Grades 3–5 · Earth & Space
What makes a volcano erupt?
A volcano is connected to places underground where hot, partly molten rock can collect. This material is magma. It contains liquid rock, crystals and dissolved gases, rather than being a pool of ordinary fire.
In the diagramRising magma and expanding gas can eruptRockStorageCracksGas and magma escape through a surface routeOne conceptual route; not all rising magma erupts.
Magma can rise because it is less dense than surrounding rock, and pressure can help force it through cracks. As it gets closer to the surface, the surrounding pressure usually falls. Some dissolved gas comes out of the magma and forms bubbles. The bubbles can expand as pressure falls further.
If magma opens a path to the surface, an eruption can begin. Runny magma that releases gas readily may flow out as lava. Thick, sticky magma that holds expanding gas can break into fragments during an explosive eruption. Volcanic ash is made of tiny rock and glass fragments, not wood ash.
The drawing follows one gas-rich route. Some magma stays underground and never erupts, and volcanoes do not all have the same shape or eruption style. Gas escape, magma properties and the surrounding rock all matter.
About this explanation
Starting knowledge: Know that magma is hot partly molten rock beneath the surface and lava is magma erupted outside.
Sources
- U.S. Geological Survey — About Volcanoes
- U.S. Geological Survey — Impacts & Mitigation: Gases & Vog