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What is Volcanic Ash? - Definition, Uses & Effects

Instructor: Amanda Robb
In this lesson, we'll cover what volcanic ash is and how it's made. We will also go over uses for volcanic ash, as well as the positive and negative effects of this substance.

Volcanic Paradise?

Hawaii is often considered a tropical paradise. Its islands have soft, white sand beaches and white capped waves reaching out to the horizon. However, Hawaii wasn't always a peaceful vacation destination. It was actually formed by massive volcanic eruptions, spewing hot, molten rock from the earth's core, along with rock and volcanic ash. Volcanic ash is a dangerous and important part of volcanic eruptions, and it will be the focus of this lesson. Today, volcanoes continue to erupt on the Big Island, such as the one below.

A volcano erupts in Hawaii. The cloud of smoke is volcanic ash.
Hawaiian volcano

What are Volcanoes?

To understand volcanic ash, we need to first think a little bit about their source, volcanoes. Volcanoes are areas of the earth where large pieces of the earth's crust, called tectonic plates, move apart. Volcanos are usually elevated from the surrounding land and resemble mountains. During an eruption, the liquid rock inside the earth, or lava, spews out in a massive eruption. The lava becomes magma when it is exposed to the surface. A diagram of volcanic eruptions is shown below.

Volcanic eruptions occur when tectonic plates spread apart
volcanic eruptions diagram

What is Volcanic Ash?

Why does lava explode out of the earth, instead of just oozing out? Well, gases inside the volcano expand as temperatures rise from the molten core of the earth moving towards the surface. The expansion of the gases increases the pressure inside the volcano immensely. The result is a violent eruption of gases, lava, rock, and volcanic ash.

Volcanic ash is a combination of fine particles made of rock, tiny strands of supercooled lava called volcanic glass, and minerals. The particles are usually smaller than an inch in diameter, but can contain powdered rocks from the volcano's exterior shattered during an eruption. The result is fine, sand-like particles and clouds of ash raining from the sky. Below is an image of volcanic ash erupting from Mount Redoubt in Alaska.

Volcanic ash falls from volcanic eruptions of as a cloud of powdered rock
Mount Redoubt

Effects of Volcanic Ash

Volcanic ash can be a serious side effect of volcanic eruptions. Ash clouds rain down on surrounding areas, sometimes coating the earth in feet of ash. Areas close to the volcano have been recorded as having 5-7 feet of ash coating the surface! Wind can carry fine particles of ash away from the eruption site, extending the damage to surrounding communities.

Volcanic ash can also cause thunder and lightening storms, and if it is carried high enough into the atmosphere it can deflect light from the Sun, cooling temperatures on earth and creating a volcanic winter. When volcanic ash is carried into the atmosphere, the rock particles can also create acid rain, which erodes the land it precipitates to, such as the forest shown below.

Acid rain from volcanic ash destroys surrounding areas of land
acid rain effects

Rock shattered from the eruption, volcanic ash, and magma can create a pyroclastic flow, which is an avalanche of these materials pouring from the eruption site. The force from the pyroclastic flow can be impossible for humans to outrun and may completely destroy buildings in the area. Below, a pyroclastic flow rolls towards a town.

Pyroclastic flow moves towards a town
pyroclastic flow

Examples of Volcanic Eruptions

There are many examples of volcanic eruptions in our planet's history, and volcanic eruptions still occur today. One eruption with a notable amount of volcanic ash was the eruption of Mount Vesuvius in Pompeii, Italy in 79 A.D. Feet of ash rained down on the city. The volcanic ash created a dark cloud and pyroclastic flows that entombed the city forever, preserving artifacts for humans to study many years later, helping us learn about ancient civilizations. Below, bodies in Pompeii are preserved by a volcanic ash coating.

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