Volcanism: The greatest volcanic hazards
There is always a certain amount of danger with volcanic eruptions. How severe the potential danger of an eruption is depends not only on its strength, but also on the nature of the eruption. The least amount of danger comes from red-hot lava. Lava lakes and lava flows only rarely kill people. But fast moving lava flows can do it. They can also destroy homes and entire towns.
In case of an explosive eruption tephra is able to endanger people close to the volcano. An impact of a lava bomb is mostly deadly,if persons hit directly.
Rising ash clouds pose a serious hazard to air traffic. The volcanic ash can cover fields and can lead to bad harvests. Ash eruptions in connection with heavy rain can cause lahars. These are a type of mudflow and debris flow which can bury whole communities. Wet ash is so heavy that roofs can collapse.
However, most dangerous are pyroclastic flows. These avalanches of lava debris and volcanic ash race to the valley on a super hot gas cushion, destroying everything in its path. Pyroclastic flows are caused by the collapse of large clouds of ash, or by the collapse of lava domes. A lava dome is a highly viscous lava flow that blocks the crater of the volcano increasing the pressure inside the volcano and possibly causing extremely powerful explosions. Pyroclastic flows destroyed the citys of Pompeii and Herculaneum during the eruption of Mount Vesuvius in 79 AD. In recent years, pyroclastic flows led to huge disasters on Montserrat and in Indonesia.
Similar strong explosions occur when in the vent lava reacts with water. It is for this reason that glaciated volcanoes pose a great danger. The melt water can also cause floods and lahars.
Last, but not least, large eruptions can emit huge ammounts of sulfur aerosols. These aerosols rise into the stratosphere and can affect the global climate. In 1815 the eruption of Mount Tambora led to a global temperature decline. Worldwide bad harvests and famine were the consequences.
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