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Geology and geography

The staircase character of the falls consists of a two-step waterfall formed by three layers of basalt. The steps are 35 and 40 metres (115 and 131 ft) in height. The columnar basalt rock sequences are part of the 1,000-metre-thick (3,300 ft) Serra Geral formation within the Paleozoic - Mesozoic Paraná Basin. The tops of these sequences are characterized by 8–10 m (26–33 ft) of highly resistant vesicular basalt and the contact between these layers controls the shape of the falls. Headwater erosion rates are estimated at 1.4–2.1 cm/year (0.55–0.83 in/year). Numerous islands along the 2.7-kilometre-long (1.7 mi) edge divide the falls into many separate waterfalls and cataracts, varying between 60 and 82 m (197 and 269 ft) high. The number of these smaller waterfalls fluctuates from 150 to 300, depending on the water level. About half of the river's flow falls into a long and narrow chasm called the Devil's Throat (Garganta del Diablo in Spanish or Garganta do Diabo in Portuguese).

The Devil's Throat canyon is 80–90 m (260–300 ft) wide and 70–80 m (230–260 ft) deep. Left of this canyon, another part of the river forms 160–200 individual falls, which merge into a single front during the flood stage. The largest falls are named San Martín, Adam and Eva, Penoni, and Bergano.

About 900 m (2,950 ft) of the 2.7 km (1.7 mi) length does not have water flowing over it. The water of the lower Iguazu collects in a canyon that drains into the Paraná River, a short distance downstream from the Itaipu Dam. The junction of the water flows marks the border between Brazil, Argentina, and Paraguay. Some points in the cities of Foz do Iguaçu, Brazil, Puerto Iguazú, Argentina, and Ciudad del Este, Paraguay, have access to the Iguazu River, where the borders of all three nations may be seen, a popular tourist attraction for visitors to the three cities.

The Iguazu Falls are arranged in a way that resembles a reversed letter "J". The Argentina–Brazil border runs through the Devil's Throat. On the right bank is the Brazilian territory, which is home to more than 95% of the Iguazu River basin but has just over 20% of the jumps of these falls, and the left side jumps are Argentine, which make up almost 80% of the falls.