Volcanoes are mountains formed around vents or fissures in the Earth’s crust that release lava, ash, rock, and gases. When volcanoes erupt, they release heat from the Earth’s interior. The Earth’s core contains decaying radioactive elements that release heat when they break down. This heat drives the convection currents in the mantle that move the tectonic plates of the crust. It also melts rock. Melted rock, called magma, collects deep underground in magma chambers. Volcanoes form over these magma chambers.
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Where and How Volcanoes Form
Magma chambers and the volcanoes above them form in three places: hot spots, divergent plate boundaries, and convergent plate boundaries. Hot spots are especially hot places in the mantle.
Scientists aren’t sure what causes hot spots, but they can track volcanoes formed by them. The island chain of Hawaii is an example of volcanoes formed over a hot spot. At divergent plate boundaries, two tectonic plates move away from each other, and magma fills the space between them. The Mid-Ocean Ridge is an example of a volcano chain along a divergent plate boundary. At convergent plate boundaries, one tectonic plate is subducted under another plate. As the plate is forced into the mantle, the rock melts and collects in a magma chamber. The volcanoes along the Ring of Fire are examples of volcanoes formed at convergent plate boundaries.
What is Magma?
Magma is a combination of liquid rock, crystallized minerals, solid rock, and dissolved gases. It rises up through cracks in rocks because it is less dense than the rocks around it. When the magma can’t rise any farther, it collects in a magma chamber. As magma fills the chamber, the pressure from the gases within it grows. Eventually, the pressure becomes so great that it explodes and breaks the rock around it. Magma is forced through the volcano’s vents, where it erupts from the volcano as lava.
Volcanic Eruptions
The force of the eruption can shoot hot liquid lava into the air. The lava in the atmosphere cools so quickly that it crystallizes into ash. Volcanic ash is made up of tiny pieces of rocks, minerals, and volcanic glass. Millions of tons of ash can fall from the sky for days after a volcanic eruption. The ash is extremely harmful to plants, animals, and humans. It can irritate people’s airways, bury plants and animals, and block out the sun. When Mt. Vesuvius erupted nearly 2,000 years ago, the ash destroyed the city of Pompeii and killed around 16,000 people.
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Over time, the volcanic ash helps people because it is full of the carbon and nitrogen that plants need to grow. Volcanic ash makes the soil around a volcano especially fertile, so many farming communities settled near volcanoes. Today, large cities sit in the shadows of volcanoes. For example, Seattle, Washington, is very close to Mt. Rainer, a dormant volcano.
Dormant Volcanoes
A dormant volcano is a volcano that has not erupted in the past 10,000 years, but is still connected to a magma chamber and may erupt in the future. An active volcano has erupted within 10,000 years and may erupt again. An extinct volcano is not connected to a magma chamber and will not erupt again.
How Volcanoes Form
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There are also different types of volcanoes based on how they formed. Cinder cone volcanoes are the most common type of volcano. These volcanoes usually have a single vent that releases fragments of lava called tephra. The tephra solidifies in the atmosphere and falls around the volcano as cinders that build up around the volcano’s vent. These volcanoes are small and form quickly over months or years. Paricutin in Mexico is one of the most famous cinder cone volcanoes. It erupted for nine years and grew to almost 1,500 feet.
Stratovolcanoes are sometimes called composite volcanoes because they are built from alternating layers of lava, ash, and stone. They have steep sides around a small crater at the summit. Unlike cinder cone volcanoes, stratovolcanoes can have multiple vents leading out of the volcano. These volcanoes have violent eruptions because of the high pressure within their magma chambers. When Mt. St. Helens, a stratovolcano, erupted in Washington State on May 18, 1980, 230 square miles of forest were destroyed, 520 million tons of ash was released into the air, and 57 people died.
Shield volcanoes are the largest volcanoes, but their sides are not steep. These volcanoes form as lava oozes from a central vent and spreads down the sides. Shield volcanoes’ wide bases and flat summits make them look like a medieval knights’ shield, which is how they got their name. Hawaii is home to the largest shield volcanoes on Earth. Mauna Loa reaches 13,681 feet above sea level but extends 55,770 feet to its base under the ocean, making it taller than Mt. Everest.
How Volcanoes Form and Change the Surface of the Earth
Volcanoes also change the surface of the Earth. They grow into mountains, cover the land around them with ash and lava. When the lava cools, it becomes the igneous rock that makes up most of the Earth’s crust. Our world would look very different without volcanoes.
Volcanoes Reading Passage
Help your students learn about volcanoes with this reading passage. Video links and comprehension questions are also included. Click on the picture below to check out the reading passage on Teachers Pay Teachers.
The Geosphere
Middle School Science
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