Volcanic Eruptions and Hazards

When the pressure inside a volcano grows to a point where the overlying surface can no longer contain is, an eruption occurs. Erupts are as unique as each volcano, from gentle lava flows and fountains to destructive pyroclastic eruptions.

There are many hazards and dangers associated with volcanic activity. These hazards can be local, or even global in scale.

Learn more about two main forms of eruptions, and some of the hazards associated with each.


Explosive Eruptions

Explosive eruptions occur when volatile (unstable and reactive) magma and volcanic gases are released. Magma can be under extremely high pressures due to its volume or its content of dissolved gases. Explosive eruptions are the most hazardous, involving large volumes of tephra and gases emitted from the volcano. Lahars, pyroclastic flows, ashfall, and many other volcanic hazards are commonly associated with explosive eruptions. Explosive eruptions are very dangerous. Basically, any form of hazard can occur. Here are some of the main hazards that can result from explosive eruptions:

Climate Change
Lava Flows
Pyroclastic Flows
Volcanic Gases

Experience national parks where explosive eruptions have occurred.

Aniakchak National Monument and Preserve, Alaska
Lake Clark National Park and Preserve, Alaska
Katmai National Park and Preserve, Alaska
Mount Rainier National Park, Washington


Effusive Eruptions

Effusive eruptions are generally considered to be gentler than explosive eruptions. During an effusive eruption, large volumes of lava pour from a vent onto the ground. These lava flows can vary in volume, area covered, thickness, length, and composition. Though effusive eruptions are generally associated with lava pouring out of a vent, some eruptions can include fantastic displays of fire and fury. Lava fountains can reach hundreds of meters into the air. Here are some of the hazards that can result from effusive eruptions:

Lava Flows
Volcanic Gases

Experience national parks where effusive eruptions have occurred.

Hawaii Volcanoes National Park
Haleakala National Park
Wrangell-St. Elias National Park and Preserve



An unpredictable nature mixed with an ability to release massive amounts of energy make volcanoes extremely hazardous. The types of hazards that threaten an area will vary based on the type of volcano. Some hazards are local in scale, while others can affect the entire planet.


Tephra is the term used to describe solid or molten rock fragments of any size ejected from a volcano. The smallest fragments (less than 2 millimeters in diameter) are called ash. Mid-sized fragments (2 to 64 millimeters in diameter) are called lapilli. Anything ejected from a volcano that is larger than 64 millimeters in diameter is called a block if it was ejected in a solid form and a volcanic bomb if it was ejected in a liquid or semi-solid form.

Tephra clouds can rise thousands of meters into the atmosphere and blanket areas hundreds of kilometers downwind of the eruption with ash. The dense tephra clouds can cause periods of darkness, even at mid-day, affecting plant, animal and human activity. Plants whose leaves are blanketed with ash are unable to perform photosynthesis or respiration. Animals experience disruption in their daily routine due to darkness at unusual times. Animals and humans can experience respiratory distress from inhaling tephra.

Darkness caused by tephra clouds reduces visibility on highways. Tephra deposits can short-circuit electric transformers and power lines. Tephra clouds are extremely dangerous to aircraft that fly into or near them. A large enough accumulation of ash can cave in roofs and collapse structures in addition to inhibiting travel on roads. The tephra clouds generate lightning, which can create interference for communications and also start fires.

Volcanic Gases

Magma contains dissolved gases. During and between eruptions, these gases are released. When an effusive eruption occurs, the magma is fluid enough that the gases can escape from it easily. In the more explosive eruptions, the thick magma traps gases until they build pressure and are released explosively.

Increased gas emission is one of the first signs of activity around a volcanic vent, and often precedes eruptions. For hundreds or even thousands of years after an eruption, gases can still be emitted from fumaroles.

Steam (water vapor) is the largest component of volcanic gas. Carbon dioxide (CO2), sulfur compounds, and chlorine compounds are other volcanic gases.

Wind controls the distribution and concentration of volcanic gases. Even low concentrations can cause damage to some plants and animals downwind. Close to a vent where concentrations are high severe damage to eyes and respiratory systems of humans and animals can occur. Acid rain is created when sulfur dioxide gas reacts with water droplets in the atmosphere. It causes corrosion of metal and stone, as well as harming vegetation. Carbon dioxide is heavier than air, and can collect in basins or low-lying areas. Sometimes concentrations in these valleys and pockets can reach lethal levels and suffocate animals and people.

Volcanic smog, called "vog" for short, is a hazardous mix of carbon dioxide, sulfur dioxide and other volcanic gases that can collect downwind of a volcano and cause a persistent air pollution problem. Vog aggravates respiratory problems and causes acid rain.

Lava Flows

Lava flows can burn, crush, or bury anything in their path. Lava flows are slow enough that they are rarely a threat to human life, and people can evacuate the area where the flow is heading and avoid danger. However, lava can travel at varying speeds. The path and distance of a lava flow can be predicted with knowledge of variables such as rate at which lava is flowing from the vent, fluid properties of the lava (viscosity), volume of lava, slope steepness of the volcano and surrounding landscape, channel geometry, and any obstructions to flow.

An 'a'a lava flow from Kilauea Volcano in Hawaii set fire to a house in its path. Lave flows move slow enough that people can be warned to evacuate their homes before the lava reaches them. Lava flows cause large amounts of property damage and loss in Hawaii.

Lava from an eruption of Kilauea Volcano flowed over this road in Hawaii Volcanoes National Park in Hawaii. This road was formerly part of the Chain of Craters Road, but after the eruption, several sections of the road had to be rebuilt.

Pyroclastic Flows

The word pyroclastic comes from Greek words - pyro meaning "fire" and klastos meaning "broken." Pyroclastics are the materials formed when magma and rock are fragmented during explosive eruptions and ejected from the volcano. Volcanic ash is the smallest pyroclastic fragment. Lapilli, blocks and bombs are larger.

Pyroclastic flows are avalanches of high-temperature rock, ash, and gas that race downslope from a volcano during explosive eruptions. These flows are extremely dangerous because they can reach temperatures of 1500 degrees Fahrenheit and speeds up to 450 miles per hour. At that temperature and speed, they can obliterate anything in their path.

A pyroclastic flow is also called Nuée Ardente, which means "glowing cloud" in French. Depending on the concentration of gases and red-hot incandescent materials, the pyroclastic flow can have a glowing appearance. The flows can originate from the mouth of the volcano, like a pot boiling over. They can also be the result of the fallout from the eruption column.

Pyroclastic surges are low-density versions of the pyroclastic flow. A "hot" pyroclastic surge is made up of dry gas and rock debris with temperatures above 100 degrees Celsius. "Cold" surges contain water and rock debris at temperature below 100 degrees Celsius.


Lahar is an Indonesian word, used to describe the debris flows and mudflows that originate from a volcano. Lahars contain rock debris and water. Mudflows contain less rock fragments and more water, and flow more cohesively. Debris flows are coarser and have a higher density of rock. They are more viscous and flow much less smoothly.

Because lahars have a high density of rock fragments and originate on a slope, they have high internal strength and can exert enough force to uproot trees, carry boulders and smash buildings as they flow downstream. Lahars can vary greatly in size, from centimeters wide and deep flowing less than 1 meter per second, to hundreds of meters wide and tens of meters deep flowing at tens of meters per second. Large, high-velocity lahars can reach many kilometers from a volcano.

Lahars are triggered by large landslides of water-saturated debris. This saturation is caused by heavy rainfall on volcanic deposits, sudden melting of snow and ice near volcanic vents from radiant heat on the flank of a volcano or from pyroclastic flows, or from water released in the breakout of glaciers and crater lakes.

Lahars can be either hot or cold, depending on the temperatures of the water and debris that they contain. Hot or cool, historically lahars are one of the most devastating volcanic hazards because they pose threats of burial and impact by debris.


When pyroclastic materials (gas and tephra) are forced out of a volcano in an explosive eruption, the blast can be extremely forceful. Volcanic blasts can be directed vertically (straight up), laterally (sideways along the ground), or at some angle between. Lateral blasts are most devastating because all of the energy of the blast is directed across land.

There are three zones associated with a lateral blast. The direct blast zone is the area that experiences the maximum force of the blast. Usually everything in this zone is completely obliterated - either burned or carried away. The channelized blast zone is an intermediate zone, where the blast still carries enough force and energy to level anything in its path, even whole forests. The last zone, the seared zone is a fringe area where trees and structures are left standing, but still affected by the heat and gas content of the blast.

The direction of the blast can be illustrated by the position of the felled trees and other debris. The action of a volcanic blast knocking down trees and buildings is called a "blow-down."


Tsunamis are also referred to as tidal waves, although they are unrelated to tides. In Japanese, Tsunami means "harbor wave." These waves can devastate coastal communities. A tsunami is a large, fast seismic ocean wave that is triggered by underwater earthquakes exceeding magnitude 7.5 on the Richter Scale. Landslides, volcanic eruptions, or other disturbances on the ocean floor also trigger tsunamis.

During any of these events great volumes of water are displaced. The water sloshes back and forth in the ocean for several hours. In deep and open ocean waters, a tsunami consists of a series of waves that are a few feet high, and a hundred or so miles apart. Tsunami waves can travel thousands of miles from their origin. They move at speeds near 600 miles per hour, close to the speed of a commercial aircraft.

As the waves near the shore, they bunch together, grow taller, and slow down. At the shore, tsunami waves may crest higher than 100 feet and have velocities of thirty miles per hour. The waves reach the shore in series, and frequently, the first is not the worst. Many coastal cities have extensive warning systems to alert residents of approaching tsunamis.

Climate Change

Climatologists have been studying the interactions between volcanoes and Earth's global climate. It is thought volcanic particles in the air are responsible for some degree of global cooling. The extent of the cooling depends on the force of the eruption, the height of the ash column, the volume of ash and gas emitted, and the location of the volcano in relationship to Earth's atmospheric currents.

The effects of some volcanoes can be substantial. For example, the eruption of Indonesia's Tambora volcano in 1815 led to 1816 being called "the year without a summer" as far away as Europe!

The sulfate particles emitted by volcanoes can increase the strength of CFC's (compounds that deplete the ozone layer) in the atmosphere. The effects of volcanic emissions on atmosphere and climate changes are measured and monitored by instruments on satellites. Remote sensing plays a large role in our knowledge of volcanic clouds.

The image shows how the tephra and volcanic gases emitted in the early 1990s by Mount Pinatubo in the Philippines traveled all around the globe. The red and yellow areas show where the volcanic ash was present in the atmosphere. Atmospheric currents carry particulates emitted from the volcano around the globe, affecting weather patterns and temperatures on the other side of the world.



Introduction to Volcanism

Eruptions and Hazards

Landforms and Features

Monitoring and Forecasting

Volcanisim in National Parks

Challenge Your Understanding


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