John is a retired librarian who writes articles based on material gleaned mainly from obscure books and journals.
Geysers and Hot Springs
There are many places around the world that provide constant evidence of the fact that a considerable amount of heat is generated at various depths within the Earth’s crust. When water comes into contact with these heat sources it can be forced to the surface and then give rise to some dramatic and spectacular sights.
Depending on local circumstances, the results can be hot springs, geysers, boiling mud pools, or fumaroles– jets of steam. Here are some notable examples from around the world:
600,000 years ago an ancient volcano collapsed, leaving a huge caldera in what is now northwest Wyoming. The magma chamber that fed the volcano is still there, about three miles below the surface, and it is inevitable that another massive eruption will take place at some time – probably within the next few thousand years. The magma heats the rocks above it, and it is these hot rocks (at a depth of around 700 feet) that supply the energy for Yellowstone’s dramatic spectacles.
As water from rainfall percolates through the rocks it eventually comes into contact with the heat source and collects in underground reservoirs that also provide pressure due to the depth at which they form. The water is therefore “superheated”, which means that it reaches a temperature of around 200 degrees Celsius (390 degrees Fahrenheit) but without turning into steam. This only happens when the water is forced upwards and the pressure is released.
The result is Yellowstone’s famous geysers – around 200 of them – that periodically burst out of surface lakes in jets of hot water and steam. If its underground reservoir is constantly replenished with water, a geyser will erupt regularly, and the most famous geyser of all gets its name ("Old Faithful") from the fact that it has erupted – on average – every 67 minutes for hundreds of years. The geyser can reach a height of 170 feet, leaving steam droplets hanging in the air for several minutes after it has fallen back.
Hot water can reach the surface in less violent but just as spectacular forms. The water dissolves minerals from the rocks as it rises, and the rapid evaporation that takes place when the surface is reached leads to the minerals being deposited on the surface. This process is seen to great effect at Mammoth Hot Springs, where two tons of carbonate minerals are deposited every day.
The name Yellowstone derives from the color of the rocks that are produced by this process, although much of the coloration is due to the action of algae in pools of water that form at various temperatures.
Yellowstone’s boiling mud pools also exhibit various colors.
The island of Iceland, which sits on top of the mid-Atlantic ridge, was created by volcanism and it provides evidence of this on a daily basis. It is the only nation in the world that obtains all its domestic and industrial energy from geothermal sources. It is also the source of the word “geyser”, this being the name (originally “Geysir”) of the first such recognized phenomenon, described in writing in 1294.
Strokkur is 50 miles east of Reykjavik, the capital city of Iceland. The name (which translates as “churn”) refers to a geyser that erupts every eight minutes. It was first noticed after an earthquake in 1789, but it stopped after another earthquake in 1896. The conduit that conveys hot water to the surface was cleared by local people in 1963 and the geyser has performed on schedule ever since.
The heat source at Strokkur is only 75 feet below the surface, but this still provides enough pressure to prevent the water in the reservoir from boiling despite being at a temperature of 120 degrees Celsius (250 degrees Fahrenheit). However, when it is released it ascends rapidly and more water is allowed to take its place and begin to be heated to a similar temperature. The short conduit and constant refilling of the reservoir account for Strokkur’s busy activity. Not surprisingly, Strokkur is a major tourist attraction.
This area of hot springs and fumaroles lies on the northern shore of the Bay of Naples, opposite Mount Vesuvius. This has been known as an area of high volcanic activity for thousands of years, not least because of the catastrophic eruption of Vesuvius in AD 79 that led to the destruction of Pompeii and Herculaneum. Vesuvius last erupted in 1944, and future eruptions are certain.
Solfatara is a crater that lies within an ancient caldera that is known as the Phlegrean Fields. There is clearly a heat source not far down because there are about forty hot springs and fumaroles here that produce a continual mist of volcanic vapors. These vents have been studied intensely in the hope that changes in the composition of the emitted gases can be related to the frequency and severity of earthquakes and thus provide a means of giving an early warning of future problems.
The largest fumarole at Solfatara is called Bocca Grande (“Big Mouth”). In classical times it was regarded, not unreasonably, as the entrance to Hades.
Of particular concern is the fact that Solfatara and Vesuvius are surrounded by built-up areas comprising Naples, Pozzuoli, and other communities. Around three million people could be adversely affected by a major earthquake or eruption.
This well-known area of hydrothermal activity is on the North Island of New Zealand, 65 miles east of the city of Hamilton. It is the remnant of a massive eruption in AD 180, although there were no humans present to witness it at the time.
New Zealand is at the south-western end of the “Ring of Fire” that surrounds the Pacific Ocean and is the result of tectonic plate movements.
There are seven active geysers that erupt in a regular sequence suggests that the reservoirs feeding them are linked. It is steam leaking from these reservoirs that gives rise to the pools of boiling mud for which Rotorua is probably best known. The mud consists of black sulfide, white silica, and kaolin clay. Bubbles of gas are continually forming and bursting in the mud, which is therefore in constant motion.