Volcanic Gases and Their Hazards: The Invisible Threat
When we picture a volcanic eruption, we think of lava and ash. But volcanoes also release vast quantities of gas, and these invisible emissions are among their most far-reaching and underappreciated hazards. Volcanic gases can poison local air, damage crops, cool the global climate, and, in rare and terrifying cases, kill silently without any eruption at all. Understanding these gases is essential to understanding the full danger volcanoes pose.
What volcanoes emit
Volcanic gas is dominated by water vapour, but it also contains carbon dioxide, sulphur dioxide, hydrogen sulphide, hydrogen chloride, hydrogen fluoride, and other compounds. These gases are dissolved in magma under pressure and escape as the magma rises and erupts. The mix and quantity of gases a volcano releases reveal much about its state and the hazards it poses.
Sulphur dioxide and acid rain
Sulphur dioxide is one of the most significant volcanic gases. In the atmosphere it reacts to form sulphuric acid, producing acid rain and a hazy pollution known in Hawaii as vog, or volcanic smog. Persistent emissions can damage vegetation, harm health, and reduce air quality over wide areas, as seen at degassing volcanoes like Masaya in Nicaragua.
Carbon dioxide: heavy and deadly
Carbon dioxide is colourless and odourless, and because it is denser than air it can collect in low-lying areas, displacing oxygen. Near volcanoes, it can accumulate in hollows, basements, and valleys, posing a suffocation hazard. In rare cases, as at Lake Nyos in Cameroon in 1986, vast quantities released from a crater lake have flowed into valleys and suffocated thousands.
Fluorine and the poisoning of land
Volcanic ash can carry fluorine, which settles on grass and water and is taken up by grazing animals. In sufficient quantities, fluorine causes a fatal disease in livestock, as seen catastrophically during the 1783 Laki eruption in Iceland, which killed much of the country's livestock and contributed to a devastating famine. This makes gas-rich eruptions a threat to agriculture as well as to people.
Cooling the climate
The gases of large eruptions can reach into the stratosphere and affect the global climate. Sulphur dioxide injected high into the atmosphere forms a haze of tiny particles that reflect sunlight, cooling the planet for months or years. The 1815 Tambora eruption caused the year without a summer, and Pinatubo in 1991 measurably lowered global temperatures, showing the planetary reach of volcanic gas.
A silent and invisible danger
What makes volcanic gases especially insidious is that they are often invisible and can strike without warning. The deadly carbon dioxide of Lake Nyos gave no sign before it flowed; the fluorine of Laki was carried far on the wind. This invisibility means that monitoring gas is a vital part of volcano surveillance, even at volcanoes that are not erupting.
Monitoring volcanic gas
Scientists monitor volcanic gases using ground-based sensors, aircraft, and satellites that measure emissions of sulphur dioxide and other compounds. Changes in gas output can be an early warning of rising magma and impending eruption, while persistent emissions reveal the ongoing hazard a volcano poses to the air, the land, and the people nearby.
Explore on the map
From the vog of Hawaii and the degassing of Masaya to the deadly gases of Lake Nyos, volcanic gases are a hazard at volcanoes worldwide. Explore these volcanoes on the interactive map — filter by region to see where the invisible emissions of volcanoes shape the air, the land, and even the climate.