Volcanoes

A volcano is an opening, or vent, through which material from below Earth's surface erupts. Molten rock underground is called magma; once it reaches the surface it is called lava. Eruptions can also release gases, steam and fragments of rock collectively called tephra. As erupted material accumulates around vents, it forms volcanic terrain. Volcanoes are not all the same shape: they include cones, broad shields and lava domes, as well as submarine forms.

Magma is generally less dense than surrounding solid rock and can rise through cracks in Earth's crust. Gas dissolved in magma also contributes pressure. Eruption style depends partly on the magma's composition and viscosity, which affect how easily gas escapes: relatively fluid magma can release gas and flow as lava, while thicker magma may retain gas until pressure drives an explosive eruption. Explosions can fragment magma into tephra, from fine ash to larger pieces.

Geologists commonly describe cinder cones, composite volcanoes (also called stratovolcanoes), shield volcanoes and lava domes. Cinder cones are built from fragments ejected around a vent; stratovolcanoes develop from alternating layers of lava and fragmented material; and shield volcanoes grow through repeated, relatively fluid lava flows that make broad, gently sloping forms. Many volcanoes lie around the Pacific's Ring of Fire. Others occur over hotspots within tectonic plates; the U.S. Geological Survey identifies Hawaiian and Yellowstone volcanoes as examples.

Volcanic hazards extend beyond flowing lava. Explosive eruptions can produce ash fall and fast-moving currents of hot gas and rock fragments; ash may be carried well beyond the volcano. When volcanic material mixes with water, it can form lahars that move down valleys. The risks differ from one volcano and eruption to another, so geological surveys and observatories monitor activity and share information. The Smithsonian Institution's Global Volcanism Program documents Earth's active volcanoes and eruptions over the last 12,000 years.