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 Volcanic Eruptions   

                           Many countries around the world live with risks from                               volcanic eruptions. 

Unless you live in Hawaii where

Kilauea erupts almost constantly, you

probably don’t think much about

volcanoes. But, the continental

United States has quite a few

volcanoes and “hot spots” which

indicate potential volcanic activity.

Learning Objectives

To understand why volcanoes are a major natural disaster, you should be able to:

  • Describe how volcanoes form at three different types of tectonic locations.  

  • Name at least three examples of volcanoes formed at three different types of tectonic locations.

  • Explain how natural disasters can be caused by volcanoes. 

  • Describe five types of material expelled from volcanoes and how they may be health risks.



ash—a mixture of rock, minerals and glass particles expelled from a volcano.

convergent plate boundary—where two tectonic plates are moving toward each other.

divergent plate boundary—where two tectonic plates are moving apart.

hotspot—an isolated region of volcanic activity usually not associated with a plate boundary.

lahar—volcanic mudflow formed from hot ash and water.

magma—hot, molten material inside the earth.

pyroclastic flow—a flow of hot gas and ash from a volcano.

volcano— an opening in the earth’s crust through which lava, steam and ash flow; a mountain formed by volcanic activity.

volcanologist—a scientist who studies volcanoes.

Why and Where Volcanoes Form

   Volcanoes and the eruptions that build them can only occur where magma (molten material from the mantle) reaches the surface of the Earth.

This happens at three locations:

1) At divergent plate boundaries where two plates are moving apart.

   Most eruptions on planet Earth occur underwater at divergent plate boundaries that split apart ocean floors. Magma erupts and lava flows across the ocean floor. These eruptions are not seen and do not endanger people, crops or buildings, and many people may not even know about them. But, underneath every ocean on Earth is lava that comes up from the mantle.


Did you know?  

   Mid-ocean ridge material is continually pushed further away from the plate boundary and magma rises to the top. The material can create landforms.

   Iceland was formed from volcanic eruptions at the Mid-Atlantic Ridge. Iceland is volcanically active. One-third of the magma that has erupted on earth for the last 10,000 years came up in Iceland.


  Examples of volcanoes at divergent plate boundaries are those in the Mid-Atlantic Ridge where the North American and Eurasian plates are moving away from each other. Volcanoes are also formed the East Pacific Rise, where the Pacific Plate moves away from the Nazca, Cocos and North American plates. 

2) At convergent plate boundaries where two plate collide

   Magna rises to the surface when two plates move toward each other and one is forced under the other. This creates subduction zones of molten material. Heat from the mantle melts rocks that are forced under and this magma can rise to the surface.

                                                          Because most of these volcanoes                                                           are on land, they cause much                                                                  destruction and loss of life. One                                                                eruption of the Nevado Del Ruiz                                                              volcano in Columbia, South                                                                      America in 1985 killed 23,000 people and destroyed numerous villages and farmland.

Image: Town of Amero, Columbia destroyed by volcanic mud flows (lahars), 1986. Image: Wikipedia.


   There are three possible types of collisions, two of which produce eruptions:

· Ocean to ocean collisions such as the Pacific plate and the ocean edge of the Australian plate,

· Ocean to continent collisions such as the Pacific and South American plates, and

· Continent to continent such as where the Indian plate slams into the Eurasian plate.


   When an ocean plate collides with

any other plate, the ocean plate is

subducted under the other plate.

In the case of a collision with a

continent, the sediment on the

seafloor is carried down. At a depth of

about 100 km, the slab of sediments gets hot enough to melt, creating magma that rises up through the continental crust. It erupts on the surface. Because the magma contains water from the sediments, the erupted magma doesn’t just flow across the surface, but explosively erupts. Lava is torn into small pieces forming ash.

   Examples of volcanoes formed at

convergent plate boundaries are

Mount Fuji in Japan, the Columbia,

South America volcano, Mt. St. Helen’s

volcano in Washington state and

Vesuvius volcano which buried the city

of Pompeii in 79 AD. 

   The volcanic ash that buried the city of Pompeii preserved the bodies of humans and dogs, roads and rooms of homes so well that the way of ancient Roman life can be seen.

3) At hotspots

   A hotspot forms when magma in the mantle is hotter at certain regions and is therefore rises through the mantle and crust to erupt at the surface. These regions are usually not associated with plate boundaries.

   Hawaii is an example of landforms and volcanoes created by a hot spot. As the Pacific Plate moved over the Hawaii hotspot a trail of mountains that pushed to the surface now stretch across the Pacific. These form the Hawaii Islands.

   Yellowstone is also over a hotspot as the North American plate moves over the stationary hotspot. It is one of the world’s largest active volcanic systems. Several huge volcanic eruptions have occurred there in the past few million years. Future eruptions are likely.

Yellowstone is classed as a supervolcano. Other supervolcanoes include Long Valley in California, Toba in Indonesia and Taupo in New Zealand.

Dangers and Health Risks from Volcanoes

   Lava from volcanoes destroy property, bury buildings, and burn homes and forests. Fragments of hot rock and lava may erupt more suddenly and explosively. They can spread faster and farther. Large blocks of hot rock rain down on nearby homes and towns.

   Finer ash and dust particles travel

great distances; ashfalls can blanket

the countryside several meters thick. 

The ashfall from the Mt. St. Helen’s

eruption in Washington state was

measured halfway across the United

States. Homes, farms, forests and

cars can be buried under hot ash.

   Lahars (a combination of volcanic ash and water) are another danger. Lahars from volcanic eruptions flow downhill and can flood towns downstream of the volcano. Entire towns can be destroyed by lahars.

   Pyroclastic flows are a mixture of hot gases and fine ash. These are much different from a rain of cinders and ash. Pyroclastic flows can be over 1,000 degrees C (Celsius) in the interior of the cloud and the cloud can rush down the mountains at speeds of 100 km per hour.

   Volcanoes also emit toxic gases such as carbon monoxide, various sulfur gases and hydrochloric acid. People have been killed by these poisonous gases before they recognized the danger.

   As you might expect, there are health risks associated with a volcanic eruption.  Burns from hot cinders and ash raining down on people, falls and injuries from efforts to escape the effects of the eruption, breathing problems from volcanic ash in the air are serious health consequences. Injuries also occur after the eruption as people try to clean up the fallen ash and burned property.

Check Your Understanding—Volcanoes

1. Describe three locations where magma rises to form volcanoes.


2. Name at least three examples of volcanoes formed at the volcanic tectonic locations.


3. Explain how natural disasters can be caused by volcanoes.


4. Describe five types of material expelled from volcanoes and how they may be health risks.

© Wheeling University, 2023. All rights reserved.




Image of erupting volcano with lava and ash pouring out
Picture of town of Amero, Columbia with mudflows covering the land
diagram of converging plate boundaries showing subduction zone
picture of the a house in Pompeii after the volcano destruction
picture of a house buried up to the roof in volcanic ash
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