Your turn
Each lab is one step from the video. Change things and see what happens.
Each lab isolates one part of the process. The models are simplified to show the idea, not to forecast real eruptions.
Drag, tap and slide.
Drag a magma blob deep down and let go. Does it ever stay down?
Magma is less dense than the solid rock around it, so it is buoyant. Push a blob down and release it.
Push the hot blob down. Let go. Where does it go?
Move the magma up the pipe. What happens to its bubbles as the squeeze gets weaker?
Lower confining pressure lets dissolved water vapor and CO₂ exsolve into bubbles. Move the magma toward the surface.
Move the magma up. Watch the bubbles grow!
Slide from runny to sticky. What do the bubbles do?
Silica-rich magma is viscous, so bubbles cannot escape and gas pressure builds until it fragments the melt.
Make the magma runny, then sticky. What happens to the bubbles?
Set the gas and the stickiness, then press erupt. Can you make all four kinds?
Combine gas content and viscosity: effusive flows, fountains, a Plinian column or a lava dome, and watch the landform change.
Tap each place to see how its magma forms.
Divergent boundaries, subduction zones and hot spots make magma in different ways. Tap each one.
Tap a place to see the hot rock come up.
A volcano is a place where melted rock comes up out of the ground. Some volcanoes are gentle. Others go boom!
Deep under the ground it is very hot. Some rock melts into a thick, hot goo called magma. Magma is lighter than the rock around it, so it floats up, like a blob in a lava lamp.
Magma is full of gas, like a fizzy drink. Deep down, the heavy rock squeezes the gas in. When magma moves up, the squeeze gets weaker and bubbles appear.
Runny magma lets its bubbles pop out, so the lava flows gently, like in Hawaii. Sticky magma, like honey, traps the bubbles. They push and push, and then, BOOM! Out comes ash.
Under the ground it's called magma. When it comes out of the top, we call it lava. When lava cools down, it turns into new rock.
Deep inside Earth it is hot enough for some rock to slowly melt into a thick, flowing liquid called magma. Magma is lighter than the solid rock around it, so it rises and collects in underground magma chambers. Eventually some of it pushes up through cracks and vents to the surface. Once it is out, we call it lava.1
What really drives most eruptions is gas. Magma holds dissolved gases, mostly water vapor and carbon dioxide. As magma rises, the pressure on it drops, and the gas comes out as bubbles that push the magma upward, a lot like opening a shaken bottle of soda.234
Whether that eruption is gentle or explosive depends mostly on how sticky the magma is and how much gas it holds.13
Magma is molten or partly molten rock.6 It doesn't come from Earth's core, and there's no hidden ocean of melted rock under our feet. Magma forms when parts of the upper mantle partly melt.3 This usually happens about 50 to 200 kilometers down.15
To picture where that is, think of a boiled egg. Earth's crust is the thin, brittle shell: only about 5 km thick under the oceans and about 30 km under the continents. Below it lies the mantle, a hot, dense layer of semi-solid rock about 2,900 km thick.8
Magma is less dense than the solid rock around it, so it floats upward, like the wax blobs in a lava lamp or air bubbles in syrup.12 It creeps up in slow blobs or squeezes through cracks. Sometimes it stops and pools in a magma chamber a few kilometers below the surface.3
They are the same material in different places. Scientists call molten rock magma while it's underground and lava once it breaks through the surface.1 When lava cools, it hardens into new rock (igneous rock). Over many eruptions that rock builds the volcano itself.9
Deep down, the enormous weight of the rock above keeps magma's gases dissolved. As magma rises, that squeeze gets weaker, so the gases come out of the liquid and form bubbles.3 The bubbles take up much more room and push hard on the magma, helping force it up and out, sometimes to great heights.2 The USGS calls dissolved gases "the driving force that causes most volcanic eruptions."4
The soda comparison comes straight from volcanologists. In a closed bottle, pressure keeps the carbon dioxide dissolved. Open it and the pressure drops, so bubbles form. Shake it first and the foam rushes out.23 Speed matters too. If magma rises slowly, gas can leak away quietly. If magma rises fast, its gas has less time to escape, so an explosive eruption is more likely.3
What finally sets an eruption off? Common triggers include fresh magma being pushed into the chamber, a build-up of gas pressure, or the sudden removal of the rock on top.3 At Mount St. Helens on May 18, 1980, an earthquake set off a giant landslide. It took the weight off the magma inside, and the volcano blasted sideways and then upward.5
The key is viscosity: how thick and sticky, or thin and runny, the magma is.3 The biggest control on viscosity is silica. In silica-rich magma the silica links into long chains that tangle "like long strands of spaghetti," so it hardly flows.3
The general rule: the stickier the magma and the more gas it holds, the more explosive the eruption.3
| Magma type | Silica | Temperature | Stickiness | Typical eruption |
|---|---|---|---|---|
| Basaltic | about 45–55% | about 1,200 °C (2,200 °F) | runny | gentle lava flows and fountains (Hawaii) |
| Andesitic | in between | 800–1,000 °C | medium | in between the two |
| Rhyolitic | about 65–75% | 750–850 °C | very sticky | the most explosive, with ash falls and pyroclastic flows |
Figures from Britannica.6 Notice that the hottest magma is the gentle one. Explosiveness comes mainly from stickiness and gas; temperature matters because it changes stickiness.
Volcanologists often name eruption styles after famous volcanoes.10 Here are the main ones, several important eruption styles:
At 8:32 a.m. a magnitude 5.1 earthquake shook loose the volcano's bulging north side. It collapsed in the largest debris avalanche in recorded history.5 With the weight suddenly gone, the gassy magma inside burst out. In less than 15 minutes the ash cloud rose more than 24 km (15 miles). The eruption went on for 9 hours, and winds carried 520 million tons of ash east across the United States. The ash cloud circled Earth in 15 days.5
A volcano is, strictly speaking, the opening (vent) that magma comes out of. The word also means the hill or mountain that builds up around it from hardened lava and ash.7 The kind of eruption decides the shape:9
Volcanoes aren't scattered randomly. Most sit along the edges of the giant rocky plates that make up Earth's surface.718
The Ring of Fire is the zone around the Pacific Ocean where the Pacific Plate meets many surrounding plates. It is the most volcanically active zone in the world.18
There are about 1,350 potentially active volcanoes worldwide, not counting the long volcanic ridges on the ocean floor. About 500 of them have erupted in recorded history. The United States has 161.13 The Smithsonian's Global Volcanism Program says at least 20 volcanoes are probably erupting at any given moment. From 2010 to 2024, an average of about 73 volcanoes were active each year.14
Often, yes, at least roughly. Before erupting, volcanoes usually show warning signs of "unrest." If scientists spot these in time, they can warn the people nearby.19 Common signs include small earthquakes caused by moving magma, and the ground swelling or bulging.35 Before its 1980 eruption, Mount St. Helens had more than 10,000 earthquakes, and its north side bulged outward about 2 meters a day.5
Yes. Over time, volcanic rock breaks down into some of the most fertile soils on Earth. The heat of young volcanic areas powers geothermal energy. And many metal ores, such as copper, gold and silver, formed from magmas in the roots of old volcanoes.20
Deep underground, some rock melts into magma. Magma is lighter than the rock around it, so it rises. Gas dissolved in the magma forms bubbles as the pressure drops, and those bubbles push the magma up and out of the volcano, where it is called lava.
Two things work together: buoyancy (magma is lighter than solid rock, so it rises) and gas pressure (dissolved gases like water vapor and carbon dioxide bubble out as magma nears the surface). Fresh magma filling a chamber, or a landslide that suddenly removes weight, can trigger the final push.
It depends mostly on how sticky (viscous) the magma is and how much gas it holds. Runny, low-silica magma lets gas escape easily, so lava flows out gently, as in Hawaii. Sticky, silica-rich magma traps gas until it bursts out violently, shattering the magma into ash, as at Mount St. Helens.
Volcanoes grow where magma reaches the surface again and again. Many eruptions add layers of lava or ash, building the volcano over time. Most form at plate boundaries, where plates pull apart or one plate sinks under another, and some form over hot spots such as Hawaii.
They are the same melted rock. Scientists call it magma while it is underground and lava once it reaches Earth's surface.
Lava flows downhill until it cools and hardens into new rock, such as basalt. Over many eruptions this rock builds up the volcano itself and can even create new land, like the Hawaiian Islands.
Mauna Loa in Hawaii is the largest active volcano. It rises more than 4 kilometers above sea level, and measured from its base on the sea floor its summit is about 17 kilometers high.