Water plays the most important role in this respect. It is not only the most abundant volatile phase but also the strongest agent. Additionally, a number of other volatile species occur in magmas, including carbon dioxide, sulphur, chlorine, traces of the heavy halogens bromine and iodine, noble gases and nitrogen compounds.
These substances are now strongly represented in the media. As a result of resources being used by humans, they are being increasingly introduced into the atmosphere and are therefore deemed to have an influence on climate change. However, there are also natural sources. Gases evaporate from the surfaces of the oceans and are released from volcanic systems. Through entrainment into the atmospere they modify its chemical composition.
During non-eruptive volcanic phases, magmatic gases diffuse in the local to regional near-surface layers. Often they are also fed from sources that are not detectable by human eye. Such invisible gas emission occurs where magma remains trapped below the Earth’s surface, and expels its gases during cooling and solidification in the crust. Surrounding ecosystems are affected by such chemical changes.
If large explosive eruptions produce high eruption columns of 10 to 40 km, the volcanic gases are carried as far as the stratosphere. Such great heights are guarded places for gas particles: they are higher up than the water cycle of precipitation, evaporation and cloud formation. Several years may therefore pass before the gases are washed out.
An all-embracing material cycle
Highly gas-enriched magmas, and therefore also the most explosive eruptions, occur predominantly in subduction zones. Sources for the gases are marine sediments and seawater, which are transported into the Earth at convergent plate boundaries together with the subducting plate. Part of these volatiles are recycled back to the Earth's surface by subduction zone volcanism. The remainder is entrained into the depths of the Earth, where it leads to chemical modification of the Earth's mantle. Because of this mantle enrichment, melts with elevated gas contents can also form where they are purely sourced by mantle material, for example at mantle plume.
Quantifying the gas emission of an eruption
The gas output of a past eruption is not directly measurable, because the gases have escaped into the atmosphere and can therefore no longer be directly measured. It is nevertheless possible to infer the amount of gases released during an eruption. Rapidly-quenched tephras serve as source of information. For this purpose, two parameters need to be quantified.
Firstly, information on the amount of rock material that was produced during an eruption is required. Influenced by the wind during or shortly after an eruption, ash particles in the atmosphere can be transported over hundreds of kilometres. To constrain the volume of erupted material, the tephra fan is mapped in the field, documenting its spatial distribution and its thicknesses at the different locations. In addition to the on-land evidence (usually covering proximal to medial distances from the vent), drill cores obtained from the sediment layers of the ocean floor permit to take the distal deposits into account. All data are presented on an isopach map. In this way, the total volume of ejected magma is determined.
Secondly, it must be determined how much gas was contained in the magma prior to the eruption. Melt inclusions preserved in minerals serve as witness of pre-eruptive magma compositions. Melt inclusions are tiny droplets of magma that become trapped in a crystal during crystal growth upon cooling of the melt. By enclosing the inclusion, the host crystal seals the tiny batch of melt from the magma chamber processes. While gases from the main magma body can escape in open system processes, a melt inclusion reflects the composition of a melt at the time of its entrapment. The inclusions are usually not bigger than a few tenths of micrometres. In the laboratory, the mineral grains are polished to expose the inclusions. Subsequently, the gas contents in the inclusion glasses are measured using in-situ high-resolution analytical techniques.
The pre-eruptive volatile concentrations are then compared to the residual gas that is still chemically bound in the matrix glass of the pumice or scoria. The difference is scaled by the total amount of the ejected material. The result is the total amount of the gases which were released into the atmosphere during the eruption.
Text: Dr. Heidi Wehrmann,GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel