Iguazu Falls, a spectacular natural boundary between Argentina and Brazil, represents the largest waterfall system on Earth. Spanning approximately 2.7 kilometers in width, this immense series of cascades comprises over 275 individual falls, with the tallest drops reaching 82 meters. The sheer scale and volume of water flowing over the falls set the stage for a geological history that explains their formation and ongoing transformation.
The Basalt Plateau Foundation
The existence of Iguazu Falls is dependent on a massive, ancient geological structure known as the Serra Geral Formation. This foundation consists of layers of dark, dense volcanic rock called basalt, created by one of the largest volcanic events in Earth’s history. These flood basalts, part of the greater Paraná Traps, erupted approximately 130 million years ago during the Mesozoic Era, covering vast areas of the South American continent.
The successive lava flows stacked up to a thickness exceeding 1,000 meters, forming a high plateau. At the falls themselves, the uppermost layer acts as a protective caprock, typically measuring between 20 and 30 meters thick. This hard basalt resists the erosive force of the Iguazu River much more effectively than the materials beneath it, which is the foundational reason the waterfalls can maintain their height and form. The cooling of this lava also resulted in distinctive vertical fractures called columnar jointing, which later played a role in the initial stages of erosion.
Structural Weakness and the Initial Breach
The location where the falls first began was dictated by pre-existing structural flaws in the basalt plateau. The falls are believed to have originated at the confluence of the Iguazu River and the much larger Paraná River, a position now located 21 kilometers downstream. This initial site was likely an abrupt drop created by differential erosion between the two river systems.
The massive layers of basalt were intersected by a network of joints, cracks, and fault lines, often running in NW–SE and SW–NW directions. These structural weaknesses created zones of reduced resistance that the flowing water could exploit. When the Iguazu River encountered one of these major fractures near the confluence, the water began to cut down rapidly. This breach allowed the river to erode through the hard basalt caprock and expose the softer, less resistant sedimentary rocks lying beneath.
The Mechanism of Upstream Retreat
The current shape and movement of Iguazu Falls are governed by headward erosion, a continuous retreat upstream caused by differential erosion. The river’s immense volume plunges over the hard basalt lip, striking the softer underlying sedimentary rock with tremendous force. This hydraulic power, combined with abrasive sediment, rapidly excavates the vulnerable sandstone and siltstone layers.
This constant excavation creates a large, deep plunge pool and slowly forms a cavern beneath the resistant basalt caprock. As the cavern grows, the weight of the unsupported basalt above eventually becomes too great, leading to a collapse of the rock face. This collapse causes the waterfall to move, or retreat, a short distance upstream, perpetuating the process. The characteristic horseshoe shape of the falls, including the Garganta del Diablo (Devil’s Throat), is the direct result of this headward erosion mechanism working across the river’s width.
Measuring the Ongoing Evolution
The erosional process has been at work for a significant geological time, with estimates suggesting the falls have moved continuously upstream over the last 1.5 to 2.0 million years. Geologists estimate the historical rate of retreat to be in the range of 1.4 to 2.1 centimeters per year. This slow but relentless movement is what has pushed the falls to their current position, 21 kilometers away from their original starting point at the Paraná River.
Today, the rate of this natural evolution is being influenced by human intervention within the river system. The construction of major dams, such as the Itaipu Dam on the Paraná River and other hydroelectric projects, has significantly altered the flow regime. These dams regulate the water volume, which can reduce the peak flow events that are necessary to trigger the largest basalt collapses. This alteration affects the energy available for erosion, consequently changing the long-term rate and pattern of the falls’ ongoing retreat.