I-Guazú: Great Water - Great Volcanism

Author Thomas Smith

Iguaçu Falls is one of the most spectacular natural sceneries in the world, caused by a wide-spread volcanic event in conjunction with continental rifting in the Early Cretaceous. This rifting episode has also meant a lot to the hydrocarbon potential on both sides of the Atlantic Ocean.

The main trail on the Brazilian side of the falls terminates along an elevated walkway that is completely surrounded by falls. Here, the spray can soak you in minutes, while feeling and hearing the thunderous roar of the water. Photo: Tom Smith

"Poor Niagara" explained United States' First Lady Eleanor Roosevelt upon first seeing Iguaçu Falls. No other names than "Great Water" therefore fit like the original to describe this grand spectacle of cascading waters.  

Located on the borders of Argentina, Brazil and Paraguay, Iguaçu Falls are not the largest (Victoria Falls in southern Africa has that distinction) or the tallest (Angel Falls, Venezuela), but split into about 270 discrete falls and large islands it is certainly one of the most spectacular natural formations in the world.  

What makes the falls special for ordinary tourists is their easy access. There are walkways extending out along many of the falls on both the Argentine and Brazilian sides, allowing visitors a close and intense experience with spell binding views of these ‘great waters'.

The area is protected by two national parks. Iguaçu National Park on the Brazil side of the river consists of 1,700 km², while the Iguazú National Park on the Argentina side encompasses 668 km². They were designated UNESCO World Heritage Sites in 1986 and 1984, respectively. Cartography: GeoPublishing/NASA

Formation of the falls

To understand how these falls formed, we have to go back more than 130 million years ago (Ma) to the Early Cretaceous. At that time, the earth's landmass consisted of one very large continent called Pangaea. This supercontinent started to break apart from forces created by internal currents in the mantle. This rifting phase would eventually create the continents we see today.  

Among the new continents, South America and Africa started to separate in the Late Jurassic (for details about this process see GEO ExPro, vol. 5, no 4, pp 62-66). At first, minor uplift and thinning of the continental crust ensued as extensional forces started to tear apart this huge landmass. At the same time, massive outpouring of lava flows erupted from volcanoes and fissures that formed along the rifting zones. These flood basalts filled the early phases of subsidence along the southern-most part of the South American Plate, particularly onshore in the Parana Basin where the falls are found today.  

This rifting event also created the huge potential for oil development on both sides of the Atlantic margins including the supergiant fields recently discovered in the ultra-deep water provinces along the Brazilian Atlantic margin (see GEO ExPro v. 5, no. 5 pp. 22-28).  

The Parana Continental Flood Basalt Province (known as the Serra Geral Formation in Brazil) is one the world's largest covering an area over 1,200,000 km² in southern Brazil, northern Argentina, Paraguay, and Uruguay. The lavas were extruded during the Early Cretaceous period between 138 and 127 Ma and reach a maximum thickness of 1.7 km.

From the elevated walkway views of the U-shaped ‘Garganta do Diablo’, or Devils Throat, the largest single falls along the giant cataract come into view. Photo: Tom Smith

The formation of the falls through erosion at the base is explained on a blackboard in the visitor centre on the Argentinean side. Photo: Halfdan Carstens

"The flood basalts form very hard, flat lying and erosional resistant layers that the major rivers of the region meandered across over time," says Professor Dr. Breno Leitão Waichel of the State University of the West of the Paraná. "In fact, of all the major South American rivers, the Paraná has the most perfect adjustment to the area's structure and it developed after the outpouring of the Serra Geral basalts. In the Iguaçu Falls area the volcanic pile is about 1,000 m thick."  

"In general, the basalts are pahoehoe flows (with the smoother, ropy style surfaces) approximately 30 m thick. Each flow has an internal 3-part division with a base, core, and vesiculated top."  

"The alignments (or fractures) related to the river paths took shape during the cooling of the lithosphere after the lava extrusions. This faulting is probably part of the basement structure that formed around the time the magmatism of the Serra Geral occurred."  

"A major fault along the Paraná riverbed (north-south structure) became evident and was identified as a stunning uplift around 80 Ma. The Iguaçu River is found along an east-west alignment with much of the present morphology being the result of water sculpturing during the Tertiary and Quaternary," says Dr. Breno Leitão Waichel.  

As the flat lying basalt flows (at least 11 individual flows in this region) were gradually uplifted by faulting, erosion started to create the falls we see today. The energy of the water falling over the abrupt faulted step creates a depression at its base. Over time, the higher energy zone at the base of the step gradually erodes back into the lower layers. Without support, the upper layers crumble progressively upstream. Large blocks of the upper lava layers can be seen at the base of the falls. The edge of the basalt cap recedes only 3 mm per year. This erosional process continues today 23 km from the original starting position at the confluence with the Paraná River.

Flood Basalts – a Common Phenomenon Throughout geological history, enormous amounts of basalt have erupted over very short periods of time. Thousands of cubic km of lava poured out in a matter of a few hundred years or even days and cover 100,000s of km². Possibly the largest of these volcanic provinces are the 2000m thick Early Tertiary Deccan Traps of India originally covering 1.5 million km².   Some of the oldest flood basalts occurred in the Archaean and Proterozoic such as the Coppermine basalts found along the north-western Canadian Shield and the Keevenawan basalts of southern Canada and northern Minnesota.   Examples of much younger flood basalts can be found in the 15 Ma Columbia River Basalts in the states of Oregon, Washington, and Idaho, USA. One major period of volcanic activity occurred when the Gondwana supercontinent fragmented into Europe, Asia, North and South America, Africa, Australia, and Antarctica during the Jurassic Era. Basalts were erupted along spreading axes (mid-oceanic ridges) resulting in the formation of new oceanic crust and transverse faults extending into the continents which covered large areas with flood basalts. The volcanic bedrocks can easily be studied at the beaches along the river downstream from the falls. Photo: Halfdan Carstens

Visiting the falls

Plan at least two days to truly see the falls and environments around them. The area is open year-around, the photos shown in this article were taken in early May. The high season is in January and February when most Brazilians and Argentineans are on holidays. May through July are the rainy months, when the volume of water going over the falls is at its maximum. Viewing and hiking the falls trails from both the Argentina and Brazil sides is a must. Boat, rafting, and rainforest tours will top off your trip.  

The town of Puerto Iguazú on the Argentina side of the park is a good place to stay with frequent scheduled bus service to the park. A short walk away from the center is Tres Fronteras offering the unusual view of two other countries, Brazil and Paraguay, at the confluence of the Iguaçu and Paraná Rivers. Each country displays their own obelisk painted with the colors of their country flag.  

Local buses can take you south on a day trip to the Minas de Wanda where tours of the mines are conducted for the public, along with a museum and shop displaying the gorgeous geodes and minerals.

Water everywhere. Photo: Halfdan Carstens

Geodes - Amethyst The Paraná Basin flood basalts, besides hosting one of the world's largest waterfalls, also contain some of the world's largest known vesicles or preserved gas bubbles. According to Dr. Waichel, "the bubbles appear when the magma rises and undergoes decompression much like the bubbles you see when you open a carbonated drink. In several localities the vesicles can reach more than a meter in length. Over time, water percolating through the basalt precipitated the spectacular crystals along the surface of these vesicles."   "Most of the filling of the vesicles are varieties of α-quartz with minor amounts of euhedral crystals of calcite and late forming gypsum," says Dr. Waichel. "The colors of the quartz (purple amethystine quartz) are due to trace amounts of iron in their formation."   Literally tons of these geodes are mined from Brazil, Argentina and Uruguay every year and grace museums, offices, and homes around the world. At the Mines de Wanda, the geodes occur and are mined along the top of one of the last lava flows in the sequence. Note the 3 m thick weathered soil surface above the solid basalt in the background that has been exposed since the Cretaceous indicating how weathering resistant these basalts are. Photo: Tom Smith This modest-sized geode will take 3 to 4 weeks to chisel out of the solid basalt. Photo: Tom Smith

How to get there This spectacular sight, which rivals both Niagara and Victoria Falls, is approximately two hours by air southwest from Rio de Janeiro, or the equivalent flying north from Buenos Aires. Iguaçu Falls are easily reached from either Foz do Iguaçu on the Brazilian side or Puerto Iguazú on the Argentinian side. The falls can also be reached comfortably by overnight buses from either capital.