A stratovolcano, also known as a composite volcano, is a tall, conical volcano built up by many layers (strata) of hardened lava, tephra, pumice, and volcanic ash. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile and periodic, explosive eruptions and quiet eruptions. The lava that flows from stratovolcanoes typically cools and hardens before spreading far due to high viscosity. The magma forming this lava is often felsic, having high-to-intermediate levels of silica (as in rhyolite, dacite, or andesite), with lesser amounts of less-viscous mafic magma. Extensive felsic lava flows are uncommon, but have travelled as far as 15 km (9.3 mi).

Stratovolcanoes are sometimes called "composite volcanoes" because of their composite layered structure built up from sequential outpourings of eruptive materials. They are among the most common types of volcanoes, in contrast to the less common shield volcanoes. Two famous stratovolcanoes are Krakatoa, best known for its catastrophic eruption in 1883 and Vesuvius, famous for its destruction of the towns Pompeii and Herculaneum in 79 A.D.

Stratovolcanoes are common at subduction zones, forming chains along plate tectonic boundaries where oceanic crust is drawn under continental crust (Continental Arc Volcanism, e.g. Cascade Range, central Andes) or another oceanic plate (Island arc Volcanism, e.g. Japan, Aleutian Islands). The magma that forms stratovolcanoes rises when water trapped both in hydrated minerals and in the porous basalt rock of the upper oceanic crust, is released into mantle rock of the asthenosphere above the sinking oceanic slab.

The release of water from hydrated minerals is termed "dewatering," and occurs at specific pressures and temperatures for each mineral, as the plate descends to greater depths. The water freed from the rock lowers the melting point of the overlying mantle rock, which then undergoes partial melting and rises due to its lighter density relative to the surrounding mantle rock, and pools temporarily at the base of the lithosphere. The magma then rises through the crust, incorporating silica-rich crustal rock, leading to a final intermediate composition (see Classification of igneous rock). When the magma nears the top surface, it pools in a magma chamber under or within the volcano. There, the relatively low pressure allows water and other volatiles (mainly CO2, SO2, Cl2, and H2O) dissolved in the magma to escape from solution, as occurs when a bottle of carbonated water is opened, releasing CO2. Once a critical volume of magma and gas accumulates, the obstacle (rock blockage) of the volcanic cone is overcome, leading to a sudden explosive eruption.

The impact of the June 1991 eruption of Mount Pinatubo was global. Slightly cooler-than-usual temperatures were recorded worldwide and brilliant sunsets and sunrises were attributed to the particulates this eruption lofted high into the stratosphere. The aerosol that formed from the sulfur dioxide (SO2) and other gasses dispersed around the world. The SO2 mass in this cloud - about 22 million tons - combined with water (both of volcanic and stratospheric origin) formed droplets of sulfuric acid, blocking a portion of the sunlight from reaching the troposphere and ground. The cooling in some regions is thought to have been as much as 0.5 °C. An eruption the size of Mount Pinatubo tends to affect the weather for a few years; the material injected into the stratosphere gradually drops into the troposphere where it is washed away by rain and cloud precipitation.

A similar, but extraordinarily more powerful phenomenon occurred in the cataclysmic April 1815 eruption of Mount Tambora on Sumbawa Island in Indonesia. The Mt. Tambora eruption is recognized as the most powerful eruption in recorded history. Its volcanic cloud lowered global temperatures by as much as 3.5 °C. In the year following the eruption, most of the northern hemisphere experienced sharply cooler temperatures during the summer months. In parts of Europe and in North America, 1816 was known as "The Year Without a Summer," which caused a brief but bitter famine.

Apart from possibly affecting climate, volcanic clouds from explosive eruptions also pose a hazard to aviation safety. For example, during the 1982 eruption of Galunggung in Java, British Airways Flight 9 flew into the ash cloud, suffering temporary engine failure and structural damage. During the past two decades, more than 60 airplanes, mostly commercial jetliners, have been damaged by in-flight encounters with volcanic ash. Some of these encounters have resulted in the power loss of all engines, necessitating emergency landings. Luckily, to date no crashes have happened because of jet aircraft flying into volcanic ash. Ashfall is a threat to health when inhaled, and is also a threat to property with high enough accumulation. Greater than 30 cm (12 in) of accumulation is sufficient to collapse most buildings.

Since the year A.D. 1600, nearly 300,000 people have been killed by volcanic eruptions. Most deaths were caused by pyroclastic flows and mudflows, deadly hazards that often accompany explosive eruptions of subduction-zone stratovolcanoes. Pyroclastic flows are fast-moving, avalanche-like, ground-hugging incandescent mixtures of hot volcanic debris, ash, and gases that can travel at speeds in excess of 100 miles per hour (160 km/h).

Approximately 30,000 people were killed by pyroclastic flows during the 1902 eruption of Mont Pelee on the island of Martinique in the Caribbean.

In MarchÐApril 1982, three explosive eruptions of El Chichon Volcano in the State of Chiapas, southeastern Mexico, caused the worst volcanic disaster in that country's history. Villages within 8 km (5.0 mi) of the volcano were destroyed by pyroclastic flows, killing more than 2,000 people.

Mudflows (also called debris flows or lahars, an Indonesian term for volcanic mudflows) are mixtures of volcanic debris and water. The water usually comes from two sources: rainfall or the melting of snow and ice by hot volcanic debris. Depending on the proportion of water to volcanic material, mudflows can range from soupy floods to thick flows that have the consistency of wet cement. As mudflows sweep down the steep sides of composite volcanoes, they have the strength and speed to flatten or bury everything in their paths. Hot ash and pyroclastic flows from the 1985 eruption of the Nevado del Ruiz Volcano in Colombia, South America, melted snow and ice atop the 5,390-m-high Andean peak; the ensuing mudflows buried the city of Armero, killing 25,000 people.

Lava flows from stratovolcanoes are generally not a significant threat to people because the highly viscous lava moves slowly enough for people to move out of the path of flow. The lava flows are more of a property threat.

However, not all stratovolcanoes have viscous lava. Mount Nyiragongo is dangerous because its magma has an unusually low silica content, making it quite fluid (even when comparing to Hawaiian lava) and having lower viscosity. Compounded by the very steep slope of Nyiragongo gives the lava the ability to flow at up to about 100 km/h (62 mph).