Sideromelane is a vitreous basaltic volcanic glass, usually occurring in palagonite tuff, for which it is characteristic. It is a less common form of tachylite, with which it usually occurs together; however it lacks the iron oxide crystals dispersed in the glass, and therefore appearing transparent and pure, with yellow-brown color, instead of tachylite opaque black. It forms at higher temperatures and with more rapid chilling. Presence of sideromelane indicates higher temperature of the lava, and solidifying of the flow closer to the vent, probably by rapid quenching in a wet environment.

Sideromelane often forms during explosions of submarine volcanoes and subglacial volcanoes, and often occurs as fragments embedded in a palagonite matrix, forming hyaloclastite deposits. Sideromelane is a mafic rock.

Reticulite from Kilauea volcano in Hawaii
Sideromelane from Hawaii

Basalt (US: , UK: ) is a mafic extrusive igneous rock formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or a moon. More than 90% of all volcanic rock on Earth is basalt. Basalt lava has a low viscosity, due to its low silica content, resulting in rapid lava flows that can spread over great areas before cooling and solidification. Flood basalt describes the formation in a series of lava basalt flows.

Easter Island

Easter Island (Rapa Nui: Rapa Nui, Spanish: Isla de Pascua) is a Chilean island in the southeastern Pacific Ocean, at the southeasternmost point of the Polynesian Triangle in Oceania. Easter Island is most famous for its nearly 1,000 extant monumental statues, called moai, created by the early Rapa Nui people. In 1995, UNESCO named Easter Island a World Heritage Site, with much of the island protected within Rapa Nui National Park.

It is believed that Easter Island's Polynesian inhabitants arrived on Easter Island sometime near 1200 AD. They created a thriving and industrious culture, as evidenced by the island's numerous enormous stone moai and other artifacts. However, land clearing for cultivation and the introduction of the Polynesian rat led to gradual deforestation. By the time of European arrival in 1722, the island's population was estimated to be 2,000–3,000. European diseases, Peruvian slave raiding expeditions in the 1860s, and emigration to other islands, e.g. Tahiti, further depleted the population, reducing it to a low of 111 native inhabitants in 1877.Chile annexed Easter Island in 1888. In 1966, the Rapa Nui were granted Chilean citizenship. In 2007 the island gained the constitutional status of "special territory." Administratively, it belongs to the Valparaíso Region, constituting a single commune of the Province Isla de Pascua. The 2017 Chilean census registered 7,750 people on the island, of whom 3,512 (45%) considered themselves Rapa Nui.Easter Island is one of the most remote inhabited islands in the world. The nearest inhabited land (around 50 residents in 2013) is Pitcairn Island, 2,075 kilometres (1,289 mi) away; the nearest town with a population over 500 is Rikitea, on the island of Mangareva, 2,606 km (1,619 mi) away; the nearest continental point lies in central Chile, 3,512 kilometres (2,182 mi) away.

Easter Island is considered part of Insular Chile.

Horizon Guyot

Horizon Guyot is a presumably Cretaceous guyot (tablemount) in the Mid-Pacific Mountains, Pacific Ocean. It is an elongated ridge, over 300 kilometres (190 mi) long and 4.3 kilometres (2.7 mi) high, that stretches in a northeast-southwest direction and has two flat tops; it rises to a minimum depth of 1,443 metres (4,730 ft). The Mid-Pacific Mountains lie west of Hawaii and northeast of the Line Islands.

It was probably formed by a hotspot, but the evidence is conflicting. Volcanic activity occurred during the Turonian-Cenomanian eras 100.5–89.8 million years ago and another stage has been dated to have occurred 88–82 million years ago. Between these volcanic episodes, carbonate deposition from lagoonal and reefal environments set in and formed limestone. Volcanic islands developed on Horizon Guyot as well and were colonised by plants.

Horizon Guyot became a seamount during the Coniacian-Campanian period. Since then, pelagic ooze has accumulated on the seamount, forming a thick layer that is further modified by ocean currents and by various organisms that live on the seamount; sediments also underwent landsliding. Ferromanganese crusts were deposited on exposed rocks.


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.

MIT Guyot

MIT Guyot is a guyot in the Pacific Ocean that rises to a depth of 1,323 metres (4,341 ft). It has a 20-kilometre-long (12 mi) summit platform and formed during the Cretaceous in the region of present-day French Polynesia through volcanic eruptions.

The volcano was eventually covered by a carbonate platform resembling that of a present-day atoll which was colonized by a number of animals. A major volcanic episode disrupted this platform, which subsequently redeveloped until it drowned in the late Albian.


Mafic is an adjective describing a silicate mineral or igneous rock that is rich in magnesium and iron, and is thus a portmanteau of magnesium and ferric. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include basalt, diabase and gabbro. Mafic rocks often also contain calcium-rich varieties of plagioclase feldspar.

Chemically, mafic rocks are enriched in iron, magnesium and calcium and typically dark in color. In contrast the felsic rocks are typically light in color and enriched in aluminium and silicon along with potassium and sodium. The mafic rocks also typically have a higher density than felsic rocks. The term roughly corresponds to the older basic rock class.

Mafic lava, before cooling, has a low viscosity, in comparison with felsic lava, due to the lower silica content in mafic magma. Water and other volatiles can more easily and gradually escape from mafic lava. As a result, eruptions of volcanoes made of mafic lavas are less explosively violent than felsic-lava eruptions. Most mafic-lava volcanoes are shield volcanoes, like those in Hawaii.


Microlites are minute crystals in an amorphous matrix. In igneous petrology, the term microlitic is used to describe vitric (glassy, non-crystalline, amorphous) matrix containing microscopic crystals. Microlitic rocks are a type of hypocrystalline rocks. Unlike ordinary phenocrysts, which can be seen with little or no magnification, microlites are generally formed in rapidly cooled (quenched) basaltic lava, where cooling rates are too high to permit formation of larger crystals.

Microlites are sometimes referred to as “small quench crystals”. They form more easily in basaltic lava eruptions, which have relatively low viscosity. Low viscosity permits rapid nucleation and ion migration, necessary for crystal formation. The high silica content of rhyolitic lavas gives them much higher viscosities. Such lavas tend to form glass (obsidian) when they cool rapidly from a fully melted liquid state; though many obsidians also contain microlites. Low viscosity mafic magmas must be quenched very rapidly from a high temperature to form glass that is free of any crystalline content.Microlites have been found in volcanic ash collected from Hawaiian lava fountains, where rapid cooling favors their formation. Sideromelane is a light brown basaltic glass, also formed in these eruptions, with and without microlites.


A mineraloid is a naturally-occurring mineral-like substance that does not demonstrate crystallinity. Mineraloids possess chemical compositions that vary beyond the generally accepted ranges for specific minerals. For example, obsidian is an amorphous glass and not a crystal. Jet is derived from decaying wood under extreme pressure. Opal is another mineraloid because of its non-crystalline nature. Pearl, considered by some to be a mineral because of the presence of calcium carbonate crystals within its structure, would be better considered a mineraloid because the crystals are bonded by an organic material, and there is no definite proportion of the components.

Pahvant Butte

Pahvant Butte (also Pavant Butte) is a butte formed by a dormant volcano in the west-central portion of Utah, United States.


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.

Sand Mountain Volcanic Field

The Sand Mountain Volcanic Field (also known as the Sand Mountain Field) is a volcanic field in the upper McKenzie River watershed, located in the United States in Oregon. Part of the Cascade Volcanic Arc, it lies southwest of Mount Jefferson and northwest of Belknap Crater and Mount Washington. Its highest elevation is 5,463 feet (1,665 m).

Active during the Holocene epoch, the Sand Mountain Field began erupting about 4,000 years ago. The field includes 23 basaltic and basaltic andesitic cinder cones and lava flows, for a total of 42 volcanic vents within two roughly north–south trending alignments. Its total dense-rock equivalent eruptive output is 0.22 to 0.29 cubic miles (0.92 to 1.21 km3), with subfeatures including a tephra field and a lava tube system. Future activity is possible, but according to the Volcano Hazards Program of the United States Geological Survey, the threat from the field itself is low.

The field lies within the Willamette National Forest near Santiam Pass. It supports some forested areas, which grow well on lava flows with tephra that serves as suitable soil for stands of Douglas fir and western hemlock and less commonly western red cedar. The surroundings represent an area of geological interest, and scoria has been quarried from one of the field's volcanic cones for highway supplies. Nearby tourist destinations include a ski resort, Tamolitch Falls, and Clear Lake.

Ubehebe Craters

Ubehebe Craters is a volcanic field in California. In northern Death Valley, it consists of 14-16 craters in a 3-square-kilometre (1.2 sq mi) area. The largest of these craters is the 800 metres (2,600 ft) wide and 235 metres (771 ft) deep Ubehebe Crater, but many of these craters are partially buried and thus poorly recognizable. Additional volcanic features present include a remnant of a scoria cone as well as a tuff cone.

The Ubehebe Craters are associated with a fault system that runs across them. The region has been affected by volcanism for the last 10 million years. The volcanic field is part of the Death Valley National Park and is accessible to tourists. The fault system is within the tectonically active Basin and Range Province physiographic region

Various estimates have been put forward for the age of the craters. Recent research has shown that the Ubehebe Craters all formed in a single phreatomagmatic eruption episode about 2,100 years ago, making it one of the most recent volcanic events in southern California. The risk of renewed volcanic activity, however, is low.

Volcanic ash

Volcanic ash consists of fragments of rock. minerals and volcanic glass, created during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to refer to all explosive eruption products (correctly referred to as tephra), including particles larger than 2 mm. Volcanic ash is formed during explosive volcanic eruptions when dissolved gases in magma expand and escape violently into the atmosphere. The force of the gasses shatters the magma and propels it into the atmosphere where it solidifies into fragments of volcanic rock and glass. Ash is also produced when magma comes into contact with water during phreatomagmatic eruptions, causing the water to explosively flash to steam leading to shattering of magma. Once in the air, ash is transported by wind up to thousands of kilometres away.

Due to its wide dispersal, ash can have a number of impacts on society, including human and animal health, disruption to aviation, disruption to critical infrastructure (e.g., electric power supply systems, telecommunications, water and waste-water networks, transportation), primary industries (e.g., agriculture), buildings and structures.

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


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