Victoria Falls Geology: How the Zambezi Cut the Smoke That Thunders

Above Victoria Falls, the Zambezi is a broad, slow river, as wide as the falls themselves, dotted with palm-covered islands. Then, without warning, the whole river drops into a crack in the earth. The spray rises hundreds of metres into the air, and below the falls the water squeezes through a narrow chasm and runs away through a zigzag of deep gorges. It is one of the great sights of Africa, but it is also one of the clearest lessons in geology anywhere on the continent, because the gorges downstream record where the falls used to be. Victoria Falls geology explains not only how the falls look today, but how they have moved, and where they are going next. On Days 12 and 13 of our Southern Africa Tour, the group spends two days at Victoria Falls with a local guide.

The smoke that thunders: names for the falls

Long before it had an English name, the waterfall had several African ones. The best known is Mosi-oa-Tunya, a Lozi name usually translated as the smoke that thunders. The Tonga call it Shungu Namutitima, boiling water, and in isiNdebele it is aManz’ aThunqayo. The falls are also a sacred place. For the Leya people of Mukuni, whose communities live near the falls on the Zambian side, the falls are the home of the river god Leza, and rituals for rain, cleansing, harvests and fertility have been held at shrines around them.

The Scottish missionary David Livingstone became the first European on record to see the falls in mid-November 1855, usually dated to the 16th. Makololo canoe men took him to a small island on the lip of the falls near the northern bank, now called Livingstone Island, and he named the waterfall after Queen Victoria. In 1989, the falls were inscribed by UNESCO as a World Heritage Site under both names, Mosi-oa-Tunya / Victoria Falls, shared by Zambia and Zimbabwe.

Lava, cracks and a river: how Victoria Falls formed

The story begins with volcanic rock. Around 180 million years ago, as the supercontinent Gondwana began to break apart, enormous floods of lava poured out across southern Africa. In this area they built up the Batoka Basalt, a thick pile of dark, layered lava flows. As the lava cooled and shrank, it cracked into joints, and later earth movements added further lines of weakness. Many of these cracks run roughly east to west, across the line of today’s river, and over time some were filled with softer material that erodes far more easily than the basalt around it.

The Zambezi did the rest. Flowing across the basalt plateau, the river found these weak lines. Water poured into a crack, scoured out the soft filling and deepened it into a chasm, until the whole river was falling over one long wall of rock into the slot below. That is what you see today: the falls are not a cliff at the end of a valley, as most waterfalls are, but the upstream wall of a crack running straight across the river.

The zigzag gorges: following the falls back in time

Below the falls, the river leaves the chasm through a narrow outlet and runs away through a series of gorges set at sharp angles to one another, like a zigzag. Each of these gorges marks a place where the falls once stood. When the river breaks through the wall of the current chasm along a weakness, it begins cutting back along the next crack upstream, and a new line of falls forms there. The old chasm is left behind as a gorge. UNESCO counts eight gorges in the sequence, and the gorge system as a whole continues for more than 100 kilometres downstream.

How long has this taken? Estimates differ widely, and depend on what is being measured. Some interpretations put the retreat through the nearer zigzag gorges at around 100,000 years. A study drawing on dated archaeological sites estimated that the falls retreated about 20 kilometres over roughly 250,000 to 300,000 years, a few centimetres a year on average. The wider process that cut the full length of the gorges is thought by the IUCN to have taken around two million years. However the numbers are reckoned, the principle is the same: the falls have been walking slowly upstream.

And they are still doing it. At the western end of the falls, the section called the Devil’s Cataract, on the Zimbabwe side, is lower than the rest and is cutting back along a line of weakness. UNESCO describes it as the starting point for the next waterfall. In a timescale far beyond our own, today’s falls will be another gorge in the zigzag.

Measuring the curtain

The falls are about 1,708 metres wide, and at their highest the water drops about 108 metres. UNESCO describes them as the largest curtain of falling water in the world. From west to east, the falls are divided into named sections: the Devil’s Cataract, the Main Falls, Livingstone Island, the Horseshoe Falls, the Rainbow Falls, which are the highest, the Armchair Falls and the Eastern Cataract. About three-quarters of the length can be seen from the Zimbabwe side, including the Devil’s Cataract and the Main Falls; the Eastern Cataract is in Zambia.

The volume of water changes enormously through the year. On average about 1,088 cubic metres of water a second pass over the falls. At the height of the flood season, around April, that can rise to as much as 500 million litres a minute, and in some years far more. By November, at the end of the dry season, flow falls to around a tenth of the April level. The spray changes with it: at peak flow it can rise around 500 metres and be seen from tens of kilometres away, sometimes so dense that it hides the falls entirely.

The river’s flow depends on rain falling hundreds of kilometres away in the Zambezi’s upper catchment, so it varies from year to year as well as by season. In 2019, after a severe drought, the flow over the falls was the lowest in about a century. Years like that are a reminder that the falls are not a fixed monument but a living river doing its work.

What will the falls look like in late August and September?

Lower and clearer than at their peak. The tour visits in late August or early September, when the Zambezi is falling after its April flood. The spray is lighter, so you see much more of the rock face and the shape of the chasm, which is ideal for understanding the geology. Water still flows over the falls on both sides in September, but on the Zambian side flow can drop sharply and the Eastern Cataract is often a dry cliff by this time. The Zimbabwe side, with the deeper Devil’s Cataract and Main Falls, keeps its water longest.

The rainforest made by spray

Along the edge of the chasm opposite the falls on the Zimbabwe side grows a strip of rainforest, sustained by spray in a region that is otherwise dry for much of the year. Ferns, palms, figs and ebony trees grow here, and UNESCO describes the spray-fed forest as a fragile ecosystem. A path of a couple of kilometres runs through it, with numbered viewpoints looking across at the falls, from the Devil’s Cataract at the western end to Danger Point at the eastern end.

The bridge and the Big Tree

Just below the falls, the Victoria Falls Bridge crosses the Second Gorge, linking Zimbabwe and Zambia. It was designed by G. A. Hobson with Ralph Freeman, built by the Cleveland Bridge company and opened in 1905 as part of Cecil Rhodes’s ambition for a railway from the Cape to Cairo. Its steel arch spans 198 metres and the deck stands about 128 metres above the river. Rhodes is said to have wanted the train passengers to feel the spray.

A short distance from the river on the Zimbabwe side stands the Big Tree, a baobab some 24 metres tall with a girth of more than 22 metres, protected by Zimbabwe’s national museums authority. It is often said to be thousands of years old, but radiocarbon dating published in 2021 put its age at around 1,150 years, which is still remarkable.

Can we see the lunar rainbow?

Possibly, but it is not part of the tour programme and depends on the calendar. On clear nights around the full moon, moonlight shining through the spray can produce a faint rainbow, and the rainforest on the Zimbabwe side opens for special evening visits on the night before, the night of and the night after each full moon. The effect is strongest from about June to August, when there is plenty of spray; by September it is weaker and visible from fewer viewpoints. If your dates coincide with a full moon, ask your Tour Leader whether an evening visit can be arranged.

Which side of the falls will we see?

The tour is based on the Zimbabwe side, at Victoria Falls town, with two days of guided visits to the falls and rainforest, including the Big Tree if time allows. The two sides offer different views: Zimbabwe faces most of the falls across the chasm, while Zambia offers close views from Livingstone Island and from the Knife Edge Bridge in Mosi-oa-Tunya National Park, which looks across to the Eastern Cataract and down into the Boiling Pot. Crossing between the two involves a border, so your Tour Leader will confirm which viewpoints the group visits on each day.

How much walking is involved, and will we get wet?

The rainforest path runs for a couple of kilometres along the edge of the chasm, with short side paths to each viewpoint; some are uneven or have steps, and the paths can be slippery where spray falls. In late August and September the spray is much lighter than in the flood season, but a light rain jacket or poncho is still useful near the Main Falls. The tour is rated Level 2 (Moderate), which means managing three to five hours of activity a day and walking up to eight kilometres on uneven ground.

Victoria Falls on the Southern Africa route

After two days at the falls, the group transfers on Day 14 to Chobe National Park in northern Botswana, on the Chobe River, a tributary of the Zambezi. There, the river becomes the best place to watch wildlife: our guide to the Chobe River cruise explains how to read the river from a boat, and our article on Chobe National Park elephants looks at the great herds that gather along it in the dry season. For a broader introduction to the region, see our guide to exploring Southern Africa, and our overview of the Southern Africa cultural journey shows where the falls sit on the full route.

It is easy to stand at Victoria Falls and simply stare; few places reward it more. But it is worth turning round, too, and looking down the gorge, because the zigzag of chasms below is the falls’ own history, written in basalt. The smoke that thunders has been moving upstream for a very long time, and it is still on its way.