What defines a complex volcano, and what hazards does it present?

A complex volcano, also known as a compound volcano, is defined by its multiple vents and mixed eruption styles, presenting hazards such as pyroclastic flows, lahars, and lava flows.

A complex volcano is a volcanic structure that is characterised by more than one eruption centre or vent, which can result in a variety of eruption styles. This is in contrast to simple volcanoes, which typically have a single vent and a more predictable eruption style. Complex volcanoes are often the result of changes in magma composition, tectonic activity, or the interaction of magma with water or ice. They can include a combination of stratovolcanoes, shield volcanoes, lava domes, and other volcanic features.

The hazards presented by complex volcanoes are diverse due to their varied eruption styles. One of the most dangerous hazards is a pyroclastic flow, which is a fast-moving current of hot gas and volcanic matter that can reach speeds of up to 700 km/h and temperatures of about 1,000 degrees Celsius. Pyroclastic flows can destroy everything in their path and are often responsible for the most fatalities during a volcanic eruption.

Another significant hazard is a lahar, which is a type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris, and water. Lahars can be triggered by heavy rainfall or the rapid melting of snow and ice during an eruption, and they can travel great distances from the volcano, causing widespread destruction.

Lava flows are also a common hazard associated with complex volcanoes. While they are typically slower-moving than pyroclastic flows and lahars, lava flows can still cause significant damage by burning and burying everything in their path. They can also lead to secondary hazards such as forest fires and the release of toxic gases.

In addition to these direct hazards, complex volcanoes can also cause indirect hazards such as ash fall, which can damage buildings, disrupt transportation, and cause respiratory problems; and volcanic gases, which can contribute to climate change and acid rain. Therefore, understanding the nature and behaviour of complex volcanoes is crucial for hazard assessment and risk management.