Silfra () is a

rift In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-grabe ...

formed in the

Mid-Atlantic Ridge The Mid-Atlantic Ridge is a mid-ocean ridge (a divergent or constructive plate boundary) located along the floor of the Atlantic Ocean, and part of the longest mountain range in the world. In the North Atlantic, the ridge separates the North A ...

– the divergent tectonic boundary between the

North American North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Ca ...

and

Eurasian Eurasia (, ) is the largest continental area on Earth, comprising all of Europe and Asia. Primarily in the Northern and Eastern Hemispheres, it spans from the British Isles and the Iberian Peninsula in the west to the Japanese archipelago a ...

plates – and is located in the

Þingvallavatn Þingvallavatn (), anglicised as Thingvallavatn,The spelling ''Pingvallavatn'' is wrong as the letter “p” should never be used to represent the letter “þ” (thorn). is a rift valley lake in southwestern Iceland. With a surface of 84 km² ...

Lake in the

Þingvellir Þingvellir (, anglicised as ThingvellirThe spelling ''Pingvellir'' is sometimes seen, although the letter "p" is unrelated to the letter thorn (letter), "þ" (thorn), which is pronounced as "th".) was the site of the Althing, Alþing, the annual ...

National Park in

Iceland Iceland ( is, Ísland; ) is a Nordic island country in the North Atlantic Ocean and in the Arctic Ocean. Iceland is the most sparsely populated country in Europe. Iceland's capital and largest city is Reykjavík, which (along with its ...

Formation

Silfra lies in the

valley and within the Þingvellir National Park. The valley, and Silfra itself, were formed by the divergent tectonic drift of the

and

plates. The plates drift about farther apart every year, building up tension between the plates and the earth mass above. This tension is relieved through periodic major

earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...

s at approximately ten-year intervals, which have caused cracks and

fissure A fissure is a long, narrow crack opening along the surface of Earth. The term is derived from the Latin word , which means 'cleft' or 'crack'. Fissures emerge in Earth's crust, on ice sheets and glaciers, and on volcanoes. Ground fissure A ...

s to form in Þingvellir valley; Silfra lies at the rim of the

Lake and is one of the largest and deepest of these fissures. The Silfra fissure intercepts a major aquifer, which feeds multiple springs at its base.Water in Silfra
Silfra.org – website

Boulder In geology, a boulder (or rarely bowlder) is a rock fragment with size greater than in diameter. Smaller pieces are called cobbles and pebbles. While a boulder may be small enough to move or roll manually, others are extremely massive. In ...

s and rocks falling into the widening cracks have formed caves within the fissures.Geology of Silfra
Silfra.org – website

Hydrology

Silfra is spring fed by

groundwater Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidat ...

originating as

meltwater Meltwater is water released by the melting of snow or ice, including glacial ice, tabular icebergs and ice shelves over oceans. Meltwater is often found in the ablation zone of glaciers, where the rate of snow cover is reducing. Meltwater ca ...

from

Langjökull Langjökull (, Icelandic for "long glacier") is the second largest ice cap in Iceland (953 km2), after Vatnajökull. It is situated in the west of the Icelandic interior or Highlands of Iceland and can be seen clearly from Haukadalur. Its ...

, Iceland's second largest

glacier A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires distinguishing features, such a ...

, about fifty kilometres north of the

Lake. In the distant past, this Langjökull meltwater ran through a river directly and unimpeded into the Þingvallavatn Lake. This river was blocked a few thousand years ago by lava flows from the Skjaldbreiður

volcano A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates ...

causing the meltwater to pond and seep underground into the

porous Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...

lava rock to form an

aquifer An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials ( gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characteri ...

. This water then percolates through the aquifer for thirty to a hundred years before emerging from the fissure springs in the Þingvallavatn Lake fifty kilometres to the south. The emerging, highly filtered groundwater is exceptionally clear and

potable Drinking water is water that is used in drink or food preparation; potable water is water that is safe to be used as drinking water. The amount of drinking water required to maintain good health varies, and depends on physical activity level, a ...

Scuba diving

Scuba diving Scuba diving is a mode of underwater diving whereby divers use breathing equipment that is completely independent of a surface air supply. The name "scuba", an acronym for "Self-Contained Underwater Breathing Apparatus", was coined by Chri ...

and

snorkeling Snorkeling ( British and Commonwealth English spelling: snorkelling) is the practice of swimming on or through a body of water while equipped with a diving mask, a shaped breathing tube called a snorkel, and usually swimfins. In cooler waters ...

in Silfra is popular because of its clear water and location within the continental rift. There are three main dive sites: Silfra Hall, Silfra Cathedral and Silfra Lagoon. The Cathedral is a long fissure with visibility almost from end to end. Wednesday, 23 October 2019 Shallow at the entry points and at the ends of the fissure, Silfra descends to a maximum depth of but diving to this depth is seldom done as it requires

technical diving Technical diving (also referred to as tec diving or tech diving) is scuba diving that exceeds the agency-specified limits of recreational diving for non-professional purposes. Technical diving may expose the diver to hazards beyond those normally ...

skills. The water temperature is between but can be comfortably dived using a

dry suit A dry suit or drysuit provides the wearer with environmental protection by way of thermal insulation and exclusion of water, and is worn by divers, boaters, water sports enthusiasts, and others who work or play in or near cold or contaminate ...

References

{{Recreational dive sites, fresit Geology of Iceland Southwest Iceland Rifts and grabens Underwater diving sites in Iceland

Formation

Hydrology

Scuba diving

See also

References