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How Does a Volcano Form?

Jose is a nature enthusiast that likes researching and writing about the natural world and how it works.

Volcanoes are some of the world's most awe-inspiring and terrifying natural features.

Volcanoes are some of the world's most awe-inspiring and terrifying natural features.

What Is a Volcano?

A volcano is a geological rupture in the earth's crust triggered by pressure, temperature, and other natural forces in the planet's interior. These forces drive gasses and hot liquid known as magma out through a volcano's orifice, which is referred to as a "vent." Once outside of the vent, these erupted materials break down, harden, or condense as they cool.

How Do Volcanoes Work?

A volcano forms when pressure, temperature, and other natural forces push magma out of a magma chamber (a large, underground pool of liquid rock) until it erupts as lava on the surface of the earth or as a boiling gush under the ocean. When the molten rock (magma) reaches the planet's surface—whether on land or at the bottom of the ocean—it immediately begins to cool and harden.

Over thousands or millions of years, the many layers of cooled magma that have erupted from a volcano may form a steep-sided cone around its vent. Most of the planet's most famous volcanoes share this cone-like appearance.

The formation of volcanoes has contributed to the configuration of many of the planet's varied landscapes. Volcanoes form most commonly at the convergent or divergent boundaries of tectonic plates. Some form on mid-ocean ridges, where tectonic plates are spreading apart. Others form near subduction zones, where one tectonic plate is sinking into the earth's mantle underneath another tectonic plate. Sometimes, volcanoes form closer to the center of a tectonic plate above a magmatic "hotspot."

Volcano Formation via Plate Tectonics

The earth’s crust is composed of enormous sections of rock called tectonic plates. Tectonic plates resemble puzzle pieces and are constantly moving toward, against, and alongside one another. Volcanoes often form in the areas where tectonic plates make contact. The friction created by the movement between two plates can melt solid rock in the mantle and turn it into magma. This hot, molten rock creates great pressure, and over time, it finds its way up to the surface of the crust through fractures. Once magma reaches the surface of the earth, it is called lava.

Approximately 1500 volcanoes around the world are considered "active." Of these, nearly 90% lie in the "Ring of Fire," which is a perimeter of oceanic volcanoes that circle the Pacific Ocean. These active volcanoes were formed by the subduction of oceanic plates under continental plates.

In a subduction zone, an oceanic plate submerges into the mantle under a continental plate. The friction created by the plates' movement creates magma. When this magma reaches the earth's surface, a volcano is formed. A typical example of this type of volcano is Mount Etna on the east coast of Italy.

This diagram illustrates the structure of a typical volcano.

This diagram illustrates the structure of a typical volcano.

The Parts of a Volcano

  1. Ash Plume
  2. Central Vent
  3. Volcanic Ash Rain
  4. Layers of Basaltic Lava
  5. Earth's Crust
  6. Magma Chamber

What Is a Hotspot?

Although many volcanoes form underwater in subduction zones or at divergent plate boundaries, some form on land. Areas where volcanoes that form on land away from tectonic plate boundaries are known as hotspots. These volcanoes are formed when hot magma rises through intrusions in the crust known as mantle plumes. Hotspots form above stationary regions of extremely hot magma under the earth’s crust.

After a mantle plume forms a volcano on a hotspot, the continental plate above will continue to drift until the newly formed volcano is no longer situated above the hotspot. An unscathed area of crust is then exposed to the same mantle plume, and a new volcano may be created. Over time, a single hotspot can form a string of volcanoes on a single plate. These strings are called volcanic chains. The Hawaiian Islands formed in this fashion and are probably the most well-known example of the volcanic chain phenomenon.

What Is a Volcanic Vent?

The cracks or openings through which magma is forced out of magmatic chambers and up through the crust are called volcanic vents. Magma gets ejected as lava through these openings. When internal heat and pressure inside the earth force magma out through a vent, incandescent jet sprays of molten lava, rocks, and gases can reach hundreds of meters high into the sky. They may send volcanic material into the sky intermittently or incessantly for an extended period of time. Over time, the volcanic fragments build-up around a vent and gradually form layers of volcanic material.

What Is a Flood Basalt?

Much of the earth's crust is made of basalt—a common igneous rock comprised of cooled lava. Nearly all of the ocean floor is made of basalt as well. Flood lava spreads over flat land, and as it piles up, it forms thick basaltic lava plateaus known as flood basalts. One of the largest flood basalts ever created on the surface of the earth is the Columbia River Plateau, which covers much of Idaho, Oregon, and Washington in the United States.

This table explains the differences between some common types of volcanoes and volcanic phenomena.

This table explains the differences between some common types of volcanoes and volcanic phenomena.

Types of Volcanoes

Volcanoes appear in a wide range of sizes and varieties. Some of the more common types of volcano are described below.

Shield Volcanoes

Shield volcanoes are some of the largest on earth and are characterized by their gentle slopes. Shield volcanoes usually have slopes of just a few degrees near their bases. Their slopes max out at around 10 degrees and tend to flatten around their summits. The gentle slopes of shield volcanoes are the result of the low viscosity of their basaltic lava flows. This type of lava can flow over long distances before it cools down and solidifies. 17 km (10.5 mile)-high Mauna Loa in Hawaii is a well-known example of a shield volcano with a gentle slope.

Composite or Stratovolcanoes

Composite volcanoes, also known as stratovolcanoes, tend to form steeper slopes due to the higher viscosity of their basaltic lava. This type of lava flows slowly due to its high-density composition. Over time, high-viscosity lava builds up to create steep-sided, conical mountains. During their formation process, these volcanoes' lava eruptions flow slowly and harden quickly, creating thick layers of basalt that build upon one another with each successive eruption. Composite volcanoes are mostly found on the continental plates. 4.4 km (2.7 mile)-tall Mount Rainier in Washington and 3.7 km (2.3 mile)-tall Mount Fuji in Japan are both composite volcanoes.

Hotspot Volcanoes

Hotspot volcanoes form over regions of underlying mantle that are extremely hot compared to their surroundings. When a vent finds its way to the surface from an underlying magma chamber, the crust ruptures and a hotspot volcano is formed. The overlying layer of crust that hosts the hotspot volcano may move due to tectonics causing a new area of crust to overlay the superheated magma chamber. This can result in additional volcanoes being formed in a chain over time. The Hawaiian Islands are a volcanic chain, each having formed over the same hot region in the earth's mantle.

Cinder Cone Volcanoes

Cinder cones are the smallest type of volcano and are formed as small rock fragments (tephra) and ash solidify around a cylindrical vent to form a circular cone. Cinder cones normally have a bowl-shaped crater at their summit and rarely rise above 300 meters (985 feet). They are often found around the periphery of larger volcanos like shield or composite volcanoes.

Lava Dome Volcanoes

Lava domes are formed from high viscosity basaltic lava that barely flows at all before cooling and hardening. This type of lava often creates a volcanic dome over and around its vent. When the dome hardens, it can trap gases, creating a buildup of pressure. When the pressure is high enough, the dome may be blown to pieces in a violent eruption. A lava dome started developing on Mt. St. Helens in Washington shortly after its famed 1980 eruption.

Submarine Volcanoes

Submarine volcanoes, or volcanoes that lie on the ocean floor, are very common. Many form at great depths, rendering them unable to reveal their explosive presence due to the extreme weight and cooling effect of ocean water above them. Others that form in more shallow waters may reveal their presence by blowing steam and rocky debris above the surface of the sea. Sometimes, submarine volcanoes may form steep pillars over their volcanic vents. Some grow so large that they reach the ocean surface and form new islands. The San Juan Islands in the state of Washington may have formed due to these types of eruptions.


Calderas are huge, circular, topographic depressions that are formed when a volcanic eruption empties a deep magma chamber, causing the overlying land to collapse. Calderas can range in size from 5 km (3.1 miles) to up to 50 km (31 miles) wide. Over time, an old magma chamber may fill with magma again, causing the floor of the caldera to rise. It may remain like this, or repeated eruptions may reshape the caldera over and over again to produce what is known as a resurgent caldera. Crater Lake in Oregon, which is an 8 km (5 mile)-wide, 600 meter (1970 feet)-tall caldera, was formed in this way.

Check your knowledge about volcanoes

For each question, choose the best answer. The answer key is below.

  1. What is a volcano?
    • An rupture in the Earth's crust
    • A fracture on the flanks of a mountain
  2. What is a volcanic vent?
    • An underground magma chamber
    • A hole in the earth's crust through which magma is ejected
  3. What type of lava forms shield volcanoes?
    • Low-viscosity lava
    • High-viscosity lava
  4. What type of volcano develops a steep slope?
    • A composite volcano
    • A shield volcano

Answer Key

  1. An rupture in the Earth's crust
  2. A hole in the earth's crust through which magma is ejected
  3. Low-viscosity lava
  4. A composite volcano

Interesting Facts About Volcanoes

  • Approximately 90% of all active volcanoes are located in the Pacific "Ring of Fire."
  • The crusts of Mercury, Venus, Earth, Mars, and the Moon (the inner planets) are made of basaltic rock.
  • More than 95% of the Earth’s first 16 km (10 miles) of crust is made of igneous rock (basalt) formed by lava eruptions.
  • Mt. Etna in Italy has been erupting for more than 3,500 years. The first eruption occurred in 1,500 BCE.
  • Kilauea, a young shield volcano in Hawaii, has been erupting continuously since 1983.
  • The Hawaiian islands formed over a hotspot in the middle of the Pacific Plate.

Questions & Answers

Question: What is the difference between a volcano and a mountain?

Answer: A mountain is a landform that stretches above the surface of the Earth that can be steep like a peak or not as steep. Volcanoes are similar to mountains; however, the pressure inside a magma chamber creates an orifice through which lava and hot gases erupt into the atmosphere.

Question: How are volcanoes formed?

Answer: The continuous eruption of lava forms volcanoes, and it may take several hundred years for them to develop.

Question: What happens after a volcano has errupted?

Answer: After a volcanic eruption, lava usually continues to flow causing fires and changing nearby landscapes. Ash often travels far away, affecting airplanes and crops and landing in rivers and lakes in other regions; ash, may also affect humans and animals.

While lava and ash from a volcano may damage technology and humans, it may also provide some benefits, including nutrients that plants and trees need to grow.

Question: How do volcanoes form on the ocean floor?

Answer: They are formed due to the activity of the earth´s lithosphere by plate tectonics. The natural activity of the planet, such as tidal changes, earth´s rotation, gravitational pull cause tectonic plates to shift and collide. This movement of tectonic plates underwater causes underwater volcanoes to form where two tectonic plates meet, most commonly at convergent and divergent boundaries.

Question: How are mud volcanoes formed?

Answer: This volcano is a mixture of mud, water and gases that are ejected during an eruption; this type of eruption does not eject lava and is not driven by magmatic activity. It may result from the perforation made by mud diapir on the earth´s crust or the bottom of the ocean.

Mud volcanoes vary in size from just a few meters to several hundred km high and 10 km wide.

Question: Do earthquakes happen after volcanoes erupt?

Answer: Not always do earthquakes happen after volcanoes erupt. Earthquakes are driven by plate tectonics, the same as volcanism. When an earthquake occurs, it may be due to plate tectonic movement in the earth´s crust. While an eruption can produce crust movement, it is not as strong as when the earth´s plates slide past each other.

Question: What is a dormant volcano?

Answer: Dormant volcanoes are those that may remain inactive for long periods of time, but might erupt suddenly at any moment.

Question: Why is the earth's interior so hot?

Answer: The interior of the earth obtains its heat energy from three sources; ther primordial heat or the heat from when the earth first formed, the frictional heat or the heat produced by the sinking material to the center of the earth and the radioactive decay of isotopes in the crust and mantle of the earth.

Question: How is lava formed?

Answer: Lava is made underground at around 160 km (100 miles) or more underground where temperatures are hot enough to melt rock. Molten rock underground is called magma, once it is erupted out through a volcano, It´s called lava.

Question: Is Mount Everest an active volcano?

Answer: Mount Everest is not a volcano. The Himalayas, where Mount Everest is located, were formed by seabed being forced together by tectonism

© 2012 Jose Juan Gutierrez


fbshhgzjklhg on May 12, 2015:

hay volcano are good

Joy on March 09, 2015:

How do the plate cause the volcano to form

Jose Juan Gutierrez (author) from Mexico City on September 17, 2012:

Yes, Scandinavia is close to Iceland, which is a region of high volcanic activity. I´ll check on Laacher Volcano in Germany.

Thanks for reading and commenting on this hub.

Gbite on September 17, 2012:

Quote from starbright: "Our nearest volcano is probably on Iceland I believe."

Iceland's volcanoes would be closest to Scandinavia yes because most of them are active, but if you want an even closer volcano you should look up Laacher See Volcano in Germany. It is a dormant volcano for the time being, roughly the same size as Mount Pinatubo Volcano in the Philippines, which made the larges eruption recorded in the 20th century.

whowas on August 02, 2012:

That is a fascinating and comprehensive account of the science of volcanoes.

I have never personally witnessed a volcano erupting. However, I have spent time in the south of Italy very close to Vesuvius and indeed walked the desolate streets of the ruined city of Pompei. There are human bodies on display that have been effectively fossilized in cooled down volcanic lava and ash. A very impressive and moving experience.

Jose Juan Gutierrez (author) from Mexico City on August 01, 2012:

Sometimes, volcanic eruptions can change the scenic landscape much beautifully. Unfortunately, most eruptions have caused distress and loss of lives.

It is good to be aware of the dangers of these natural phenomena, so as to react accordingly. There have been eruptions which have killed people in less than 4 hours. That is the case of Armero in Colombia.

Lucy Jones from Scandinavia on August 01, 2012:

The most beautiful and / or interesting 'acts of nature' are very often the most dangerous. Stay safe unvrso.

Jose Juan Gutierrez (author) from Mexico City on August 01, 2012:

Thanks for spending your time to read this hub. I live close to one volcano called "Popocateptl" which was dormant for a while, but has been showing signs of activity recently. Even though I live 35 miles from it, I wouldn't want to witness an explosion.

Lucy Jones from Scandinavia on August 01, 2012:

Very interesting subject, which until I'd read your hub I didn't know much about. Our nearest volcano is probably on Iceland I believe. An eruption is quite fascinating to see and although I haven't seen one first hand, I have visited Pompeii, which is particularly interesting as the devastation that Vesuvius left in its wake hundreds of years ago, can still be seen. Thanks so much for sharing - it's renewed my interest to find out more. Voted up as interesting.

Jose Juan Gutierrez (author) from Mexico City on August 01, 2012:

Thanks, for your comment.

Dora Weithers from The Caribbean on August 01, 2012:

Now I can talk intelligently about magma and lava. Thanks to your well-presented hub and the video as well. My recent hub on the active volcano on the Caribbean island of Montserrat (https://msdora.hubpages.com/hub/Montserrat-Before-... even means more to me now.

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