Volcano
A volcano is an opening in Earth's crust through which magma, gas, and ash erupt, often building a mountain from the accumulated material.
Definition
A volcano is a place where melted rock, ash, and gas from deep underground reach the surface, often building a mountain from the material they erupt. Shield volcanoes such as those in Hawaii are broad and gently sloped, built from runny basaltic lava, while composite or stratovolcanoes such as the Cascades are steep and erupt explosively because their magma is thicker and gassier, and most volcanoes form at subduction zones, at divergent boundaries, or over hot spots. Eruption style depends on magma viscosity, which rises with silica content and falling temperature, and on dissolved volatiles such as water and $CO_2$, whose escape as magma rises drives fragmentation; the Volcanic Explosivity Index ranks eruptions logarithmically from gentle Hawaiian effusions (VEI 0 to 1) to Plinian blasts (VEI 5 to 6) and caldera-forming supereruptions (VEI 8, over $1{,}000\ \text{km}^3$).
Example
Mount St. Helens in Washington blew its top off in 1980, while Kilauea in Hawaii oozes red lava that flows down to the ocean, and Yellowstone is a hot-spot supervolcano whose caldera spans about $70\ \text{km}$. Mount Pinatubo's 1991 eruption, a VEI 6 event, injected roughly $20$ million tonnes of $SO_2$ into the stratosphere and cooled global average temperature by about $0.5^\circ C$ for two years, and Yellowstone's last caldera-forming eruption, $640{,}000$ years ago, spread ash across much of the western and central United States.
Key Insight
Volcanoes are not all violent: some erupt with huge explosions while others leak lava slowly for years, and the difference comes down to how much silica and trapped gas the magma holds, thick silica-rich magma bursting once gas pressure builds, runny basalt letting gas escape gently. Over the long run, volcanoes are also the source of the atmosphere and oceans, since billions of years of outgassing of water vapor, $CO_2$, and nitrogen built the air we breathe, with volcanic $CO_2$ still balancing the carbon that weathering removes.