Strokkur Geysir (Geyser)

Icelands "Strokkur Geysir" is one of the most persistent geysers in the world and lies in the Haukadalur valley at the base of Laugarfjall hill about 50 m from the site of the famous, but rarely active, "Geysir" which is the source of the generic name given to these kind of hot springs. The term Geysir (Engl. Geyser) is itself derived from the icelandic word "geysa" which means to gush.

The small geothermal area in which the geyser lies is one of the major tourist attractions in iceland. Strokkur was first reported in 1789, after an earthquake which probably created or unblocked its conduit. Eruptions were regular until 1896, after which a renewed earthquake inactivated the geyser. In 1963, local people succeeded in unblocking the plumbing system and Strokkur has been active ever since. First records of geyser activity in the area date back to 1294, when the great Geysir is thought to have become active.


	Classic eruptions of Strokkur Geysir

Different Stages of Eruptions:

Eruptions usually occur with intervals of between 5-10 minutes and involve a single burst reaching a height of up to 30 meters, although occasionally up to 5 bursts in relatively quick succession are observed. Prior to eruptions, the pool is full and gently pulsates up and down. The eruption commences when a pulse of steam rising from below pushes the water in the pool upwards forming a large dome (or bubble) of water through which the steam bursts and expells much of the water in the pool skywards. After regular eruptions the pool is empty but becomes rapidly refilled by flowback and water rising from underneath. Following smaller irregular eruptions which are also frequently observed, the pool may remain almost full and renewed activity may occur very rapidly (as little as 10 secs by own observations).


Water swells upwards forming dome (or bubble) shape	Steam bubbles visible rising beneath surface


Bubble surface breached	Water spouts up through middle of bubble


Geyser erupts water violently upwards	Pool is emptied by regular eruptions


Pool refills after eruption(s)	Water from eruptions partially flows down channel to top right

Geysir and Strokkur have long interested travellers. In 1894, the area was sold to James Craig who fenced in the geyser site and charged admission. However, this was only short-lived and access to the site became free again the following year. The site currently belongs to the icelandic state and is frequently visited as part of the Golden Circle tour which takes visitors to a number of sites in southern iceland.


Asymetric eruption - bubble often emerges off-center	Bubble bursting in front of tourist infrastructure

How does a geyser work? Basically, a geyser requires a heat source (cooling magma), a source of water, permeable rocks through superheated waters can rise and a pressure-tight (i.e. non-permeable) chamber or series of chambers where pressure can build up prior to eruption. Most geysers are found in highly silicic rhyolitic rocks. Superheated waters dissolve silica from these rocks at temperatures of around 300'C which exist in the high-pressure environment deep under the surface. Silica exsolves again and is deposited when the superheated waters boil or cool as pressure and temperature fall, respectively as they rise towards the surface. The deposited "geyserite" (largely hydrated silicon dioxide (a form of opal)) seals the geysers plumbing system, allowing it to pressurize.

Strokkur geysir is located in an area with extensive geyserite deposits, yet is surrounded by minor travertine sinter formations (as can be found at e.g. Mammoth Hot Springs at Yellowstone), which can however perform a similar sealing function. Travertine formation is fostered by carbon dioxide gases rises from below which dissolve calcium out of the underlying rocks forming calcium carbonate. This is transported to the surface dissolved in the geothermal waters and is deposited there once the waters evaporate.

In an active geyser system, superheated waters rise into the geysers chamber(s), whilst cooler near-surface waters (may) also enter the chamber from above. These initially effectively cap the hotter water body below keeping it pressurized and allowing it to maintain temperatures well above boiling point under atmospheric pressure conditions. As the chamber fills, the superheated water body transfers heat to the cooler water above. Eventually, the upper water body reaches boiling point. As heating continues water vapour (gas) formation becomes more vigorous and water may start to be pushed out of the vent (at Strokkur this is seen as a rising and falling of the pool). Eventually, the overflow and / or increasing concentration of bubbles decreases the mass of the upper water body, thus reducing the pressure acting on the body of superheated water below. This depressurization causes violent "run-away" vaporization of the water below, resulting in water and steam being violently expelled upwards, causing the eruption. Eruptions at Strokkur are very short, each involving a single or series of discrete thrusts.

In the geothermal system feeding Strokkur, subsurface temperatures of about 240'C are thought to exist, probably at a depth of 1 km and below.

Strokkur Geysir. Geyser in Iceland, Bubble forming at beginning of eruption

Strokkur Geysir. Geyser in Iceland, Bubble bursting at beginning of eruption

Sequence of three images from same eruption - Note first signs of bubbles in left image

Further Images:

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Geyserite platform with pool of the great "Geysir", situated 50m north of Strokkur. Not frequently active (apparently once every 24 h in 2010) although can be reactivated following earthquakes or artificially following priming with detergents or reduction in the water level by drainage. Artificial priming is however today frowned upon for environental reasons.

Strokkur Geysir is just over an hours drive along paved roads from Icelands capital Reykjavic. For those without an own car, several companies offer coach trips to the site. Restaurant and hotel facilities are available just over the road from Strokkur Geysir, and a small camping site is nearby.