Strokkur

Deep under the ground, the Earth is hot. It heats the water.

Bubbles of gas gather down below. When they rise into the tube, the water bursts up into the air. Then the pool fills up again.

The Geysir area lies in Iceland's western volcanic belt, which explains its heat. Under Strokkur's pool is a narrow tube more than 20 meters deep, full of water that stays close to boiling.

Scientists found that gas gathers in a pocket called a bubble trap, about 24 meters underground, beside the tube. Bubbles keep escaping from it and collapsing in the tube, heating the water more and more until it erupts. Each time, the geyser goes through the same four steps: eruption, refilling, gathering gas, then bubbles bursting in the tube.

The Geysir field lies in Iceland's western volcanic belt, which accounts for its geothermal activity. Strokkur's water is constantly around boiling point and wobbles between eruptions.

In June 2018, Eibl and colleagues used seismometers, video cameras, water pressure sensors and a tiltmeter. They found that every eruptive cycle passes through four phases: (i) eruption, (ii) refilling of the conduit, (iii) gas accumulation in a bubble trap, at a mean depth of 23.7 ± 4.4 m and 13–23 m west of the conduit, and (iv) bubbles leaving the trap and collapsing in the conduit at shallow depth. The collapses release latent heat, first into the trap and then into the conduit, until the water reaches the conditions for an eruption. The authors say their data provide documented evidence of a bubble trap beneath a pool geyser.

Scientists listened to Strokkur for a whole year.

They counted more than seventy thousand eruptions! Sometimes the geyser erupts once. Sometimes it erupts a few times in a row.

From June 2017 to June 2018, scientists buried four seismometers, instruments that feel ground shaking, close to Strokkur. In one year they recorded 73,466 eruptions.

Most were single eruptions, but sometimes Strokkur erupted two, three or even six times in quick succession. After a single eruption, the next one came on average 3.7 minutes later. After six in a row, the wait was 16.4 minutes. The scientists could predict roughly when the next eruption would come, but not how big it would be.

Eibl and colleagues (2020) ran four broadband seismometers about 39 to 96 m from Strokkur from 27 June 2017 to 6 June 2018 and built a catalog of 73,466 eruptions: 50,135 single eruptions and sets of up to six in quick succession, with only one sextuple. The number of eruptions fell exponentially from singles to sextuples, while the mean waiting time after an eruption rose linearly, from 3.7 min after a single to 16.4 min after a sextuple.

They concluded that the waiting time after an eruption can be predicted, but not the type or amplitude of the next eruption. The first seismic recording at Strokkur was made in summer 1967, with one geophone placed as close as possible to the boiling pool.

Around the hot springs, the ground is warm.

Some special little plants grow there. Mosses grow on the warm, wet ground too.

The Geysir area is not only steam and rock. Birch trees grow around it, along with flowers such as self-heal and creeping thyme.

The warm ground near the hot springs is home to some special plants. Two of them, blue water speedwell and adder's tongue, are classified as species in some danger. Several kinds of moss also grow on the warm ground.

Protection of the Geysir area aims to preserve its geological formations, hot springs, microorganisms and unique vegetation, which the Environment Agency of Iceland calls singular on a national and worldwide scale. Birch trees mark the landscape, with vascular plants including self-heal, creeping thyme, hairy stonecrop, marsh cudweed and buckhorn.

Two geothermal vascular plants are classified as species in some danger: blue water speedwell (Veronica anagallis-aquatica) and adder's tongue (Ophioglossum azoricum). Several geothermal mosses also grow here. The area's crust of silica sinter, called geyserite, is protected too, and visitors must stay on the marked walkways partly so that it can be protected and possibly renewed.

The water in Strokkur is boiling hot.

Stay on the marked paths. Never touch the hot water. Never throw anything into a hot spring.

The water in Strokkur's pool is close to boiling, and the crust around the hot springs is protected. Everyone must follow the marked walkways, both for safety and to protect the crust.

The area's rules forbid throwing stones, coins or soap into the hot springs, cooking food in them, or writing or scratching on the rocks. Watch Strokkur from the path, hold a grown-up's hand, and wait for the next eruption.

The Geysir area has been a protected natural monument since 17 June 2020, covering 1.2 km² and managed by the Environment Agency of Iceland. Its rules forbid disturbing geosites (including any inscriptions), the hot spring crusts, soil and stones; changing water levels or flow; cooking in the hot springs, except the region's traditional bread baking; and throwing stones, coins, soap or anything else into them. For the protection of the sinter crust and for safety, all visitors must follow the marked walkways.

The area's 2022–2031 management plan says drones need the agency's permission and must not fly lower than 50 m over Strokkur, because they could be hit by the jet when the geyser erupts. The plan also describes laying the area's main path where the present path to Strokkur runs, and adding viewing platforms.