Events that cause death, injury and damage to property and infrastructure are known as natural hazards. Volcanic eruptions can swiftly become natural hazards which can quickly lead to major loss of life and widespread damage. Primary hazards that are associated with volcanoes include; pyroclastic flows, Tephra, Lava flows and volcanic gases. Pyroclastic flows are waves of very hot gases and tephra with flow with high velocities. The pyroclastic flow that was seen at the eruption of Mount St Helens, USA in 1980 was a result of dense gas and ash combinations. It travelled a total of 25km north of the volcano and had a top temperature of 700’c. The flow was slowed down by ridges around the volcano but it contributed to a $450 million in damages to forestry. Tephra – the solid material that is ejected in a volcanic eruption of varying grain size (these can be volcanic bombs or ash) can often be found in the pyroclastic flows.
The ash can fly for long period of time, for example the ash that was erupted from the Mount Pinatubo eruption of 1991 was tracked by satellites going round the globe several times. Lava flows are commonly found in Iceland and Hawaii. They can be halted by spraying cold water on them (Heimaey, Iceland) but most of unstoppable. In 1990 a lava flow from Puu Oo crater in Hawaii buried the village of Kalapana. When they can’t be stopped they create millions of dollars in damage and severe loss of life. After the primary hazards can come more hazards that are triggered by the volcanic eruption.
Secondary hazards include lahars, Jӧkulhlaups (flooding) and volcanic landslides that can lead to tsunamis. Lahars occurs when heavy rain starts to fall as well as ash or when the release of crater lakes mix with pyroclastic flows. These are mixtures of water, rock, sand, mud and ash that flow down the valleys leading away from the volcano. Lahars are fast moving, can flow long distances and unfortunately follow the valleys in which settlements and populations are often found. The lahars that occurred in the USA after Mount St Helens eruptions flowed into the Columbia River and reduced its depth from 40 feet to 14 feet. This also occurred at the shipping canal to Portland which was blocked by the sediment deposition and 31 ships were stranded.