A volcanic dam is a type of natural dam produced directly or indirectly by volcanism

Volcanism

Volcanism is the phenomenon connected with volcanoes and volcanic activity. It includes all phenomena resulting from and causing magma within the crust or mantle of a planet to rise through the crust and form volcanic rocks on the surface....

, which holds or temporarily restricts the flow of surface water in existing streams, like a man-made dam

Dam

A dam is a barrier that impounds water or underground streams. Dams generally serve the primary purpose of retaining water, while other structures such as floodgates or levees are used to manage or prevent water flow into specific land regions. Hydropower and pumped-storage hydroelectricity are...

. There are two main types of volcanic dams, those created by the flow of molten lava

Lava

Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...

, and those created by the primary or secondary deposition of pyroclastic material

Pyroclastic rock

Pyroclastic rocks or pyroclastics are clastic rocks composed solely or primarily of volcanic materials. Where the volcanic material has been transported and reworked through mechanical action, such as by wind or water, these rocks are termed volcaniclastic...

 and debris. This classification generally excludes other, often larger and longer lived dam-type geologic features, separately termed crater lake

Crater lake

A crater lake is a lake that forms in a volcanic crater or caldera, such as a maar; less commonly and with lower association to the term a lake may form in an impact crater caused by a meteorite. Sometimes lakes which form inside calderas are called caldera lakes, but often this distinction is not...

s, although these volcanic centers may be associated with the source of material for volcanic dams, and the lowest portion of its confining rim may be considered as such a dam, especially if the lake level within the crater is relatively high.

Volcanic dams generally occur world-wide, in association with former and active volcanic provinces, and are known to have existed in the geologic record, in historic times and occur in the present day. Their removal or failure is similarly recorded. The longevity, and extent varies widely, having periods ranging from a few days, weeks or years to several hundred thousand years or more, and dimensions ranging from a few meters to hundreds, to several thousand.

The emplacement, internal structure, distribution and longevity of such dams can be related variously to the amount, rapidity and duration of (primary

Primary energy

Primary energy is an energy form found in nature that has not been subjected to any conversion or transformation process. It is energy contained in raw fuels, and other forms of energy received as input to a system...

) geothermal energy released, and the rock material made available; other considerations include the rock types produced, their physical and toughness characteristics, and their various modes of deposition. Depositional modes include gravity flow of molten lava at the surface, gravity flow or fall of pyroclastics through the air, as well as the redistribution and transportation of those materials by gravity and water.

Lava dam

Lava dams are formed by lava

Lava

Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...

 flowing or spilling into a river valley in sufficient quantity and height to temporarily overcome the explosive nature (steam) of its contact with water, and the erosive force of flowing water to remove it. The latter depends on the quantity of water flow and stream gradient. The lava may flow during numerous successive or repetitive eruptions and may emanate from single or numerous vents or fissures. Lava of this nature, like basalt

Basalt

Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...

 usually is associated with less explosive eruptions; more viscous lavas with lower mafic

Mafic

Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...

 content, like dacite

Dacite

Dacite is an igneous, volcanic rock. It has an aphanitic to porphyritic texture and is intermediate in composition between andesite and rhyolite. The relative proportions of feldspars and quartz in dacite, and in many other volcanic rocks, are illustrated in the QAPF diagram...

s and rhyolite

Rhyolite

This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.Rhyolite is an igneous, volcanic rock, of felsic composition . It may have any texture from glassy to aphanitic to porphyritic...

s can also flow, but tend to be more closely associated with eruptions of greater explosiveness and the formation of pyroclastics.

Once initially established, continued lave flow creates a steeper upstream face as it battles the rising water, but with most lava flowing unimpeded downstream covering the now-dried river bed and its alluvial

Alluvium

Alluvium is loose, unconsolidated soil or sediments, eroded, deposited, and reshaped by water in some form in a non-marine setting. Alluvium is typically made up of a variety of materials, including fine particles of silt and clay and larger particles of sand and gravel...

 sediments, sometimes for miles. Thus emplaced, the shape of a lava dam resembles an elongated blob, wedged in the valley bottom. At the same time, the water continues to flow, the lake continues to rise and accumulate sediment, which previously had migrated unimpeded downstream. Sediment filling, over-topping, downward erosion, waterfalls and under-cutting inevitably follow, unless an alternative outlet is established, for water and sediment elsewhere in the drainage.

Large examples of lava dams from the geologic record include those repeatedly developed from the western side of the Grand Canyon, with the largest remnant now termed Prospect Dam

Prospect Dam (Arizona)

Prospect Dam is the term used to describe a former lava dam in the Grand Canyon of Arizona, United States. It was the oldest and tallest of several former lava dams in the Grand Canyon. It was formed about 1.2 million years ago as lava flowed down the walls of the Grand Canyon, and temporarily...

, and in several locations within the Snake River

Snake River

The Snake is a major river of the greater Pacific Northwest in the United States. At long, it is the largest tributary of the Columbia River, the largest North American river that empties into the Pacific Ocean...

 drainage. The former 'Lake Idaho', which existed for more than 6.5 million years, filled the western portion of the behind such a structure and created the western section of the Snake River Plain

Snake River Plain

The Snake River Plain is a geologic feature located primarily within the state of Idaho in the United States of America. It stretches about westward from northwest of the state of Wyoming to the Idaho-Oregon border. The plain is a wide flat bow-shaped depression, and covers about a quarter of Idaho...

, and accumulated 4000 ft of lake sediments. Other locations include near American Falls, Idaho

American Falls, Idaho

American Falls is a city in and the county seat of Power County, Idaho, United States. The population was 4,111 at the 2000 census. It is part of the Pocatello, Idaho Metropolitan Statistical Area.-History:...

, and numerous others. Many of these were overtopped, washed out or skirted by the outburst flood

Glacial lake outburst flood

A glacial lake outburst flood is a type of outburst flood that occurs when the dam containing a glacial lake fails. The dam can consist of glacier ice or a terminal moraine...

 originating from ancestral Lake Bonneville

Lake Bonneville

Lake Bonneville was a prehistoric pluvial lake that covered much of North America's Great Basin region. Most of the territory it covered was in present-day Utah, though parts of the lake extended into present-day Idaho and Nevada. Formed about 32,000 years ago, it existed until about 14,500 years...

.

Many other examples exist globally including, Caburgua Lake

Caburgua Lake

Caburgua Lake is located 23 km northeast of the city of Pucón, in the Araucanía Region of Chile. Huerquehue National Park lies to the east of the lake. Like Villarrica Lake, it is part of Toltén River basin...

 in Chile, Mývatn

Mývatn

Mývatn is a shallow eutrophic lake situated in an area of active volcanism in the north of Iceland, not far from Krafla volcano. The lake and its surrounding wetlands have an exceptionally rich fauna of waterbirds, especially ducks...

 in Iceland and Lake Reporoa

Reporoa caldera

The Reporoa caldera is a 10 km by 15 km caldera in New Zealand's Taupo Volcanic Zone. It formed some 230,000 years ago, in a large eruption that deposited the approximately 100 km3 Kaingaroa Ignimbrites. It contains three rhyolitic lava domes and the active Reporoa geothermal field...

 in New Zealand. Examples in western Canada and others in northwestern United States include, Lava Lake

Lava Lake

Lava Lake is a lava dammed lake located in northwestern British Columbia, Canada. The lake lies within the Nisga'a Memorial Lava Beds Provincial Park, which includes fishing, hiking and other features.-Formation:...

 and The Barrier

The Barrier

The Barrier is a lava dam retaining the Garibaldi Lake system in southwestern British Columbia, Canada. It is over 300 m in thickness and about 2 km wide where it impounds the lake....

, which still impounds Garibaldi Lake

Garibaldi Lake

The Garibaldi Lake volcanic field is a volcanic field, located in British Columbia, Canada. It was formed by a group of nine small andesitic stratovolcanoes and basaltic andesite vents in the scenic Garibaldi Lake area immediately north of Mount Garibaldi was formed during the late Pleistocene and...

, and Lava Butte

Lava Butte

Lava Butte is a cinder cone located in central Oregon, USA, just west of US Highway 97 between the towns of Bend, Oregon, and Sunriver, Oregon. It is part of a system of small cinder cones on the northwest flank of Newberry Volcano, a massive shield volcano which rises to the southeast...

.

Pyroclastic dam

Pyroclastic dams are created in an existing drainage either by their direct emplacement or by the accumulation of widely variable pyroclastic particles, broadly termed tephra

Tephra

200px|thumb|right|Tephra horizons in south-central [[Iceland]]. The thick and light coloured layer at center of the photo is [[rhyolitic]] tephra from [[Hekla]]....

. Unlike lava dams, which are formed by coherent, molten liquid gravity surface flow, filling the valley bottom directly and solidifying rapidly from the outside inward, pyroclastic dams are produced by less coherent air-born gravity currents or falls of tephra particles from the atmosphere, which solidify on the surface more slowly from the inner portion outward; pyroclastics are also deposited both in the valley bottom and widely distributed on the adjacent slopes. Their air-born nature is less restricted to the immediate drainage and they may roil over drainage boundaries; their particulate components allow for continued redistribution after initial placement by gravity and water. The explosiveness of pyroclastic eruptions, both laterally and vertically range from fiery surges

Pyroclastic surge

A pyroclastic surge is a fluidized mass of turbulent gas and rock fragments which is ejected during some volcanic eruptions. It is similar to a pyroclastic flow but it has a lower density or contains a much higher proportion of gas to rock ratio, which makes it more turbulent and allows it to rise...

, to hot flows, to warm falls

Pyroclastic fall

A pyroclastic fall is a uniform deposit of material which has been ejected from a volcanic eruption or plume such as an ash fall or tuff. Pyroclastic air fall deposits are a result of:...

 of tephra; the former may tend to emplace a dam directly while the latter tends to enhance placement or provide additional material. Unless violently expelled and generally speaking, larger sized tephra falls closest to the crater and smaller tephra landing farther away, with its distribution more highly influenced by prevailing wind velocity and direction.

Once initially established, a pyroclastic dam's continued longevity remains a balance between its slowly consolidating hardness and toughness, and the amount and velocity of flowing water's erosive capacity to remove it from its outset. Unconsolidated tephra is quickly moved by precipitation and flowing water in drainages, at times creating a lahar

Lahar

A lahar is a type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris, and water. The material flows down from a volcano, typically along a river valley. The term is a shortened version of "berlahar" which originated in the Javanese language of...

. Upstream of the dam this material would rapidly accumulate to fill the lake, and downstream it would tend to erode its slopes and base. The often rapid accumulation of unconsolidated pyroclastic material on steep sideslopes tends to be inherently unstable over time; pyroclastic dams may be emplaced by the landsliding of such material into rivers and streams. Pyroclastic material, given sufficient time to consolidate or 'weld' into hard rock, produce assemblages variously classified as ignimbrites, variously brecciated or agglomerated

Agglomerate

Agglomerates are coarse accumulations of large blocks of volcanic material that contain at least 75% bombs...

, along with various types of tuff

Tuff

Tuff is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered...

s and volcanic ash

Volcanic ash

Volcanic ash consists of small tephra, which are bits of pulverized rock and glass created by volcanic eruptions, less than in diameter. There are three mechanisms of volcanic ash formation: gas release under decompression causing magmatic eruptions; thermal contraction from chilling on contact...

, and are mostly of felsic

Felsic

The word "felsic" is a term used in geology to refer to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium....

 composition.

While evidence of pyroclastic dams occur within the geologic record, they are best known and studied in relation to recent and current volcanic eruptions. Examples world-wide include associations with El Chichon

El Chichón

El Chichón, also known as El Chichonal is an active volcano in Francisco León Municipality in northwestern Chiapas, Mexico. Its only recorded eruptive activity was on March 29, April 3 and April 4, 1982 , when it produced a one km-wide caldera that then filled with an acidic crater lake...

 in Mexico, and the Karymsky Volcano in Russia. The caldera lake

Taal Lake

Taal Lake is a freshwater lake in the province of Batangas, on the island of Luzon, Philippines. The lake is situated within a caldera formed by very large eruptions between 500,000 and 100,000 years ago. It is the country's third largest lake...

 associated with Taal Volcano

Taal Volcano

Taal Volcano is a complex volcano located on the island of Luzon in the Philippines. Historical eruptions are concentrated on Volcano Island, an island near the middle of Lake Taal. The lake partially fills Taal Caldera, which was formed by powerful prehistoric eruptions between 140,000 to 5,380 BP...

, which was previously open to the East China Sea was permanently closed by a pyroclastic dam during the 1749 eruption, and remains in equilibrium at a higher level to this day, while the pyroclastic dam comprising the low rim of crater Lake Nyos

Lake Nyos

Lake Nyos is a crater lake in the Northwest Region of Cameroon, located about northwest of Yaoundé. Nyos is a deep lake high on the flank of an inactive volcano in the Oku volcanic plain along the Cameroon line of volcanic activity...

 in Cameroon is considered less stable.

Hazards

The erosion or failure of volcanic dams by, like all forms of natural dams, can produce catastrophic

Disaster

A disaster is a natural or man-made hazard that has come to fruition, resulting in an event of substantial extent causing significant physical damage or destruction, loss of life, or drastic change to the environment...

 flood

Flood

A flood is an overflow of an expanse of water that submerges land. The EU Floods directive defines a flood as a temporary covering by water of land not normally covered by water...

s, debris flow

Debris flow

A debris flow is a fast moving, liquefied landslide of unconsolidated, saturated debris that looks like flowing concrete. It is differentiated from a mudflow in terms of the viscosity and textural properties of the flow. Flows can carry material ranging in size from clay to boulders, and may...

s and associated landslide

Landslide

A landslide or landslip is a geological phenomenon which includes a wide range of ground movement, such as rockfalls, deep failure of slopes and shallow debris flows, which can occur in offshore, coastal and onshore environments...

s, depending on the size of the impounded lake

Lake

A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...

.