Volcanic Deposits: Definition & Explanation

Instructor: Maria Howard

Maria is a teacher and a learning specialist and has master's degrees in literature and education.

Discover what happens to the landscape after a volcano erupts. Learn how deposits from volcanoes create mountains, islands and even precious minerals.

Vesuvius Blows Its Top

You may have heard of the historic volcanic eruption of Mount Vesuvius near Naples, Italy in 79 A.D. It was kind of a big deal, with its violent surge of lava, ash and gases destroying the nearby towns of Pompeii and Herculaneum, covering them both in over six feet of ash alone.

By Friedrich Federer, 1850. Painting of Pompeii as the artist imagined it looked like in 79 A.D.
Painting of Pompeii

Sure, other volcanoes have erupted since then--heck, even Vesuvius has erupted since then. The reason our attention often returns back to that early Vesuvius blast, though, is because of what it left behind. 18th-century excavations revealed a carbonized city, preserved for historians and volcanologists (the people who study volcanoes). Ash and steam acted like a kind of wet plaster, keeping the two towns just as they were at the time of the eruption.

What they found were ordinary items, like bowls of carbonized walnuts, figs and eggs and a loaf of bread marked with a local baker's stamp. Most memorable are the preserved casts of people and even dogs, caught in the act of trying to escape Vesuvius' destruction. Together, they create a time capsule of what happened almost 2,000 years ago and provide valuable clues about how volcanoes dramatically change the land around them.

What Are Volcanic Deposits?

Volcanic deposits are the result of materials like rock and gases emitted from a volcanic eruption. Volcanoes are cracks in the Earth that allow melted or partially melted rock called magma to break through to the surface. (Though as soon as this molten rock does hit the surface, it is no longer called magma, but is referred to as lava.)

Lava flows from the Kamoamoa fissure, part of the Kilauea volcano in Hawaii.
Photo of lava flowing from a volcano in Hawaii

Flowing lava, ash, rocks and even gases are all emitted in volcanic eruptions, causing destruction and altering ecosystems. But, in the long run, volcanic deposits are part of a larger process of creating new landforms, richer soil for farming and a diverse number of new rocks.

Creating New Land

Deposition is the process of rock and tiny particles of rock called silt building up in a particular place. This buildup can create new land. In 1943, a farmer in Paricutin, Mexico, found a crack in his cornfield spitting out hot steam. (Sometimes, volcanoes begin as nothing more than small cracks in the ground.) After about a day, enough rocky materials had been expelled from the crack to form a hill the height of a 13-story building.

When lava cools, it hardens and forms an additional layer of rock on top of mountains already created by previous explosions. The type of lava, thick or thin, gives the mountain its shape, with the thinner lava creating a wider, flatter mountain and thicker lava giving it a more dome-like shape.

In addition to changing the landforms above ground, ranges of volcanoes deep beneath the ocean called mid-ocean ridges steadily alter the ocean floor. The ocean water keeps these volcanoes from making the same kind of fiery eruptions as their on-land versions. Instead, lava flows out at a gentle pace, creating rounded shapes of pillow lava. In places where the tectonic plates are tearing apart, oceanic volcanoes provide new material for the Earth's crust in a process called seafloor spreading.

3D imaging of a deep-sea volcano near the Northern Mariana Islands in the Pacific Ocean.
3D imaging of volcano under the ocean

Islands, like the chain which makes up the U.S. state of Hawaii, are sometimes formed when undersea volcanoes grow high enough to make their way above the surface of the ocean. An eruption off the coast of Iceland created the new island of Surtsey in 1963, and a growing Hawaiian volcano called Loihi is about 914 meters (3,000 feet) away from breaking the surface of the Pacific.

Ash, Dust, Blocks and Bombs

Pyroclastics are fragments of hot rock, ranging from blocks of lava the size of houses to much smaller rock fragments called ash to powdery dust. Sometimes even small masses of liquid rock known as bombs are discharged from a volcano, which seems like an appropriate name. Some volcanoes throw up enough ash that it can affect the weather and, at times, lower the Earth's temperature. Large eruptions in Iceland and Japan in 1783 led to a 'Little Ice Age,' with countries seeing much colder winters for the next few years.

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