📚 Study helper: This page helps you learn and practice for school. Read it, listen to it, and try to say the four steps in order!
A volcano is an opening on the surface of a planet or moon.
Hot material from deep inside can escape through the opening.
When the material escapes, it causes an eruption.
Deep inside the Earth, it is so hot that some rocks slowly melt.
The melted rock is called magma.
Here is how magma gets out.
Elementary: A Volcano Erupts, Step by Step
Let us go slowly. A volcano erupts in four steps.
Count the steps with me. One, two, three, four!
Step 1. Rock melts deep down
Deep within the Earth it is so hot that some rocks slowly melt.
The melted rock becomes a thick flowing substance called magma.
Step 2. Magma rises and waits in a pocket
Magma is lighter than the solid rock around it.
So the magma rises up.
It collects in pockets under the ground. The pockets are called magma chambers.
Step 3. Magma pushes through vents and cracks
After some time, some of the magma pushes through vents and fissures in the Earth's surface.
A vent is an opening. A fissure is a crack.
Step 4. Pop! An eruption happens
A volcanic eruption occurs!
Magma that has erupted is called lava.
So remember this. Under the ground, it is magma. Out of the volcano, it is lava.
Two kinds of eruptions
Kind 1. Runny magma flows out
Some magma is thin and runny.
Gases can escape easily from runny magma.
When this type of magma erupts, it flows out of the volcano.
Lava flows move slowly enough for people to get out of their way.
Kind 2. Sticky magma blasts out
Some magma is thick and sticky.
Gases cannot escape easily from sticky magma.
Pressure builds up. Then the gases escape all at once, and the magma blasts into the air.
The U.S. Geological Survey compares the gases to the gas in a bottle of soda. Shake the bottle. Take your thumb off the top. Whoosh! The gas bursts out.
In the air, the magma breaks apart into pieces called tephra.
The tiniest pieces of tephra are ash.
The volcano builds a mountain
Lava, ash, and other material collect around the opening.
Layer piles on layer.
Over time, the layers build a mountain.
Scientists watch volcanoes
Scientists study volcanoes and watch them closely.
When a volcano gets more active, scientists share warnings.
The warnings help keep people safe.
Middle School: How a Volcano Erupts, With More Detail
A volcano is most commonly a conical hill or mountain built around a vent.
The vent connects with reservoirs of molten rock below the surface of the Earth.
The word volcano also means the opening, or vent, itself, through which molten rock and gases are expelled.
Volcanoes are built by the accumulation of their own eruptive products — lava and tephra, which is airborne ash and dust.
Why the magma rises
Heat in the Earth's upper mantle can melt rock locally. Small, isolated blobs of magma form first.
The blobs collect and rise through conduits and fractures. Some re-collect in larger pockets, or reservoirs, a few miles beneath the Earth's surface, the U.S. Geological Survey says.
Driven by buoyancy and gas pressure, the molten rock — which is lighter than the surrounding solid rock — forces its way upward.
Mounting pressure within the reservoir may drive the magma further upward through weak zones in the Earth's crust. If it breaks through, an eruption begins.
The gases decide how it erupts
All magmas contain dissolved gases. As magma rises to the surface, the confining pressure is reduced, and the dissolved gases are liberated either quietly or explosively.
If the lava is a thin fluid, the gases may escape easily, and the molten rock may pour from the vent as non-explosive lava flows.
But if the lava is thick and pasty, the gases will not move freely. They build up tremendous pressure and ultimately escape with explosive violence, shooting dense clouds of lava fragments into the air.
The U.S. Geological Survey says tephra can range in size from tiny particles of ash to house-size boulders.
The USGS teaching guide glossary says ash is fragments less than 2 millimeters (about 1/8 inch) in diameter of lava or rock blasted into the air by volcanic explosions.
The USGS also says ash erupted into the sky falls back to Earth like powdery snow, but snow that won't melt.
Three ways magma rises, says NASA Space Place
NASA Space Place lists three reasons why magma might rise and cause eruptions onto Earth's surface.
One: pieces of Earth's crust called tectonic plates slowly move away from each other, and magma rises up to fill in the space. Underwater volcanoes can form this way.
Two: the plates move toward each other, and part of Earth's crust can be forced deep into its interior. The high heat and pressure cause the crust to melt and rise as magma.
Three: magma rises over hot spots — hot areas inside Earth. These areas heat up magma. The magma becomes less dense, and when it is less dense it rises.
Active, dormant, or extinct
NASA Space Place sorts volcanoes into three groups.
Active volcanoes have had recent eruptions or are expected to have eruptions in the near future.
Dormant volcanoes no longer produce eruptions, but might again sometime in the future.
Extinct volcanoes will likely never erupt again.
On Earth, NASA Space Place says, the erupted material can be liquid rock, ash, cinders, and/or gas.
Signs before an eruption, and warnings
The U.S. Geological Survey says volcanoes often show signs that they are getting ready to erupt days to months in advance.
Scientists monitor seismic activity, ground movements, and gas emissions to detect subtle changes that may herald the next eruption.
When volcanic activity increases, the USGS issues advisories, warnings, and, whenever possible, specific forecasts about eruptions and their potential impacts.
Volcanoes beyond Earth
NASA Space Place says there are volcanoes all around our solar system, but only a few places besides Earth — like some of the moons of Jupiter, Saturn, and Neptune — have active ones today.
NASA's Voyager 1 spacecraft took the first close-up pictures of Jupiter's moon Io in 1979, NASA Space Place says. The pictures showed a massive plume erupting from its surface. It was the first time an erupting volcano had been found anywhere besides Earth.
NASA Space Place also says Mars is home to the largest volcano in our solar system, and Venus is covered with volcanoes.
On Earth, NASA Space Place notes, volcanoes are not all bad. They provide important nutrients for plants and animals and create beautiful mountain ranges and islands.
Volcano: The USGS teaching guide says a volcano is a vent, or opening, in the Earth's surface through which magma erupts. It is also the landform that is constructed by the eruptive material.
Volcano, NASA's wording: NASA Space Place says it this way: a volcano is an opening on the surface of a planet or moon that allows material warmer than its surroundings to escape from its interior.
Magma: Magma is molten rock that forms within the upper part of the Earth's mantle and crust. It contains liquids, crystals, and dissolved gases.
Lava: Lava is the term used for magma once it has erupted onto the Earth's surface.
Erupt: To erupt is to burst out. A volcanic eruption occurs when magma pushes through vents and fissures in the Earth's surface and comes out.
Crater: A crater is the circular depression containing a volcanic vent.
Ash: Ash is tiny fragments of lava or rock blasted into the air by volcanic explosions.
📚 Study helper: Great job! Now try this. Say the four steps in order: rock melts into magma, magma rises and collects, magma pushes through vents and cracks, and an eruption occurs. Then answer this: magma outside the volcano gets a new name. What is it? Lava!
U.S. Geological Survey, CoreFacts — How do volcanoes erupt?: https://www.USGS.gov/media/audio/how-do-volcanoes-erupt
U.S. Geological Survey — Volcanoes: The Nature of Volcanoes: http://pubs.usgs.gov/gip/volc/nature.html
U.S. Geological Survey — Volcanoes! Teaching Guide (grades 4–8, with glossary): https://pubs.usgs.gov/gip/70043848/teaching-guide.pdf
U.S. Geological Survey, Cascades Volcano Observatory — Who We Are and What We Do: https://www.usgs.gov/observatories/cascades-volcano-observatory/who-we-are-and-what-we-do
NASA Space Place — What Is a Volcano?: https://spaceplace.nasa.gov/volcanoes2/en/