Often the misconception people have is that lava is the most deadly thing about an erupting volcano! But this is just not true. Hardly any human has been killed by a lava flow – they are relatively slow-moving and while they can destroy huge tracks of farming land, houses, roads, and infrastructure, even on the most active volcanoes, you can outrun all the lava flows. Now occasionally some people get way too close to a lava flow either just to experience its fury, or maybe for some scientific study and have a mishap and fall into the flow. But this is such a rare event and is due to them taking massive risks rather than the danger posed by the flow itself.
But volcanoes do kill!
Often large volcanic eruptions kill many people. In recent historic times, we have seen eruptions kill many thousands – the largest being the eruption of Mount Tambora in Indonesia in 1815 which took the lives of over 250,000 due to northern hemisphere crop failures. In 1883 the tsunami generated by the explosive eruption of Krakatoa killed around 36,000 people. And less than 100 years ago the eruption of Mount Lamington in Papua New Guinea killed 2,942.
But if lava is not the culprit – what is it about volcanic eruptions that are so dangerous?
These six things are the most deadly hazards from volcanoes:
The silent volcanic killer of volcanic eruptions, volcanic gasses can roll down the slopes of a volcano displacing fresh air and killing all the animals in its path. Carbon dioxide is the major culprit – heavier than air, colorless, and odorless gas. Other gases, like Hydrogen sulfide – (rotten egg gas) can also cause issues. In 1986 Lake Nyos in Cameroon, Africa – a volcanic lake – release a thick cloud of carbon dioxide in the middle of the night, which flowed down the slope concentrating in valleys and suffocated 1742 sleeping people. People died up to 20 km away from the lake.
When a volcano erupts it releases a huge amount of gases into the atmosphere. Some of those gases react with the air and sunlight to form microscopic droplets called aerosols. These droplets, mostly sulphuric acid, can stay in the upper atmosphere for many months and even years blocking out a proportion of sunlight reaching the Earth’s surface. In the very largest eruptions, such as that of Tambora Indonesia in 1815, the reduced sunlight in the northern hemisphere was so great that crops failed across Europe in the summer. This was to become known as the infamous ‘year without summer’. Hundreds of thousands of people starved to death because of the effect of the eruption of aerosols.
Artists of the time painted landscapes – like this one by Turner – showing diffused light and deep red sunsets. Writers started to dabble in what we call gothic themes – such as Mary Shelly with Frankenstein.
In more recent times, eruptions producing these aerosols have affected aircraft that fly at these altitudes, pitting windows and paintwork and costing the industry millions of dollars in repairs.
3. Ash & Tephra
The fine rock fragments that come from explosive volcanoes can cover buildings, forests, and farmland in deep deposits. The finest of these materials is called ash and is 2 mm or less in size. This material can be blown by winds for many miles. Larger material is called tephra and may be confined closer to the eruptive vent. Large accumulations of both can cause roofs to collapse and can, in some cases, completely bury homes. Ash can also cover and destroy arable land, forcing famine on local communities. It can pollute drinking water and cut off roads and other infrastructure.
In 1973 an eruption on the island of Heimaey completely blanked parts of the town of Vestmannaeyjar in ash. The eruption lasted 157 days and buried houses close to the previously unknown vent. Recently the ash was removed from a house to reveal the contents 40 or more years later and is a museum you can visit.
When volcanic rock material mixes with water, often from melting snow and glacier ice on the flanks of a volcano, a debris flow called a lahar can develop. These are like flowing concrete and roll down valleys around the flanks of composite volcanoes and incorporate materials for the valleys (rocks, trees, water) as they flow and can increase their size dramatically downslope. And, they can travel fast – 100 km/hr or more wiping out everything in their path – houses, bridges, roads, rail lines, power poles.
Mount Rainier in Washington State USA has produced some dramatic lahars in the past. Around 5600 years ago a lahar swept down the slopes of the volcano and covered areas on which parts of Seattle are now built. The potential for future lahars into the Seattle suburbs is high and is considered to be the greatest risk posed by the volcano.
5. Pyroclastic Flows
This devastating high-density hot mix of lava blocks, ash, pumice, and gas can roll down the slopes of a volcano at terrifying speeds destroying everything in its path. The heat of the flows – often in excess of 300 degrees celsius causes fires and melt any snow or ice that they pass over. When they stop the heat of the material can weld all the fragments together to form a dense rock (ignimbrite).
Pyroclastic flows normally occur from the collapse of an eruption cloud that can no longer hold its own weight as it moves away from the volcanic vent but can also form from the boiling over of an eruptive event from the crater. They can travel across the water on a superheated bed of steam, and also flow underwater.
They are deadly! Nother survives a pyroclastic flow and even people and animals who are close to the flanks of a flow can be killed by the heat.
Walls of water can be pushed out from the flanks of volcanoes during explosive events right at sea level. The generated tsunami can travel across ocean basics and destroy towns and villages thousands of kilometers away from the original eruption. Underwater eruptions that are close to the ocean surface can also generate tsunamis, as can large landslides or slumps from the flanks of volcanoes that may and may not be related to volcanic eruptions.
The Krakatoa, Indonesia eruption of 1883 created a tsunami wave that killed over 36,000 people. Most probably the tsunamis were caused by huge pyroclastic flows collapsing to the ocean, as well as the initial explosion.