Victoria Falls, known locally as Mosi-oa-Tunya (“The Smoke that Thunders”), is one of the world’s most spectacular waterfalls. Situated on the Zambezi River, it marks the border between Zambia and Zimbabwe. The immense curtain of falling water and the deep, narrow gorge represent a geological story of power and relentless change. The dramatic landscape seen today results from the river interacting with a specific, fractured foundation of volcanic rock over vast stretches of time.
The Ancient Foundations: Basalt and the Rift Valley
The stage for the formation of Victoria Falls was set approximately 180 million years ago during the Jurassic period. At that time, volcanic eruptions across Southern Africa laid down successive, thick layers of dark, fine-grained volcanic rock known as basalt, part of the extensive Karoo Supergroup. The basalt plateau underlying the area is estimated to be at least 300 meters thick.
This hard, resistant basalt forms the bedrock over which the Zambezi River flows. As the lava cooled, it developed a network of internal cracks, known as joints or fissures. Subsequent tectonic movements, associated with the break-up of Gondwanaland and the developing East African Rift System, further fractured this bedrock.
These stresses created two dominant sets of weaknesses: one running roughly east-west, perpendicular to the Zambezi’s flow, and another running north-south. Over time, these open cracks became in-filled with softer materials, such as sedimentary clays and sandstones. This fractured foundation of alternating hard basalt and soft infill determined where the river would eventually cut its path, as the infill is significantly less resistant to erosion.
Establishing the Timeline: Geological Age
While the basalt bedrock dates back about 180 million years, the current configuration of Victoria Falls is geologically young. The entire zigzagging gorge system below the falls is a much more recent development. Scientists estimate that the Batoka Gorge, which stretches over 100 kilometers downstream, has been carved by the river over the last 1.4 to 2.5 million years.
The Zambezi River’s course was altered about 15 million years ago following tectonic uplift in the region. This led to the river capturing new drainage systems, and the eventual joining of the upper and lower Zambezi systems initiated the rapid gorge incision.
The present location of Victoria Falls is the most recent of a series of waterfalls that have retreated upstream over time. Evidence suggests the falls have been in at least eight distinct positions, marked by the broad, steep walls of the gorges below. The current waterfall is estimated to be approximately 100,000 years old.
The Mechanism of Retreat: Erosion Along Fracture Zones
The process that created the gorge system is a slow but powerful form of headward erosion, where the waterfall retreats upstream by exploiting the structural weaknesses in the basalt. The Zambezi River initially encountered one of the major east-west fracture zones in the basalt plateau. Since these joints were filled with easily eroded sandstones and clays, the river rapidly cut down through this softer material.
This action undermined the hard basalt cap that forms the lip of the falls, causing huge blocks of the rock to collapse into the chasm below. The constant flow of water plunging into the gorge further exacerbates this process, a form of hydraulic action that scours out the softer infill from the fractures. This repeated collapse and erosion along the east-west joint created the first line of the waterfall, which was essentially a wide, deep trench.
The falls do not retreat uniformly; instead, the river exploits the secondary, north-south joints that intersect the main falls line. The water concentrates its erosive power along one of these narrow, transverse fissures, cutting a deep, narrow channel upstream, which acts as a localized point of accelerated back-cutting. This concentrated erosion eventually causes the main volume of the river to be diverted into the new, narrow channel, abandoning the broader, older falls line.
Once the river has cut back far enough along the narrow, north-south fissure, it eventually intercepts the next major east-west fracture zone. At this point, the narrow channel rapidly widens as the river exploits the new line of soft infill, creating a new, broad waterfall upstream from the previous one.
This cyclical process of cutting back along a narrow joint and then widening along a transverse fracture is what has created the distinctive, tight zigzag pattern of the Batoka Gorge. Each bend in the gorge marks a former, temporary location of the waterfall before it “jumped” backward to the next weakness. The current Devil’s Cataract section of the falls demonstrates this ongoing mechanism, as it is actively cutting back along a prominent joint system, preparing the way for the next major shift in the falls’ position.