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Volcanic outgassing transfers gases from magma into surrounding rock, water or an atmosphere. High pressure can keep volatile components dissolved in magma; ascent and decompression can separate a gas phase. Bubbles may expand, escape through connected paths or help drive an eruption.

Measurements of gas composition and flux probe processes that occur below direct view. The signal also depends on storage, permeability and interaction with groundwater, so a change in surface emissions need not simply equal a change in magma supply. Repeated background measurements make anomalous changes interpretable.^[ref]