Hyaloclastite is a volcaniclastic accumulation or breccia consisting of glass (from the Greek hyalus) fragments (clasts) formed by quench fragmentation of lava flow surfaces during submarine or subglacial extrusion. It occurs as thin margins on the lava flow surfaces and between pillow lavas as well as in thicker deposits, more commonly associated with explosive, volatile-rich eruptions as well as steeper topography. Hyaloclastites form during volcanic eruptions under water, under ice or where subaerial flows reach the sea or other bodies of water. It commonly has the appearance of angular flat fragments sized between a millimeter to few centimeters. The fragmentation occurs by the force of the volcanic explosion, or by thermal shock and spallation during rapid cooling.

Several minerals are found in hyaloclastite masses. Sideromelane is a basalt glass rapidly quenched in water. It is transparent and pure, lacking the iron oxide crystals dispersed in the more commonly occurring tachylite. Fragments of these glasses are usually surrounded by a yellow waxy layer of palagonite, formed by reaction of sideromelane with water.

Hyaloclastite ridges, formed by subglacial eruptions during the last glacial period, are a prominent landscape feature of Iceland and the Canadian province of British Columbia. Hyaloclastite is usually found at subglacial volcanoes, such as tuyas, which is a type of distinctive, flat-topped, steep-sided volcano formed when lava erupts through a thick glacier or ice sheet.

In lava deltas, hyaloclastites form the main constituent of foresets formed ahead of the expanding delta. The foresets fill in the seabed topography, eventually building up to sea level, allowing the subaerial flow to move forwards until it reaches the sea again.[1]

Hyaloclastite between pillows of lava in Montana
Lava enters pacific
Pahoehoe lava enters the Pacific at Hawaii Volcanoes National Park, the Big Island of Hawaii


  1. ^ Naylor, P.H., Bell, B.R., Jolley, D.W., Purnall, P. & Fredsted, R. 1999. Palaeogene magmatism in the Faeroe-Shetland Basin: influences on uplift history and sedimentation. In: Fleet, A.J. & Boldy, S.A.R. (eds) Petroleum Geology of Northwest Europe: Proceedings of the 5th Conference, 545-558. Geological Society, London.
Ash Mountain (British Columbia)

Ash Mountain is the highest summit in the Tuya Range of the Stikine Ranges in northcentral British Columbia, Canada, located immediately north of High Tuya Lake at the north end of Tuya Mountains Provincial Park. It is one of the six tuyas clustered close to Tuya Lake. The base of the volcano comprises pillow lava and hyaloclastite indicating that the volcano formed beneath ice or under a large lake. The volcano comprises loose debris as well as dikes of basaltic rock that intruded into the volcanic pile. Other tuyas in the area include Tuya Butte, South Tuya and Mathews Tuya, although most of the group of tuyas are unnamed.

Brown Bluff

Brown Bluff is a basalt tuya located on the Tabarin Peninsula of northern Antarctica. It formed in the past 1 million years, which erupted subglacially within an englacial lake. The volcano's original diameter is thought to have been about 12-15 kilometers, and probably formed by a single vent. Brown Bluff is subdivided into four stages: pillow volcano, tuff cone, slope failure, and hyaloclastite delta and into five structural units.The volcano is named "Brown Bluff" because of its steep slopes and its brown-to-black hyaloclastite.

Caribou Tuya

Caribou Tuya is a basaltic subglacial mound in far northwestern British Columbia that began eruptive activity under glacial ice during the Fraser glaciation (25 to 10 ka). Like Ash Mountain and South Tuya, sections of the subglacial mound reveal a consistent stratigraphic progression from pillow lavas to hyaloclastite deposits from the base upward. Locally the sections are capped by subaerial basaltic lava flows. Samples of the glassy pillow basalts and hyaloclastites along with crystalline basalt flows were collected at Caribou Tuya. The volcano is believed to have formed and last erupted during the Pleistocene Epoch.


Dallasite is a breccia made of subequant to rectangular or distinctly elongate, curvilinear shards that represent the spalled rims of pillow basalt (see: Hyaloclastite). This material is commonly partly altered to chlorite, epidote, quartz and carbonate, for which the local term 'dallasite' has been coined. The stone dallasite is named after Dallas Road, Victoria, British Columbia. It is considered the unofficial stone of British Columbia's capital city. Dallasite is found in Triassic volcanic rocks of Vancouver Island and is considered the third most important gem material in British Columbia.


Eiríksjökull (Icelandic for "Eirík's glacier") is a glacier north-west of Langjökull in Iceland, with an area of 22 km2 (8.5 sq mi) reaching a height of 1,675 m (5,495 ft), making it the largest table mountain in Iceland. Rising over 1,000 m (3,300 ft) above its surrounding, the lowest 350 m (1,150 ft) of a hyaloclastite (móberg) tuya formed presumably by a single subglacial volcanic activity is capped by a 750 m (2,460 ft) thick lava shield. It is currently dormant or extinct in terms of volcanic activity.

Hyaloclastite Dam

Hyaloclastite Dam was a 1,201 ft (366 m) high lava dam that occupied the Grand Canyon of the U.S. state of Arizona. It formed during the Pleistocene epoch when basaltic lava flows from the Uinkaret volcanic field blocked the Colorado River. The dam failed catastrophically 160,000 years ago to produce the largest known outburst flood on the Colorado River in the Grand Canyon. A slump block of basalt is the remaining remnant of the Hyaloclastite Dam.


Lomilik is a seamount in the Western Pacific Ocean, within the exclusive economic zone of the Marshall Islands. It lies to the west of Anewetak atoll and is named after the best fishing site of Anewetak atoll.Lomilik has a 40-by-15-kilometre-wide (24.9 mi × 9.3 mi) summit terrace with the proper summit at circa 1,500 metres (4,900 ft) depth; a scarp separates the two and small hills reach depths of 1,350 metres (4,430 ft). The summit terrace is covered by rocks with ooze in between. A notch in the southern flank of Lomilik was probably created by a landslide. It is part of the Magellan Seamounts and consists of a Cretaceous volcano with a thin layer of carbonate rocks and ferromanganese. Lami seamount lies northwest of Lomilik.The rocks found on Lomilik consist of basalt and limestone. Fluorapatite, hyaloclastite, mudstone, phosphorite and siltstone have been identified in rocks from the seamount. Manganese nodules have been found on Lomilik and the manganese crusts on the seamount reach thicknesses of over 10 centimetres (3.9 in); the thickest crust recovered from an ocean is a 18 centimetres (7.1 in) thick ferromanganese crust from Lomilik recovered in 1989. The deposits on Lomilik could potentially be mined.

Mount Haddington

Mount Haddington is a massive 1,630 m (5,350 ft) high shield volcano comprising much of James Ross Island in Graham Land, Antarctica. It is 60 km (37 mi) wide and has had numerous subglacial eruptions throughout its history, forming many tuyas. Some of its single eruptions were bigger in volume than a whole normal-sized volcano. Old eruption shorelines are widespread on the volcano's deeply eroded flanks.

Haddington formed along the Larsen Rift dominantly during the Miocene and Pliocene epochs but more recent eruptions have produced tuff cones on its slopes. The youngest tuff cones and pyroclastic cones on the eastern slope are situated below the summit icecap and may have formed in the last few thousand years. Effusive eruptions have created large deltas composed of hyaloclastite breccia and lava flows.Mount Haddington was discovered on December 31, 1842 by the Ross expedition, a voyage of scientific exploration of the Antarctic from 1839 to 1843 led by James Clark Ross. Ross named the mountain after the Earl of Haddington, then First Lord of the Admiralty.

Nazko Cone

Nazko Cone is a small potentially active basaltic cinder cone in central British Columbia, Canada, located 75 km west of Quesnel and 150 kilometers southwest of Prince George. It is considered the easternmost volcano in the Anahim Volcanic Belt. The small tree-covered cone rises 120 m above the Chilcotin-Nechako Plateau and rests on glacial till. It was formed in three episodes of activity, the first of which took place during the Pleistocene interglacial stage about 340,000 years ago. The second stage produced a large hyaloclastite scoria mound erupted beneath the Cordilleran Ice Sheet during the Pleistocene. Its last eruption produced two small lava flows that traveled 1 km to the west, along with a blanket of volcanic ash that extends several km to the north and east of the cone.

Ne Ch'e Ddhawa

Ne Ch'e Ddhawa (also known as Wootten's Cone) is a volcano located 7 km (4.3 mi) upstream from Fort Selkirk in the Fort Selkirk Volcanic Field of central Yukon, Canada. It has been described as a cinder cone or a subglacial mound. The volcano erupted subglacially during the late Pleistocene, erupting hyaloclastite tuffs, breccias, and pillow breccias.

New Senator Caldera

The New Senator Caldera is a large Archean caldera complex within the heart of the Blake River Megacaldera Complex, Quebec, Canada. It has a diameter of 15-30 kilometers and is made of thick massive mafic sequences. The caldera complex has inferred to be a subaqueous lava lake during the early stages of the caldera's development. Gabbro sills represent lava lakes, which are common in mafic summit calderas. These subaqueous lava lakes are large units with a change in grain size from coarse to fine grained and a hyaloclastite top. The Kiwanis (Norands) intrusion, a high-level synvolcanic magma chamber, intrudes felsic rocks, and is in turn cross-cut by basaltic dikes and sills.

The Glenwood fault forms the eastern margin of the New Senator Caldera.


Palagonite is an alteration product from the interaction of water with volcanic glass of chemical composition similar to basalt. Palagonite can also result from the interaction between water and basalt melt. The water flashes to steam on contact with the hot lava and the small fragments of lava react with the steam to form the light colored palagonite tuff cones common in areas of basaltic eruptions in contact with water. An example is found in the pyroclastic cones of the Galapagos Islands. Charles Darwin recognized the origin of these cones during his visit to the islands. Palagonite can also be formed by a slower weathering of lava into palagonite, resulting in a thin, yellow-orange rind on the surface of the rock. The process of conversion of lava to palagonite is called palagonitization.

Palagonite soil is a light yellow-orange dust, comprising a mixture of particles ranging down to sub-micrometer sizes, usually found mixed with larger fragments of lava. The color is indicative of the presence of iron in the +3 oxidation state, embedded in an amorphous matrix.

Palagonite tuff is a tuff composed of sideromelane fragments and coarser pieces of basaltic rock, embedded in a palagonite matrix. A composite of sideromelane aggregate in palagonite matrix is called hyaloclastite.

Phreatomagmatic eruption

Phreatomagmatic eruptions are volcanic eruptions resulting from interaction between magma and water. They differ from exclusively magmatic eruptions and phreatic eruptions. Unlike phreatic eruptions, the products of phreatomagmatic eruptions contain juvenile (magmatic) clasts. It is common for a large explosive eruption to have magmatic and phreatomagmatic components.

South Tuya

South Tuya, also called Southern Tuya, is a tuya clustered around Tuya Lake in the Northern Cordilleran Volcanic Province in British Columbia, Canada. The base of South Tuya comprises hyaloclastite and pillow lava indicating that the volcano formed beneath a large lake or beneath ice.

Sturgeon Lake Caldera

Sturgeon Lake Caldera is a large extinct caldera complex in Kenora District of Northwestern Ontario, Canada. It is one of the world's best preserved mineralized Neoarchean caldera complexes, containing well-preserved mafic-intermediate pillow lavas, pillow breccias, hyaloclastite and peperites, submarine lava domes and dome-associated breccia deposits. The complex is some 2.7 billion years old with a minimum strike length of 30 km (19 mi).


Stóra-Björnsfell is an elongated medium-sized tuya located in Iceland. It contains pillow lava, hyaloclastite and sheet lava. The volcano formed when a subglacial eruption occurred beneath an ice sheet during the last ice age. Stóra-Björnsfell's pillow lava appears originally to have erupted from a fissure.

Subglacial mound

A subglacial mound (SUGM) is a type of subglacial volcano. This type of volcano forms when lava erupts beneath a thick glacier or ice sheet. The magma forming these volcanoes was not hot enough to melt a vertical pipe right through the overlying glacial ice, instead forming hyaloclastite and pillow lava deep beneath the glacial ice field. Once the glaciers had retreated, the subglacial volcano would be revealed, with a unique shape as a result of their confinement within glacial ice. They are somewhat rare worldwide, being confined to regions which were formerly covered by continental ice sheets and also had active volcanism during the same period. They are found throughout Iceland, Antarctica and the Canadian province of British Columbia.

Subglacial mounds can be mistaken for cinder cones because they may have a similar shape. An example of this confusion is Pyramid Mountain in the Wells Gray-Clearwater volcanic field of east-central British Columbia, Canada.

Subglacial volcano

A subglacial volcano, also known as a glaciovolcano, is a volcanic form produced by subglacial eruptions or eruptions beneath the surface of a glacier or ice sheet which is then melted into a lake by the rising lava. Today they are most common in Iceland and Antarctica; older formations of this type are found also in British Columbia and Yukon Territory, Canada.

During the eruption, the heat of the lava from the subglacial volcano melts the overlying ice. The water quickly cools the lava, resulting in pillow lava shapes similar to those of underwater volcanoes. When the pillow lavas break off and roll down the volcano slopes, pillow breccia, tuff breccia, and hyaloclastite form. The meltwater may be released from below the ice as happened in Iceland in 1996 when the Grímsvötn caldera erupted, melting 3 km3 of ice and giving rise to a large glacial lake outburst flood.

The shape of subglacial volcanoes tends to be quite characteristic and unusual, with a flattened top and steep sides supported against collapse by the pressure of the surrounding ice and meltwater. If the volcano eventually melts completely through the ice layer, then horizontal lava flows are deposited, and the top of the volcano assumes a nearly level form. However, if significant amounts of lava are later erupted subaerially, then the volcano may assume a more conventional shape. In Canada the volcanos have been known to form both conical and nearly level shapes. The more distinctly flat-topped, steep-sided subglacial volcanoes are called tuyas, named after Tuya Butte in northern British Columbia by Canadian geologist Bill Mathews in 1947. In Iceland, such volcanoes are also known as table mountains.

Volcanic glass

Volcanic glass is the amorphous (uncrystallized) product of rapidly cooling magma. Like all types of glass, it is a state of matter intermediate between the close-packed, highly ordered array of a crystal and the highly disordered array of gas. Volcanic glass can refer to the interstitial, or matrix, material in an aphanitic (fine grained) volcanic rock or can refer to any of several types of vitreous igneous rocks. Most commonly, it refers to obsidian, a rhyolitic glass with high silica (SiO2) content.

Other types of volcanic glass include:

Pumice, which is considered a glass because it has no crystal structure.

Apache tears, a kind of nodular obsidian.

Tachylite (also spelled tachylyte), a basaltic glass with relatively low silica content.

Sideromelane, a less common form tachylyte.

Palagonite, a basaltic glass with relatively low silica content.

Hyaloclastite, a hydrated tuff-like breccia of sideromelane and palagonite.

Pele's hair, threads or fibers of volcanic glass, usually basaltic.

Pele's tears, tear-like drops of volcanic glass, usually basaltic.

Limu o Pele (Pele's seaweed), thin sheets and flakes of brownish-green to near-clear volcanic glass, usually basaltic.

Types of basalts
Basalts by tectonic setting
Basalts by form and flow
Basalts by chemistry
Important minerals


This page is based on a Wikipedia article written by authors (here).
Text is available under the CC BY-SA 3.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.