Some fundamentals about Crater Lakes:


A crater lake is a highly dynamic and complex system where several processes occur simultaneously.

The lake is acting like a huge chemical reactor trapping most of the volatiles degassed by the shallow magma body.

The lake chemistry is affected by fluid-rocks interaction, dilution by meteoric waters, evaporation, overflow, seepage or recycling of lake waters into the sub-surface hydrothermal system.

Seepage of the lake waters into the hydrothermal system is necessary for an efficient transportation of heat and chemicals from the magma to the lake and to the surface of the earth.

Crater lakes act as huge chemical traps or condensers for magmatic volatiles. Therefore, they represent an ideal situation for long term monitoring of gas and heat fluxes released by a shallow magma.


This is a general and simplified model of a crater lake from a passively degassing volcano.


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2. A crater lake is also a calorimeter trapping most of the heat released by the magma.

Many crater lakes are in a steady-state equilibrium. The temperatures of the lake waters are constrained by the balance between heat input and output.

Heat balance box model (adapted from Brantley, Agustsdottir and Rowe (1993).

3. Chemical features of crater lakes

The discharge of magmatic gases (SO2 , H2S, HCl and HF) into a crater lake frequently lead to highly acidic sulfate-chloride waters with pH= 0-1. The disproportionation of magmatic SO2 in these waters is the most significant process responsible for this acidity by forming sulfuric acid and elemental sulfur S°.

These acid lakes are too acidic to convert and store CO2 gas as bicarbonate ions (HCO3-). Notable exceptions are lakes Nyos and Monoun (Cameroon) where the emission of acid gases (SO2, H2S, HCl) has now ceased. These lakes are almost completely devoid of sulfates and chlorides but contain huge amounts of dissolved carbon dioxide gas.

Université Libre de Bruxelles. Last modification: January 22, 2002

Webmaster: A. Bernard