Volcanic Picture Glossary
Volcano Picture Glossary. Over 90 volcanic terms and features of volcanoes illustrated with volcano images. Find definitions for the types of volcano,volcanic eruptions,volcanic rocks and volcanic hazards. Stratovolcano,pyroclastic flow,pahoehoe lava and volcanic ash cloud are all here.
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Aa aa lava is a basaltic lava but at a lower temperaure than Pahoehoe at around 1000C. The lower temperature means that it is more viscous and flows as a blocky lava flow consisting of blocks called clinker. It moves slowly and creeps forward the top of the flow falling over the front face. The core of the flow is a paste like thick lava. Once cooled the surface is very rough and treachorous to walk on.
Acid or felsic magmas are enriched in silicon,oxygen,aluminium,sodium and potassium. The common minerals are quartz ( SiO2),muscovite (mica) ,orthoclase feldspar and plagioclase feldspars and are light in colour and are lower density than darker iron rich minerals. Acid or Felsic magmas solidify form rocks with a silica content less than 63%. Acid/Felsic magmas solidify in the crust commonly as granite and erupt as rhyolite lavas. Within an intrusion crystals grow large within the magma chamber. Pumice and Obsidian are eruptive forms of the same composition. 
Andesite is an intermediate extruisve lava with 52-63%silica content. It conatins Plagioclase feldspar,pyroxenes and hornblende , it may also contain smaller quantitiies or biotite and hornblende. It has a dark grey appearence and like most lavas it has small crystals.
Phenocrysts ( large crystals formed already in the melt) are present. Gas vesicles called amygdales are often filled wit a mineral called zeolite.Andesite is named after the Andes mountain range in South America where it is a common volcanic lava. Andesite occurs as long lava flows from Stratovolcanoes in subdction zone settings.It erupts between 900 C and 1100 C a similar temperature to basalt but much higher than Rhyollite.
Back Arc basins are an elongate zones of extnsion behind the arc associated with a subduction zone. As the the oceanic crust is subducted the trench pulls or rolls back away from the overlying plate- trench rollback. The extension allows basalts to erupt in a a similar way to mid ocean ridge basalt the main difference being that the basalts are have a higher water content derived from the loss of fluids from the mantle wedge. Spreading rates vary from 1-2cm / year to as much as 15cm/year.
A Benioff zone is a descending zone of earthquakes which follows the path of a descending slab of oceanic crust beneath a subduction zone. As the slab descends earthquake foci become deeper as the slab moves downwards. Earthquakes have been detected at a depth of 700km. Named after Hugo Benioff a seismologist from California.
Black smokers are sea bed hydrothermal vents which are rich in dissolved minerals. When the superheated mineral saturated water enters the cold sea the minerals are precipitated out which form a tall chimney structure, one chimney in the Pacific reached 40m tall. Black smokers are rich in sulphurs and sulphides which can form ore deposits eg manganese. Blacksmokers were originaly discoverd by a small submarine called Alvin in 1977. Not ony were they a completely new geological discovery but the smokers were found to support their own ecosystems of organisms which relied on the warm nutrient rich waters rather than a photosynthesis based ecosystem. The deepest blacksmokers are found 5000m down in the Cayman trough.
Found where large amounts of magma have been supplied to near surface magma chmaber. If the volcano dramatially erupts and the chamber is explosively evacuated then the roof of the chamber will collapse forming a caldera. An often rouded depression with a flat bttom and steep cliff sides will be left. A lake or sea may fill the depression. 
Cinder cones are compsed of rock fragments whcih have been ejected from a vent possibly as molten lava , cooled on hteir descent and droped downwind of a volcanic vent. Often made of scoria and tephra ( small volcanic rock fragments) they from quickly perhaps over a few days. They can 100 of metres tall. The rock fragments are cooled quickly and glassy in texture. They require low viscosity lava to form such as basalt.
Composite and Stratovolcanoes are one and the same. They are the volcano diagram that we all drew at school with layers of ash and lava. They are normaly located above
subduction zones and erupt explosively. Viscous lava, ash,tephra and pyroclastic flows build up a steep sided cone with a crater(s) at the summit. The voclano can erupt a variety of lava types from black basalt, through andsite and sticky dacite to the eruptive equivalent of granite- rhyolite. This variety of lavas is due to cooling and crystallisation of the magma within the volcano changing the composition of the melt when it is erupted.
The volcano may have smaller parasitic cones. Domes of lava may form in the summit craters.
Mt.St.Helens and Mt.Rainier in the Cascades are both Stratovolcanoes. 
Where two plates slide past each other in opposite directions.
The boundary is called a strike slip fault or transform boundary. The movement is not smooth and enormous tension is stored along the fault until it releases suddenly. This sudden release of energy forms ripples throught the earths crust called seismic waves or earthquakes. There are no volcanoes along this kind of boundary. Famous Examples include the San Andreas Fault in California
Where new crust is created.
A long ridge of submarine volcanoes which erupt to form new oceanic crust and cause the oceans to widen. The ridge forms a plate boundary and the basalt lava which erupts spreads outwards from the ridge to form the new ocean floor.
As the lava erupts the iron bearing minerals are aligned with the earths magnetic poles allowing age and magnetic reversals to be plotted.
On land a constructive margin starts as a rift with volcanoes which if successful can split apart a continent to form a new oceanic crust.
Where two continental sections of crust collide.When subduction of ocanic crust occurs continents are pulled together as the intervening ocean sinks back into the earths crust. Eventually the continental landmasses collide adn one is thrust under the other. Thrusting and folding occurs causing uplift and mountains such as the Himalayas and AlpsThe mountains are made from the marine sediments which once lay between the continents prior to collision.The route of the mountain chain often melts and granie intrusion occurs.
A circular depression on the earths surface caused by volcanic activity. 
Classically found at the summit of a volcano craters can also be found on the flanks of volcanoes. A crater will have a vent which is connected to a magma chamber from which lava can erupt. Hydrothermal activity in the form of fumaroles is common. Crater lakes may also form which may be highly acidic and hot ie. White island New Zealand. Should the magma chamber become evacuated rapidly during an eruption the it may collapse to form a caldera. Some volcanoes have multiple craters on the summit area such as Mt.Etna.
Continental crust is composed of silica rich minerals and has a largely granitic compostion
(quartz,feldspars,micas) it also includes surface sediments and metamorphic rocks. It is typically 35-40km thick but can be thicker in mountain ranges and thinner in areas of extension. It is very bouyant and forms the main land masses.
Continental crust along with oceanic crust forms the earths plates and the movement of the plates carries the continents around the surface of the earth.As it is so bouyant it is difficult to destroy and some sections of the crust are incredibly old up to 4 billion years old, from a period called the Archaen - Precambrian.
Opposite is Half dome, in Yosemite National Park USA. Within this photo all you can see is granite the main constituent of continental crust. Granite is formed in destructive and collisional settings. It does not erupt but rises as plutons ( huge blobs shaped like balloons) of magma which rise through and consume the crust above.
A cryptodome literally means hidden dome and refers to a lava dome which is hidden within the structure of the volcano. Most lava domes are extruded in the crater area of the volcano. If however the rising magma cannot ascend to the surface then the dome will build within the volcano causing the volcanoes slopes to bulge. The most infamous example is of Mt St Helens where a cryptodome caused the the northern slopes of the volcano to bulge. An earthquake dislodged the mountain covering the dome causing a landslide which dramatically released the pressure on the magma causing a massive lateral blast that extended for 20 miles.
Dacite lava forms a light grey rock which contains 63 to 68% silica and the minerals plagioclase,pyroxene and amphibole/hornblende .If the magma is gaseous then Plinian style eruptions can occur when low in gases dacite is extruded as a lava dome. If this collapses then pyroclastic flows can occur. It erupts at a temperature between 800 and 1000 C. Dacite is found above subduction zones particularly on ocean/continent boundaries.Mt.St Helens.Mt Pinatubo and Mt.Unzen have all erupted Dacite.
Destructive margins and Subduction Zones Explosive volcanoes occur at subduction zones. As Oceanic crust moves away from the mid ocean ridges it gets colder and more dense. Eventually it will sink back into the mantle. As it sinks back into the mantle fluids are released which cause melting in the wedge of mantle above. The molten rock gradually rises towards the surface through cracks and weaknesses in the crust above. Eventually it collects in a magma chamber from which it then erupts as a volcano.
The magma the volcano produces will be quite "sticky" or viscous due to high levels of silica picked up from the crust on the ascent and from fractional crystallisation. This is where more siica rich minerals crystallise out first as the magma cools in the magma chamber.
Subduction zone volcanoes are often called strato-volcanoes or composite volcanoes because they erupt a mxture of lava an ash. Eruptions are usually explosive. The Andes give their name to Andesite the rock type which is the predominant volcanic rock type. Rhyolite,Dacite and occasiionally basalt are also erupted.
A dike is a sheet like structure of intrusive rock. Magma exploits a weakness in rock such as a bedding plain in sedimentary rock or structural weakness and injects magma. Rings of dykes often surround volcanoes and dike swarms can accompany larger intrusions. Dikes are vertical or near vertical and although narrow may extend for several kilometres. They are by defintion younger than the rocks through which they pass. Compositions range from dolerite a, finer grained version of gabbro,to granitic dikes called pegmatites which have very large crystal sizes and mineraliation. Ophiolite complexes ,such as the Lizard Peninsular in Cornwall, show a high concentration of dolerite dikes.
An Ophiolite is a section of Oceanic crust which has been thrust onto the the land. Basalt,dolerite and Gabbro can all be present.
Domes or lava domes are extrusions of very viscous lava typically dacite or rhyolite.To form a dome the lava has to have the consistency of toothpaste. The dome of lava can grow rapidly and may even engulf the original crater. As the dome grows it may become unstable and collapses then pyroclastic flows can and eruption columns can occur.Mt St Helens has extruded a lava dome within the 1980 crater. More recently the Soufriere Hills volcano on Montserrat has had an extended period of dome building and collapse since 1995. The pyroclastic flows generated have devastated most of this tiny Caribbean Island.
Effusive eruptions are characterised by outpourings of lava onto the surface.These eruptions can be prolonged and erupt large volumes of rock. Basalt lava flow are very fluid and can flow with the appearence of water, Andesite is more viscous and produces short flows , Dacite and Rhyolite are very viscous and often form lava domes. The Pu oo oo vent on Kilauea has been erupting basalt flows since 1983.
Rocks which have been erupted onto the surface by volcanic activity. Extrusive rock are characterised by a fine groundmass with individual crytals to small to discern .Rapid cooling causes this plus the explosive nature of some eruptions. Pyroclastic material such as ash,tephra,pumice,scoria and bombs are included . In some instances cooling is so rapid that no crsytallisation can occur and the melt resembles glass.
Fissures and rifts are elongate cracks in the earths surface.Fissures are smaller than rifts. They typically form where the crust is under tension and is pulled apart allowing molten rock to pour out. Basalt is the predominate rock type found and fountains or curtains of "fire" can spectacularly erupt. Eruptions can pour out large volumes of basalt and eventually focus on one or two main vents.
A flank eruption is an eruption on the slopes of the main volcanic cone. Effusive flank eruptions of basalt are common on shield volcanoes . Explosive flank eruptions can also be called lateral blasts. Parasitic or secondary cones can pepper the sides of volcanoes.
Mt etna has numerous secondary eruption centres with 60 flank eruptions in the last 400 years and 5 in the last ten.
Flood basalt or trap basalt is a result of extensive eruption of basaltic lava. The cause of these vast outpourings is thought to be continental rifting combined triggered by a mantle plume/hotspot beneath. As the crust exends decompression of the mantle occurs which causes large volumes of basaltic melt to spill onto the earths surface. Volcanic cones tend not to be formed as the lava is too fluid and flows away from any vents. The Deccan traps,Siberian Traps and Columbia River Basalts are 3 extensive flood basatlt provinces.
When magma cools it begins to crystallise as the crystals seperate from the magma the composition of the melt changes.The melt becomes increasingly rich in silica as it is the silica rich minerals that crystallise last. Through this process the melt becomes less dense and rises further within the crust. A basalt melt can change to andesite then dacite. If rising magma assimilates continental crust on its ascent then large amounts of Si rich melt can be generated leading to emplacment of granite. Within a granitic pluton the top can be highly rich in the lighter Si rich minerals of Quartz and Feldspar whereas the base of the intrusion can dark and rich in Fe and Mg minerals.
Fumaroles are vents from which volcanic gases escape, carbon dioxide,sulphur dioxide,hydrochloric acid,hydrogen sulhide and water vapour. They occur along cracks,fissures and weaknesses. Mineral deposits can form as the hot gas hits the cooler air. Changes in the temperature of fumaroles can indicate changes to the activity levels of the volcano.Sulphur deposits occur around fumaroles as bright yellow deposits and crystals.
Where rainwater or groundwater comes into contact with hot rocks below the surface and is heated. The water rises to the surface through fractures and cracks. Hot springs,geysers and fumaroles (small outlets of steam and gas). The water may have a high mineral content and be acidic due to dissolved gases such as sulphur dioxide. Rocks can be corroded and crumble in hydrothermal areas. On the plus side natural Spas are often in hydrothermal areas.
Nuee Ardente literally means "Glowing cloud" in French and the term was first used to describe the fast moving clouds of hot,ash and dust which devastated the twon of St.Pierre during the eruption of 1902 on Martinique.The term is an alternative for the volcanic hazard pyroclastic flow.
A pyroclastic flow is a fast moving ground hugging surge or avalanche of hot ash,dust aand rocks released from a volcano. The flow appears as a billowing grey cloud moving at great speed down the slopes of a volcano. They consist of hot ash,rocks,dust and gas at a temperature of 100OC and can move at speeds of up to 100km/hour. The grey cloud is the cooled exterior of the cloud and thermam imaging equipment would show them to be very hot. Dome collapse and eruption colomn collapse are the main causes. They can even travel across water. Vesuvius, Soufriere Hills.Mt.Pelee and Mt.St.Helens all produce Pyroclastic flows.
Hawaiianeruptions are characterised by the long flowing rivers of basaltic lava which gradully build up into vast shield volcanoes. Lava fountains, fissure eruptions and lava flows are the main characteristics. eruptions are effussive rather than explosive and are generally much much safer. Basalt can form two distinctive lava flows one called "pahoehoe" in Hawaiin or rope like and "aa aa" which popular myth says is the sound you make if you walk on it bare foot-its very sharp.
Pahoehoe is very fluid and flows like custard or treacle, it can form channels which then roof over to form lava tubes. Aa aa is a slower moving flow with a steep front and is blocky and jagged.
A hot jet or column of mantle which rises up from deep within the earth, spreads out below the crust and pierces it to form volcanoes. The plume remains stationary as the plates move over the top .Exact cause is not known.
Chains of volcanoes occur where the plume pierces oceanic crust eg. Hawaii and on land vast flood basalts and supervolcanoes can occur eg Yellowstone.
he surface. Hot mud pools can also occur. The waters and mud are often rich in minerals and are have been enjoyed for their health benefits fro thousands of years. Left is Mammoth Hot Springs in Yellowstone. Banff Hot Springs in Canada and The Blue Pool in Iceland are famous examples.Water warmed by volcanic activity within the ground. Geysers, fumaroles,mud pots/pools and springs are are are surface hydrothermal features. On the sea bed black and white smokers are hot mineral rich rich jets of water which support their own ecosystems.
The water for hydrothermal systems is normally sourced from ground or sea water which has pentrated the earths crust and become warmed by the magmatic bodies or hot rocks below, it then rises to the surface dissolving minerals on its rise.
An ignimbrite is the rock formed by a pyroclastic flow it is commonly of Dacitic or Rhyolitic composition. Ignimbrite is composed poorly sorted of ash,pumice and lithic fragments. The ash wll consist of glass shards.It exhibits flow banding often with fragments of pumice. 
An island arc is a chain of volcanic islands formed over a subduction zone where oceanic crust is subducting beneath oceanic crust.The volcanoes form an arc shaped line curving away from the trench. When cold dense oceanic crust converges with less dense oceanic crust it sinks back down into the mantle at a trench and is reincorporated into the mantle.
Mantle convection currents combine with slab pull to keep the plate moving downwards. A pattern of earthquakes with increasing depth follows the descending slab- Benioff zone. As the plate descends it takes sea sediments with it or scrapes them off as an accretionnary prism. The sediments and minerals in the slab release fluids which cause the mantle above to melt and magma to rise to the surface. Where it erupts it forms a volcano.
Fluids released from the slab enable the melting of the mantle wedge aboveto produce a calc-alkaline melt. Volcanoes tend to erupt explosively. Notable Island Arcs include The Antlles Arc in The Caribbean and The Aleutian Arc in Alaska.
A lahar is a volcanic mudflow consisting of fast flowing mixture of volcanic debris and water. The flow has the consistency of cement and once it drys out is very hard. They form where water from a craterlake,melted glacier, river or heavy rain mixes with either new eruption material or old ash deposits. Lahars can either be hot or cold. They can travel great distances along existing valleys far from their original source.
Lapilli is a form of tephra which is rocky material that falls to earth during a volcanic eruption. Lapilli literlly means little stones in Latin and are from 2mm to 64mm. Any bigger and they are volcanic bombs.
Lapilli is formed when droplets of molten lava are ejected during an eruption and cool as they pass throught he air. They take on a rounded shape caused by the movement through the air. Lapilli Tuffs are where large amounts of lapilli become welded together with residual heat. They are often formed from rhyolitic,andesitic and dacitic lavas. Accretionary Lapilii is where ash particles which bonded together within the eruption cloud- a bit like volcanic hail.
Molten rock which erupts onto the earths surface. Viscosity is dependent on temperature and silica content. Low silica hot ( 1200 C ) lavas ie.basalt flow quickly cooler higher silica lavas such as dacite and rhyollite have a higher viscosity. Basaltic lavas produce lava fountains or prolonged effusive eruptions. High silica lavas can form lava domes and tend to erupt explosively.
The most common lava on the planet is basalt which makes up the oceans floor.
Lava columns are formed when a molten basaltic lava flow cools. As the lava cools it shrinks and forms polygonal columns of basalt.
The Giants Causeway in N Ireland is extensive area of columnar basalt formed during the rifting which created the North Atlantic Ocean.
Lava domes are composed of highly viscous lavas with high silica contents such as dacite or rhyolite. They are typically found in the craters of stratovolcanoes such as Mt.St. Helens or Soufriere Hills. The lava is exuded rather than erupted and is squeezed out like thick toothpaste. If they become unstable and collapse pyroclastic flows can be produced or tall eruption clouds.
A lava flow is molten rock ( lava) which has flowed over earthe surface. Basaltic lava forms the classic image of a lava flow - rivers of molten rock. More silica rich lavas can form flows but they tend to not flow far from the vent. On Hawaii s big island molten basaltic lava flows into the sea.
Lava fountains occur mainly with basaltic lava. When gas bubbles in the lava expand rapidly lava fountaining can occur. Lava fountains can reach 100m or even 500m. Fountains erupt from vents,craters and fissures. Etna and Kilauea are famous for their lava displays.
Lava Lakes are form in craters or depressions and are when molten lava is ponded. Basalt is the most common type of lava lake as it is highly fluid. The surface of the lake is cooled by the atmosphere and can appear solid but is only cm s thick. The surface can be cracked and the red hot lava glows through.As the molten lava below convects it is recycled and created- not unlike plate tectonics on a mini scale.
Lava plateaus are extensive areas of flood basalt which are formed by the extensive eruption of basaltic lava over a large area. Rifting of the continental crust is often responsible as the crust thins and allows magma from the mantle to erupt over an extensive area of fissures. The eruptions are often non xplosive and continue for a long time building lava flow upo lava flow.
The Colombia river Flood Basalts are situated between the Casacades and Rocky Mountains in the NW USA. Over 10-15 million years during the Miocene ( 5-23 Ma ago) a 6000 feet thickness of basaltic lava was built up. As the lava erupted the crust sank into the cavity left by the eruption.
When basalt erupts under water it extrudes in pillow shaped lobes.As the the basalt emerges an outer skin forms as the hot rock makes contact with the cold sea. This crust breaks as more basalt is pushed out like toothpaste. If pillow lava s are found in the field then geologists know that the area was once covered by water.Aa aa lava is a basaltic lava but at a lower temperaure than Pahoehoe at around 1000C. The lower temperature means that it is more viscous and flows as a blocky lava flow consisting of blocks called clinker. It moves slowly and creeps forward the top of the flow falling over the front face. The core of the flow is a paste like thick lava. Once cooled the surface is very rough and treachorous to walk on.
Pahoe hoe
Pahoehoe is an Hawaiian word which means smooth or rope like.Pahoe hoe lava is composed of basalt ,is very fluid and has a temperature of 1200C. The ropey surface texture takes the form of nestling loops of lava. Pahoehoe flows quickly and forms lava flows which can run for miles within roofed over lava tubes. Lava tubes form when the edges of the flow cool and form a channel, eventually the channel roofs over and the lava carries on flowing until it runs out leaving an empty tube.
The eruptive characteristics of lava depend on chemical composition , in particular silica content,temperature and volatile content. Basalt has a low silica content and a high eruptive temperatur it is therefore has a low viscosity and can form long fast moving lava flows.In contrast stickier lavas such as Rhyolite and Dacite are extruded as "toothpaste" like lava and often form domes. Stickier lavas tend to be mor explosive as volatiles cannot(gas,water bubbles) cannot escape so easily. 
Lavas are the eruptive equivalent of intrusive igneous rocks. Chemical and mineral contents are the same and it is texture and crystal formation which makes them appear different. Igneous rocks cool slowly within a magma chamber and crystals are able to grow . Lavas cool quickly and crystals do not have time to grow large giving a finer texture. Larger crystals from the magma chamber are erupted with the lava and are are called phenocrysts.
The table below gives examples such as Rhyolite is the eruptive form of granite. The word sin pink are the names of the minerals. Quartz is Si O2 (silica oxide) silica plays a major role in the colour,density and viscosity of igneous and volcanic rocks. More silia lighter,less dense and viscous less silica and maore Iron darkcolour,high density aand low viscosity.
Beneath a volcano as magma rises through the crust it pools as a mass of molten rock.The chamber is not an underground cave but can either be an area of fractured rock or a complete body of molten rock within which convection currents operate. The magma will either be erupted or may cool in situ causing crystals to form and eventually solidification. Lighter Si rich minerals will float to the top and heavier iron rich minerals to the bottom. Magma chambers can be detected up 10km below the surface. Granite,grano-diorite,dioritr and gabbro are the coarseley crystalline rocks found in solidified magma chambers.
A 2900km thick layer of the earths structure between the core and the crust. The boundary of the crst and mantle is called the MOHO. The mantle has two main layers the upper and lower mantle. With the earths crust the very top of the uppermantle forms the Lithosphere which is around 200km thick.
The upper mantle is at temperatures of 500-900C but the rocks are solid due to the pressure they are under.However the mantle is said to be "plastic" in nature can flow -like warm plasticine.
The main rock type is called peridotite which mainly consists of the minerals olivine and pyroxene.
MORB is a direct product of the mantle and has a high olivine and pyroxene content.Within the mantle convection currents occur caused by the enormous amount of heat given out from the earths core. Although the mantle is solid it can flow due its" plastic" nature. Convection currents push and pull the tectonic plates over the mantle causing continents to collide and subduction zones.
In the 1960s hidden chains of volcanoes were discovered under the sea. At these Mid Ocean Ridges or constructive margins oceanic crust is created at chains of volcano which form the boundary between two plates. As new crust is formed the older crust is pushed away a process known as sea floor spreading. The iron minerals in the basaltic lava align with changes in the earths magnetic field creating magnetic "stripes". Pillow lavas are erupted forming ocean floor,these are fed by an extensive network of dykes fed by gabbroic magma chambers. To see these characteristics on land you need to find an ophiolite , a slice of oceanic crust which has thrust onto the land. The Lizard Peninsula in Cornwall or the Troodos Ophiolite in Cyprus.Pyroclastic means" fire of broken rocks".Nuee Ardente is the French version.
The flow appears as a billowing grey cloud moving at great speed down the slopes of a volcano. They consist of hot ash,rocks,dust and gas at a temperature of 100OC and can move at speeds of up to 100km/hour. The grey cloud is the cooled exterior of the cloud and thermam imaging equipment would show them to be very hot. Dome collapse and eruption colomn collapse are the main causes. They can even travel across water.
Vesuvius, Soufriere Hills.Mt.Pelee and Mt.St.Helens all produce Pyroclastic flows.
Obsidian is a black volcanic glass with a rhyolitic composition. Obsidian has no crystal structure and has a conchoidal fracture which gives very sharp edges- like glass. It is formed when rhyolite lava cools so rapidly that crystals cannot form. Rhyolite lava is rich in silica and would normally appear light in colour , it is minute crystals of magnetite .
Oceanic crust is formed at constructive margins or mid ocean spreading ridges and is around 10km thick.It last around 200ma before being subducted. It is mafic in composition which means its direct origins are from the mantle and is very high in iron and magnesian.Ths mafic melt forms two main rock types MORB or mid-ocean ridge basalt, which erupts from the spreading ridge volcanoes and gabbro which is the crystalline version of basalt formed in the magma chamber below the ridge. It is very difficult to research the ridge , submarines and seismic surveys being the main tools. However fragments called Ophiolites ,which have been scraped off onto the surface during collision, have helped geologists stay dry and still look at oceanic crust. The surface is composed of basaltic pillow lavas beneath this are sheeted dykes which supply the lava to the surface.Beneath this section is the solidified magma chamber made from crystalline gabbro and beneath this the mantle peridotites.
Pahoehoe is an Hawaiian word which means smooth or rope like.Pahoe hoe lava is composed of basalt ,is very fluid and has a temperature of 1200C. The ropey surface texture takes the form of nestling loops of lava. Pahoehoe flows quickly and forms lava flows which can run for miles within roofed over lava tubes. Lava tubes form when the edges of the flow cool and form a channel, eventually the channel roofs over and the lava carries on flowing until it runs out leaving an empty tube.
A paroxysm is a sudden, violent and dramatic volcanic eruption. 
The pelean style of eruption is charcterised by fast moving pyroclastic flows or surges whcih incinerate everything i their path. Large volumes of dust,ash.rock and gas are ejected from the volcano then career down slopes as Nuee Ardente or pyroclastic flows. The style is named after Mt.Pelee in the Carribbean which erupted in 1902 killing 28 000 people.
Volcanic glass strands which are fomed from basaltic lava flows and areuptions. From Stretching or blowing out of basaltic volcanic glass from lava fountains,cascades and turbulent lava flows. Hairs can be 2m long and travel 10 s of kilometers. Pele is the Hawaiin Goddess of the volcano and legend has it lives within the crater of Kilauea, 
Occur where rising magma meets with groundwater and instant evaporation and consequent expansion to water vapour occurs. Violent explosions and eruptions follow which disentegrate the lava and surrounding rocks.
When basalt erupts under water it extrudes in pillow shaped lobes.As the the basalt emerges an outer skin forms as the hot rock makes contact with the cold sea. This crust breaks as more basalt is pushed out like toothpaste. If pillow lava s are found in the field then geologists know that the area was once covered by water.
Plinian eruptions are named after Pliny the Younger who wrote a detailed accoount of the eruption of Vesuvius in AD79. He likened the tall eruption cloud to an umbrella pine- tall slender trunk and large spreading top.Eruption clouds can extend well above 11km into the atmosphere and produce vast quantities of ash and gas.When the column looses its upward momentum it collapses back down the slopes of the volcano creating pyroclastic flows. Ash and pumice fall may be deep enough to bury homes,collapse roofs and turn day to night. Lahars may form where ash and water mix burying everything in their path.
Pompeii and Hercalaneum were famously buried in AD79 and the last eruption of Vesuvius was in 1944.
Mt. Pinatubo, in the Phillipines,was the worlds last dramatic plinian erption in 1991. Due to the proximity of an american airbase the voclano wa closely monitored and 1000s of locals evacuated to safety. Although the lahars and refugee camps caused many fatalities.
A Pluton is a mass of molten igneous rock which is less dense than the surrounding rocks and rises through the crust. It is sometimes called a diapir or montgolfier ( as its appeareance resembles a balloon). Plutons are commonly made of granitoid rocks with high silica contents which gives them a low density such as granite and granodiorite. As plutons cool slowly crstal sizes can be large and contain even larger crystals called phenocryts. As the pluton rises pieces of country rock can be incorporated into the melt- xenoliths.Convection currents within the pluton circulate a crystal mush and these evidence of these flow can be seen where the pluton is exposed at the surface. They are mostly found in subduction zone and collision zone settings.
Pumice is porous volcanic rock characterised by its network pores and bubbles held in a light coloured volcanic glass. Pumice has a very low density and can even float on water. All types of magma (basalt, andesite, dacite, and rhyolite) can form pumice. Pumice is formed when liquid gases dissolved in the magma suddenly expands to form a froth or foam. Pumice is this solidified foam.
Pyroclastic means" fire of broken rocks".Nuee Ardente is the French version. The flow appears as a billowing grey cloud moving at great speed down the slopes of a volcano. They consist of hot ash,rocks,dust and gas at a temperature of 1000C and can move at speeds of up to 100km/hour. The grey cloud is the cooled exterior of the cloud and thermam imaging equipment would show them to be very hot. Dome collapse and eruption colomn collapse are the main causes. They can even travel across water. Vesuvius, Soufriere Hills.Mt.Pelee and Mt.St.Helens all produce Pyroclastic flows.
Rhyolite is the eruptive equivalent of granite and has a silica content of 68%. Indicative minerals are Quartz,Feldspars and Micas and it erupts at around 700-850 which makes it one of the cooler lavas! Rhylite is associated with volcanic activity in continental settings such as subduction,collision and sub continetal hotspots. Erutions are often violent due to the high viscosity making it difficult for gases to escape gradually. Rhyolite can also erupt effusively to form Obsidian (Volcanic glass) or pumice.Flow banded may be present
A rift valley is formed by extension within the crust. Extension thins the crust causing the central area to drop down forming a graben. If after futher extension the crust becomes thin enough then volcanic activity can occur such as the East African Rift Valley (Kilamanjaro). Continental break up starts with rifting and may eventually form a new ocean as the continent is pulled apart. Rifts do not always herald the start of a new ocean. The North Sea is a failed rift and graben wich did nt quite make ocean. 
Scoria is ejected from a volcano and can be of basaltic to andesitic compositon. Its texture is full of gas bubbles set in a dark coloured lava rock caused by the escape of volcanic bubbles during the volcanic eruption. It can rain down on the surrounding slopes creating new cones.It can be black,red or even blue and has a low density but does not float. It is used a lot in road building.
A seamount is a an under mountain which does not emerge from the sea and is therefore not an island. They are usually under water volcanoes and can form long chains of which string out across the ocean floor.
Hawaii has an active volcanic seamount, Loihi, which will be the newest island in Hawaii. The Hawaiian islands are the emergent parts of a 3600m mile long chain of seamounts and atolls called the Hawaii-Emporer Seamount chain.The chain has 80 identified volcanoes.The Hawaiian islands are formed by a hot spot ( mantle plume) which pierces the crust above as the Pacific plate moves over the top. As the volcanoes move away from the hot spot they become inactive and erode beneath the waves forming seamounts.
A Seismograh or seismometer is an instrument for measuring the movement of the earth caused by earthquakes. Volcanoes produce magmatic quakes as magma moves within the crust. Monitoring the depth,frequency and strength of these quakes can help predict whena volcanic eruption may occur. In the past seismographs consisted of a trace drawn on a roll of paper nowadays more modern examples are all digital and on a computer.
Shield volcanoes have gentle slopes and large diameters. They are formed by the accumulation of low viscosity basalt lava flows. Because the flows have low viscousity they flow a long way. Lava tubes form which enable molten lava to flow long distances from the vent. They form where large volumes of basalt lava are erupted over along period of time from one location such as over a hot spot/mantle plume.
They are called shield volcanoes because they look like a warriors shield.Silicic magmas are those with a high percentage of silica, over 65%, granite and rhyolite come into this category. Granitic plutons will crystallise slowly and larger crstals will grow. Indicative minerals are
- Quartz Si O2
- Alkali Feldspar KAlSi308
- Plagioclase Feldspar NaAlSi3O8
- Biotite (Mica) K(Mg,Fe)3AlSi3O10(F,OH)2
- Muscovite (Mica)KAl2(AlSi3O10)(F,OH)2
- Amphibole/Hornblende (Ca,Na)2–3(Mg,Fe,Al)5(Al,Si)8O22(OH,F)2.
Within the magma flow patterns caused by convection currents align the crystals. Silicic magmas occur in subduction zones beneath continental crust,continental collision where fractional crystallisation occurs. This where lighter minerals float to the top of the magma chamber and more dense Fe/Mg minerals sink to the bottom.
A Skylight is an opening in the roof of a lava tube. Basaltic lava can be seen flowing through the tube. Skylights in old lava tubes provide light and moisture to enable plants to grow. 
Mt Etna on Sicily blows the best smoke rings. At 100-200m across , the smoke rings appear white and appear to roll in on themselves as they move upwards. It is thought they are formed when gases are rapidly expelled through a narrow conduit. 
Solfatara literally means sulphur earth in latin and refers to an area where large amounts of sulphurous fumaroles are situated. The original Solfatara is iin Puzzuoli, near Naples, and is situated in the Campi Flegrei volcanic area.It is a shallow dormant volcanic crater which formed 4000 years ago and last had a phreatic eruption 1198.
The Campi Fegrei or Phlegraean Fields is a 8 mile whide caldera to the west of Naples which contains 24 seperate craters. 37,000 years ago an enormous eruption here is thought to have aided in the extinction of Neanderthal man and the expansion of modern humans in Asia and Europe.
Constant basaltic fountaining produces a spatter cone where globules of molten lava build up around the eruption vent and solidify. In some cases these can roof over to form a feature called a hornito. Longer lived eruptions produce spatter cinder cones. 
Composite and Stratovolcanoes are one and the same. They are the volcano diagram that we all drew at school with layers of ash and lava. They are normaly located abovesubduction zones and erupt explosively. Viscous lava, ash,tephra and pyroclastic flows build up a steep sided cone with a crater(s) at the summit. The voclano can erupt a variety of lava types from black basalt, through andsite and sticky dacite to the eruptive equivalent of granite- rhyolite. This variety of lavas is due to cooling and crystallisation of the magma within the volcano changing the composition of the melt when it is erupted. The volcano may have smaller parasitic cones. Domes of lava may form in the summit craters.
Strombolian eruptions are characterized by the intermittent explosion or fountaining of basaltic lava from a single vent or crater. Each episode is caused by the release of volcanic gases, and they typically occur every few minutes or so, sometimes rhythmically and sometimes irregularly. The lava fragments generally consist of partially molten volcanic bombs that become rounded as they fly through the air.
Stromboli in the aeolain islands of Italy is on eof the worlds most reliable and cintinually active volcanoes- worth the walk!Where oceanic crust is destroyed. When cold dense oceanic crust converges with less dense oceanic or continetal crust;sinks back down into the mantle at a trench and is reincorporated into the mantle. Mantle convection currents combine with slab pull to keep the plate moving downwards.This can form a destructive boundary 1000km s long. A pattern of earthquakes with increasing depth follos the descending slab- Benioff zone. As the plate descends it takes sea sediments with it or scrapes them off as an accretionnary prism. The sediments and minerals in the slab release fluids which cause the mantle above to melt and magma to rise to the surface. Where it erupts it forms a volcano.
Where a volcanc eruption occurs underneath a glacier or icesheet. The heat of the eruptio can cause large scale melting and result in floods , lahars and pilow lavas. Flat or table topped volcanoes ,called Tuyas, can form . Pillow lavas can form as lava enters the subglacial meltwater, then phreatomagmatic activity deposits a breccia then lava flows over the top. Iceland has numerous subglacial volcanoes. In 1996 the Grimsvotn volcano beneath tthe Vatnajokull Icesheet erupted melting the icesheet above melt. The water was stored beneath the ice sheet until it lifted the icesheet and burst through the base of the glacier and out across the coastal plane at a rate of 50 000 m3/s. This is called a jokulhlaup or glacial burst.
A supervolcano usually forms over a hot spot and under thick crust. They often appear on the surface as a large depression called a caldera which can be 10 s of km across. The depression is sometimes filled with a lake as at Taupo. The caldera is formed as the magma inside is violently blown out of the magma chamber below. Initially this is from the edges then the whole caldera floor collapses, leaving a flat bottomed depression surrounded by steep sides.
The map shows the track of the Yellowstone hot spot. 
A crack,fissure,crack or circular opening in the earths crust through which molten rock,ash and gases erupt and escape. Vents can be elongate fissures which erupt lava fountains,craters on the summit of volcanoes and subsiduary cones. Vents are responsible for the growth of volcanoes. producing shield volcanoes,stratovolcanoes on a large scale and hornitos and fumaroles on a small scale.
Lava bombs (<64mm) are when partially molten lava is ejected from a volcano. As the lava passes through the air it can become aerodynamically shaped. Types o fbomb are breadcrust bombs, ribbon bombs, spindle bombs (with twisted ends)and spheroidal bombs. 
All volcanoes emit gases. The most common gas is water vapour followed by carbon dioxide and sulphur dioxide. The rate of sulphur dioxide emmision is an indicator of volcanic activity levels. Sulphur dioxide high in the atmosphere has a cooling effect on the earth. Carbon dioxide has a warming effect and is a greenhouse gas. Volcanoes probably supplied the earth with its initial atmosphere, warming the environment enough that life could survive. If magma is fluid then gas can easily escape, such as basalt. If magma is more viscous then gas cannot so easily escape and may be released violently. Volcanoes emit many other gas including the Noble gases,chlorine,methane,CO and hydrogen.
Volcanic ash is volcanic rock whch has been violently disentegrated in a volcanic eruption. Ash consists of mineral,rock and volcanic glass fragments smaller than 2mm. It can resemble wheat flour is highly abrasive. A violent volcanic eruption can blanket an area in this "grey snow" however it does not melt, can collapse roofs,block aeroplane engines and if when wet can set like concrete. If Ash mixes with flowing water a lahar or volcanic mud flow can form.
A volcanic plug is a hardened magma which blocks the main vent of a volcano. The plug can cause pressure to build up within the volcano as new magma rises. Eventually a violent volcanic eruption can occur as the plug is blasted out of the volcano, Erosion can remove the softer rocks leaving the plug as a prominant pinnacle of solid rock. 
The VEI has 8 levels of volcanic eruption each level being 10x greater than the last, similar to the Richter Scale, it is a measure of the size of volcanic eruption and is based on the volume of erupted pyroclastic material (ashfall, pyroclastic flows), height of eruption column and how long the eruption lasts. The scale shows how some known volcanic eruptions appear on the scale. Even our largets modern times eruptions are small compared to pre-history eruptions.Volcanic glass occcurs where magma cools so rapidly that crystals are unable to form. Like normal glass it is halfway between a solid and a gas but not a liquid fluid. It will shatter,break, have conchoidal fracture and produce sharp edges. Obsidian is a rhyolitic glass which appears black, formed when rhyolitic lava cools rapidly.Pumice is considered glass as it has no crystal structure,Peles hair : thin strands of glass,Peles tears droplet of glass,Polagonite basalitc glass with a ow silica content. Violent volcanic explosions can produce large volumes of ash which will include very fine volcanic glass. 
A volcanic lake or crater lake forms in a crater or caldera or depression left by a volcanic eruption. Crater or volcanc lakes can form in the crates of active volcanoes. Fumarole activity can make the lake highly acidic,hot and a deep green colour. Gases can be trapped in the water, in 1986 1800 people were killed when gas escaped from the volcanic lake at Lake Nyos,Cameroon. If no fumarolic activity is present then Volcanic Lakes can be be very pure. Calderas hold enormous lakes , Crater Lake is 594m deep the deepest lake in the USA. The crater lake on New Zealands White Island is one of the most acidic lakes in the world.
White smokers are hydrothrmal vents associated with mid-ocean ridges. Sea water percolates through cracks in the ocean floor,heated by magma then returns to the surface. Similar to black smokers in their origin accept they consist of lighter coloured elements such as barium , calcium and silicon. Tall chimneys of minerals an form around the vent and grow at a rate of 9m /year ( they collapse too). The water temperature is cooler than black smokers.
Xenolith means "foreign rock" in Greek. A xenolith is a rock fragment that has been enveloped by an igneous rock. This can happen as an intrusion pushes through country rock, or an eruption rips fragments of the vent wall or picked up by the base of a lava flow. Xenoliths can be igneous,metamorphic or sedimentary. 
Yellowstone is a supervolcano caused by a Hot spot or mantle plume underneath the American continent. Most continental crust is 20 to 30 miles thick, here it is just 2 caused by the rising of a plume of hot mantle underneath the crust. As the North American plate has moved over the Hot spot it has punched holes through the continental crust. Unlike Hawaii the crust here allows sporadic violent eruptions 100,000s years apart although some quieter eruptions of lava have occured.
The volcano has had 3 major eruptions in the past 2 miilion years and averages an eruption every 600,000 years or so. The last eruption around 660 000 years ago devastated 1000 sq miles and ejected 240 cubic miles of material. The caldera from this eruption is 85km by 45 km and is 1km deep - in fact so big you don't know you are in it! The caldera was gradually filled in after the last eruption by rhyolite lava flows which only stopped 75,000 years ago.