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On 10 January 1977 the walls of Mount Nyiragongo’s crater in the Democratic Republic of Congo cracked open, and the lava lake inside drained in less than an hour. The molten rock was so runny that it raced down the upper slopes at up to 60 kilometers per hour, killing at least 50 people in nearby villages.
Nyiragongo is an extreme case, but it shows how little a volcano has to do with the picture of a quiet cone. The facts below range from the ocean floor to Yellowstone’s geysers, and include one eruption that changed the weather on another continent.
1. Most volcanoes on Earth are actually underwater
The mid-ocean ridge system stretches about 65,000 kilometers, more than 90 percent of it underwater. It sits where tectonic plates pull apart, and molten rock rises to the seafloor and erupts, mainly as basalt, creating new oceanic crust. NOAA describes the system as a continuous range of underwater volcanoes.
That is part of why counts of volcanoes on land look small. The U.S. Geological Survey lists about 1,350 “potentially active” volcanoes, roughly 500 of which have erupted in historical time, but that tally leaves out the continuous belts of ocean-floor volcanism along spreading centers like the Mid-Atlantic Ridge.
2. Some volcanic eruptions create their own lightning
Scientists have identified more than one way an eruption cloud can make lightning. Near the vent, ash particles rub against each other and build up static charge, a process called triboelectricity, and Japan’s Sakurajima is a well-studied example. High in the atmosphere, ice can form in a thinning ash cloud that also carries water vapor, and colliding ice crystals generate charge much like they do in an ordinary thundercloud.
The second mechanism was described for Chile’s Calbuco volcano after its April 2015 eruption, when lightning struck about 100 kilometers from the volcano. Plume lightning is sometimes nicknamed a “dirty thunderstorm” when meltwater and ice join the ash, as at Iceland’s glacier-capped Eyjafjallajökull.
3. There’s a “supervolcano” under Yellowstone National Park
Yellowstone has had three giant caldera-forming eruptions, about 2.1 million, 1.3 million, and 631,000 years ago, separated by roughly 600,000 to 800,000 years. The most recent one created the caldera visible today. According to the U.S. Geological Survey, the volcano has since produced mostly lava flows, with the latest about 70,000 years ago.
The park’s hot springs and geysers are the surface sign of the heat below. Yellowstone has about 500 geysers, more than half the world’s total, and over 10,000 hydrothermal features in all. The National Park Service says a catastrophic eruption is unlikely in the foreseeable future, and the area is monitored continuously.
4. Volcanic soil is some of the most fertile on the planet
Take Indonesia as an example. Soils formed from volcanic ash, called Andisols, cover about 3 million hectares there, developing from andesitic and basaltic rock. Heat and heavy rainfall speed up weathering, producing deep soils with dark, organic-rich topsoil, and a research summary calls most Indonesian Andisols among the most productive soils in the country.
Tea, coffee, and cocoa grow on the lower and middle slopes of volcanoes in Sumatra and Java, and areas near volcanoes there support more than half of Indonesia’s people. Fertility has a catch, though: these ash soils bind phosphate strongly, so phosphorus can be unavailable to plants even when it is present in the soil.
5. Some volcanoes have lava lakes that never fully cool
Persistent lava lakes are rare. Mount Erebus in Antarctica, Erta Ale in Ethiopia, Kīlauea’s Halemaʻumaʻu crater in Hawaii, and Nyiragongo have all hosted them in recent decades. Nyiragongo’s was first confirmed scientifically in 1948 and, before 1977, had a surface at about 3,250 meters above sea level.
“Never” really means “until the volcano drains it.” Nyiragongo’s lake emptied in 1977 and again in 2002, when a 13-kilometer fissure sent lava through the city of Goma, covering about 13 percent of it, leaving around 120,000 people homeless and killing about 245. Kīlauea’s lake vanished in 2018 and returned in December 2020.
Tambora, 1815: the eruption behind a summer that never came
Mount Tambora on the Indonesian island of Sumbawa erupted in April 1815 with a Volcanic Explosivity Index of 7, an eruption often described as the most powerful in recorded human history. Sulfur dioxide reached the stratosphere and formed a haze that spread around the globe, and average global temperatures fell by roughly 0.4 to 0.7 degrees Celsius.
In 1816, remembered as the “Year Without a Summer,” frost hit New England in June and snow fell in Albany, New York, on 6 June. Cool, wet weather ruined harvests in the British Isles and Germany, and food prices rose. The gloomy weather also kept a group of writers indoors at Lake Geneva, where Mary Shelley began the story that became Frankenstein.
If you’re into how extreme our planet’s geology gets, you might also enjoy our piece on ancient Egypt’s engineering mysteries. The same shifting plates are behind earthquakes, and the ridges described above are one small part of what lies beneath the oceans.
If this topic interests you, it’s worth checking out our piece on facts about dogs and cats.
Want to explore geology further?
Rocks, Minerals & Gems Field Guide — identify real specimens on hikes or trips.
Volcano Science Experiment Kit — a hands-on way to see eruption mechanics up close.
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Nyiragongo’s lake drained twice in 25 years, and Tambora’s sulfur haze dimmed a summer continents away; a volcano’s reach is measured in hours at the crater and in seasons across the world.