Mofette

A mofette is a type of cold volcanic gas vent that emits carbon dioxide (CO₂) and other gases at temperatures typically below 100°C, usually in regions of extinct or dormant volcanic activity. Unlike hot fumaroles, which release steam and high-temperature gases, mofettes discharge cool gases that seep gently through cracks, fissures, or springs in the ground.
The term mofette originates from the Italian word “mofeta”, meaning a noxious exhalation, reflecting the suffocating nature of the carbon dioxide emitted. Mofettes are important geological features as they indicate residual volcanic activity and serve as surface expressions of degassing processes within the Earth’s crust.

Nature and Characteristics

Mofettes are characterised by the emission of gases, primarily carbon dioxide, often accompanied by small amounts of nitrogen (N₂), methane (CH₄), hydrogen sulphide (H₂S), and water vapour. Unlike active volcanic vents, these gases emerge at low temperatures, typically between 20°C and 60°C, though some may be slightly warmer depending on depth and geothermal gradient.
Key characteristics include:

• Temperature: Below 100°C (usually much cooler than fumaroles).
• Gas Composition: Predominantly carbon dioxide (up to 95%), occasionally with trace volcanic gases.
• Appearance: Often invisible, though bubbling may occur where gas escapes through water or mud.
• Location: Commonly found in volcanic or tectonically active regions with recent magma or geothermal systems.
• Pressure: Gas emission is often slow and diffuse, unlike the forceful eruptions of fumaroles or geysers.

Because CO₂ is heavier than air, it tends to accumulate in low-lying areas near mofettes, creating potentially hazardous conditions for animals and humans.

Formation and Process

Mofettes are formed by post-volcanic degassing processes that continue long after volcanic eruptions have ceased. The sequence involves:

1. Residual Magmatic Activity:
   • Deep-seated magma or hot rock masses continue to release gases as they slowly cool.
   • These gases migrate upward through cracks and fractures in the crust.
2. Gas Separation and Cooling:
   • As gases rise, they lose heat and pressure.
   • Steam condenses, leaving mainly cold carbon dioxide and minor gases to escape at the surface.
3. Surface Emission:
   • The gases emerge gently from soil pores, fissures, or through groundwater, sometimes creating carbonated springs or gas-rich pools.

This phenomenon marks the final stage of volcanic activity, when the magma beneath the surface has largely solidified but continues to release trapped gases.

Types of Mofette Occurrence

Mofettes appear in several geological settings:

1. Volcanic Fields:
   • Associated with extinct or dormant volcanoes, such as the Eifel region in Germany or Auvergne in France.
2. Geothermal Areas:
   • Occur in areas of subsurface heat flow and residual magmatism, often near hot springs or fumaroles.
3. Fault Zones:
   • Found along tectonic fractures that serve as pathways for deep-seated gases to reach the surface.
4. Lake or Spring Emissions:
   • Gas bubbles may escape through water bodies, forming carbonated mineral springs used in therapeutic spas.

Distribution and Examples

Mofettes are distributed globally, particularly in regions of recent volcanic activity or geothermal significance. Notable examples include:

• Eifel Mountains (Germany):
  • One of the most studied mofette regions; features gas-emitting vents such as the “Laacher See” and the “Wallenborn Mofette.”
  • CO₂ emissions are monitored as indicators of ongoing mantle degassing beneath Western Europe.
• Auvergne (France):
  • The Chaine des Puys volcanic field hosts several mofettes that release CO₂ and nitrogen gases through soil and springs.
• Italy:
  • Central and southern regions, including the Alban Hills near Rome and the Phlegraean Fields near Naples, contain numerous mofettes associated with extinct volcanoes.
• Czech Republic (Karlovy Vary and Františkovy Lázně):
  • Famous for carbonated mineral springs fed by mofette gases used in therapeutic treatments.
• Iceland and Hungary:
  • Notable for geothermal mofettes linked with volcanic terrains.
• East African Rift Valley:
  • CO₂-rich springs and vents occur along active tectonic rifts, marking ongoing crustal degassing.

Ecological and Environmental Impact

While mofettes are natural geological phenomena, they pose certain environmental hazards due to their emission of dense, colourless, and odourless carbon dioxide:

• Toxicity: CO₂ displaces oxygen in low-lying areas, leading to asphyxiation risks for animals and occasionally humans.
• Soil Chemistry Alteration: High CO₂ concentrations affect soil pH and vegetation, creating zones of limited plant growth (sometimes termed “death zones”).
• Indicator of Geothermal Activity: Continuous gas emission provides valuable data for volcanic monitoring and climate studies.

In some cases, mofettes also support unique microbial ecosystems adapted to high-carbon environments.

Distinction from Related Features

Thus, while fumaroles and solfataras signify active geothermal or volcanic activity, mofettes represent late-stage degassing where thermal energy has largely dissipated.

Scientific and Economic Importance

1. Geochemical Studies:
   • Mofettes provide insights into the composition of gases escaping from the Earth’s crust and mantle.
   • Used in studying CO₂ flux, volcanic hazards, and geothermal systems.
2. Volcanic Monitoring:
   • Continuous measurement of CO₂ emissions helps predict volcanic reactivation or seismic activity in dormant regions.
3. Medical and Therapeutic Uses:
   • Carbon dioxide-rich mofette gases are used in balneotherapy, believed to improve blood circulation and treat cardiovascular ailments.
   • Famous spas in Germany, France, and the Czech Republic utilise mofette springs for therapeutic purposes.
4. Climate Research:
   • Natural CO₂ emissions from mofettes contribute to the global carbon cycle and are studied in relation to greenhouse gas budgets.