Volcanoes are responsible for a large number of deaths, but lava is not the only danger associated with these hazards. Mount Vesuvius (Naples, Italy) is infamous for its violent explosion thousands of years ago in 79 AD when a pyroclastic flow travelled over the Roman countryside and engulfed the cities of Pompeii and Herculaneum . It was not until the 18th century that we uncovered the shocking remains of these towns beneath over 10 feet of ash and the casts of people preserved within it.
We know more about volcanic hazards over the past century because they have been better monitored and documented. We have seen enormously violent explosions, effusive lava, fast pyroclastic flows, ash, landslides, toxic gases, and more!
The most dangerous type of volcanic hazard are pyroclastic flows. These flows are a mix of lava blocks, pumice, ash, and hot gases between 400 to 1,300°F! The turbulent cloud of ash and gas races down the steep flanks at high speeds at an average of 60 mph (much faster than people can run) into the valleys where farmlands grow and cities thrive .
Pyroclastic flows can often be expected of stratovolcanoes that contain felsic or intermediate magma. This magma is silica-rich and contains volatile gases that make it highly viscous. Therefore, these volcanoes often have very violent eruptions that are accompanied by a pyroclastic flow.
There are numerous examples of deadly pyroclastic flows. In 2014, the Mount Ontake pyroclastic flow in Japan killed 47 people. The flow was caused by magma heating groundwater into steam, which then rapidly ejected with ash and volcanic bombs. Some were killed by inhalation of toxic gases and hot ash, while volcanic bombs struck others . In 1902, on the Caribbean Island Martinique, Mount Pelee erupted with a violent pyroclastic flow that destroyed the entire town of St. Pierre and killing 28,000 people in moments .
Video 4.3.1 What are lahars and how are lahars formed? How does the composition influence the lahar behavior? (3:34)
A lahar is an Indonesian word for a mudflow that is a mixture of water, ash, rock fragments, and other debris that moves down the mountainside of a volcano (or other nearby mountains covered with fresh ash). They form from the rapid melting of snow or glaciers on volcanoes or sometimes in combination with a new eruption and heavy thunderstorm, as seen at Mt. Pinatubo.
Lahars move like a slurry of concrete, but they can move extremely fast at speeds up to 50 mph. Part of the reason they are so deadly is that they are slurry-like; they easily capture materials in their wake and they can travel very long distances like a flash flood .
During the 1980 Mount St. Helens eruption, lahars reached 17-miles (27 km) down the North Fork of the Toutle River. Another scenario played out when a lahar from the volcano Nevado del Ruiz, Colombia, buried a town in 1985 and killed about 25,000 people .
Tephra and Ash
Tephra is heavier than ash, so it will fall closer to the volcano’s crater and vent. Large masses of tephra sometimes erupt from volcanoes and can pose deadly hazards to anyone nearby. These are called volcanic bombs .
Ash is much finer, but it is dangerous. It can travel much longer distances away from the volcano, and that can cause more widespread issues in nearby towns and cities. A build-up of ash can collapse the roof of a building or home, and the microscopic minerals within ash will cause respiratory illnesses such as silicosis. Inhaling ash is extremely hazardous because much of it contains microscopic volcanic glass particles . Imagine inhaling tiny shards of glass!
Ash will also interfere with transportation services farther away from an eruption. For example, the 2010 Eyjafjallajökull volcanic eruption in Iceland created a gigantic ash cloud that caused a significant air travel disruption in northern Europe. No one was hurt, but the cost to the world economy was estimated in the billions of dollars .
Magma contains dissolved volatile gases. As magma rises toward the surface, the pressure that keeps it in the magma chamber will start to decrease, which allows those gases to escape. Think of this process like twisting the cap of a soda bottle; the first thing to rise and escape in that initial hissing noise is gas! 
The types of gases that are commonly released from volcanoes include greenhouse gases such as carbon dioxide (CO2), sulfur dioxide (SO2), hydrogen sulfide (H2S), methane (CH4), and water vapor (H2O). There are also toxic and acidic gases present at volcanoes such as HF, HBr, and HCl. After gigantic volcanic explosions, some volcanic gases such as sulfur dioxide will become sulfate aerosols in the atmosphere. These aerosol particles block sunlight coming toward Earth’s surface and cause the planet to become cooler .
The volcanic gases can be both toxic and suffocating, and these gases sometimes are released from a volcano without an accompanying eruption. Gases were released from the Oku Volcanic Plain in Lake Nyos, Cameroon, and the carbon dioxide suffocated almost 2,000 people in 1986 .
an area on the Earth's surface where lava, ash, and/or volatile gases erupt and eventually solidify into rock.
molten rock that has erupted at the Earth's surface due to volcanic processes.
an extreme natural event that is a threat to life and property
A mixture of super-heated gas, ash, volcanic glass, and rock fragments that rapidly moves downslope from 60 to over 400 mph.
volcanically ejected rock fragments or particles that are less than 2 mm.
Eruption style in which magma flows from volcanoes and associated cracks in the ground as runny lava.
A vesicular, extrusive igneous rock that is felsic. Pumice is light-colored and low density, and it often floats atop water.
A tall and steep conically-shaped volcano built upon past layers of hardened ash and lava flows. Also known as a composite volcano.
originating from a feldspar and silica-rich magma/lava composition.
A composition of igneous rocks, magma, and lava that is between felsic and mafic.
molten rock that can be found beneath the Earth's surface.
a compound composed of silicon and oxygen in the formula SiO2.
Gases dissolved within magma or lava.
A fluid (such as lava or magma) that is highly resistant to flow or movement due mostly to composition.
A hazardous molten rock ejected by volcanic eruptions that is at least more than 2.5 inches in diameter.
A mudflow, or type of mass movement, that is a mixture of water, ash, rock, sediment, and debris that rapidly and suddenly moves downslope. Lahars are a commonly associated hazard with volcanic eruptions.
ejected rock and particle fragments from volcanic eruptions.
A lung disease that is caused by breathing in silica (SiO2).
A category of gas molecules that can absorb infrared radiation from sunlight and cause heat to remain trapped within a planet's atmosphere.
Fine particles, liquid droplets, or gases that are ejected into the atmosphere by man-made pollution or volcanic eruptions. Aerosols have the ability to reflect sunlight back toward space and cause the planet to experience colder temperatures.