Volcano

A volcano is a vent or fissure in the crust of a planetary-mass object. On Earth, these openings allow hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.

This process is known as volcanism. Understanding volcanoes is essential because they shape our planet and influence our atmosphere. Large eruptions can release ash and sulfuric acid droplets into the air. These particles can obscure the Sun and cool the Earth's troposphere. Historically, these events have led to volcanic winters and catastrophic famines.

Most volcanic activity occurs because of plate tectonics. Earth's lithosphere, or its rigid outer shell, is divided into sixteen large plates and several smaller ones. These plates move continuously due to convection in the underlying ductile mantle. Most volcanoes form at plate boundaries where plates are either converging or diverging.

At divergent boundaries, plates pull apart. This happens at mid-ocean ridges where hot mantle rock rises. As the pressure decreases, the rock undergoes adiabatic expansion and partial melting. This creates new oceanic crust through submarine volcanism.

At convergent boundaries, plates collide in a process called subduction. Usually, an oceanic plate dives beneath a continental plate, creating a deep ocean trench. This triggers flux melting, where water from the subducting plate lowers the melting temperature of the mantle. The resulting magma is often very viscous, meaning it is thick and sticky. This high silica content can cause the magma to solidify at depth or erupt violently. These zones create chains of volcanoes known as volcanic arcs. The Pacific Ring of Fire is a famous example of this system.

Volcanoes can also form away from plate boundaries through hotspot volcanism. This occurs when mantle plumes rise from the core-mantle boundary. These plumes are columns of hot material that cause decompression melting as they reach the crust. Because tectonic plates move over these stationary plumes, a chain of volcanic islands can form. The Hawaiian Islands were created this way. Another example is the Yellowstone Caldera in North America.

Additionally, sustained upwelling can cause continental rifting. This happens when the crust thins and may eventually split a continent apart. The East African Rift is a notable example of this process.

Scientists categorize volcanoes by their frequency of eruption. Active volcanoes have a history of volcanism and are likely to erupt again. Dormant volcanoes have not erupted for a long time, generally since the start of the Holocene about 12,000 years ago. While they are quiet, they are still considered seismically active. Extinct volcanoes have no magma source left and cannot erupt.

Some volcanoes are polygenetic, meaning they have multiple periods of activity. Others are monogenetic, meaning they erupt only once in their lifetime.

Volcanoes display many different structures based on the type of lava they erupt. Shield volcanoes have broad, gentle profiles. They are formed by low-viscosity basaltic lava that flows great distances.

In contrast, lava domes have steep sides. They are built by the slow eruption of highly viscous lava, such as rhyolite. Cinder cones are smaller, cone-shaped hills. They are built from small pieces of scoria and pyroclastics that accumulate around a vent.

Some volcanoes even feature fissure vents, which are long, linear fractures that release basaltic lava.

Volcanic activity is not limited to Earth. Other planets in our solar system host massive volcanic features. Venus has a very large number of volcanoes. Mars also possesses significant volcanic structures. The largest known volcano in the solar system is Olympus Mons on Mars. It is a massive shield volcano.

On some moons of Jupiter, Saturn, and Neptune, scientists observe cryovolcanism. This involves the eruption of materials that are not molten rock, but rather ice or other volatiles.