Illustrated Volcano Glossary

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. Read all

An adjective describing the chemical composition of magmas (or lavas) having relatively high content of silica (SiO2). It has little to do with the use of the term in chemistry where it is used to describe low pH values found in acids.

In petrology and volcanology, magmatic rocks (or the magmas they derived from) are often classified by their bulk silica (SiO2) content, which can typically range from around 40-75 weight %, an results in the 4 main types of magma by composition. These are, in order of increasing silica content: basaltic (45-53%), andesitic (53-62%), dacitic (62-69%), and rhyolitic (69-77%).
Rocks with high sili... Read all

Andesite is a gray to black volcanic rock with intermediate silica content, typical for lava domes and stratovolcanoes in subduction zones.

Andesite is an igneous volcanic rock containing between about 52 and 63 weight % silica (SiO2). Andesite is the extrusive equivalent to plutonic diorite - if a dioritic magma cools beneath the surface as an intrusion, it will be diorite, if the same magma erupts during a volcanic eruption, the rock formed is andesite.

Where does andesite usually occur?

Andesite and diorite are among the m... Read all

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.
Ash is produced by explosive activity when ex... Read all

Volcanic ash fall is the deposition of ash from the air from eruption plumes emitted during explosive volcanic eruptions.

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... Read all

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.... Read all

An adjective describing the chemical composition of magmas (or lavas) having relatively low content of silica (SiO2), as opposed to "acidic", and a comparably high content of iron and magnesium.

In petrology and volcanology, magmatic rocks (or the magmas they derived from) are often classified by their bulk silica (SiO2) content, which can typically range from around 40-75 weight %, and results in the 4 main types of magma by composition. These are, in order of increasing silica content: basaltic (45-53%), andesitic (53-62%), dacitic (62-69%), and rhyolitic (69-77%).
Rocks with low sili... Read all

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... Read all

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. Read all

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. Read all

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,... Read all

A depression often on the top or the flanks of a volcano usually created by explosions of lava from the vent.

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... Read all

A sudden collapse of volcanic material from an unstable side of a volcano. Debris avalanches are a particularly violent type of pyroclastic flows (in its broader meaning).

Diorite is an intrusive igneous rock of intermediate silica content (53-63%). It is the equivalent of its extrusive (volcanic) counterpart andesite.

Diorite is composed principally of the silicate minerals plagioclase feldspar (typically andesine), biotite, hornblende, and/or pyroxene. The chemical composition of diorite is intermediate between gabbro and granite. Read all

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. Read all

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.
The reason for explosive eruptio... Read all

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 ... Read all

Granite is the most well-known and one of the most common intrusive magmatic (plutonic) rock type. It is formed when an intrusion of viscous magma with high silica content (68-75 wt %) remains under the surface of the earth, where it cools and crystallizes slowly inside the crust.

Granite is the equivalent of its extrusive (volcanic) rock type rhyolite. It is granular and coarse-grained in texture. Its principal minerals are feldspars, quartz, and mica.
Granites can be predominantly white, pink, or gray in color, depending on their mineralogy. Read all

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... Read all

Hornito (Spanish) = "Little oven".
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... Read all

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.
The temperature of erupted lava ranges from around ... Read all

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 ... Read all

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. Read all

Lava domes are accumulations of highly viscous lava above or near the vent(s) where they were erupted during slow flow, like a sticky glue squeezed out or oozing out from a hole in its container.
Lava domes often take the form of a rounded hill, resembling a dome, but come in many other shapes as well.

Lava domes are very similar to thick lava flows, with the difference being that the lava once erupted from the vent almost doesn't flow at all, and piles up above the vent instead of flowing away downhill or to the side. In fact, there is a transition between thick, sticky lava flows and lava domes, depending on how viscous the lava is, how steep the terrain, the flow rate and other factors. Somet... Read all

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. Read all

Jets of fluid lava propelled into the air from an erupting vent, driven by expanding gasses.

Lava fountains are sustained jets of (usually very) fluid lava into the atmosphere. Lava fountains occur commonly on basaltic volcanoes such as Kilauea, or Etna.
The fountain gains its momentum by the expansion of gas bubbles that dissolve from the magma as pressure falls while it is rising in the conduit.
Heights, appearance, duration and erupted volumes of lava fountains can vary greatly. St... Read all

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. Read all

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.
The typical lava trees are formed when af... Read all