6.6

6.6 The magma is also composed of dissolved gases

          Magma is composed of dissolved gases which are release in the atmosphere by different ways. The most important is during an eruption but diffused gases are very important too. In fact gases may escape continuously into the atmosphere by volcanic vents, fumaroles or hydrothermal systems during an intrusion of magma.

          At high depth and high pressure, volcanic gases are totally dissolved in the molten rock. Thanks to the convective motion, the magma rises to the surface. When the pressure is lower, volcanic gases begin to create some small and rare bubbles. Their number and their size increase when the pressure continues to decrease. The size and the number of bubbles depend of the decompression rate.

          In volcanic gases, the most abundant is the water vapor (H2O). This is not a dangerous gas. Unfortunately the carbon dioxide (CO2) and the sulfur dioxide (SO2) are very important too. In rare case, the carbon dioxide is more abundant that water vapor (H2O).

Volcano
Tectonic Style
Temperature
Kilauea Summit
Hot Spot
1170°C
Erta` Ale
Divergent Plate
1130°C
Momotombo
Convergent Plate
820°C
H20 37.1 77.2 97.1
C02 48.9 11.3 1.44
S02 11.8 8.34 0.50
H2 0.49 1.39 0.70
CO 1.51 0.44 0.01
H2S 0.04 0.68 0.23
HCl 0.08 0.42 2.89
HF 0.26
Examples of volcanic gas compositions, in volume percent concentrations
(from Symonds et. al., 1994)

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