Volcanic gases are invisible and hazardous to collect therefore our knowledge of them is imprecise and limited. Normally when there is a volcanic eruption of any extent one gets as far away as possible.
Reliable data comes from non-explosive volcanoes such as Kilauea on Hawaii. The Hawaiian volcanoes have formed over a hot-spot in the Earth’s mantle. The basaltic magma that almost continually and gently erupts has provided us with samples of the volatile components of a primary magma.
The gas composition is predominately water vapor with carbon dioxide and hydrogen sulfide being next in importance. The geochemist Frauskopf estimated the average composition of magmatic gas (by volume) to be H2O 1000, CO 2 50, H2S 30, HCl 10, N2 10, HF 0.1
The average water content of fresh igneous rocks is about 1 wt % which may be taken as a lower limit for the average water content of a magma. This explains the huge clouds of steam often seen issuing during an eruption.
Although the percentage of minor gas components is very small their cumulative effect can be enormous e.g., gases from 1919 eruption had on average 0.117% HCl and 0.032% HF and was estimated to release into the atmosphere a total of 1,250,000 tons of HCl and 200,000 tons of HF.
The volcanoes of active mountain belts such as the Andes tend to be of the explosive type, erupting andesite and rhyolite lavas of high viscosity. It is very dangerous to directly sample the volcanic gases being emitted however we know the gas composition to differ significantly from that issued from basalt volcanoes of the rift valleys, the mid-Atlantic ridge and Iceland.
Where the Earth’s plates are being pulled apart the volcanoes developed here have their basaltic magma derived from the upper mantle, and are thus considered to be of primary origin. In mountain building regions, two crustal plates collide and one plate is pushed beneath the other to depths of about 100 kms forming a subduction zone. Melting of these crustal rocks occurs and the magma formed may be squeezed upwards to erupt as a volcano. The great Andean mountain chain of South America, which starts in Colombia and stretches 6500 kms to southern Chile, averages one volcano every 100 kms, with many still active today.
The gases of andesitic volcanoes are derived from melting and assimulation of prior existing crustal rocks, some of which could represent extensive regions of carbonate rocks, such as limestones and marbles, or of coal measures etc., hence CO2 would be more abundant than in gases from a primary magma.
It is known that over geological time volcanoes have brought to the Earth’s surface a vast amount of water vapor which has condensed to liquid water and so formed our oceans. The minor amounts of acidic gases have played an important role in the formation of metallic ore deposits and volcanic sulfur. The plants of the biosphere have thrived on the carbon dioxide gas freely distributed as a fertilizer.
