Ask a kid to draw a volcano and you'll get a picture of a mountain with a plume of clouds over it. Even without lava flows, a volcano is defined by hot gases. To the ancient Greeks who thought of volcanoes as the forges of the blacksmith god Hephaestus, that cloud on the mountain was smoke from his forgean aftereffect of the main action. But volcanic gases are more important than that.
The gases in volcanoes are what cause eruptions in the first place. Magma, the red-hot melt that lives underground, is more than melted minerals. It also has a volatile fraction, a portion consisting of dissolved gases. As magma rises toward the Earth's surface (which it always does because melted rock is less dense than solid rock), the volatiles come out of solution exactly like bubbles of carbon dioxide gas come out of carbonated drinks.
In fact, magma too has carbon dioxide (CO2). But the most abundant volatile is generally water (H2O). That is, the smoke from Vulcan's forge is largely steam. The third major volcanic gas is sulfur dioxide (SO2). All three of these, but mostly water, are what make lava blow up.
As magma rises through cracks in the Earth's crust, the gas bubbles in it expand rapidly and force it up even faster. This positive feedback acts as long as the bubbles hold. Gas bubbles often survive eruption and remain in solidified lava, where geologists call them vesicles. Some lavas are so full of bubbles that they float on water. As long as bubbles can break and release their gas, the magma will collapse like a soufflé, but magma always has the potential for a runaway rise with an explosive outcome. Active volcanoes, whether they're erupting or not, send up a stream of gas. If there's enough, the gas forms a white cloud as the water condenses into drops, but small amounts of gas may be invisible.
When magma is stickier, its explosive potential is greater because the gas in the bubbles is held at a higher pressure. The other thing is that sticky lava can't stretch as fast as fluid lava. Once the bubbles expand faster than the lava can handle, everything shatters and we have an explosive eruption.
Volcanic Gases in the Air
The three main volcanic gaseswater, CO2 and SO2have different effects once they leave the volcano.
Water vapor is benign; the atmosphere already has lots of it. Carbon dioxide is too, because volcanic CO2 is accounted for in the natural cycle. (Volcanic CO2 emissions, over the entire globe, amount to less than 1 percent of human emissions.) However, sulfur dioxide reacts with air and sunlight to make H2SO4, sulfuric acid, which in turn forms a white aerosol.
At low altitudes, sulfuric acid washes out in short order, but up in the stratosphere it can leave the air milky for months. The aerosol reflects sunlight into space, cooling the ground below. Large eruptions in tropical latitudes may produce enough stratospheric sulfur gases to affect the whole world for many months. In 1991, Pinatubo's eruption in the Philippines depressed global temperature by about 1°C for roughly a year.
Locally, both CO2 and SO2 can be hazardous when their concentrations are high. Because of its high density, CO2 can sometimes collect in low places and cause suffocation. A notable tree kill at Mammoth Mountain, California, in 1989 was blamed on this.
In addition, minor gases can sometimes be important. Fluorine and chlorine occur in volcanic gas as hydrofluoric and hydrochloric acid (HF and HCl), which can have nasty effects. The fluorine-rich eruption of Chile's Cerro Hudson, in 1991, is thought to have added to Antarctica's record-setting ozone hole a few months later. In 1783 the eruption of Laki, in Iceland, caused massive death among livestock from fluorine poisoning.
Why Volcanoes Are Gassy
What is gas doing underground in the first place? Volcanoes are part of the recycling process of plate tectonics. When oceanic plates enter the underlying mantle in subduction zones, they carry down the mud and water that they gathered during their time on the seafloor. The water from the mud, and the carbon compounds and sulfur compounds from organic matter in the mud, end up in the mantle. These ingredients rise back toward the surface and act as a flux, a substance that promotes melting.
There is a smaller component of volcanic gas that is thought to come from farther down in the mantle. When the Earth itself came together, more than four and a half billion years ago, its ingredients included lots of volatiles. To a geochemist, "volatiles" include a large number of elements besides hydrogen, carbon and sulfur. The inert gaseshelium, neon, argon, krypton, xenonare among them; so are mercury and solids like arsenic, copper, zinc, lead and even gold. All of these were part of the original mantle that have been working their way out ever since.
Volcanoes don't erupt showers of gold. But volcanismthe formation and movement of magmais what brings all the volatile elements into the crust, where they become concentrated into minerals and ore bodies. The gases of volcanoes are the most visible part of this great geochemical process.