How Volcanoes Form and the Different Types of Eruptions They Produce

The 1815 Eruption of Mount Tambora Lowered Global Temperatures for a Year and Caused Crop Failures Worldwide

Tambora's eruption on April 10, 1815 — the largest volcanic eruption in recorded history — ejected 160 km³ of magma, sent sulfur dioxide 43 km into the stratosphere, and caused a volcanic winter in 1816. Crop failures from abnormal frosts and cold swept Europe and North America. Tens of thousands died from famine in what became known as "the year without a summer." Krakatoa, by comparison, ejected only 21 km³. Pinatubo in 1991 ejected 10 km³ and cooled global temperatures by 0.5°C for a year. The scale difference between eruptions is not linear — it spans orders of magnitude, and so do the consequences.

A volcano is an opening in Earth's crust through which molten rock (magma), volcanic gases, and ash escape to the surface. Approximately 1,500 potentially active volcanoes exist on Earth's surface; an estimated 80% of volcanic activity occurs below the oceans, largely unobserved. Understanding why volcanoes occur where they do requires understanding plate tectonics.

The Three Tectonic Settings for Volcanism

Volcanoes are not randomly distributed. They concentrate in three geological settings, each producing a distinct type of volcanic activity:

• Subduction zones: Where an oceanic plate descends beneath a continental or oceanic plate, the subducting slab releases water (from hydrated minerals) into the overlying mantle wedge. Water lowers the melting point of mantle rock, generating magma. This magma rises through the crust to form arc volcanoes — the Cascades, the Andes, Japan, Indonesia. Subduction zone magmas are typically silica-rich (andesitic to rhyolitic) and highly explosive.
• Mid-ocean ridges: At divergent plate boundaries, seafloor spreading causes mantle rock to rise through decompression melting — the pressure drop as rock ascends causes melting without additional heat input. The resulting basaltic magma is low in silica and erupts relatively gently, building the oceanic crust that covers two-thirds of Earth's surface.
• Mantle plumes (hotspots): Columns of anomalously hot mantle material rise from deep within the mantle, melting as they approach the surface regardless of plate boundaries. Hotspot volcanism creates volcanic island chains (Hawaii), flood basalt provinces (the Deccan Traps), and isolated ocean island volcanoes. Hawaii's Kilauea has been erupting nearly continuously since 1983.

Magma Chemistry: The Key Variable in Eruption Style

Whether a volcanic eruption is a gentle lava lake or a catastrophic explosive blast depends more on magma chemistry than on eruption volume. The controlling variable is silica (SiO₂) content and dissolved gas concentration.

Silica polymerizes at high concentrations, creating long SiO₄ chain networks that dramatically increase viscosity. High-viscosity magma traps dissolved gases (primarily water vapor, CO₂, and SO₂) that can only escape explosively — like shaking a carbonated bottle. Low-viscosity basaltic magma allows gases to bubble out slowly, producing the relatively gentle fountain eruptions typical of Hawaii.

Types of Volcanic Eruptions

Volcanologists classify eruptions by their style and intensity using the Volcanic Explosivity Index (VEI), a logarithmic scale from 0 (non-explosive) to 8 (super-colossal).

• Hawaiian eruptions (VEI 0–1): Low-viscosity basaltic lava forms fire fountains and lava lakes. Lava flows can travel dozens of kilometers at several km/hour. Relatively low hazard to human life, high property destruction potential.
• Strombolian eruptions (VEI 1–2): Rhythmic, moderate explosions ejecting incandescent blobs (bombs) 100–200 m into the air. Stromboli itself has erupted almost continuously for 2,000 years.
• Vulcanian eruptions (VEI 2–3): Short, violent explosions of relatively viscous magma, producing dense ash clouds and ballistic blocks. The eruption of Soufrière Hills volcano in Montserrat (1997) destroyed Plymouth, the island's capital, with Vulcanian activity.
• Plinian eruptions (VEI 4–6): Sustained, high-velocity columns of fragmented magma and gas driven 20–45 km into the stratosphere. Named for Pliny the Younger's account of Vesuvius (79 CE). Mount Pinatubo (1991, VEI 6) injected enough aerosols to cool global temperature by 0.5°C for 18 months.
• Ultra-Plinian eruptions (VEI 7–8): Super-eruptions that collapse calderas and produce ignimbrite sheets hundreds of km³ in volume. The last VEI-8 was Toba, ~74,000 years ago. Yellowstone's last super-eruption (640,000 years ago) deposited ash across half of North America.

Pyroclastic Flows: The Deadliest Volcanic Hazard

Pyroclastic density currents — commonly called pyroclastic flows — are ground-hugging avalanches of hot gas, ash, and rock fragments that travel at 100–700 km/hour and reach temperatures of 300–800°C. Nothing in their path survives. The 1902 eruption of Mount Pelée on Martinique generated a pyroclastic flow that killed all 29,000 residents of Saint-Pierre in under two minutes. Only one man survived, protected by his jail cell.

Pyroclastic flows form when eruption columns collapse — either because the erupted material is too dense to remain buoyant, or because the magma supply rate is too high. They can travel up slopes, cross water, and deposit pumice and ash sheets meters thick across hundreds of square kilometers.

Volcanic Monitoring and Eruption Prediction

Volcano observatories monitor multiple precursors to eruption:

• Seismicity: Swarms of small earthquakes signal magma movement and fracturing of host rock. LP (long-period) earthquakes indicate fluid movement in conduits.
• Ground deformation: GPS and InSAR satellite radar detect inflation of volcanic edifices as magma accumulates underground. Kilauea's summit inflated by several meters before the 2018 eruption.
• Gas emissions: Increased SO₂ and CO₂ flux signals degassing magma rising toward the surface. High-resolution FTIR spectrometers and DOAS instruments measure gas ratios that indicate magma composition and depth.
• Thermal imaging: Infrared cameras detect new heat anomalies at fumaroles and summit craters.

Eruption prediction has improved dramatically since the 1980 Mount St. Helens eruption, which was monitored but still surprised scientists with its lateral blast. Modern monitoring of Pinatubo in 1991 enabled the evacuation of 60,000 people, preventing casualties that would have numbered in the tens of thousands.