Vulcanicity and Volcanic Landforms: Terminology for UPSC Geography Optional

Vulcanicity covers everything molten rock does — forming, rising, freezing underground or erupting at the surface — and the landforms it leaves, from shield volcanoes and calderas to batholiths and geysers. UPSC now tests single named terms here: phreatic eruptions and pseudovolcanic features have each carried a full question, and neither is defined in older standard accounts.

Each entry gives an exam-ready definition first, then mechanism, examples and a sketch line. UPSC asked about volcanicity and plate tectonics in 1998, pseudovolcanic features in 2017 and phreatic eruptions in 2019; Paper II in 2021 turned to India’s volcanic soils.

Quick Revision Table

Term	Meaning in one line	Example
Vulcanicity vs volcanism	All magmatic processes vs surface eruption only	Deccan intrusions and lavas
Magma & lava	Molten rock below ground; the same rock erupted	Malani rhyolite vs Deccan basalt
Pahoehoe & aa	Smooth ropy lava; rough clinkery lava	Kīlauea, Hawaii
Pillow lava	Rounded lobes of lava quenched under water	Maradihalli, Karnataka
Columnar jointing	Polygonal columns formed as lava cools	Gilbert Hill, Mumbai
Ring of Fire	Subduction-zone volcano belt around the Pacific	Andes to Japan to New Zealand
Eruption styles	Hawaiian to Plinian classes of eruption	Stromboli; Vesuvius 79 CE
Volcanic Explosivity Index	0–8 logarithmic scale of explosive size	Pinatubo 1991, VEI 6
Active, dormant & extinct volcanoes	Classes by likelihood of eruption	Barren Island (active)
Pyroclastic materials	Fragments blown out: ash, lapilli, bombs	Eyjafjallajökull ash, 2010
Nuée ardente & pyroclastic flow	Glowing avalanche of hot gas and fragments	Mont Pelée, 1902
Lahar	Volcanic mudflow of debris and water	Armero, Colombia, 1985
Phreatic eruption	Steam blast from heated groundwater, no fresh magma	Ontake, Japan, 2014
Shield volcano	Broad, gentle cone of fluid basalt	Mauna Loa, Hawaii
Cinder cone	Small steep cone of scoria around one vent	Parícutin, Mexico
Composite (strato) volcano	Steep cone of alternating lava and ash	Mount Fuji
Lava dome	Mound of viscous lava piled over a vent	Mount St Helens dome
Parasitic (adventive) cone	Subsidiary cone on a volcano’s flank	Flank cones of Etna
Crater & caldera	Vent depression; large collapse basin	Crater Lake, Oregon
Maar & diatreme	Low-rimmed explosion crater; breccia pipe beneath	Eifel maars; Majhgawan pipe
Flood basalt	Plateau-scale stack of fissure-fed basalt flows	Deccan Traps
Batholith, laccolith, lopolith & phacolith	Large plutonic intrusions of different shapes	Sierra Nevada batholith
Sill & dyke	Concordant and discordant sheets of intruded magma	Great Whin Sill; Great Dyke
Volcanic neck (plug)	Solidified conduit left after the cone erodes	Ship Rock, New Mexico
Fumarole, solfatara & mofette	Steam, sulphur and carbon dioxide vents	Valley of Ten Thousand Smokes
Geyser & hot spring	Intermittent spouting vs steady flow of hot water	Old Faithful; Manikaran
Pseudovolcanic features	Volcano-like forms not made by eruption at that spot	Lonar crater, Maharashtra
Mud volcano	Cone of mud and gas forced up from buried sediment	Baratang, Andaman Islands

Vulcanicity, Magma and Lava

Vulcanicity vs Volcanism

Vulcanicity (vulcanism) is the whole set of processes by which magma is generated in the mantle or crust, rises, and either solidifies underground as intrusions or reaches the surface as lava, gas and fragments; volcanism in the narrower sense is only its surface part — eruption through vents and fissures and the building of volcanic landforms.

• Components: intrusive — batholiths, sills, dykes and other plutonic bodies; extrusive — central and fissure eruptions, cones, calderas, lava plateaus; post-volcanic — fumaroles, hot springs and geysers.
• How magma forms: rock melts in three ways — decompression melting as hot mantle rises beneath ridges, rifts and plumes; flux melting when water released from a subducting slab at about 100 km depth lowers the melting point of the overlying mantle wedge; heat-transfer melting when basalt ponds under continental crust and melts it to granite and rhyolite.
• Plate-tectonic pattern: divergent margins give quiet fissure eruptions of basalt (Iceland); subduction zones give explosive andesitic and dacitic volcanoes (the Ring of Fire; Barren Island); continental collision zones such as the Himalaya have little volcanism; intraplate hotspots give shields and flood basalts (Hawaii; Deccan).
• Controls on eruption style: silica content and viscosity, dissolved gas, and contact with water.
• Significance: builds new land (Iceland, Hawaii); forms fertile soils such as the black regur of the Deccan; supplies geothermal energy; causes hazards; and alters climate — Pinatubo’s 1991 sulphate veil cooled global surface temperature by about 0.5 °C.
• Sketch: a cross-section from ridge to trench to hotspot showing where and how magma forms.

Magma and Lava (Basaltic vs Acidic; Viscosity)

Magma is molten rock beneath the surface, carrying dissolved gases and suspended crystals; lava is magma that has reached the surface and lost much of its gas. Both are classed by silica content, which controls viscosity and therefore how gently or violently they erupt.

Type	Silica	Temperature	Viscosity and eruption
Basaltic (mafic, basic)	About 45–52%	About 1,000–1,200 °C	Fluid; gas escapes; effusive
Andesitic (intermediate)	About 52–63%	About 900–1,000 °C	Moderately viscous; explosive
Rhyolitic (felsic, acidic)	Above about 69%	About 650–850 °C	Very viscous; gas trapped; highly explosive

• Viscosity: rises with silica and falls with temperature; viscous magma traps gas until it fragments explosively.
• Examples: basalt — Kīlauea and the Deccan Traps; dacite — Mount St Helens, 1980; rhyolite — the Malani volcanic suite of western Rajasthan.
• Don’t confuse with: igneous rock names — basalt and rhyolite are the solid equivalents.

Pahoehoe and Aa

Pahoehoe is basaltic lava with a smooth, billowy or ropy surface formed by hot, fluid lava moving slowly; aa is basaltic lava with a rough, jagged surface of clinker formed when lava cools, loses gas or is stirred faster so its crust tears. Both terms come from Hawaiian.

• Formation: pahoehoe advances in small toes and lobes beneath a plastic skin; aa rolls forward like a bulldozer over its own broken crust.
• Key features: pahoehoe can change downstream into aa, but not back; pahoehoe feeds lava tubes; silica-rich flows form block lava.
• Examples: Kīlauea and Mauna Loa, Hawaii; both pahoehoe and aa flows are recognised in the Deccan Traps of the Western Ghats.
• Sketch: a ropy wrinkled surface beside a rubbly clinker surface.

Pillow Lava

Pillow lava is lava that has erupted or flowed under water, or under ice, and cooled into stacked, rounded, sack-like lobes, usually under a metre across, each with a glassy chilled rind and radial cracks; it is the commonest lava form on Earth because most basalt erupts at mid-ocean ridges.

• Formation: a lobe’s surface is quenched instantly by water; pressure bursts the rind and a new lobe buds off.
• Significance: proves submarine eruption; pillows in mountain belts mark ancient ocean floor (ophiolite).
• Examples: Hawaiian flows entering the sea; the pillow lavas of Maradihalli, Chitradurga district, Karnataka, about 2.5 billion years old and a National Geological Monument.

Columnar Jointing

Columnar jointing is a pattern of long, parallel, polygonal — often hexagonal — columns formed when a thick lava flow, sill or dyke cools and contracts, cracking at right angles to its cooling surfaces; it is the most visible structure of basalt landscapes.

• Formation: contraction during cooling opens cracks that propagate inward from the top and base.
• Key features: regular lower colonnade and irregular upper entablature; columns tens of centimetres across.
• Examples: the Giant’s Causeway, Northern Ireland; Devils Tower, Wyoming; Gilbert Hill in Mumbai, a column-jointed basalt monolith; the columnar volcanic rocks of St Mary’s Islands off Malpe, Karnataka, a National Geological Monument.
• Don’t confuse with: joints formed by tectonic stress or unloading.

Ring of Fire

The Ring of Fire is the horseshoe-shaped belt of volcanoes, trenches and earthquake zones, about 40,000 km long, that encircles the Pacific Ocean where oceanic plates subduct beneath surrounding plates; it holds roughly 750–915 active or dormant volcanoes — about two-thirds of the world’s — and about 90% of its earthquakes.

• Formation: subduction of the Pacific, Nazca, Cocos, Juan de Fuca and Philippine Sea plates feeds explosive stratovolcanoes by flux melting.
• Segments: Andes; Central America; Cascades; Aleutians; Kamchatka and Kurils; Japan; Philippines; Indonesia; Tonga and New Zealand.
• Examples: Pinatubo (1991); Mount St Helens (1980); Hunga Tonga–Hunga Haʻapai (2022).
• Other belts: the Alpine–Mediterranean belt (Vesuvius, Etna, Stromboli), the mid-Atlantic ridge (Iceland) and the East African Rift. India’s Barren Island lies on the Andaman–Sunda arc, which joins the Pacific belt through Indonesia.
• Sketch: a Pacific outline ringed with triangles and trench lines.

Eruptions and Their Hazards

Eruption Styles (Hawaiian, Strombolian, Vulcanian, Pelean, Plinian, Fissure)

Eruption styles are classes of volcanic eruption named after type volcanoes and ordered by increasing explosivity, which rises with magma viscosity and gas content; the same volcano can change style during one eruption.

Style	Character	Type example
Hawaiian	Fountains and flows of fluid basalt; Pele’s hair	Kīlauea
Strombolian	Rhythmic bursts of scoria and bombs	Stromboli; Barren Island
Vulcanian	Short cannon-like blasts through a plugged vent	Vulcano; Sakurajima
Pelean	Dome collapse and glowing avalanches	Mont Pelée, 1902
Plinian	Sustained gas–ash column tens of kilometres high	Vesuvius, 79 CE; Pinatubo, 1991
Surtseyan (phreatomagmatic)	Magma explodes on contact with water	Surtsey, 1963; Hunga Tonga, 2022
Fissure (Icelandic)	Curtains of lava from long cracks	Laki, 1783–84

• Plinian: named after Pliny the Younger (Gaius Plinius Caecilius Secundus), whose letters described Vesuvius burying Pompeii and killing his uncle.
• Fissure: Laki poured out about 15 km³ of basalt; its gases and the famine that followed killed about a fifth of Iceland’s people.
• Sketch: a row of small cones with columns increasing in height from Hawaiian to Plinian.

Volcanic Explosivity Index

The Volcanic Explosivity Index (VEI) is a 0–8 scale of the size of explosive eruptions, devised by Christopher G. Newhall and Stephen Self in 1982, based on erupted tephra volume, eruption-column height and observed style; from VEI 2 upward each step represents a tenfold increase in ejected volume.

• Thresholds: VEI 0 below 10,000 m³; VEI 4 above 0.1 km³; VEI 6 above 10 km³; VEI 8 above 1,000 km³.
• Examples: Kīlauea effusions 0–1; Ontake 2014, VEI 3; Mount St Helens 1980, VEI 5; Hunga Tonga 2022, VEI 5 (some estimates 5–6); Pinatubo 1991, VEI 6; Tambora 1815, VEI 7; Toba, about 74,000 years ago, VEI 8.
• Limits: measures explosivity, not lava volume or sulphur release, so effusive Laki scores low despite its climatic impact.

Active, Dormant and Extinct Volcanoes

Active, dormant and extinct volcanoes are classes based on eruptive history and the likelihood of future eruption: an active volcano is erupting or has erupted in historical or Holocene time and shows unrest; a dormant volcano is quiet but expected to erupt again; an extinct volcano is not expected to erupt.

• Working rule: many volcanologists count any volcano that erupted in the last 11,700 years (the Holocene) as potentially active.
• Caution: Vesuvius was thought extinct before 79 CE; Pinatubo woke in 1991 after about 500 years.
• Indian examples: Barren Island is active — first recorded eruption 1787, reawakened in 1991 after about 140 years, erupted in 1994–95, 2005–07 and 2017–19, and almost continuously since 2022, with ash plumes about 3 km high reported in October 2025; Narcondam is generally treated as dormant, with reports of mud and smoke in 2005.

Pyroclastic Materials (Tephra, Ash, Lapilli, Bombs)

Pyroclastic materials (tephra) are fragments of magma and rock blown out by explosive eruptions, classed by size into ash (under 2 mm), lapilli (2–64 mm) and bombs or blocks (over 64 mm); bombs are molten clots shaped in flight, blocks are solid rock.

• Varieties: pumice — frothy, light, silica-rich; scoria (cinder) — dark, vesicular basaltic fragments.
• Rocks: ash consolidates into tuff; coarse fragments into agglomerate or volcanic breccia; pyroclastic-flow deposits into ignimbrite.
• Hazards: ash loads collapse roofs, ruin crops and damage jet engines — the April 2010 Eyjafjallajökull eruption in Iceland closed much of European airspace for days.
• Sketch: a column with bombs falling near the vent, lapilli farther out and ash drifting downwind.

Nuée Ardente and Pyroclastic Flow

A pyroclastic flow is a ground-hugging, gravity-driven current of hot gas and volcanic fragments, often several hundred degrees Celsius and faster than 100 km per hour; a nuée ardente (“glowing cloud”) is the incandescent variety produced by collapse of a lava dome or eruption column.

• Coined by: Antoine François Alfred Lacroix named the nuée ardente after the Mont Pelée disaster of 8 May 1902, which destroyed Saint-Pierre, Martinique, and killed about 28,000–29,000 people.
• Types: dense pyroclastic flows follow valleys; dilute pyroclastic surges can spill over ridges.
• Examples: Vesuvius, 79 CE, burying Herculaneum; Unzen, Japan, June 1991, which killed 43 people, among them the volcanologists Maurice Krafft, Katia Krafft and Harry Glicken.
• Hazard link: the deadliest eruption process, because it cannot be outrun.

Lahar

A lahar is a fast-moving slurry of volcanic debris and water that flows down the valleys of a volcano like wet concrete, formed during or long after an eruption; the word is Javanese and was introduced to geology in 1922 by Berend George Escher.

• Triggers: crater-lake breakouts; eruptions melting snow and ice; heavy rain on loose ash; earthquakes and flank collapses.
• Key features: speeds of tens of metres per second; can travel more than 100 km; bury valleys under metres of debris.
• Examples: Kelud, Java, 1919, when crater-lake water killed more than 5,000 people; Nevado del Ruiz, Colombia, 13 November 1985, when a small eruption melted summit ice and lahars buried Armero, killing about 23,000; Pinatubo, where typhoon rains turned 1991 ash into lahars for years.
• Mitigation: sabo dams, acoustic flow monitors, hazard zoning.
• Don’t confuse with: non-volcanic debris flows.

Phreatic Eruption (and Phreatomagmatic Eruption)

A phreatic eruption is a steam-driven explosion that occurs when groundwater or surface water is heated by magma, hot rock or a hydrothermal system and flashes to steam, blasting out fragments of pre-existing rock, mud and gas without erupting new (juvenile) magma; it is usually small but sudden, and one of the deadliest events for people near craters.

Mechanism

1. Heat from a magma body warms water in a hydrothermal system beneath the crater.
2. Mineral deposits seal the system, or water is trapped beneath impermeable rock, so pressure builds.
3. A pressure rise, or a sudden pressure drop from an earthquake, landslide or lake drainage, lets superheated water flash to steam, expanding more than a thousandfold.
4. The blast shatters the cap rock and ejects blocks, ash of altered rock and gas, leaving an explosion crater.

Consequences

• Ballistic blocks and base surges near the vent; ashfall of altered rock.
• Toxic gases — carbon dioxide and hydrogen sulphide; about 140 people died from gas during a phreatic event on Java’s Dieng Plateau in 1979.
• Lahars and acid floods from crater lakes.
• Little warning: precursors are weak, so tourists and climbers are exposed.
• Precursor role: phreatic blasts often announce magmatic eruptions, as at Mount St Helens in March 1980.

Examples

• Ontake, Japan, 27 September 2014: a VEI 3 phreatic blast at midday on a busy autumn weekend killed 58 climbers, with 5 more never found; no alert had been raised.
• Whakaari/White Island, New Zealand, 9 December 2019: 22 of the 47 people on the island died; alert level 2 had been in force since November.
• Others: Kusatsu-Shirane, Japan, 2018; the Biscuit Basin hydrothermal explosion at Yellowstone, 23 July 2024.

Phreatic vs phreatomagmatic

A phreatomagmatic eruption involves direct contact of magma with water, so juvenile magma fragments are ejected; it builds maars and tuff rings and, at sea, Surtseyan cones — Surtsey (1963) and Hunga Tonga–Hunga Haʻapai (15 January 2022), whose plume reached about 57–58 km. A purely magmatic eruption is driven by gas in the magma alone.

• Sketch: a crater over a hydrothermal system with a sealed cap, steam expansion arrows and ejected blocks.

Volcanic Cones, Craters and Plateaus

Shield Volcano

A shield volcano is a very broad, gently sloping volcano, shaped like a warrior’s shield lying face up, built by countless flows of fluid basaltic lava from a summit vent and rift zones; its slopes are only a few degrees and its base can span over 100 km.

• Formation: low-viscosity basalt spreads far before cooling; eruptions are Hawaiian and effusive.
• Key features: summit caldera, flank rift zones, lava tubes.
• Examples: Mauna Loa, Hawaii, the largest active volcano on Earth, which erupted in November–December 2022; Kīlauea; Skjaldbreiður, Iceland (“broad shield”); Olympus Mons on Mars.
• Don’t confuse with: a flood basalt, which has no central cone.
• Sketch: a wide, low profile with a summit caldera.

Cinder Cone

A cinder cone is a small, steep, conical hill of loose scoria (cinders), usually less than about 300 m high, built by Strombolian eruptions from a single vent, with slopes near the angle of repose of about 30°–35° and a summit crater; most erupt only once.

• Formation: gas-rich basaltic fountains throw out clots that cool in flight and pile around the vent.
• Key features: permeable, easily eroded; lava often escapes from the base.
• Examples: Parícutin, Mexico, which rose in a cornfield on 20 February 1943 and grew to 424 m before activity ended in 1952; Wizard Island in Crater Lake; the central cone of Barren Island.
• Sketch: a steep symmetrical cone with a crater and a lava flow breaching its base.

Composite (Strato) Volcano

A composite volcano (stratovolcano) is a tall, steep, often symmetrical cone built of alternating layers of lava flows and pyroclastic material from repeated eruptions of viscous andesitic or dacitic magma; it forms the classic explosive volcanoes of subduction zones.

• Formation: explosive phases lay down ash and cinders; effusive phases add lava that armours the cone.
• Key features: concave profile steepening towards the summit; radial dykes; parasitic cones; prone to flank collapse, pyroclastic flows and lahars.
• Examples: Fuji (3,776 m), Mayon, Vesuvius, Kilimanjaro, Cotopaxi; Mount St Helens lost about 400 m of height in 1980; Barren Island and Narcondam in the Andaman Sea.
• Sketch: a steep cone cut open to show layers of lava and ash around a central pipe.

Lava Dome

A lava dome is a steep-sided, rounded mound of viscous lava — usually dacite or rhyolite — that piles up over a vent because it is too stiff to flow far; domes grow inside craters or on flanks and are dangerous because their collapse produces pyroclastic flows.

• Types: endogenous — grows by swelling from within; exogenous — grows by outpourings at the surface; plug dome — solid lava pushed up as a spine, like the spine of Mont Pelée in 1902–03.
• Examples: the domes built in Mount St Helens’ crater in 1980–86 and 2004–08; Unzen, Japan; Soufrière Hills, Montserrat; Lassen Peak, California.
• Don’t confuse with: a shield volcano, which is broad and fluid-built.

Parasitic (Adventive) Cone

A parasitic cone (adventive or lateral cone) is a subsidiary cone built on the flank of a larger volcano by eruptions from a branch of the main conduit or from a flank fissure, rather than from the summit vent.

• Formation: as a volcano grows tall, magma finds easier exits through fractures in its flanks.
• Key features: often cinder cones in lines along rift zones; flank eruptions come closer to towns, as when Etna’s 1669 flank lava reached Catania.
• Examples: the hundreds of flank cones on Etna, Sicily; Shastina on Mount Shasta, California; cinder cones on the flanks of Mauna Kea.
• Sketch: a large cone with small cones on its sides fed by branching pipes.

Crater and Caldera

A crater is a funnel-shaped depression, usually less than 1 km across, at the top of a volcanic vent, formed by explosion and by collapse around the vent; a caldera is a much larger basin — from a few to tens of kilometres wide — formed mainly by collapse of the volcano’s summit into a magma chamber emptied by a large eruption.

• Origin debate: the view that calderas are blasted out has given way to collapse, set out by Howel Williams (1941); explosion alone cannot remove so much rock, and the ejecta are mostly new magma.
• Types: Crater Lake type — collapse after a Plinian eruption; Hawaiian type — collapse as magma drains laterally; resurgent calderas whose floors later rise, such as Toba and Yellowstone.
• Examples: Crater Lake, Oregon, formed by collapse of Mount Mazama about 7,700 years ago, 594 m deep; Toba, Sumatra, about 100 km long; Kīlauea’s summit caldera, which collapsed by about 500 m in 2018; Barren Island, whose active cone stands inside a caldera.
• Sketch: a small summit crater beside a wide caldera with a lake and a new cone.

Maar and Diatreme

A maar is a broad, low-rimmed volcanic crater cut below the surrounding ground surface, formed by phreatomagmatic explosions where rising magma meets groundwater and often filled by a lake; a diatreme is the carrot-shaped pipe of shattered rock and volcanic breccia that lies beneath a maar.

• Formation: repeated explosions deepen the crater and fill the pipe with mixed country-rock and magma fragments.
• Examples: the maars of the Eifel, Germany, from whose local word the term comes; the Ukinrek Maars of Alaska, formed in 1977; kimberlite diatremes, the source of diamonds, at Kimberley in South Africa and the Majhgawan pipe near Panna in Madhya Pradesh.
• Don’t confuse with: a caldera, formed by collapse, or an impact crater, a pseudovolcanic feature.
• Sketch: a shallow crater lake above a funnel-shaped breccia pipe.

Flood Basalt (Large Igneous Province)

A flood basalt is a vast accumulation of nearly horizontal basalt flows, often kilometres thick, erupted from long fissures in geologically short pulses and building plateaus of stepped “trap” topography; the largest form large igneous provinces, defined by M. F. Coffin and Olav Eldholm (1992) as covering more than 100,000 km².

• Formation: a plume head melting beneath the lithosphere feeds dyke swarms and fissure eruptions (mantle plume).
• Examples: the Deccan Traps, erupted about 66 million years ago, now covering about 500,000 km² and perhaps once 1.5 million km², more than 2 km thick in the Western Ghats; the Siberian Traps (about 252 million years), linked to the end-Permian extinction; the Columbia River basalts (17–6 million years); the Paraná–Etendeka province; the Rajmahal Traps of Jharkhand.
• Debate: the Chicxulub impact is now seen as the main cause of the extinction at the end of the Cretaceous, with Deccan volcanism a contributing stress.
• Landforms: step-like scarps (Swedish trapp, stair), mesas and deep gorges.
• Sketch: stacked flat lava flows forming steps, fed by vertical dykes.

Intrusive Forms

Batholith, Laccolith, Lopolith and Phacolith

Batholiths, laccoliths, lopoliths and phacoliths are bodies of magma that solidified underground, distinguished by size and shape: a batholith is a huge, deep-seated, discordant granite mass; laccoliths, lopoliths and phacoliths are concordant bodies shaped like a dome, a saucer and a lens respectively.

Body	Shape and relation to strata	Example
Batholith	Huge, steep-walled, floor unseen, exposed over more than 100 km²; plutonic	Sierra Nevada batholith; Closepet Granite, Karnataka
Laccolith	Mushroom or lens, flat floor, arches overlying beds (Greek lakkos, cistern)	Henry Mountains, Utah, described by Grove Karl Gilbert (1877)
Lopolith	Large saucer sagging in the middle (Greek lopas, dish)	Bushveld Complex, South Africa
Phacolith	Lens in the crest or trough of a fold	Corndon Hill, Shropshire

• Landforms: unroofed batholiths form granite domes, tors and inselbergs — the domes of the Ranchi plateau; laccoliths form domed hills with radial drainage.
• Significance: batholiths are the roots of old mountain belts; lopoliths such as the Bushveld host platinum and chromium.

Sill and Dyke

A sill is a tabular sheet of igneous rock intruded parallel to the bedding of the host rocks (concordant); a dyke is a tabular, wall-like sheet that cuts across the bedding (discordant), usually steep, and often occurs in parallel or radial swarms that fed surface eruptions.

• Landforms: a hard sill forms scarps, ridges and waterfalls where it crops out; a hard dyke stands as a wall-like ridge, a softer one weathers into a trench.
• Examples: the Great Whin Sill of northern England, with High Force waterfall and Hadrian’s Wall along its scarp; the Palisades Sill on the Hudson; the Great Dyke of Zimbabwe, about 550 km long; the Deccan dyke swarms of the Narmada–Tapi belt; mica-peridotite and lamprophyre sills in the Jharia and Raniganj coalfields, which baked coal into natural coke.
• Sketch: a sill between beds and a dyke cutting across them, with a dyke ridge at the surface.

Volcanic Neck (Plug)

A volcanic neck (plug) is a steep, isolated tower of solidified magma or volcanic breccia that once filled the conduit of a volcano, left standing when erosion has removed the softer cone and surrounding rock.

• Formation: resistant conduit fill outlasts the weaker ash layers and country rock around it, so the neck grows taller as the land is lowered.
• Examples: Ship Rock, New Mexico, rising about 480 m above the plain with wall-like dykes radiating from it, formed about 27 million years ago; the Castle Rock of Edinburgh; the rock pinnacles of Le Puy-en-Velay, France.
• Sketch: a tower with radiating dyke ridges on a plain, the eroded cone outlined above it.

Post-Volcanic and Pseudovolcanic Features

Fumarole, Solfatara and Mofette

A fumarole is a vent emitting steam and volcanic gases, sometimes at several hundred degrees Celsius; a solfatara is a fumarole rich in sulphur gases that deposits yellow sulphur; a mofette is a cooler vent emitting mainly carbon dioxide. All three mark waning — or reawakening — volcanic heat.

• Key features: water vapour dominates fumarole gas; carbon dioxide, being heavy, collects in hollows around mofettes and suffocates animals.
• Examples: the Valley of Ten Thousand Smokes, Alaska, formed after the 1912 Novarupta eruption, the largest of the twentieth century; the Solfatara crater at Pozzuoli in the Campi Flegrei, Italy, which gave its name to the class.
• Significance: changes in gas output and temperature are used to forecast eruptions.

Geyser and Hot Spring

A hot spring is a steady discharge of groundwater heated by hot rock at depth; a geyser is a hot spring that erupts intermittently, throwing up a column of water and steam when water in a narrow, constricted conduit is superheated and flashes to steam. The word comes from Geysir in Iceland.

• Conditions: a heat source, abundant water and a narrow pipe lined with silica sinter (geyserite).
• Examples: Old Faithful and Steamboat Geyser, the tallest active geyser, in Yellowstone, which holds about half the world’s geysers; Strokkur, Iceland; Pōhutu, Rotorua, New Zealand.
• Indian examples: hot springs without true geysers — Manikaran in the Parvati valley, Himachal Pradesh; Tapovan, Uttarakhand; Puga valley in Ladakh, a geothermal field; Tattapani, Chhattisgarh; Bakreshwar, West Bengal. Many are fed by deep circulation along faults rather than by active volcanoes.

Pseudovolcanic Features

Pseudovolcanic features are landforms that resemble volcanic forms — cones, craters, domes or flows — but were not produced by a magmatic eruption at that spot; they arise instead from meteorite impact, steam explosions beneath lava flows, the forcing up of mud, gas or salt, or ground freezing and liquefaction. Definitions vary: a narrow use covers only forms with no magmatic link at all, while a broad use adds rootless cones fed by lava from elsewhere.

Types

Type	Process	Example
Impact crater	Hypervelocity meteorite impact	Lonar, Maharashtra; Meteor Crater, Arizona
Rootless cone (pseudocrater)	Lava crosses wet ground; trapped water explodes as steam	Skútustaðagígar, Lake Mývatn, Iceland
Mud volcano	Gas-charged mud forced up from buried sediment	Baratang, Andaman Islands
Gas-emission crater	Blow-out of gas from thawing permafrost	Yamal Peninsula, Siberia
Salt dome and salt glacier	Buoyant salt rising and spreading at the surface	Salt domes of the Zagros, Iran
Sand volcano (sand blow)	Liquefied sand vented during earthquakes	Rann of Kachchh, 2001
Hydrothermal explosion crater	Steam blast in a geothermal field	Mary Bay, Yellowstone

• Lonar: a crater 1.83 km wide and 150 m deep in Deccan basalt in Buldhana district, long thought to be volcanic; shatter cones and shock-melted glass (maskelynite) proved an impact. Its age is disputed — about 50,000 years by one method, about 570,000 years by argon dating. It became a Ramsar site in November 2020.
• Rootless cones: no conduit connects them to a magma chamber; they are also found on Mars.
• Gas-emission craters: first spotted in 2013–14; 17 were known by 2020, linked to permafrost thaw.
• Significance: misreading such forms distorts hazard maps and geological history; mud volcanoes point to hydrocarbons; gas craters are a new Arctic hazard.
• Sketch: a true volcano with a conduit to a magma chamber beside an impact crater and a mud volcano with no magmatic root.

Mud Volcano

A mud volcano is a cone, dome or pool from which a mixture of mud, water and gas — chiefly methane — is forced to the surface from overpressured clay-rich sediments at depth, driven by the weight of overlying strata, tectonic squeezing and gas buoyancy rather than by magma.

• Features: cones from a few metres to hundreds of metres high; gryphons — steep cones under 3 m; salses — bubbling mud pools; methane sometimes ignites.
• Distribution: about 1,100 on land, nearly 400 of them in Azerbaijan; many more on the sea floor; common in accretionary prisms and oil basins.
• Examples: the Lusi mud volcano in East Java, which began on 29 May 2006, flooded about 4 km², killed 14 and displaced about 24,000 people; the Makran coast of Pakistan, where the 2013 Balochistan earthquake raised a mud island off Gwadar; Baratang Island in the Andamans — India’s only mud volcanoes, on the Andaman accretionary wedge — which erupted on 18 February 2003 and again in 2005 after the 2004 earthquake (accretionary wedge).
• Sketch: a mud cone with a crater above a buried gas-charged clay layer and a fracture feeder.

PYQs Built on These Terms

• Describe phreatic eruptions and their consequences. (2019)
• Write a note on pseudovolcanic features. (2017)
• Discuss the concept of volcanicity and show how the theory of Plate Tectonics explains the mechanism of volcanism and volcanic eruptions. (1998)
• Discuss the economic significance of the volcanic soils of India. (Paper II, 2021)

Frequently Asked Questions

What is the difference between a phreatic and a phreatomagmatic eruption?

A phreatic eruption is a steam explosion of heated groundwater that throws out only old rock, mud and gas, with no new magma, as at Ontake in 2014. A phreatomagmatic eruption happens when magma itself meets water, so fresh magma fragments are ejected, as at Surtsey or Hunga Tonga in 2022. Both are sudden and violent.

What is the difference between a crater and a caldera?

A crater is a small depression, usually under a kilometre across, at the mouth of a volcanic vent. A caldera is a much larger basin, often several to tens of kilometres wide, formed when the summit collapses into a magma chamber emptied by a big eruption. Crater Lake in Oregon and Toba in Sumatra are calderas.

Is Lonar crater volcanic?

No. Lonar crater in Maharashtra lies within Deccan basalt and was long assumed to be volcanic, but shatter cones and shock-melted glass show it was made by a meteorite impact. It is therefore a classic pseudovolcanic feature: it looks like a volcanic crater but no eruption formed it.

Which is the only active volcano in India?

Barren Island in the Andaman Sea is India’s only confirmed active volcano. It has erupted repeatedly since its first recorded activity in 1787, reawakened in 1991 and has been almost continuously active since 2022. Nearby Narcondam is generally treated as dormant. Both lie on the Andaman–Sunda volcanic arc above the subducting Indian Plate.

What is the difference between a sill and a dyke?

A sill is a sheet of intruded magma that runs parallel to the layers of the host rock, while a dyke cuts across them, usually steeply. Sills form flat-topped scarps and waterfalls, like the Great Whin Sill in England; dykes form wall-like ridges or trenches, like the Deccan dyke swarms along the Narmada.

Why are shield volcanoes gentle and composite volcanoes steep?

Shield volcanoes are built of fluid basaltic lava that spreads far before solidifying, giving slopes of only a few degrees, as on Mauna Loa. Composite volcanoes erupt viscous, gas-rich magma that piles up close to the vent in alternating layers of lava and ash, producing steep cones such as Fuji and Mayon.