How do volcanoes erupt?

A volcano is a vent in Earth’s crust that lets molten rock, volcanic gases, and sometimes ash reach the surface. The molten rock below is called magma when it is still underground and lava once it comes out. The trigger is simple: pressure builds in a magma chamber beneath the volcano until the rock above can no longer hold it, and the magma pushes or blasts its way to the surface.

The deeper driver is plate tectonics. Most volcanoes line up along plate boundaries, especially the “Ring of Fire” that circles the Pacific. Where an oceanic plate dives under another plate in a subduction zone, water released from the sinking slab lowers the melting point of the overlying mantle rock, generating magma that rises and feeds arc volcanoes like those in the Cascades, the Andes, and Japan. At mid-ocean ridges, plates pull apart and magma wells up to create new seafloor, much of which never builds an island. Hotspot volcanoes, like Hawaii and Yellowstone, sit in the middle of plates where a deep mantle plume delivers heat from below.

Why some eruptions are violent and others are quiet comes down to magma chemistry and gas content. USGS explains that runny, low-silica basalt magma lets dissolved gases escape easily, so it tends to flow as lava, the kind you see in Hawaii. Sticky, high-silica magma traps gas, which builds pressure until it explodes, the kind that produces the towering ash clouds of Mount St. Helens in 1980 or the 1991 eruption of Mount Pinatubo. Mount St. Helens killed 57 people and flattened hundreds of square miles of forest. Pinatubo’s aerosol cloud lowered global temperatures by about 0.5 °C for the next year or two.

The actual mechanics are layered. As magma rises, dissolved water and carbon dioxide come out of solution and form bubbles, the same way a soda fizzes when you open the bottle. If the magma is fluid, those bubbles slip away gently. If the magma is sticky, they expand explosively and shred the magma into ash and pumice. Once an eruption is underway, lava flows, pyroclastic flows of hot gas and ash, ashfall, lahars of volcanic mud, and volcanic gases are the main hazards. Pyroclastic flows are the deadliest; they can travel over 60 mph and reach temperatures above 700 °C, which is why hazard maps around active volcanoes draw restricted zones around valleys that channel them.

Volcanoes are also closely watched because of the secondary effects. The 2010 eruption of Eyjafjallajökull grounded European flights for a week because volcanic ash melts inside jet engines. Sulfur dioxide plumes can affect air quality hundreds of miles downwind, and large eruptions can shift global rainfall patterns for months. Long-arc risks include climate shifts; the 1815 Tambora eruption caused the “Year Without a Summer” in 1816, with crop failures in Europe and snow in June in New England.

Volcanic soils are a quieter upside. Repeated ashfall weathers into mineral-rich, well-draining soils that support surprisingly dense agriculture, which is why places like Naples, the Andes, Indonesia, and the Pacific Northwest have historically thrived on the flanks of dangerous peaks. Geothermal energy is another benefit; Iceland, the Philippines, Kenya, and parts of California all tap volcanic heat for power.

That is why the USGS Hawaiian Volcano Observatory, the Alaska Volcano Observatory, and the Smithsonian Global Volcanism Program run constant monitoring networks of seismometers, GPS stations, gas sensors, and webcams. The aim is to catch the warning signs — increased earthquakes, ground swelling, rising sulfur dioxide, and changes in fume chemistry — and give nearby communities time to evacuate before the magma finds a way out. For more on the seismic side, see the earthquake explainer and the weather versus climate guide.