middle block between the two parallel normal faults to slip downward.
The faulted middle block can only be slipped downward when it would be able to displace the magma lying below the crustal blocks. If this process is accepted then the formation of the rift valley must be followed by volcanic activities because the displaced magma would try to ascend through the faults.
Sometimes, the mechanism may be so sudden that there may be sudden violent volcanic eruption, but the observations of several deep rift valleys denote the fact that rift valley formation is not necessarily always associated with volcanic eruptions. The observations and several experiments have revealed the fact that already existing volcanic activities and active volcanoes ceased to operate at the time of the formation of rift valleys.
It might have become possible only when the exit of the ascent of magma would have been plugged due to faulting activity. This explanation is also refuted on the ground that if we accept the mode of formation of a rift valley due to horizontal tensional forces and resultant pulling of bounding faulted side blocks of two normal faults apart, then the upwelling of magma in the form of lava cannot be stopped, rather the pouring of lava can be stopped due to compressive forces. Thus, the tensional hypothesis of the origin of the rift valleys is rejected on this ground.
Category # 2. Compressional Hypothesis:
In order to remove the difficulties of the tensional hypothesis of the origin of the rift valleys compressional hypothesis was postulated by a number of scientists e.g. Wayland, Baily Willis. Waren D. Smith. E.C. Bullard etc. Wayland through his studies of Lake Albert and Ruwenzori section and Baily Willis based on his studies of Dead Sea have postulated the concept that the rift valleys are not formed by tensional forces but are formed due to compressional forces at greater depth.
Due to intense compression the side blocks are thrown up along the thrust faults in the form of horsts. These up-thrown blocks are called over-thrusting rift blocks. The middle portion is forced to slip downward because of the pressure resulting from the rising side blocks Thus, the downward slipping middle portion between two faults is called rift block which is narrow upward but broader downward.
In other words, the rift block gradually broadens out downward. Thus, the rift valleys are formed due to slipping of middle block or rift block downward between two rising side blocks caused by thrust faulting under the impact of convergent compressional forces.