A type of basaltic lava characterized by a rough or rubbly surface composed of broken lava blocks called clinker. The Hawaiian word was introduced as a technical term in geology. The loose, broken, and sharp, spiny surface of an ʻaʻā flow makes hiking difficult and slow. The clinkery surface actually covers a massive dense core, which is the most active part of the flow. As pasty lava in the core travels downslope, the clinkers are carried along at the surface. At the leading edge of an ʻaʻā flow, however, these cooled fragments tumble down the steep front and are buried by the advancing flow. This produces a layer of lava fragments both at the bottom and top of an ʻaʻā flow. Accretionary lava balls as large as 3 metres (10 feet) are common on ʻaʻā flows. ʻAʻā is usually of higher viscosity than pāhoehoe

Accretionary Prism

A sedimentary body formed by which ocean floor sediments and trench-fill sediments provided from the land accrete to the landward slope of trench by off-scraping accreation and underplating accreation. Oceanic plate subduction is responsible for such accretion.


An extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and dacite. Characteristic of subduction zones, andesite represents the dominant rock type in island arcs.



Basalt is a dark-colored, fine-grained, igneous rock composed mainly of plagioclase and pyroxene minerals. It most commonly forms as an extrusive rock, such as a lava flow, but can also form in small intrusive bodies, such as an igneous dike or a thin sill. It has a composition similar to gabbro, its extrusive counterpart. Basalt underlies more of Earth’s surface than any other rock type and is also very common on the Moon. It is most commonly formed at divergent plate boundaries such as mid-ocean ridges but can also be found at other volcanic centres.

Black Smoker

A hydrothermal vent that can be found on the ocean floor. It is a crack in the planet’s surface from which geothermally heated water comes out. Hydrothermal vents are commonly found near volcanically active places, areas where tectonic plates are moving apart, ocean basins, and hotspots. The super heated water is rich in minerals and is home to spectacular extreamophiles which live on the surrounding rock.



A large volcanic crater, usually formed by a major eruption leading to the collapse of the mouth of the volcano. Calderas can be tens of kilometres wide.


An unusual igneous rock that contains carbonate minerals at levels that exceed 20 percent (by volume). Almost exclusively, associated with continental rift-related tectonic settings. The majority of carbonatites are Proterozoic or Phanerozoic in age. Today Ol Doinyo Lengai volcano, in the Great Rift Valley, Tanzania is the world’s only active volcano that erupts natrocarbonatite lava with nearly no silica content. It erupted the lowest temperature lava in the world, at 500-600 °C (930-1,100 °F).

Cinder Cone

These are the classic, cone-shaped peaks we commonly associate with a lava-spewing eruption. Eruptions from cinder cones are pretty small potatoes, as far as volcanic eruptions go. They tend to be small, hill-sized volcanoes that range in height from tens to hundreds of meters high and they can build up over short periods of a few months to a few years. Cinder cones are characterized by their steeply angled sides and conical shapes.


The pipe/vent at the heart of a volcano where material travels to the surface.

Contact Metamorphism

A type of metamorphism where rock minerals and texture are changed, mainly by heat, due to contact with magma usually as it intrudes on the bed rock through a dike or sill.


A volcano that erupts volatiles such as water, ammonia or methane, instead of molten rock. These substances are usually liquids form plumes, but can also be in vapour form. After eruption, cryomagma condenses to a solid form when exposed to the very low surrounding temperature. They are common on other bodies in our Solar System such as Triton and Io.


Igneous rocks formed by the accumulation of crystals from a magma either by settling or floating. Cumulate rocks are named according to their texture; cumulate texture is diagnostic of the conditions of formation of this group of igneous rocks. Typically produced by precipitation of solid crystals from a fractional crystallisation. These accumulations typically occur on the floor of the magma chamber, although they are possible on the roofs if anorthite plagioclase is able to float free of a denser mafic melt.


Decade Volcano

A list of 16 volcanoes identified by the International Association of Volcanology and Chemistry of the Earth’s Interior (IAVCEI) as being worthy of particular study in light of their history of large, destructive eruptions and proximity to populated areas.

Decompression Melting

When a portion of the earth’s mantle is moved upwards to an area of lower pressure the reduction in overlying pressure enables the rock to melt, leading to magma formation. This is an important part in magma generation at divergent plate boundarys and mantle plumes.


Are form when magma intrudes into a crack then crystallizes as a sheet intrusion, either cutting across layers of rock or through an unlayered mass of rock.

Draw Back

If the first part of a tsunami to reach the coast is a trough, rather than a wave crest, the water along the shoreline is dragged back dramatically, exposing parts of the shore that are normally underwater and stranding many marine creatures; this is called draw back. Drawback can be a warning that a tsunami is approaching the shoreline but the problem is that the time between the sea water receding and the full tsunami moving in is usually only seconds or, at best, a few minutes.



A type of volcanic eruption in which lava steadily flows out of a volcano onto the ground. Effusive eruption differs from explosive eruption, wherein magma is violently fragmented when expelled from a volcano.

Evolved Magmas



A process by which a solid solution phase unmixes into two separate phases in the solid state. Exsolution occurs only in minerals whose compositions vary between two or more pure endmember compositions.


In its vary basic terms an extinct volcano is one which volcanologists believe is extremely unlikely to erupt again. Most use the ideal that it has not erupted in the past 10,000 years and/or the volcano no longer has a lava supply; for example due to plate motion it is no longer over a hot spot.

Extrusive Igneous Rock

Once a melt erupts on the Earth’s surface it is no longer termed magma but lava. Once this lava cools it forms an extrusive igneous rock such as basalt of andesite. An easy was to remember is the melt EXited the Earth and is therefore in EXtrusive. INtrusive rocks on the other hand solidify IN the Earth




Fire Fountain

A continuous spray of disrupting magma through a vent to form a persistent fountain of molten magma above the vent. The fountain, which may rise to 200 m, is supported either by the hydrostatic pressure of magma in the upper levels of the main volcanic superstructure, or by expanding gas released from the magma during the eruption. Fall-out from the column produces a spatter rampart around the vent and if the accumulation rate is high the molten spatter may coagulate to form a flow of  lava (a ‘clastogenic flow’).


A linear volcanic vent through which lava erupts, usually without any explosive activity. The vent is often a few meters wide and may be many kilometers long. Iceland is often famed for its fissue eruptions such as Bardabunga in 2015.

Flood Basalt

A type of large-scale volcanic activity, both in terms of extent and duration, that can occur on land or on the ocean floor. A flood basalt may continue to erupt for tens of thousands – possibly millions – of years and the lava can cover hundreds of thousands of kilometres. Large plateaux and mountains can result from the huge volume of newly surfaced rock. The huge volume of lava is accompanied by a similarly large release of volcanic gases such as sulphur dioxide and carbon dioxide. These can affect climate and cause acid rain, so flood basalts are thought to be a potential cause of mass extinctions.

Fractional Crystallisation



An opening in a planet’s crust, often in the neighborhood of volcanoes, which emits steam and gases such as carbon dioxide, sulfur dioxide, hydrogen chloride, and hydrogen sulfide.


Geothermal Gradient

Geothermal gradient is the rate of increasing temperature with respect to increasing depth in the Earth’s interior. Away from tectonic plate boundaries, it is about 25 °C per km of depth (1 °F per 70 feet of depth) near the surface in most of the world.





Intrusive Igneous Rock






An extreamly dangerouse consequence of volcanic activity, they are 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.


A size classification term for tephra, which is material that falls out of the air during a volcanic eruption or during some meteorite impacts. Lapilli (singular: lapillus) means “little stones” in Latin. By definition lapilli range from 2 to 64 mm (0.08 to 2.52 in) in diameter.

Large Igneous Province




Lava Dome





Magma Chamber


Mantle Plume


Mantle Wedge


Mid-Ocean Ridge



Nuée Ardentes





A basaltic lava that has a smooth, billowy, undulating, or ropy surface. These surface features are due to the movement of very fluid lava under a congealing surface crust. A pāhoehoe flow typically advances as a series of small lobes and toes that continually break out from a cooled crust. It also forms lava tubes where the minimal heat loss maintains low viscosity. The surface texture of pāhoehoe flows varies widely, displaying all kinds of bizarre shapes often referred to as lava sculpture. With increasing distance from the source, pāhoehoe flows may change into ʻaʻā flows in response to heat loss and consequent increase in viscosity. Pahoehoe lavas typically have a temperature of 1,100 to 1,200 °C (2,010 to 2,190 °F). Most lava flows on the Earth are less than 10 km (6.2 mi) long, but some pāhoehoe flows are more than 50 km (31 mi) long.

Partial Melting


Pillow lava

Lava structure typically formed when lava emerges from an underwater vent or subglacial volcano or a lava flow enters the ocean. However, pillow lava can also form when lava is erupted beneath thick glacial ice. The viscous lava gains a solid crust on contact with the water, and this crust cracks and oozes additional large blobs or “pillows” as more lava emerges from the advancing flow. Since water covers the majority of Earth surface and most volcanoes are situated near or under bodies of water, pillow lava is very common.

Phreatic Eruption


Primary Melts



A type of extrusive volcanic rock, produced when lava with a very high content of water and gases (together these are called volatiles) is extruded (or thrown out of) a volcano. As the gas bubbles escape from the lava, it becomes frothy.


Pyroclasts (or ” tephra ‘) are any volcanic fragment that was hurled through the air by volcanic activity. A pyroclastic eruption is one in which the great majority of activity involves fountaining or explosions

Pyroclastic Flow

A pyroclastic flow (also known scientifically as a pyroclastic density current) is a fast-moving current of hot gas and rock (collectively known as tephra), which reaches speeds moving away from a volcano of up to 700 km/h (450 mph). The gas can reach temperatures of about 1,000 °C (1,830 °F). The are possibly one of the most deadly of volcanic hazards.



Rift Zone

A rift zone is a feature of some volcanoes, especially shield volcanoes, in which a linear series of fissures in the volcanic edifice allows lava to be erupted from the volcano’s flank instead of from its summit.


An igneous, volcanic rock, of felsic (silica-rich) composition (typically > 69% SiO2—see the TAS classification). It may have any texture from glassy to aphanitic to porphyritic. The mineral assemblage is usually quartz, sanidine and plagioclase (in a ratio > 2:1—see the QAPF diagram).



An extremely vesicular basaltic lava with very small (< 1mm) vesicles.

Shield Volcano

A broad domed volcano with gently sloping sides, characteristic of the eruption of fluid, basaltic lava. Some of the largest single volcanoes are shield volcanoes shuch as those of the Hawaiian Islands and Olympus Mons on Mars.


A tabular sheet intrusion that has intruded between older layers of sedimentary, beds of lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet. This means that the sill does not cut across preexisting rocks, in contrast to dikes, discordant intrusive sheets which do cut across older rocks. Sills are fed by dikes, except in unusual locations where they form in nearly vertical beds attached directly to a magma source


This would be the most iconic of volcanic shapes; tall, conical volcanoes composed of layers of hardened lava, tephra, and volcanic ash. These volcanoes are characterized by a steep profile and periodic, explosive eruptions. The lava that flows from them is highly viscous, and cools and hardens before spreading very far. Mount Fiji is probably the most iconic stratovolcano.

Strombolian Eruption

A type of volcanic activity which produces frequent, moderate eruptions. The lava is basaltic, but sufficiently viscous for entrapped gases to build up a pressure which is released in continuous small explosions. The style was characterised and named after Stromboli volcano in Italy.

Subduction Zone

Where two plates collide and the lighest (usually older oceanic crust) is forced beneath (subducts) the other. Subduction zones tend to be marked by deep oceanic trenches closely followed by volcanic arcs.



Fragmented material produced by a volcanic eruption regardless of composition, fragment size or emplacement mechanism.

Tuff Rings






A classification of igneous rock. Ultramafic have  a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed of usually greater than 90% mafic minerals (dark colored, high magnesium and .



Magma contains small amounts of dissolved gas (water, CO2 etc) which is released as pressure is removed. Magmas formed by melting of mantle rocks have generally low volatile contents, but those formed by partial melting of crustal rocks are often volatile-rich. A high volatile content decreases viscosity (like adding water to treacle), and is probably the main factor in enabling some highly viscous (but also volatile-rich) melts to reach the surface at all. The release of gas during eruption is particularly likely to be explosive if the magma is both viscous (as gas is released, so viscosity is immediately increased) and volatile rich.