Pyroclastic Flow

Pyroclastic flow is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that moves away from a volcano reaching speeds of up to 300 km/h and temperatures of between 700°C and 1,000 °C. At the lower layer, the flow moves in a sheet form as a layer of thick ash and gas. In the upper layer an ash cloud forms. Pyroclastic flows normally touch the ground and move fast downhill, or spread laterally under gravity. Their speed depends upon the density of the current, the volcanic output rate, and the gradient of the slope.

The Eruption Column

This primary hazard results directly from the eruption and varies in size depending on the type of eruption. Plinian eruptions can reach 20-40km in height. The lower section is known as the gas thrust region and forms due to the pressure release of trapped gas within the magma chamber. Above this convective currents lift the column higher before it mushrooms out into the umbrella region

Ash Fall

Ash fall from ash clouds covers the largest area and is made up of tephra, which is fine pieces (2mm) of pulverized rock, minerals and volcanic glass. Ash fall on the surface can reach meters in height and clog rivers and lakes. When combined with heavy rainfall from storms, fast-moving mudflows called lahars result. Acid rain is also s secondary hazard associated with ash fall. In addition, the air is choked with as, which can lead to both humans and animals to asphyxiate. Heavy ash fall can also cause weak roofs in homes and barns to collapse.

Lava Flow

Lava flows are the least hazardous of all processes in volcanic eruptions. How far it travels depends on the temperature, silica content, extrusion rate, and slope of the land. Slower lava flow is associated with composite volcanoes due to its high silica content and viscosity. Faster flowing lava such as a lava and pahoehoe lava are low viscosity so flow much further from the volcano spreading over larger surface areas

Pahoahoa Lava

A a Lava

Lahars

Lahars are a type of mudflow composed of a slurry of pyroclastic material, rocky debris, and water. The material flows down from a volcano, typically along a river valley. Lahars vary in size and speed. Small lahars are less than a few meters wide and several centimetres deep may flow a few meters per second. Large lahars hundreds of meters wide and tens of meters deep can travel at fast speeds. With the potential to flow at speeds up to 100 kilometres per hour, and flow distances of more than 300 kilometres, a lahar can cause catastrophic destruction in its path.

Landslides

The surface of volcanoes is unstable and made mostly of loose rock. This means they are easily susceptible to landslides. Volcanic gases create acidic groundwater, which contributes to the rocks' breakdown, increasing the weakness and instability in the volcano flank, and further increasing the likelihood of landslides.

Glacial Outburst Floods

A glacial outburst flood can be triggered by geothermal heating and occasionally by a volcanic subglacial eruption. Sometimes referred to as jökulhlaups based on their Icelandic name they emerge from hydrostatically-sealed lakes with floating levels far above the threshold, their peak discharge can be much larger than that of a marginal or extra-marginal lake burst. They cause rapid and devastating floods in and around the immediate location of the volcano and glacier.