The mental image most people have of a volcano is probably Pompeii — the Roman city buried under ash and pyroclastic flow when Vesuvius erupted in 79 CE. Or perhaps Krakatau in 1883, or the eruption of Mount St. Helens in 1980, which removed the top 400 metres of the mountain in seconds and flattened forests across a radius of 30 kilometres. Volcanoes as destroyers: sudden, catastrophic, indifferent.
This is true and it is also incomplete. The same processes that cause those catastrophes have been building the planet's surface for four and a half billion years. Hawaii exists because of a volcanic hotspot in the middle of the Pacific plate. Iceland is entirely volcanic in origin, still growing as new lava reaches the surface between the North American and Eurasian plates. The fertile soils of Campania around Vesuvius, which made that region one of the most agriculturally productive in the ancient Mediterranean, exist because of centuries of volcanic deposits. The terror and the productivity come from the same source.
How They Form
Most volcanoes exist at the boundaries of tectonic plates — the large sections of the Earth's crust that move slowly across the mantle below. Where two oceanic plates converge, one typically subducts beneath the other — driven down into the mantle, where it melts. The molten rock, lighter than the surrounding mantle, rises, and where it reaches the surface, a volcano forms. This is the mechanism behind the Pacific Ring of Fire, an arc of volcanic and seismic activity that traces the boundaries of the Pacific plate from the Andes and Central America through Alaska, Japan, the Philippines, Indonesia, and New Zealand.
Where tectonic plates pull apart — at mid-ocean ridges — magma wells up to fill the gap. The Mid-Atlantic Ridge, running down the middle of the Atlantic Ocean, is a continuous chain of undersea volcanoes along which new ocean floor is constantly being created. Iceland sits directly on this ridge and receives volcanic material from both the ridge spreading and an additional hotspot below it — one of the reasons it is so geologically active.
Hotspots are a third mechanism: plumes of unusually hot material that rise through the mantle and punch through the crust regardless of plate boundaries. As tectonic plates move over fixed hotspots, chains of volcanic islands are formed. The Hawaiian island chain — which extends for thousands of kilometres across the Pacific, with only the big island of Hawaii currently sitting over the active hotspot — is the classic example.
What Eruptions Do to the World
Large volcanic eruptions inject enormous quantities of sulphur dioxide into the stratosphere, where it reacts with water vapour to form a reflective aerosol that can measurably reduce global temperatures for months to years. The eruption of Mount Tambora in Indonesia in 1815 was the largest in recorded history. It injected so much material into the stratosphere that the following year, 1816, became known as the Year Without a Summer. Crops failed across Europe and North America. Famine followed. The unusual gloomy weather that summer influenced Mary Shelley to write Frankenstein and Lord Byron to write a poem literally titled "Darkness."
The eruption of Pinatubo in the Philippines in 1991 — much smaller than Tambora but still the second-largest eruption of the twentieth century — reduced global average temperatures by approximately 0.5 degrees Celsius for two years. This is the same order of magnitude as some proposals for deliberate solar geoengineering, which gives a sense of scale to what a large eruption can do.
The Fertility Paradox
Despite the destruction, volcanically active regions often have some of the most fertile agricultural land on Earth. Volcanic soils — derived from the weathering of lava and ash over time — are rich in minerals, particularly phosphorus and potassium, that many other soils lack. The slopes of Etna in Sicily are intensively farmed. The highlands of Java are densely populated and agriculturally productive in part because of volcanic soil enriched over centuries of eruptions. The same Vesuvius that destroyed Pompeii is surrounded by vineyards producing wine that draws on minerals deposited over 2,000 years of intermittent activity.
People have always settled near volcanoes despite the risk, and this has usually not been irrational. In agricultural societies before synthetic fertiliser, volcanic soil was an extraordinary resource — the difference between marginal and productive farming was often the difference between starvation and surplus. The risk calculation, made against a background of infrequent eruptions on a human timescale, often came out in favour of staying.
Supervolcanoes and What They Are Not
The term "supervolcano" gets attached to Yellowstone with alarming regularity in popular science coverage, typically in connection with predictions of imminent catastrophe that scientists in the field tend to find overblown. The US Geological Survey defines a supervolcano as a volcanic system capable of producing an eruption of at least 1,000 cubic kilometres of material — a threshold that would cause global climate disruption. The last Yellowstone supereruption was roughly 640,000 years ago. Current monitoring shows no evidence of activity suggesting an imminent eruption of any kind.
What Yellowstone currently is: a geothermal system of extraordinary richness, responsible for the geysers, hot springs, and mud pots that make it one of the most spectacular national parks on Earth. The same volcanic heat that makes it a theoretical hazard is what makes it, right now, one of the most geologically alive and visually remarkable places a person can visit. The planet has not finished making itself. Volcanoes are where you can see it happening.