Geothermal activity is caused by the transfer of heat from depth to the earth's surface. Listed below are the surface manifestations of geothermal activity.
1) Warm Ground
Warm ground represents a low level of geothermal activity.
The ground temperature is raised at 1 m depth but not at the surface.
Warm ground is not visible on infrared images but changes to vegetation can be identified.
Examples of warm ground areas.
Fuzhou (South China)
Ngawha (New Zealand)
Steam Vents, Kilauea volcano (Hawaii)
Puhimau Thermal area, Chain of Craters Road, Kilauea Volcano (Hawaii)
2) Hot Steaming Ground
Hot ground is the result of underground thermal conduction.
Hot vapours rise near the surface but are not actually discharged. The vapours
condense and drain away without being released to the atmosphere.
A thin layer of steam may condense under moist air conditions. If the air is dry then no steam is observed.
3) Hot Pools
Hot pools are the result of hot water or steam heating a pool of groundwater.
Hot pools may be calm, ebulliant (effervescent) or boiling.
Examples of Hot Pools
Waiotapu (New Zealand)
4) Hot Lakes
These lakes fill hydrothermal depressions in geothermal areas. They are a subclass of volcanic lakes.
Examples of Geothermal Hot Lakes
Rotokawa Lake (Rotorua, New Zealand)
Frying Pan Lake (Okataina, New Zealand)
Sulfur Lake (Iamelele, PNG)
Salt Lake (Iamelele, PNG)
Oyumuma (Kuttara, Hokkaido, Japan)
Golovnin (Kurile Islands, Russia)
Bannoe (Uzon, Kamchatka, Russia)
Lake Dal'nee (Uzon, Kamchatka, Russia)
5) Hot Springs
Hot springs are the most common type of geothermal activity.
They are located where water from a geothermal system reaches the surface. Some springs in Icelend pulsate and release water in cycles.
Inferno Crater (Waimangu, New Zealand) discharges water every 5 weeks when it overflows.
A fumarole is a steam discharge from a hydrothermal or volcanic system.
A solfatara contains sulfur emissions. Soffioni emit boric acid. Fumaroles can burn, so be careful when approaching them.
Composition of Fumarole Emissions
Steam is the most abundant emission from fumaroles.(60% by molar concentration)
Steam originates from both groundwater and magma.
Chloride is added from seawater.
Noisy fumaroles - discharge rates >20m/s
Quiet fumaroles - discharge rates <20m/s
A geyser is a vent from which hot water and steam are violently emitted.
They are very rare but well known and extensively studied.
Requirements for geyser formation include fractured rocks and boiling water at a shallow depth.
Regular eruptions (Old Faithful, Yellowstone, USA)
Rainy season eruptions (Rajabasa, Sumatra, Indonesia)
8) Hydrothermal Eruptions
Hydrothermal eruptions are caused by catastrophic discharges of water close to the boiling point. It is a phreatic eruption. No ash, incandesence, or clasts are erupted. Hydrothermal eruptions may be caused by a reduction in the overlying pressure.
Examples of hydrothermal eruptions
Waiotapu (New Zealand)
Rotarua (New Zealand)
Kawah Komojang Field (Java, Indonesia)
Yangbajing (Tibet) Drilling induced
A seepage is a general term which describes any subsurface discharge of warm fluids from a geothermal area.
Seepages may occur into rivers or lakes. A river seepage may be indentified by differing non reactive constituents above and below the seepage outlet.
Examples of seepage.
Mokai (New Zelaland) River seepage.