volcanic lakes

Volcanic lakes occur in volcanic areas young and old, in both active and extinct volcanoes.

How do volcanic lakes form?

Volcanic lakes are generally formed by one of three mechanisms:
1. explosive excavation (crater lakes)
2. collapse (caldera lakes)
3. blockage of common waterways (rivers, streams) by mudflows, lava flows or ash

Why are lakes relatively common in volcanic areas?

 A disturbance of the drainage pattern will lead to an accumulation of water in low areas, with or without a surface outlet. Many crater lakes lack an outlet and only lose water through evaporation or subsurface leakage. Most volcanic lakes receive an input of hydrothermal or volcanic fluids. In exceptional cases, lakes may be intruded by new magma and volcanic explosions can occur in or below the lake. Some volcanic lakes are fed by the effluents of surface hot springs or fumarolic vents. In high areas, lakes of glacial origin may become volcanic lakes through activity and inputs of nearby volcanoes.
Under what conditions do volcanic lakes exist?
Volcanic lakes range in composition from common meteoric waters to those with a strong volcanic imprint, and temperatures can vary from ambient to close to boiling. Some subglacial lakes can be warmer than ice and have no direct contact with the atmosphere.

Lakes can only stably exist when the water influx = water outflux. This steady state condition can be determined by simple lake hydrology.  In warm lakes, evaporation may play a significant role in determining the size or level of the lake at steady state.  In such cases, one can gain insight into lake dynamics through energy budget analyses.  For example, by calculating energy losses and gains from evaporation and radiation, it is possible to estimate the volcanic energy input into a lake.

Natural hazards associated with volcanic lakes:
Lahar Flows
Volcanic eruptions through lakes can cause failure of a retaining wall and a resulting flood of the lake waters. Many eruptions through lakes lead to massive lahar flows (hot mudflows), as is common at Kelut volcano in Indonesia.  A tunnel was dug to that lake to control the lake level and volume to prevent major disasters.
Lake Explosions
True lake explosions may be thermal in origin (sudden catastrophic boiling event) or may be related to catastrophic degassing of the lake waters. Catastrophic degassing occurs in a special class of lakes (e.g., Monoun, Nyos, both in Cameroon) where the bottom waters have high dissolved CO2 contents. A variety of triggers can cause an overturn of the lake, and a sudden decrease in hydraulic head may lead to explosive degassing of the lake waters. Almost 2000 people perished in 1986 during the Lake Nyos explosion. Most victims were asphyxiated in the cold CO2 cloud that travelled as a gaseous density current from the lake down the valleys.

In glaciated regions, subglacial eruptions can create large lakes that may escape through floodchannels causing glacier bursts or jokuhlaups, as happens periodically in Iceland.

Natural Pollution
Some very hot and concentrated crater lakes emit whitish clouds of HCl gas, forming acid aerosols in humid air. Many volcanic lakes leak acid and/or toxic fluids into the local watersheds where the water may be used for household or irrigation purposes. The environmental chemistry of volcanic lakes (natural pollution) is thus a rapidly emerging field. Fluorosis is a common disease in many areas with an abundance of volcanic fluids, and at the moment we can only guess at the effects of heightened levels of toxic elements such as the heavy metals, Li, As and Tl in local surface waters.