During a volcanic eruption, hot molten rock (magma), gas, and ash rise from deep underground and burst out through cracks or a main vent at the surface, changing the landscape and atmosphere around the volcano.

What sets off an eruption?

  • Deep underground, rock melts into magma because of high heat and pressure near tectonic plate boundaries.
  • Magma is full of dissolved gases (like water vapour and carbon dioxide) that expand as it rises, increasing pressure inside the volcano.
  • When that pressure becomes stronger than the surrounding rock, fractures and vents open and magma is forced towards the surface, triggering an eruption.

Step‑by‑step: what happens during an eruption?

  1. Magma rises
    • Magma moves up from a source region into a magma chamber beneath the volcano, often causing the ground to swell and crack before and during the eruption.
  1. Pressure breaks the surface
    • As gases in the magma bubble and expand, they push the molten rock upward through conduits and fissures, sometimes cracking surface rocks and widening the vent.
  1. Lava, ash, and gases burst out
    • The volcano ejects a mixture of lava, ash, rock fragments (bombs and blocks), and hot gases through the main crater or side vents.
 * In explosive eruptions, gas-charged magma shatters into fine ash and forms towering eruption columns that can reach high into the atmosphere.
  1. Dangerous flows race down the slopes
    • Pyroclastic flows—fast, dense clouds of hot gas, ash, and rock—can surge down the volcano at speeds up to hundreds of kilometers per hour and are the most lethal hazard.
 * Lahars (volcanic mudflows) form when ash and debris mix with water from rain, rivers, or melted snow and ice, flowing like wet concrete along valleys.
  1. After the main burst
    • Activity can shift to quieter outpourings of lava, which spread downslope and cool into new rock layers.
 * As magma is withdrawn, the ground can subside and fracture, sometimes forming craters or calderas where the surface collapses.

Different “styles” of volcanic eruption

  • Magmatic eruptions
    • Driven mainly by decompression of gas in magma, which propels lava, ash, and bombs out of the vent.
* Can range from gentle lava flows to highly explosive events depending on magma thickness and gas content.
  • Phreatic (steam‑blast) eruptions
    • Occur when groundwater or surface water contacts hot rock or magma, flashes to steam, and explodes, blasting out steam, ash, and rock fragments without much fresh magma.
  • Phreatomagmatic eruptions
    • Involve direct interaction of water and magma, producing especially mobile, water-rich pyroclastic surges called base surges.

What comes out of a volcano?

  • Lava: Molten rock that reaches the surface, then flows and solidifies into new crust.
  • Tephra: All solid material thrown into the air—ash, lapilli, volcanic bombs, and large blocks.
  • Gases: Mainly water vapour, carbon dioxide, sulfur dioxide, and other gases that can affect air quality and climate.
  • Pyroclastic flows and surges: Hot, fast-moving clouds that can devastate areas tens of kilometers away.

Impacts on people and the environment

  • Immediate hazards
    • Incineration, asphyxiation, impact, and burial from pyroclastic flows and ash, plus building damage from ground shaking, fissuring, and heavy ash fall.
* Lahars can destroy bridges, roads, and settlements far downstream from the volcano.
  • Longer‑term effects
    • Ash can disrupt air travel and damage crops, water supplies, and infrastructure.
* Large eruptions can influence climate by injecting ash and gases into the atmosphere, sometimes cooling global temperatures, while emitted carbon dioxide contributes to warming.

Information gathered from public forums or data available on the internet and portrayed here.