(Intro)

The word volcano probably calls to mind flowing lava, tropical islands, and maybe the tragic destruction at Pompeii. But volcanoes can be bigger, badder, and have wider-ranging effects than you might expect. The aftermath of huge volcanic eruptions can cause other natural disasters or widespread cooling, and they've even altered the DNA of a disease-causing bacterium.

A volcano is a vent that allows molten rock – called magma when it's underground and lava above ground – to be squeezed out onto Earth's surface. And when they form, it's literally Earth-changing. When two sections of the Earth's crust, called tectonic plates, move away from each other, there's room for magma to flow up and make new crust. That's how the mid-ocean ridges were formed! Or, when two tectonic plates move toward each other, the denser plate might sink below the other one, and melt as the temperature increases. In both cases, sometimes magma seeps up to the surface through vents or cracks called fissures – and, hello, volcano!

Volcanoes can also form over hotspots – super hot areas below the Earth’s crust, which can melt tectonic plates. And as the plates move, the hotspots stay put. So they can form a chain of volcanoes, like the Hawaiian islands.

The basic signs of volcanic activity are lava flows and heat – but that's only the beginning. More explosive eruptions can spew lots of particles and gases into the atmosphere which, as you might imagine, can mess stuff up. Now, some of those chemicals are well-known greenhouse gases, like carbon dioxide. Volcanic eruptions nowadays currently release less than one percent of the carbon dioxide released by human activities. But millions of years ago, it's possible that lots of extreme volcanic eruptions contributed to global warming, and its evil twin: ocean acidification. See, carbon dioxide in the atmosphere can also dissolve in the oceans, making them highly acidic and dangerous to all kinds of marine creatures.

More recent eruptions can have big effects, too. Like, take sulfur dioxide. When it enters the stratosphere, it can react with water vapor to form a bunch of droplets of sulfuric acid, which can reflect solar radiation and cause cooling of the lower atmosphere. Depending on how big the eruption is, these sulfur aerosols can float around in the stratosphere for a while, along with all the tiny particles of ash. This cooling can affect huge regions of the globe, causing volcanic winters.

And if that doesn't sound concerning enough, big volcanic eruptions can also prevent raindrops from forming, leading to drought. See, a raindrop forms when water molecules hitch onto a tiny particle of dust, until the drop gets big enough that it can't float in the air anymore. But when there's all this extra stuff up there, like ash or other particles, the water in a cloud gets spread out over too many droplets. Each droplet takes longer to grow, so more water stays up in clouds instead of falling as rain.

And that's not the only water-related problem volcanoes can also cause tsunamis. Big underwater eruptions, volcanic collapses, or the hot debris that explodes from volcanoes into the ocean can form massive waves. The Krakatoa eruption in 1883, for example, caused the largest volcanic tsunami on record – the biggest waves were around 40 meters tall, and affected sea levels all over the world.

Basically, volcanoes can spawn all kinds of disasters. And the bigger the eruption, the bigger the consequences. So let's take a look at the most destructive volcanic eruption since humans have been on Earth: In 1815, Mount Tambora in Indonesia erupted and spewed out some 150 cubic kilometers of ash, plus lots of other junk into the atmosphere. It was incredibly destructive for the local population, but it also had global effects. The particles from that volcano hung around in the atmosphere for two years, cooling global temperatures as much as 3° Celsius, and giving 1816 the nickname: "the year without a summer." Plants need sunlight and heat to grow, and humans depend on crops for food. So, less sun, extra frost, and cold summers – even as far as North America and Europe – meant famine that spanned most of the globe. This eruption also altered the course of a disease.

The sulfur aerosols released by the eruption affected the climate in India: first there was drought, and then some late flooding. This weird weather changed the ecology in the Bay of Bengal, giving rise to a new strain of cholera bacteria – the kind that infects your intestine and causes diarrhea, dehydration, and sometimes death. Cholera used to be contained within the Ganges delta, but this mutant strain spread and took the world by storm. Today, there are millions of cases of the disease each year.

But among all the disastrous impacts worldwide, this bizarre weather also made for a wet, yucky vacation at Lake Geneva – where Mary Shelley and her author companions were staying. The gloom gave her inspiration and time to write the novel Frankenstein. So, volcanoes: not just the mother of disasters, but also the mother of monsters.

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