People who live near volcanoes have many good reasons to be worried about safety. Volcanoes can do much damage. The volcanic-triggered landslide that buried Armero, Colombia, in 1985, and the eruption of Mt. Pelée on the island of Martinique in the Caribbean in 1902, each killed about 30,000 people. Other volcanic disasters have brought the human death toll to perhaps 200,000 over the last few centuries. Compared to war, disease, or even automobile accidents, this is not a terribly high toll; however, the 200,000 people directly involved almost certainly would have appreciated enough warning to get out of harm’s way. One of the goals of modern geology is to predict volcanic hazards and to save lives and property by doing so. There are many hazards to worry about. These include:
Pyroclastic flows
Often, a volcano will produce a dense mixture of ash and hot gases (up to 1500 °F or 800 °C, and including poisons such as hydrogen sulfide). This potentially deadly mix is either forced away from the volcano (the lateral blast released by the landslide on Mt. St. Helens) or forced upward to then collapse and flow under gravity, at speeds up to hundreds of miles (hundreds of km) per hour. The deaths on Martinique were caused by such a “glowing cloud” (nuée ardente in French, where the only survivor in the whole city of St.-Pierre was a lone man locked in a heavily built prison.) See some amazing pictures in the slideshow below.
Pyroclastics
If the heat and gases don’t get you, the rocks might. People have been killed by rocks up to car-sized or bigger, called bombs, that were thrown from volcanoes. Having a car-sized rock fall on your head from a great height is not recommended. Even fine-grained ash deposits may bury and kill nearby crops. Jet aircraft are endangered by flying into ash at high speeds. From 1980-1995, ash caused an estimated $200 million in damage to the 80 aircraft that flew into eruption clouds, mostly over the Pacific. Of those, seven lost engine power and came close to crashing. Improved monitoring of eruption clouds, to provide warnings and steer the airplanes into safer air, has greatly reduced damage since then. However, to avoid crashes from the 2010 eruption of Eyjafjallajökull in Iceland, literally millions of passengers were stranded in Europe and elsewhere as flights were suspended for weeks.
Poisonous gases
Sometimes, a volcano will smother or poison victims. Pompeii and Herculaneum, the cities entombed by the eruption of Vesuvius in the year 79, have proven to be archaeological treasures, but certainly would be considered tragedies by the many people killed there and by their relatives. The people were killed before they were buried, and poisonous gases as well as great heat may have contributed to the deaths. Lake Nyos in Cameroon rests in a volcanic crater. Volcanic CO2 feeds into the bottom of the lake, but the lake typically remains stratified and does not mix. The CO2 thus builds up in the deep waters. In 1986, the lake overturned, perhaps because a landslide from the crater wall temporarily mixed the water at one end. The escaping CO2 made a great fountain like a giant erupting champagne bottle, filled the crater with CO2, and then flowed down outside the crater, killing about 1700 people through some combination of suffocation and poisoning. The lake is now being vented through large pipes, but an earthquake might break the walls and release a huge flood, and that would release much CO2 that has not been vented, and that might kill people. (Note that while CO2 can be toxic locally, in lower quantities it is not toxic, and a little is necessary for plants to grow. The flux of CO2 from all the volcanoes in the world is about 1% of the flux from human fossil-fuel burning, and there has been no significant change in that natural volcanic flux recently, so the volcanoes are not driving recent global warming.)
Landslides and mudflows
These are often less dramatic, but more dangerous cumulatively, than the explosive events. Most of the andesitic volcanoes are steep, and many are capped by very large glaciers. Mt. Rainier, for example, has 25 times as much glacier ice as Mt. St. Helens had, ready to melt and trigger mudflows after even a minor eruption. The tragedy at Armero arose from a minor eruption that triggered a big landslide. It is worth noting that Armero was built on a known, older debris-flow deposit.
Tsunamis
A large undersea eruption may move a lot of water. This water movement may form into a tsunami, a long, low wave that moves very rapidly. When a tsunami nears a shore, the water “piles up” into a short, steep wave that may be 100 feet or more high. Such waves, which can also be caused by landslides or earthquakes, may affect coasts hundreds of miles (or kilometers) from the source. The largest eruption of historical times, that of Krakatau in Indonesia in 1883, killed thousands of people on neighboring islands in this way. The great Tohoku earthquake of 2011 in Japan caused a tsunami that was over 130 feet high (40 m) at its worst, which it came ashore where people lived. We’ll look more at tsunamis in Module 4.
Right: A tsunami inundates Pago Pago in American Samoa in September 2009. (National Park of American Samoa)
Climate change
A large volcanic eruption puts a lot of sulfur gases into the stratosphere, together with ash and other materials. The sulfur eventually forms sulfuric-acid droplets, which typically remain aloft for one to a few years before falling out across much or all of the planet. While they are aloft, the sulfuric-acid droplets block some of the sunlight, cooling the planet a little. This can produce killing frosts during normal growing seasons, leading to widespread starvation in sensitive regions. The Tambora eruption of 1815 is associated with the starvation “year without a summer” of 1816. Ice cores from Greenland, Antarctica and elsewhere record volcanic fallout (the ash and sulfuric acid are preserved in the ice) and the temperature (from certain indicators including the isotopic composition of the ice), and show that big eruptions typically are accompanied by a cooling of a good chunk of a degree for a year or two, with more cooling in some places and seasons, and less in others. This isn’t a huge change, but when one killing frost can cause starvation, it may be too much. If many volcanic eruptions occurred in a short period, it might produce major climate changes; however, volcanism doesn’t seem to get organized—there is no way for a volcano in Alaska to tell a volcano in Indonesia that it is time to erupt. (Volcanoes also release carbon dioxide, which tends to warm the climate, as we will see later in the course. However, not a lot of carbon dioxide comes out in one volcanic eruption. If all the world’s volcanoes started erupting a lot faster, maybe twice as fast as normal, enough carbon dioxide would be released in “only” a few hundred thousand years to start warming the world notably. Over really short time scales of years to centuries, more volcanism would cause more cool years, because the sun-blocking effect would be much bigger than the warming-from-carbon-dioxide effect. If you greatly increased the rate at which volcanoes erupt, you would get cooling first and then warming later.)
Optional Viewing
For the first video, add optional text: Hawaiian volcanoes are usually not as dangerous as the explosive eruptions of Cascade volcanoes such as Mt. St. Helens. But that does not mean that Hawaiian volcanoes are safe! Here, a USGS film shows a lava flow slowly destroying a subdivision.
Video: Kīlauea Volcano — Pāhoehoe Flows on Kaupili Street (1:24 minutes)
Aerial view of fissure 8 of Kilauea Volcano on May 6, 2018, in Leilani Estates. After vigorously erupting for much of the day, activity on the fissure diminished around 4:00 pm. A lava flow from the fissure advanced about 1 km (0.6 mi) to the northeast. Again, this sort of eruption is normally slow enough that people can get out of the way, but it can still be dangerous.