Volcano Science Explained: How Volcanoes Form, Magma, Lava and Eruptions

What Is a Volcano?

A volcano is an opening, vent or system of fractures through which magma, volcanic gases, ash and broken rock can escape from inside Earth. The word can also describe the landform built around those openings by repeated eruptions.

Although volcanoes are commonly pictured as steep mountains with summit craters, volcanic systems take many forms. They include broad shield volcanoes, stratovolcanoes, small cinder cones, lava domes, long fissures, large calderas and submarine vents hidden beneath the ocean.

A volcano is therefore not simply a mountain filled with molten rock. It is an evolving geological system connecting:

• A source region where rock partially melts
• Zones where magma rises through the crust
• Underground reservoirs and crystal-rich storage regions
• Dikes, sills, fractures and conduits
• One or more surface vents
• Lava flows, cones, domes, craters or calderas
• Hydrothermal systems heated by magma

Many large volcanoes contain several vents connected to a shared underground system. Eruptions can occur at the summit, along the flanks or from fissures extending far beyond the central cone.

How Do Volcanoes Form?

Volcanoes form when rock beneath Earth’s surface partially melts and produces magma that rises through the crust. Because magma is often less dense than the surrounding solid rock, buoyancy can help it move upward through fractures and weak zones.

Rock does not usually melt because Earth’s mantle becomes completely liquid. Most magma forms through partial melting, in which only some minerals within hot mantle or crustal rock melt.

Three major processes can promote magma generation:

Decompression melting

Hot mantle rock can begin to melt when it rises and the pressure surrounding it decreases. This process is common at divergent plate boundaries, continental rifts and many oceanic hotspots.

Flux melting

Water and other volatile substances released from a descending tectonic plate can lower the melting temperature of overlying mantle rock. This process generates magma above subduction zones and feeds many volcanic arcs.

Heat-transfer melting

Hot magma rising from the mantle can transfer heat into surrounding crustal rocks. Some of those rocks may partially melt, changing the composition and behavior of the evolving magma.

Once generated, magma may rise directly toward the surface, stall within the crust, spread sideways, mix with other magma or cool and crystallize underground. Only a portion of the magma produced beneath volcanic regions eventually erupts.

Plate Tectonics & Volcanoes

Most active volcanoes are associated with tectonic plate boundaries. Earth’s rigid outer shell is divided into moving plates that converge, separate or slide past one another.

Volcanism is especially common at convergent boundaries and divergent boundaries. Transform boundaries, where plates slide horizontally past each other, generally produce earthquakes rather than extensive magma generation.

Subduction-zone volcanoes

Subduction occurs when one tectonic plate sinks beneath another. The descending plate carries water-bearing minerals and sediments into the mantle. As depth, temperature and pressure increase, water and other volatile substances are released into the overlying mantle.

These fluids lower the melting temperature of the mantle and promote partial melting. The resulting magma can rise through the crust and feed chains of volcanoes known as volcanic arcs.

Subduction-zone volcanism created many of the volcanoes surrounding the Pacific Ocean, including those in:

• Japan
• Indonesia
• The Philippines
• Kamchatka
• The Cascade Range
• The Andes
• New Zealand

Because subduction-zone magmas may evolve toward relatively viscous, gas-rich compositions, these volcanoes can produce powerful explosive eruptions.

Divergent-boundary volcanoes

At divergent boundaries, tectonic plates move apart. Hot mantle rises to replace the separating material, pressure decreases and decompression melting generates magma.

This process creates volcanic activity along the global mid-ocean ridge system, where new oceanic crust continuously forms. Because most ridges are underwater, much of Earth’s volcanism occurs unseen beneath the oceans.

Divergent volcanism also occurs where continents are being pulled apart, including the East African Rift. Iceland is a particularly visible example because part of the Mid-Atlantic Ridge rises above sea level and interacts with an additional source of volcanic activity beneath the island.

Explore the tectonic settings of major volcanic zones in Volcanic Regions Explained.

Mantle Plumes & Volcanic Hotspots

Not all volcanoes form directly along plate boundaries. Some volcanic chains develop within tectonic plates above regions commonly called hotspots.

A widely used explanation proposes that some hotspots are fed by columns or broader upwellings of unusually hot mantle material. As this material rises, decompression can promote partial melting and magma formation.

When a tectonic plate moves across a relatively persistent magma source, a trail of volcanoes may form. Older volcanoes are gradually carried away from the active source while new volcanoes develop above it.

The Hawaiian–Emperor volcanic chain is the classic example. The active volcanoes of Hawaii occupy one end of a much longer chain of islands and submerged seamounts recording Pacific Plate movement over millions of years.

Yellowstone also sits at the active end of a volcanic track extending across the Snake River Plain. Its volcanic system differs greatly from Hawaii because the rising magma interacts with thick continental crust.

The exact origin, depth and shape of every hotspot remain subjects of scientific research. Some may involve deep mantle plumes, while others may be influenced by shallower mantle circulation, plate stresses or pre-existing structural weaknesses.

Continue with Hawaiian Volcanoes & Hotspot Volcanism and the Yellowstone Supervolcano & Hydrothermal System.

Anatomy of a Volcano

The visible cone is only the uppermost part of a volcanic system. Beneath the surface, magma can occupy an interconnected network extending through a large volume of crust.

Important parts of a volcano include:

Magma source

The deeper region where partial melting generates magma. This source may lie in the upper mantle, lower crust or both.

Magma reservoir

An underground zone where magma accumulates, evolves and interacts with crystals, gases and surrounding rock.

Dike

A sheet-like body of magma that cuts across existing rock layers, often steeply or vertically. Dikes can transport magma toward the surface or laterally away from a central reservoir.

Sill

A sheet of magma injected approximately parallel to existing rock layers. Sills allow magma to spread sideways through the crust.

Conduit

A pathway through which magma and gases move toward an erupting vent. Conduits are rarely simple permanent pipes and may repeatedly open, seal, widen or shift.

Main vent

The primary surface opening from which lava, ash, gases or rock fragments erupt.

Flank vent

An opening on the side of a volcano. Large volcanic systems may erupt through flank vents located many kilometers from the summit.

Crater

A relatively small depression surrounding a volcanic vent, commonly formed by explosions, collapse or excavation during eruptions.

Caldera

A much larger volcanic depression formed mainly when the ground collapses after a substantial volume of magma is removed from an underground reservoir.

Learn more about large collapse structures in Calderas Explained.

What Is Magma?

Magma is hot, molten or partially molten material beneath Earth’s surface. It is usually a mixture rather than a completely uniform liquid.

Magma may contain:

• Liquid silicate melt
• Solid mineral crystals
• Dissolved gases
• Gas bubbles
• Fragments of surrounding rock

As magma rises and cools, minerals begin to crystallize. The remaining liquid can change in composition as different minerals are removed from the melt. Magma may also mix with newly arriving magma or incorporate material from surrounding crust.

These processes mean that magma stored beneath a volcano can evolve substantially before it erupts.

Main magma compositions

Volcanic magmas are commonly grouped according to their silica content and overall chemistry:

• Basaltic magma: generally hotter, lower in silica and relatively fluid
• Andesitic magma: intermediate in composition and viscosity
• Dacitic magma: relatively silica-rich and viscous
• Rhyolitic magma: high in silica and commonly very viscous

These categories do not determine eruption behavior by themselves, but they strongly influence magma viscosity, crystallization and the ability of gas bubbles to escape.

Magma Composition, Temperature and Viscosity

Viscosity describes a material’s resistance to flow. Low-viscosity magma flows relatively easily, while high-viscosity magma is much more resistant to movement.

Magma viscosity is controlled mainly by:

• Chemical composition, especially silica content
• Temperature
• The abundance and shape of crystals
• The presence of bubbles
• Water and other dissolved volatile substances

Hot basaltic magma is typically less viscous than cooler, silica-rich rhyolitic magma. Fluid magma allows gas bubbles to rise and escape more easily, often favoring lava fountains and flowing lava.

Viscous magma can impede gas escape. Pressure may then build as bubbles expand, increasing the likelihood of magma fragmentation and explosive activity.

This does not mean every basaltic eruption is gentle or every rhyolitic eruption is catastrophic. Basaltic magma can produce violent explosions when it interacts with water or accumulates gas, while some viscous magma erupts slowly to build lava domes.

Eruption style results from the combined effects of magma chemistry, gas content, ascent rate, conduit conditions and interaction with water or ice.

Magma Chambers & Magma Reservoirs

A magma chamber is commonly imagined as a giant underground cavern filled entirely with liquid magma. Real volcanic storage systems are usually more complicated.

Magma may be distributed through interconnected reservoirs, lenses, fractures and crystal-rich zones extending across different depths. Some regions may contain mobile melt, while others consist mainly of crystals with smaller amounts of liquid between them.

The term magma reservoir is therefore often more useful than the image of one enormous molten chamber.

While stored within the crust, magma can:

• Cool and crystallize
• Release or absorb gases
• Mix with newly arriving magma
• Melt or incorporate surrounding rock
• Separate into layers or chemically distinct portions
• Move into dikes, sills and smaller reservoirs

Some magma reservoirs remain active for thousands or hundreds of thousands of years. Others solidify underground without ever producing an eruption.

Changes within a reservoir can deform the ground above it, trigger earthquakes and alter volcanic gas emissions. Scientists use these signals to investigate volcanic unrest, as explained in Volcano Monitoring & Forecasting.

Volcanic Plumbing Systems

A volcanic plumbing system is the network through which magma, gases and hydrothermal fluids move beneath a volcano. It may connect deep magma-generation zones with shallow reservoirs, dikes, sills, conduits and surface vents.

These systems are dynamic. Magma does not necessarily follow the same route during every eruption.

Rising magma may:

• Move vertically through a dike
• Spread horizontally through a sill
• Collect in a shallow reservoir
• Branch into several intrusions
• Stall and crystallize underground
• Travel laterally before erupting far from the summit
• Heat groundwater without reaching the surface

Volcanic earthquakes often occur when pressurized magma or fluids fracture rock and open new pathways. Ground deformation can occur as intrusions push surrounding rocks apart or as magma enters or leaves a reservoir.

Some eruptions involve magma rising directly from depth. Others result from the reactivation of older magma stored within the crust. Newly arriving magma may heat, mix with or pressurize an existing reservoir.

Understanding this plumbing is essential because the location of surface unrest does not always reveal exactly where a future eruption will occur.

Volcanic Gases

Volcanic gases are dissolved within magma under high pressure. As magma rises and surrounding pressure decreases, those gases separate from the melt and form bubbles—a process called exsolution.

The same basic effect occurs when a pressurized carbonated drink is opened, although volcanic systems operate at vastly greater temperatures, pressures and scales.

The most abundant volcanic gas is usually water vapor. Other important gases include:

• Carbon dioxide
• Sulfur dioxide
• Hydrogen sulfide
• Hydrogen
• Carbon monoxide
• Hydrogen chloride
• Hydrogen fluoride

Gas expansion is one of the main forces driving volcanic eruptions. If bubbles escape easily, magma may erupt relatively quietly. If bubbles remain trapped within viscous magma, pressure can increase until the magma fragments explosively.

How volcanic gases reach the surface

Gases can escape through:

• Active eruptive vents
• Fumaroles
• Cracks and fractures
• Crater lakes
• Hot springs
• Porous soil
• Diffuse degassing across broad areas

Gas emissions may continue when no lava is erupting. Changes in gas composition or output can provide information about magma depth, movement and interaction with groundwater.

Volcanic gases can also create serious hazards. Carbon dioxide can accumulate in low-lying areas, sulfur dioxide can irritate the respiratory system and react in the atmosphere, and fluorine-rich ash can contaminate vegetation and water.

Hazardous volcanic smog produced when sulfur dioxide reacts in the atmosphere is commonly called vog. Explore these dangers in Volcanic Hazards Explained.

What Is Lava?

Lava is magma that has reached Earth’s surface. Once erupted, it begins losing heat, releasing gas and forming new volcanic rock.

Lava behavior depends on temperature, composition, crystal content, eruption rate, slope and surrounding terrain.

Basaltic lava

Basaltic lava is generally hot and fluid enough to travel considerable distances under favorable conditions. It commonly forms broad shield volcanoes, fissure-fed lava fields and oceanic crust.

Andesitic, dacitic and rhyolitic lava

More silica-rich lava is usually more viscous. Instead of flowing freely, it may move slowly, form thick blocky flows or accumulate around a vent to build a lava dome.

Pāhoehoe lava

Pāhoehoe is a basaltic lava surface characterized by smooth, billowy or rope-like textures. A cooling crust may form while liquid lava continues moving beneath it.

‘A‘ā lava

‘A‘ā develops a rough, broken and jagged surface as the moving flow tears apart its cooling crust. Its interior may remain molten while a mass of loose clinker advances at the flow front.

Block lava

More viscous lava can break into large angular blocks rather than producing the loose, clinker-like surface associated with ‘a‘ā.

Pillow lava

When lava erupts underwater, rapid cooling can create rounded or elongated masses called pillow lavas. These structures are widespread on the ocean floor.

Lava tubes

A lava tube forms when the surface of a flow cools and solidifies while molten lava continues moving beneath the crust. Tubes insulate the lava and can allow it to travel farther from the vent.

Lava flows are often slower than pyroclastic flows, but they can bury roads, destroy buildings, ignite vegetation and permanently reshape communities and landscapes.

What Triggers a Volcanic Eruption?

Volcanic unrest does not always lead to an eruption, and eruptions do not all begin for the same reason. Several processes may destabilize a magma reservoir or open a pathway toward the surface.

Injection of new magma

Fresh magma entering an existing reservoir can add heat, mass, gases and pressure. It may also mix with older magma and alter its density or viscosity.

Gas accumulation and expansion

As magma rises or crystallizes, dissolved gases can form bubbles. Expanding gas may increase pressure and help drive magma toward the surface.

Buoyancy and overpressure

Magma can rise when it becomes sufficiently buoyant or when pressure inside a reservoir exceeds the strength of surrounding rock.

Fracturing of surrounding rock

Tectonic stresses, earthquakes or pressure from magma can create new fractures that allow magma to migrate.

Collapse or removal of overlying material

Landslides, glacier retreat, erosion or changes in loading may alter pressure conditions around a volcanic system, although their influence varies greatly between volcanoes.

Interaction with groundwater

Magma or hot rock can rapidly heat groundwater. Expanding steam may trigger explosions even when no new magma reaches the surface.

These triggers can operate together. An eruption may begin after a sequence involving deep magma recharge, earthquake swarms, dike intrusion, gas accumulation and final conduit opening.

Volcanic Eruption Styles

Volcanic eruptions range from quiet lava effusion to enormous explosions that inject ash and gases high into the atmosphere. Eruptive behavior may also change during a single eruption.

The most important controls include:

• Magma viscosity
• Gas content
• Magma ascent rate
• Conduit geometry
• Crystals and bubbles within the magma
• Interaction with groundwater, seawater, snow or ice
• The rate at which magma is supplied

Effusive eruptions

Effusive eruptions occur when lava reaches the surface without extensive explosive fragmentation. They may produce lava flows, lava lakes, fountains and spatter around vents or fissures.

Effusive does not mean harmless. Lava can destroy infrastructure, isolate communities, release hazardous gases and cover extensive areas.

Hawaiian eruptions

Hawaiian-style activity commonly involves fluid basaltic magma, lava fountains and flowing lava. Gas generally escapes more easily than it does from highly viscous magma.

Strombolian eruptions

Strombolian activity consists of repeated bursts caused by gas bubbles rising and bursting near the surface. These explosions can throw incandescent lava fragments, bombs and scoria around the vent.

Vulcanian eruptions

Vulcanian explosions are short, forceful eruptions that eject ash, blocks and bombs. They may occur when viscous magma or solidified material temporarily seals a vent and pressure builds beneath it.

Plinian eruptions

Plinian eruptions generate sustained columns of ash, pumice and gas that can rise high into the atmosphere. Large columns may spread ash across enormous regions or collapse to produce pyroclastic flows.

Pelean and dome-collapse activity

Viscous magma may accumulate as a lava dome. If part of the unstable dome collapses, hot gas and fragmented lava can accelerate downslope as pyroclastic flows.

Phreatic eruptions

Phreatic eruptions are steam-driven explosions caused when water is rapidly heated by magma or hot rock. They may eject old rock and ash without bringing fresh magma to the surface.

Phreatomagmatic eruptions

Phreatomagmatic activity occurs when magma interacts directly with water. Rapid steam generation and fragmentation can produce fine ash, explosive jets and ground-hugging surges.

Subglacial eruptions

Eruptions beneath glaciers or ice sheets can melt large volumes of ice, produce steam explosions and trigger sudden floods or lahars.

Fissure eruptions

Fissure eruptions occur along elongated cracks rather than a single central vent. They can produce lines of lava fountains and extensive lava fields.

Volcanologists also use the Volcanic Explosivity Index, or VEI, to describe the relative scale of explosive eruptions. The index considers factors including erupted material, eruption-column height and qualitative observations, but it does not measure every aspect of an eruption’s danger.

Compare the landforms associated with these eruption styles in Volcano Types Explained.

Active, Dormant and Extinct Volcanoes

Volcanoes do not erupt at regular intervals. A system may erupt frequently, remain quiet for thousands of years or gradually lose its magma supply.

Active volcano

An active volcano is one that is erupting, has erupted within a geologically recent period or shows evidence that it could erupt again. Definitions vary between scientific organizations and geological settings.

Dormant volcano

Dormant is an informal term for a volcano that is currently quiet but still considered capable of future activity. Long periods without eruption do not necessarily mean a volcanic system is extinct.

Extinct volcano

An extinct volcano is considered unlikely to erupt again because its magma supply or geological setting is no longer active. This classification can remain uncertain when the eruption history and underground structure are poorly known.

Scientists assess a volcano’s status using:

• Past eruption deposits
• Radiometric dating
• Earthquake activity
• Ground deformation
• Gas emissions
• Heat flow
• Regional tectonic conditions
• Evidence of magma beneath the surface

A volcano may appear completely quiet at the surface while retaining heat, magma or active hydrothermal circulation underground.