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Accretionary lapilli in an ash deposit on Santorini.
Small spherical balls of volcanic ash
Accretionary lapilli are small spherical balls of volcanic ash that form from a wet nucleus falling through a volcanic ash cloud. They can flatten on hitting the ground or may roll on loose ash and grow like a snowball. -> See whole entry
Typical andesite from the Methana peninsula (Greece)
Andesite is a gray to black volcanic rock with between about 52 and 63 weight percent silica (SiO2). Andesites are typical for lava domes and stratovolcanoes.
Andesite is an igneous, volcanic rock, of intermediate composition, containing between about 52 and 63 weight % silica (SiO2).
Andesites contain crystals composed primarily of plagioclase feldspar and one or more of the minerals pyroxene (clinopyroxene and orthopyroxene) and lesser amounts of hornblende. At the lower end of the silica range, andesite lava may also contain olivine. Andesite magma ... -> See whole entry
Ash plume from an explosive eruption at Etna volcano (Italy)
Snow and ash from Hekla volcano's eruption in 2000
Volcanic ash is the term for all fine-grained volcanic products (smaller than 2 mm), normally magma or older rock fragmented during explosive eruptions.
Volcanic ash has nothing to do with fire, but is a mere definition of grain-size. Ash can range in size from sandy to extremely fine; any fragment ejected by a volcano less than 2 mm in diameter is called ash. It may consists of freshly ejected lava (usually turned into a glass shard because of rapid cooling), older fragmented rock, or small crystals.
The most common type of volcanic rock, with a relatively low silica content and typically erupted at shield volcanoes.
Basalt is the usually hard and black volcanic rock formed from (liquid) balsalitc lava. Balsaltic lava contains less than about 52 percent silica (SiO2) by weight. Because of its low silica content, it has a low viscosity (resistance to flow). Therefore, basaltic lava can quickly and easily flow more than 20 km from a vent. The low viscosity typically allows volcanic gases to escape without genera... -> See whole entry
Base surge deposits from the great Minoan eruption on Santorini (ca. 1613 BC)
Base surges are ground hugging, fast outward moving and turbulent, dilute clouds of gas and ash. They result from water magma interactions (violent steam explosions). During the 1965 eruption of Taal volcano (Philippines), such base surges were first observed. Some of them traveled 4 km and killed 189 people. Base surges were first identified during ocean nuclear weapons explosions in the Pacific.... -> See whole entry
Large ballistically ejected block on Santorini (Greece)
Volcanic blocks are solidified rock fragments greater than 64 mm in diameter. Blocks commonly are ejected during explosive eruptions and consist of older pieces of the volcano's edifice, e.g. parts of the conduit, lava domes or older lava flows.
During violent eruptions, blocks of up to several meter size can be thrown to several km distance. For example, during the Minoan eruption (ca. 1613 BC) of the Santorini volcano in Greece, meter-sized blocks were thrown to up to 7 km horizontal distance and impacted violently into the ground, some of them destroying houses of ancient settlements. The time some of these blocks spent on their trajec... -> See whole entry
Ejected fragments of fresh magma larger than 64 mm in diameter, often shaped aerodynamically during their flight.
Volcanic bombs are lava fragments larger than 64 mm in diameter that were ejected while still viscous and partially molten. Many bombs acquire rounded aerodynamic shapes during their travel through the air. Volcanic bombs include breadcrust bombs, ribbon bombs, spindle bombs (with twisted ends), spheroidal bombs, and "cow-dung" bombs. -> See whole entry
Breadcrust bomb, ca. 50 cm long, from Lokon volcano (N-Sulawesi, Indonesia)
Perfectly shaped breadcrust bomb from Nea Kameni, Santorini (Greece)
Volcanic bomb with a cracked surface, similar to bread, caused by the slow expansion of the interior gas bubbles while cooling.
A breadcrust bomb is a volcanic bomb with a cracked and checkered surface, sometimes resembling the surface of a loaf of bread. The cracks develop when the outer surface of a partially molten lava fragment cools to form a brittle surface and then subsequently cracks as the hot interior expands due to the continued growth of gas bubbles. -> See whole entry
View of the 13x8km large caldera of Santorini, Greece, formed during several Plinian eruptions including the Minoan eruption around 1613 BC.
Öraefajökull volcano, SE-Iceland. The summit caldera of this large, explosive volcano that erupted last in 1728, is occupied by a glacier, which is part of the Vatnajökull ice cap that covers most of SE Iceland.
Large crater, usually several kilometers across, formed by the collapse of the roof of a magma chamber emptied by large explosive eruptions.
A caldera is a large, usually circular depression at the summit of a volcano formed when magma is withdrawn or erupted from a shallow underground magma reservoir. The removal of large volumes of magma may result in loss of structural support for the overlying rock, thereby leading to collapse of the ground and formation of a large depression. Calderas are different from craters, which are smaller,... -> See whole entry
Dacite is an igneous, volcanic rock with a high iron content found at many lava-domes.
Dacite (pronounced /deɪsaɪt/) is an igneous, volcanic rock with a high iron content. It is intermediate in compositions between andesite and rhyolite, and, like andesite, it consists mostly of plagioclase feldspar with biotite, hornblende, and pyroxene (augite and/or enstatite). It has an aphanitic to porphyritic texture with quartz as rounded, corroded phenocrysts, or as an element of t... -> See whole entry
A lava flow is the result of an effusive eruptions (here Mt. Etna in Nov 2006)
Effusive means flowing out of lava as opposed to explosive eruptions.
If magma is sufficiently fluid and if it is not framented by expanding gasses when reaching the surface vent, it can erupt to form lava flows. This is called effusive eruption. The opposite of effusive is explosive, i.e. fragmentation of magma. -> See whole entry
Explosion of magma at Krakatau at the beginning of a vulcanian eruption
Explosive eruptions occur, when the erupting magma is ejected as fragments into the air, as opposed to effusive eruptions producing lava flows.
Explosive eruptions are called so, when the erupting magma is fragmented when exiting the conduit. The reason are expanding gasses from the magma itself or external water e.g. from an aquifer. The resulting fragments of the magma itself are called tephra, and consist in small and large pieces: Ash, lapilli and bombs are the typical products of explosive eruptions.
A fissure vent, also known as a volcanic fissure or simply fissure, is a linear volcanic vent through which lava erupts.
A fissure vent, also known as a volcanic fissure or simply fissure, is a linear volcanic vent through which lava erupts, usually without any explosive activity. The vent is usually a few meters wide and may be many kilometers long. Fissure vents can cause large flood basalts and lava channels. This type of volcano is usually hard to recognize from the ground and from outer space because it has no ... -> See whole entry
Lava fountain more than 1,000 ft tall from Mauna Ulu, a vent of Kilauea Volcano in Hawaii, on 22 Aug 1969, a spectacular example of a Hawaiian eruption. Photo by D.A. Swanson, HVO / USGS
During a Hawaiian eruption, very fluid, basaltic lava is erupted from the vent as lava fountains and forms lava flows. The fountains are driven by the expanding gasses that were contained in the magma and leave the vent as a jet. Such eruptions are common for hot-spot volcanoes such as Kilauea on Hawai'i.
The lava fountains can reach heights of several hundreds meters, sometimes even more than 1 km. They can last for hours or even days and often occur from fissure vents to form so-called curtains of fire.
The lava spatter that falls back from the fountain can form lava flows if it is still fluid enough, or build up welded spatter ramparts and cinder cones around the vent. Along with the lava foun... -> See whole entry
Volcanic hornitos are small (usually a few meters high) rootless spatter cones that form on the surface of a (usually basaltic pahoehoe) lava flow. A hornito develops when lava is forced up through an opening in the cooled surface of a flow (a skylight) and then accumulates around the opening. Typically, hornitos are steep sided and form conspicuous pinnacles o... -> See whole entry
Lava poors out from several vents inside the crater of Pu'u 'O'o, Kilauea volcano, Hawai'i
Molten rock, called magma, is called lava when it reaches the surface during a volcanic eruption. Depending on how the magma erupts, it can form lava flows, lava fountains, lava lakes, or be fragmented into scoria and ash during explosive eruptions.
In a wider use of the term, also the cooled and solidified products of erupted magma are called lava (in form of coherent lava flows, or fragmented tephra).
Magma when it erupts as lava is almost never a completely molten rock, but contains 3 phases: liquid, solid mineral crystals (xenoliths) and gas bubbles, mainly water and carbon dioxide.
Steaming lava blocks on 27 Nov 2011 near La Restinga (El Hierro) photographed from the air by Guardia Civil / INVOLCAN
Lava balloons are hollow gas-filled pieces of lava floating to the surface above effusive submarine vents. Many lava balloons have been produced and observed during the ongoing 2011-12 shallow submarine eruption at El Hierro.
Floating lava blocks could result from the detachment of pillow-lava edges followed by the ascent of blocks with sufficient gas content. It is also possible that hot, gas-rich lava fragments result from small submarine lava lakes or fountains.
A thin frozen skin of lava seals the gas cavity, and the block might then rise as a hot lava balloon. During ascent, the gas exsolves and nucleates inside ... -> See whole entry
A lava bench in formation: active lava flows covering a small beach, forming a solid cap.
A lava bench is a platform formed by new lava flows that extends the old shoreline; in particular, this can be observed on Hawaii at Kilauea volcano during times when lava is entering the ocean, forming new land.
In their young stadium, benches are highly unstable. They often are underlain only by loose material such as sand and wave-eroded rock. Young benches can collapse at any time, and it is life-threatening to stand on one. Only after a long time, when the pile of material under and in front of the bench is sufficiently stabilized, the bench can be considered new stable land. -> See whole entry
Lava flows near the coast from Kilauea volcano, Hawai'i
Lava flow on Stromboli volcano flowing down the Sciara del Fuoco and entering the sea
Lava flows are almost self-explanatory. When magma is erupted in molten or a partially molten state it often has the ability to flow. This is typically the case for basaltic volcanoes such as Hawaii and Etna whose lavas are relatively fluid. Lava flows might form either as primary flows directly flowing out of a vent from the vent or by rapid aggregation of hot fluid spatter that fall back from lava fountains to form a flow.
The appearance of lava flows can vary greatly with chemistry, flow rate, strain rate, temperature, viscosity and other factors. The most simple division can be made between aa lava that usually has a blocky appearance, since the surface of the flow breaks into spiny or blocky pieces, and pahoehoe lava flows, which form a smooth surface and often produce spectacular ropy textures. -> See whole entry
The lava lake of Nyiragongo volcano, DRCongo (Jan. 2006)
The lava lake of Nyiragongo volcano, DRCongo (Jan. 2006)
Lava lakes are accumulations of larger volumes of liquid lava above one or several vents, usually contained within a crater on the summit of the volcano.
Volcanoes with eruptions that produce long-lasting lava lakes are relatively rare; some volcanoes are famous for their lava lakes: Kilauea (Hawaii), Erta Ale (Ethiopia), Nyiragongo (at present probably the most violent lava lake in the world, about 120 meters wide), Ambrym volcano (Vanuatu), Mt Erebus (Antarctica) and a few others with sometimes smaller lakes. -> See whole entry
Lava trees on the eastern Rift zone Kilauea volcano, Hawaii
The lava coating around a tree trunk left by an invading liquid lava flow.
When a liquid lava flow invades a forest, often, the lava does not overthrow the larger trees, but flows around their trunks. At the very contact of the hot lava to the bark, a thin layer of the lava is quenched sufficiently to form an isolating coating around the trunk. The tree itself most often burns down slowly, eventually falling down onto the lava.
Mud volcanoes are no true volcanoes, but vents that erupt mud, as fine sediemtn is squeezed upwards by prezzurized water, steam and gas escaping from deeper deposits.
A mud volcano is a vent on the surface erupting mud and gas or steam, but no lava. Mud volcanoes are usually not the result of volcanic processes, but more generally related to environments where pressurized deposits at depth occur that release gas and steam, which mixes with fine-grained sediments to form mud. Temperatures are much cooler than at volcanic processes. The largest structures are 10 ... -> See whole entry
Obsidian from the Rocce Rosse lava flow on Lipari island (Italy)
Detail of obsidian with spherolites (small bubbles where the glass has crystallized around the bubble)
Black volcanic glass
Obsidian is a naturally occurring glass formed as an extrusive igneous rock. It is produced when felsic lava extruded from a volcano cools rapidly through the glass transition temperature and freezes without sufficient time for crystal growth. Obsidian is commonly found within the margins of rhyolitic lava flows known as obsidian flows, where cooling of the lava is rapid. Because of the lack of cr... -> See whole entry
Lava fountain at Etna during the 2001 eruption. The activity is phreatomagmatic in origin, which explains the large amount of ash involved in the fountain: the rising water on its way meets wet layers where the contact between water and magma produces violent fragmentation.
Phreatomagmatic lava fountain.
Volcanic activity where fresh magma AND external water are involved.
Phreatomagmatic activity means that erupting magma reacts with external water, e.g. ground water, lake water, sea water etc. In contrast, if only magma is erupted and driven only by gasses originally contained in the magma, it is called magmatic activity. If no magma itself erupts, but heated ground water drives explosions and eruptions of older material, the activity is called phreatic.
Plinian eruption of Mt. St. Helens on May 18, 1980 (USGS Photograph taken on May 18, 1980, by Donald A. Swanson)
Pumice deposit on Santorini, Greece, from the large Plinian "Minoan" eruption on Santorini in 1613 BC, showing the holes in the pumice where remants of an olive tree could be found and recovered by Tom in 2003. This material allowed the most recent and most precise dating of this eruption to date.
The most explosive and largest type of volcanic eruptions. Plinian eruptions erupt more than 1 cubic kilometer of magma often within less than a few days and produce ash columns that can reach 20-50 km height.
Plinian eruptions are large explosive events that form enormous dark columns of tephra and gas high into the stratosphere (>11 km). Such eruptions are named for Pliny the Younger, who carefully described the disastrous eruption of Vesuvius in 79 A.D. This eruption generated a huge column of tephra into the sky, pyroclastic flows and surges, and extensive ash fall. Many thousands of people evacuate... -> See whole entry
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