A complex volcano, often referred to as a compound volcano, is characterized by multiple vents and diverse eruption styles, posing significant hazards such as pyroclastic flows, lahars, and lava flows.

Specifically, a complex volcano features more than one eruption center or vent, which allows for a variety of eruption styles. This is in contrast to simple volcanoes, which generally possess a single vent and exhibit a more predictable eruption pattern. Complex volcanoes often arise from changes in magma composition, tectonic activity, or interactions between magma and water or ice. They can encompass a range of volcanic forms, including stratovolcanoes, shield volcanoes, lava domes, and other related features.

The hazards associated with complex volcanoes are particularly diverse due to their varied eruption styles. One of the most perilous hazards is the pyroclastic flow, which is a rapid current of hot gas and volcanic matter. These flows can reach speeds of up to $700 \, \text{km/h}$ and temperatures around $1,000 \, \text{°C}$. Pyroclastic flows are capable of obliterating everything in their path and are frequently responsible for the highest fatality rates during volcanic eruptions.

Another critical hazard is the lahar, a type of mudflow or debris flow composed of a mixture of pyroclastic material, rocky debris, and water. Lahars can be triggered by intense rainfall or the rapid melting of snow and ice during an eruption. These flows can travel considerable distances from the volcano, resulting in widespread destruction.

Lava flows also represent a common hazard linked to complex volcanoes. Although lava flows typically move slower than pyroclastic flows and lahars, they can still inflict significant damage by burning and burying everything in their path. Additionally, lava flows may lead to secondary hazards, such as forest fires and the release of toxic gases.

Beyond these direct hazards, complex volcanoes can induce indirect hazards, including ash fall, which can damage structures, disrupt transportation, and cause respiratory issues. Furthermore, volcanic gases can contribute to climate change and acid rain. Consequently, a thorough understanding of the nature and behavior of complex volcanoes is essential for effective hazard assessment and risk management.