Late volcanic activity



After termination of active volcanic eruptions, all volcanoes continue to produce gas, steam, sulphur and other elements at temperatures of up to 650°C for a long period of time. The steam is due to the heating up of underground water that originated on the surface.


Gas emissions are called fumaroles; if the gas is sulphurous they are called solfataras. Solfataras produce native sulphur and are active for many years. The solfatara near Naples, in Italy, for example has been releasing hot sulphurous gas for more than nine centuries, since the volcanic eruption in 1198.


The dynamic hot springs that intermittently eject a column of water and steam into the air are called geysir (singular = geyser).

This name derives from a province of Iceland and a volcano called the Great Geysir, where phenomena like these were first noticed and studied. In the Icelandic language, the word geyser literally means wild, or simply eruption of a hot spring.

Geysir belong to the secondary manifestations of volcanic activities. When a geyser violently ejects the volume of water and steam that obstructs its vent, it is said to be in eruption. Its jet of water can be 30-50 metres high but powerful geysir with jets of water of up to a height of hundreds of metres are not rare. 

The Great Geysir (Iceland) for example erupts a column of hot water at 100°C to the height of the 10th floor of a building.


Experts think that the water ejected from geysir comes from the phreatic layer and that it is heated up by steam and volcanic gases which, by means of small cracks, reach the vent of the geyser where the phreatic water collects. The volume of water is heated up gradually, but due to the shape and length of the vent, upward drifts that would render the temperature of the volume of water more or less uniform cannot occur; so the temperature in the lower part of the geyser is higher than the temperature on the surface.

After a certain period of time, the water at a relatively shallow level reaches such a high temperature that it would start to boil if it were not for the weight of the column of water above it. The temperature, however, continues to rise until, at that same level, the pressure causes boiling point to be reached. The water’s change in state from liquid to steam at that depth causes a strong upward push of the mass above. The sudden decrease of pressure in the vent lowers the water’s boiling point and much of it is suddenly transformed into steam with explosive violence: the geyser then erupts. After the eruption, the vent gradually refills with phreatic water and the process starts again.


Geysir can also be caused by man-made drillings; one example is the geyser that erupted on the island of Ischia (Italy) in 1939 during the excavation of a well for geothermic research: for some time, there were jets of hot water and steam up to a height of 18 metres.


Like the common edifice of volcanoes (cone-shaped and simple volcanoes), geysir also have: a vent, i.e. a more or less cylindrical pipe through which the mass of water and steam rises and is then brusquely ejected, and a crater, i.e. the surface opening of the vent.


Around this crater, a small edifice may form, a conic or cylindrical trunk, in which the mineral substances precipitating from the water of the geysir are deposited. The main substance here is silica, which forms deposits of geyserite (spongy hydrated silica); there are also mounds of carbonates and more rarely sulphates. The deposits around the geysir can cover a large area and even form piles of substances, either white or coloured, that create a peculiar effect and are of great morphologic interest.

Sometimes, by observing the thickness of the deposits, it is possible to guess the period in which geyser activities started in a specific area. In this way experts have estimated that the 100 geysir in the Yellowstone National Park (Wyoming, USA) have been active for at least 800,000 years.

The geysir in the Yellowstone National Park are well known worldwide. Among the best known geysir people remember: Old Faithful, which every 60-80 minutes has an eruption lasting 4 minutes when it gushes boiling water up to a height of 50 metres; the Castle, whose less regular jets sometimes reach the height of 80 metres; and the Giant, which has impressive but very irregular explosions. The intervals in the activity of geysir are not always constant and regular. Some geysir have intervals lasting only a few minutes, others even a few days.


There are also cases where jet intervals change in the course of time, as happened in Iceland with the Great Geyser, one of the most famous geysers on the island. Whereas in 1772 this geyser had an eruption interval of half an hour and ejected a column of hot water up to a height of 30-70 metres, in 1883 it erupted every 20 days. As well as the above-mentioned examples in Iceland and the Yellowstone National Park, geysir can be found in various other parts of  the world.


The Geyser Valley in the peninsula of Kamchatka is famous for its hot springs. In the valley of the Geysermaya river, there are 20 hot springs. The most famous is Velikan which ejects a column of hot water weighing 20 tons up to a height of 30 metres, and steam emissions up to a height of 300 metres. Its eruptions last about 2 minutes and recur every 4-5 hours.


There are many geysir and hot springs in New Zealand. The biggest geyser, Pohuta Geyser,  emits a jet of 30 metres every 20 minutes.  The geysir in this classic region have formed huge deposits of mainly calcareous steps. The hot springs and waterfalls near Rotorua  attract many tourists.


There are also geysir in Java, Sumatra and the Aleutian and Celebes islands; in Japan; in California (in the region of Sonoma); in Tibet (at a height of about 4,800 metres near the Tengri Nor Lake); in Chile (inside the Sochomba Alachamal volcano at a height of more than 6,000 metres); in Kenya  and in other volcanic regions.


As well as hot springs there are also boiling mud volcanoes which emit a disgusting smell.


Geysir  - Postage Stamps


Last modified: Lunedì 22 maggio 2023


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