Volcanic Hazards

A pyroclastic flow or Nuée Ardente is one of the most dangerous hazards that can come from a volcanic eruption. It is a fast-moving avalanche of hot gases, ash, and rock fragments that rushes down the slopes of a volcano. These flows can move at speeds of over 100 miles per hour (160 km/h) and reach temperatures hotter than 1,000°F (about 500°C).

What makes pyroclastic flows especially deadly is their combination of speed, heat, and force. Unlike lava flows, which move slowly enough that people can usually escape, pyroclastic flows travel so quickly that they can overtake anyone in their path. The burning hot gases and ash can destroy buildings, forests, and anything else in their way, while suffocating or burning living things instantly.

One of the most deadly pyroclastic flows in history occurred in A.D. 79.  The Roman city of Pompeii was destroyed by a massive eruption of Mount Vesuvius in southern Italy. For over 1,500 years, Pompeii lay buried and forgotten until its rediscovery, giving us a detailed look into life in ancient Rome.

The eruption began with an enormous towering cloud of ash, pumice, and gases that shot high into the sky. For hours, volcanic material rained down on the city, collapsing roofs and forcing people to flee or hide indoors. But the most devastating event came next: deadly pyroclastic flows—fast-moving avalanches of superheated gas, ash, and rock—swept down the volcano’s slopes and into Pompeii. These flows buried the city in several meters of volcanic debris and killed nearly everyone who had not escaped.

The ash that covered Pompeii actually preserved the city remarkably well. Buildings, streets, mosaics, and even the shapes of people and animals were sealed in time, creating a snapshot of daily life almost 2,000 years ago.

Today, Pompeii is one of the world’s most famous archaeological sites, offering a window into Roman civilization and a powerful reminder of the destructive force of volcanoes.

A lahar is a type of volcanic mudflow or debris flow that can be just as destructive as an eruption itself. It forms when volcanic ash, rock, and other loose material on the slopes of a volcano mix with water. This water can come from heavy rain, melting snow and ice, or even from a crater lake at the summit.

Once formed, lahars race down valleys like rivers of wet concrete. They can move at speeds of 30 to 60 miles per hour and carry massive boulders, trees, and debris with them. Because of their speed and weight, lahars can sweep away bridges, bury towns, and destroy farmland.

What makes lahars especially dangerous is that they can occur during or even long after a volcanic eruption. For example, the 1985 eruption of Nevado del Ruiz in Colombia triggered a lahar that buried the town of Armero and killed more than 20,000 people. 

A tsunami is a series of enormous ocean waves caused by a sudden displacement of water. Unlike normal waves, which are mainly caused by wind, tsunamis can travel across entire oceans at speeds of up to 500 miles per hour. When they reach shallow coastal areas, these waves slow down but grow dramatically in height, sometimes reaching over 100 feet tall, causing massive flooding and destruction.

While earthquakes are the most common cause of tsunamis, volcanoes can also trigger them in several ways (about 5% of tsunamis are caused by volcanoes). One way is through underwater (submarine) volcanic eruptions, where the rapid explosion of gas and magma pushes water upward, creating waves. Other ways include pyroclastic landslides, flank collapses, or caldera collapses.  In these cases, material from the volcano, or parts of the volcano itself, can fall into the ocean.  This sudden displacement of water can send waves racing toward nearby coastlines. A famous example is the 1883 eruption of Krakatoa in Indonesia, which caused huge tsunamis that destroyed towns and killed tens of thousands of people.

Understanding how tsunamis form helps scientists predict and warn communities, giving people a better chance to evacuate and stay safe. Volcanic tsunamis, though less common than earthquake tsunamis, are powerful reminders of the dramatic ways Earth's natural forces can interact.

Volcanic ash isn’t like the soft ash from a fireplace—it’s made of tiny, sharp fragments of rock, minerals, and volcanic glass created during an eruption. Despite being lightweight, it can be extremely dangerous for people, animals, and the environment.

One reason ash is hazardous is respiratory problems. The tiny particles can be inhaled deep into the lungs, causing coughing, difficulty breathing, and long-term health issues, especially for people with asthma or other lung conditions. Ash can also contaminate water supplies and destroy crops, harming both communities and wildlife.

Another danger comes from its weight and abrasiveness. Thick ashfall can collapse roofs, block roads, and damage machinery, including cars and airplanes. Volcanic ash is so fine and abrasive that it can grind down airplane engines, making air travel extremely risky during eruptions.

Finally, volcanic ash can block out the sun, especially during large eruptions that send ash high into the atmosphere. When enough ash and volcanic gases are released, they can spread over large areas and form a dense cloud that reduces sunlight, sometimes causing a noticeable drop in temperature and other climate related problems.

This effect can lead to “volcanic winters”, where sunlight is dimmed for days, weeks, or even months in extreme cases. Crops may fail because plants don’t get enough sunlight, and air temperatures can drop.