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Geothermal Fields, Taupo Volcanic Zone, north Island, New Zealand

Abstract

The Taupo Volcanic Zone (TVZ) is the most volcanically active region in New Zealand.  The TVZ occupies a narrow band 250 km long and 50 km wide which extends from the centre of the North Island to beyond the Bay of Plenty.

The majority of New Zealand geothermal fields are located with the TVZ and most of these fields occur in association with large basin-shaped depressions called calderas.  Calderas are remnants of volcanoes that have suffered magma chamber collapse.  Geothermal systems are responsible for the formation of several unique geological and geomorphological features.  They have been studied as pysdo-environments to replicate conditions that may have lead to the development of life on Earth, and perhaps other planetary terrestrial systems such as Mars.

Caldera Formation

Fractures in the earth’s crust provide pathways for magma close to the surface to escape which form volcanoes.  The type of volcano produced is dependent upon the tectonic environment and local geology of the area.  Beneath a volcano there is a magma chamber filled with molten magma.  Periodically this chamber is emptied of magma during an eruption phase leaving most of the chamber hollow until magma refills the chamber. 

The weight of overlying rock and earth tremors can cause the ground above the chamber to collapse forming a depression at the earths surface called at caldera.  Volcanic domes often mark the boundaries of the caldera and volcanic activity may still occur in the centre of the caldera as the are is still volcanically active.  Geothermal fields are very common along fracture zones at the edge of the caldera.

The TVZ has several calderas and dome complexes which mark the location of volcanoes that have erupted forming calderas (Lake Taupo is a huge caldera filled with water).  Consequently, geothermal field are very common in this region of active volcanic activity.

What is a Geothermal System

Geothermal systems occur where magma is located at relatively shallow depths relative to the earth’s surface.  Fractures allow heat to be rapidly transmitted to the surface.  Magma heats the surrounding underground rocks which in turn heats deep and shallow circulating groundwater.  The hot water becomes less dense and rises towards the surface taking advantage of the channelling effect of fractures and spaces within rocks.  Dependent upon the rock chemistry, overlying strata composition and volume of groundwater, the heated water reaches the surface either as steam, heated bubbling mud or boiling water and may form geysers or geothermal pools.

Temperatures, gas composition and pH for geothermal fields are variable and are dependent upon the chemical composition of the underlying magma and surrounding country rock.  Average temperatures fall between 20 degrees and 175 degrees Celsius and pH can vary between 2 and 7.

Geothermal Field Charactertistics

Collapse Craters

Often the surface above a geothermal field collapses forming a small collapsed crater (1 to 20 m wide) which can continually grow as further rock collapses around the central crater.  The initial collapse is caused by the preferential movement of boiling water and gas through fractures within the overlying rock strata.  Craters are caused by the acidic nature of rising steam and water which cause surrounding rock to chemically corrode becoming unstable. 

(if the steam and water were not acidic, different surface features such as acid-sulphate pool would form).

The chemical reaction is:  H2O (steam) + H2S (hydrogen sulphide) + 2O2 (oxygen) = H2SO4 (sulphuric acid) + H2O (water).

Steam and hydrogen sulphide gas (H2S) dissolves in steam condensate and combines with atmospheric oxygen (O2) to produce sulphuric acid (H2SO4).

If the environment is acidic (low pH) further hydrogen sulphate dissolving in acidic steam condensate, reacts with atmospheric oxygen to preciptare as sulphur on walls.  

 

 

 

 

 

 

 

Steaming Ground

When deep reservoir water boils and there are no open fractures and pathways within the rock steaming ground can occur.  An area of diffuse steaming ground is known as steaming ground, and a localised high velocity discharge is called a fumarole.  Gasses which accompany fumaroles are usually dangerous and include carbon dioxide (CO2) and hydrogen sulphide (H2S).  CO2 is especially dangerous as it is odourless and may linger in low lying areas.

 

 

 

In low-lying areas, rainwater collects and mixes with ascending steam, water and gasses to form mud pools and coloured pools. 

Acid-Sulphate Pools

In low-lying areas, rainwater collects and mixes with ascending steam, water and gasses toform mud pools and coloured pools.

These pools are known as acid-sulphate pools and are acidic with a pH of 3.  These pools are usually opaque, green or yellow in colour which is due to the presence of sulphur.

Alkali-Chloride Pools and Sinter Deposition

When fractures are present in the rock they allow for migration of deep reservoir water directly to the surface forming clear, blue coloured hot springs, pools and geysers.  The thermal fluid is very rich in silica and chloride and has a neutral pH of around 7 and 22 degrees Celsius.  As the water cools in these pools silica comes out of solution and forms silica sinter.

Silica sinter forms very slowly by the gradual deposition of silica from ascending alkali-chloride thermal water.  Silica from surrounding rocks dissolve into solution if the temperature is above 175 degrees Celsius.  Silica is released at 100 degrees celcius forming sinter.

Fumaroles & Geysers

Fumaroles are vents that only release steam and gasses.  They usually form above the thermal water table and are commonly associated with steaming ground.  The ground conditions channel the gases through narrow vents at increased velocity, often producing a hissing sound.  Carbon dioxide is the dominant discharging gas, although hydrogen sulphate can also be discharged.  Hydrogen sulphate produces acidic steam condensate which can cause underground and surface corrosion resulting in collapse craters.

Geysers discharge boiling alkali-chloride water.  Hot, clear blue, silica rich water reaches the surface from an underground reservoir through a fracture within the overlying rock strata.  The fracture in the rock is partially sealed by silica deposition, preventing continuous water flow and dividing the fracture into an upper and lower chamber.  This chamber segregation creates a build up of pressure in the lower chamber, which raises the water temperature.  

The lower chamber then boils producing steam, which pushes the water out of the upper chamber.  This overflow can be seen in vents around the geyser and is often an indication of an impeding eruption.  The empted upper chamber causes a reduction of pressure in the lower chamber, triggering a violent steam explosion (flashing).  This drives the deep boiling water through the constriction ring to the surface under high pressure, where it erupts as a plume of steam and water.  Eruptions are cyclical depending on the speed at which the chambers refill.  

Without the silica deposition at the boundary between the upper and lower chamber, the geyser would be a spring with water flowing continually to the surface.  Therefore, the three factors necessary for geyser formation are: boiling water, silica formation and a constriction ring between the two chambers.

Thermophiles and Sulphur-Eating Bacteria

There are several kinds of thermophiles (microbial mats) that inhabit geothermal regions, and each is identified superficially by their colour.  The colour of the mat has a direct relationship to the temperature of the surrounding water.

Orange mats contain microbes that like water temperatures between 35 and 59 degrees Celsius.  The orange colour is a carotenoid pigment which acts as a sunscreen.  These mats live very closer to the surface of the water a need protection against damaging ultra-violet light.

Green mats accommodate microbes which also prefer warmer water temperatures between 39 and 59 degrees Celsius.  These microbes live in deeper water which gives them protection against untra-violet light.  In the hotter faster flowing channels, long strands or streamer microbial mats form parallel to the prevailing current.  Green and orange mats photosynthesize and during this process release oxygen which becomes trapped and forms bubbles within the mat filaments.

Brown or black mats live in cooler flowing water with a temperature around 35 degrees Celsius.

Sulphur-eating Bacteria

In pools that have a high sulphur content microbes grow in vertical pillars on sulphur islands and along the pool’s rim, feeding off the sulphur.  These microbes are usually white in colour and produce living structures that resemble 15 mm high stalagmites.  The base of the structure is usually a bright orange colour caused by the high concentration of sulphur.

These microbes are in a continual race against time before silica entombs and fossilises them.

Despite living above the water line, alkali-chloride water from the pool splashes over the microbes coating them in silica.  Splash activity can be caused by wind generated waves and currents or bubbles of gas forming concentric wave rings.  If this was not enough the osmotic effect of the water being sucked up through the microbes caused silicification internally.

Hydrothermal Eruption Craters

Hydrothermal eruption craters develop if boiling groundwater has a sudden pressure decrease. 

The sudden decrease in pressure causes water to flash to steam which provides lift due to expansion.  The expansion and increased pressure causes an explosive eruption ejecting above lying strata.  The eruption throws rocks and mud into the air, some of which settle around the rim of the newly formed crater.  These deposits are called hydrothermal eruption breccias.  Afterwards, water may fill the crater forming a lake or pool.  A hydrothermal eruption crater is distinguished from a collapse crater by the presence of hydrothermal breccia and from a volcanic crater by the absence of lava or tephra (ash) deposits.

 

 

 

 

 

 

 

 

 

 

 

 

Mud Pools and Mud Volcanoes

The formation of mud pools are associated with acid-sulphate water.  Ascending gasses bubble through rainwater where it has accumulated in surface hollows.  The ascending gasses cool and interact with the surface oxygen to form acidic steam condensate (sulphuric acid).

The sulphuric acid condensate alters surface rock changing the rock’s molecular and chemical structure to a clay mineral.  Rainwater then mixes with the clay to form a very fine-grained liquid mud.  Ascending steam rich with hydrogen sulphide and carbon dioxide slowly percolate through the mud, producing concentric rings or causing large explosive mud eruptions.  

Mud pools do not boil, rather the bubbling is caused by the gas discharge.  Mud pools are always acidic with a pH between 2 and 4 and a temperature range between 30 and 100 degrees Celsius.

Mud volcanoes form as repeated layers of thick mud are thrust to the surface and stacked on onr another.

 

Palaeozoological and Palaeoenvironmental Twist 

As the microbes that form the mats grow in the thermal water, silica is constantly deposited on their bodies, coating and entombing them.  The entombed microbes eventually die and become fossilised.  Study of the fossils allows insight into conditions existing at the time of their death.

The diameter of the fossilised microbe indicates the temperature range at which it lived.  The textures preserved in the silica sinter indicate water flow conditions, speed and direction.  Streamers indicate fast flowing conditions while bubbles indicate quiet conditions.  Further studies utilising DNA analysis can provide information relating to the pH of the surrounding water at the time the microbe died.

In addition to microbes, plants  growing close to the hot springs can drown and also become fossilised if the hot spring channel migrates laterally.  Leaves, pollen and diatoms may also be blown into the water to be fossilised within the silica sinter.  These fossils provide valuable insight into what the surrounding environment was like.

Hot spring environments may have been the site for life’s origins on Earth.  By studying fossilised microbes in hot spring environments on earth we can learn how to recognise them in extinct geothermal settings and possibly on other planets such as Mars.

BELOW: Photographs that mimic conditions outlined in the text. All photographs taken in the north Island of New Zealand

 

This area is a large ancient hydrothermal eruption crater that has filled with alkali-chloride water. Water temperature are around 25 degrees Celsius and the pH ranges from 5.5 to 3.5. The colour is caused by the minerals antimony, sulphur and arsenic. The pool which has been tilted by earthquake activity directs an outflow of water over the flat which precipitates silica sinter as the water cools

Silica sinter that has been precipitated by the above pool as water outflow cools

The base of a collapse crater where discharging gases mix with mud and oil. The oil is natural and forms as high grade oil and carbon are carried towards the surface by up welling gases and steam. The oil is produced by high temperatures beneath the surface which alter buried carbon to oil and gas. The carbon initially came from wood, algal and other plant material buried

hydrothermal eruption crater lake. Suspended sulphur is responsible for the yellow-green colour which varies in intensity and shape dependant on cloud cover and the amount of reflected light. Water temperature is 23 degrees Celsius and the water had a pH of 3.9

Mud explodes from a mud crater at frequent intervals as hot gases come to the surface and expand. The frequency of discharge is variable as is the location. Temperature is around 100 degrees Celsius

Orange microbial mates (thermophiles) like water temperatures between 35 and 60 degrees Celsius. The orange colour is a carotenoid pigment which acts as a sunscreen. These mats live in open areas close to the surface in full sun and need protection from damaging ultra-violet light

Orange microbes (thermophiles) grow in vertical pillars on sulphur islands that precipitate around the rim of some pools. The microbes feed off the precipitating sulphur and there is a continual race against time before precipitating silica entombs and fossilises them. Although these thermophiles live above the water line, alkali-chloride water does splash over them coating them in silica during wind and storm events. Furthermore, the osmotic effect of the water being sucked up through the microbes cause silification internally. The vertical structures resemble stalagmites and are approximately 15 mm in height

Brown microbes (thermophiles) prefer water with a temperature lower than 35 degrees Celsius

Collapse craters are the result of gases ascending with stream from deep seated hot water reservoir, corroding the overlying rock. The gas comprises hydrogen sulphide which dissolves in the steam condensate and combines with atmospheric oxygen to produce sulphuric acid. The resulting acidic steam condensate dissolves surrounding rocks which subsequently become weakened and collapse. The black colour in this crater is caused by iron sulphides (iron oxides cause red colours when the iron sulphide combines with atmospheric oxygen). Much of the surface directly adjacent to the crater is very spongy to walk on.

This bright yellow pool which has a water temperature of between 60-75 degrees Celsius has formed on the floor of a hydrothermal eruption crater. The pool is produced by the mixing of alkali-chloride water, emanating from an adjacent pool, with acid sulphate conditions in the crater floor. This chemical combination produces an intense yellow precipitate in small pools where gas discharge maintains the necessary environment. The precipitate contains around 10% arsenic, sulphur and a small concentration of gold

The discharging gas in this image does not have enough velocity to create boiling mud. Instead, a steady gas discharge causes the mud to "plop, plop, plop". Temperature is around 30-40 degrees Celsius.

Green microbial mats (thermophiles) inhabiting water with a temperature between 35-60 degrees Celsius

 

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