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Grand Prismatic Spring, Yellowstone National Park, USA.

Hot Springs

By Angelika Jung-Hüttl

Geology illustrated | October 2017

As soon as water bubbles up out of the earth in a spring and is warmer than 20 degrees Celsius geologists refer to this as thermal, geo-thermal or hydro-thermal springs or-if the temperature is way above what we humans take as comfortable-they speak of hot springs. Except for the temperature most thermal springs are hardly distinguishable from normal springs.

Where does the hot water come from? Hot or thermal springs can be found anywhere in the world where ground water circulates in the hot depths of our planet, heats up then rises through cracks and faults in the rock and, heated up or boiling hot, flows out of the earth. This mainly happens in volcanic regions regardless of whether they are still active or long extinguished. Depending on the temperature and the mineral composition this water and the earth around the springs can take on amazing colours from intensive blue, green, yellow, orange, brown and even red.

Where do these splendid colours come from? First from the minerals which the hot water dissolves from the base of the volcanic regions, transports upwards and deposits as soon as it makes contact with cool air and oxygen. Among these are various iron and sulphur compounds mostly in minute amounts but also poisonous substances like mercury or precious metals like silver and gold. A major part in this is played by heat-loving algae and bacteria which, at home in the hot, sometimes  corrosive water, contribute to these precipitates.

The Champagne Pool is one of the earth´s particularly impressive spring basins ....

Champagne Pool, Wai-O-Tapu, New Zealand

About 900 years ago a volcanic eruption tore a crater 60 metres deep and 60 metres wide into the ground in the geothermal region named Wai-O-Tapu (Holy Water) by New Zealand’s original inhabitants, the Maoris. The crater then filled with hot mineral-rich spring water and was later dubbed the Champagne Pool  because of the bubbling water inside it. The greenish water does in fact contain a quantity of carbonic acid, which is used in making fizzy drinks.  As in an opened Champagne bottle bubbles of carbon dioxid rise to the surface and burst. The sulphur gives the Artist’s palette, as the area around the pool is called, a yellow tinge.

  • Sinter crust at the rim of Champagne Pool
  • Remnants of plants covered by bacteria and minerals

Grey-white siliceous minerals form a crust around the crater of Champagne Pool. The orangey-red precipitation is made up of arsenic and antimony compounds, sulphur and traces of mercury, silver and even gold. The heat-loving bacteria that inhabit the pool, and which coat the crust as well as the remains of plants, probably help precipitate the minerals.

The blue spring – ‚Bláhver’ in Islandic.

Bláhver, or blue spring, in the volcanic area of Hveravellir is one of the most beautiful hot springs in Iceland. The water has a temperature of around 90 degrees Celsius, and owes its delicate, light blue colour to tiny drops of silicon dioxide. As the hot, silicon-dioxide-rich water cools, quartz starts to crystallize and build up a hard white crust. This geyserite, as geologists call it, surrounds the entire basin of the spring.

Like in a different world – geothermal area of Dallol in the Ethiopian desert.

The hot springs, that well from the ground in Dallol in the desert of the north of Ethiopia, bring with them salts, iron and sulphur minerals. When the water cools on the surface, these are deposited, forming a thick crust. Sulphur and iron hydroxide dye it yellow, whilst iron oxide minerals stain it rusty brown. The green water is caustic, stinging the skin when touched.

Laguna Roja - a red lake on an high plateau in the Andes of northern Chile.

Quite unknown but particularly impressive is the Laguna Roja, situated at a height of 3,700 metres on a plane in the deserted Parinacota volcano region in the north of Chile.  Here it looks as if a giant had emptied out a bucket of paint. The blood-red water, springing from the surrounding mountains,  collects in this hollow with a temperature between 40 and 50 Celsius. The intensive colour is formed by heat-loving red algae.

Grand Prismatic Spring, Yellowstone National Park - with the boardwalk for visitors.

The most famous hot spring worldwide is the Grand Prismatic Spring in Yellowstone. Its water has a temperature of 80 degrees Celsius. Its basin has a diametre of around 80 metres and is about 40 metres deep. The water, which flows out flat around the spring, deposits geyserite. Mats of heat-loving bacteria and algae, which hold carotine, also colour the sinter crust orange and brown.

Hot spring named Morning Glory, Yellowstone National Park

The pool of ‚Morning Glory’ hot spring opens up like a blossoming flower. Along with the Grand Prismatic Spring , it is one of the best known of the roughly 10,000 hot springs in Yellowstone National Park. The water in the center of the pool – which has a diametre of around seven metres – has a temperature of almost 100 degrees Celsius. This spring pool owes the colour on its edges to heat-loving bacteria and algae.

Sinter terraces of Mammoth Hot Springs, Yellowstone National Park, Wyoming, USA.

The water which streams out of the ground of Mammoth hot springs is very rich in calcium carbonate and, as soon as it comes into contact with cold air, it deposits its load. The calc-sinter, also known as travertine, creates shallow, stepped pools over which the water flows in little cascades.

There are not only thermal springs on our planet´s continents but also several thousand metres deep down at the bottom of our oceans. They gush forth from the sea basin along the mid-ocean ridges. These mountain ranges, many thousand metres in length, run through the oceans along the boundaries of the earth's plates. Here there are also many undersea volcanoes and the said hot springs, too. Geologists call them 'black' and 'white' smokers. The water from them shoots up out of the deep seabed with a temperature of 300 degrees Celsius. The 'black' smokers transport sulphur compounds of iron, manganese, copper and zinc from the ground, the 'white' smokers on the other hand contain salt minerals like gypsum, anhydrite and silicon oxide. Both types unload their freight as soon as they make contact with the ocean water with a temperature of around 2 degrees, forming  in the course of time chimney-like structures that can reach a height of 20 metres.

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