How do we know?
↻ One of the pages with a moving picture
Scientists watch volcanoes with over 200 instruments.
- Deep underground, it is so hot that some rock melts into thick, liquid magma. It collects in a magma chamber.
- Magma is lighter than the solid rock around it, so it rises.
- Magma holds gas. As it rises, the pressure drops and the gas forms bubbles.
- The bubbles push magma out through the vent. Magma that reaches the surface is called lava.
USGS scientists in Hawaiʻi watch volcanoes with over 200 instruments. Some feel the ground swell as magma fills up below. Others record tiny earthquakes or “sniff” volcano gas.
Bottled soda is a good model. Shake it and tiny bubbles grow fast and push the drink out, just like the bubbles in magma push it up through a volcano.
At Mount St. Helens on 18 May 1980, the volcano’s side slid away and trapped gas blasted out sideways.
Wow number
2,140°F is how hot Kīlauea’s lava is as it erupts: more than 4 times the top of an oven dial (about 500°F)!
Myth or fact?
Under the ground is a giant sea of melted rock. Myth. The rock under us is almost all solid. Magma forms in small pockets.
Some volcanoes hide under the ocean. Fact. More volcanoes are on the ocean floor than on land.
Think about it
Many families live near volcanoes. Why do you think they stay?
Where in the world?
Kīlauea, Hawaiʻi where runny magma makes lava pour out. At Mount St. Helens, sticky magma held gas in until the side of the mountain slid away on 18 May 1980 — 19.4069° N, 155.2834° W.
Where these facts come from
Every number on page 19, and who measured it
This page explains how magma rises and bursts out of a volcano.
How Do Volcanoes Erupt? — The U.S. Geological Survey, the government's Earth scientists
usgs.gov/faqs/how-do-volcanoes-erupt
We checked it on 31 October 2026
This page tells us how hot Hawai'i's lava is.
How hot is a Hawaiian volcano? — The U.S. Geological Survey, the government's Earth scientists
usgs.gov/faqs/how-hot-a-hawaiian-volcano
We checked it on 31 October 2026
This page tells us what happened at Mount St. Helens on May 18, 1980.
1980 Cataclysmic Eruption — The U.S. Geological Survey, the government's Earth scientists
usgs.gov/volcanoes/mount-st.-helens/science/1980-cataclysmic-eruption
We checked it on 31 October 2026
This page tells us about the sideways blast at Mount St. Helens.
MSH Lateral "blast" [USGS] — The U.S. Geological Survey, the government's Earth scientists
pubs.usgs.gov/gip/msh/lateral.html
We checked it on 31 October 2026
This page compares an eruption to a shaken soda bottle.
Volcano Watch — Bottled soda helps us to understand volcanic eruptions — The U.S. Geological Survey, the government's Earth scientists
usgs.gov/observatories/hvo/news/volcano-watch-bottled-soda-helps-us-understand-volcanic-eruptions
We checked it on 31 October 2026
This page also explains bubbles in shaken soda, just like gas in magma.
Volcanoes — The U.S. Geological Survey, the government's Earth scientists
pubs.usgs.gov/gip/volc/text.html
We checked it on 31 October 2026
This page tells us about the scientists who watch Hawai'i's volcanoes.
About the Hawaiian Volcano Observatory — The U.S. Geological Survey, the government's Earth scientists
usgs.gov/observatories/hvo/about-hawaiian-volcano-observatory
We checked it on 31 October 2026
This page tells us how tools measure the ground swelling when magma moves.
Deformation Monitoring Tracks Moving Magma and Faults — The U.S. Geological Survey, the government's Earth scientists
usgs.gov/observatories/hvo/science/deformation-monitoring-tracks-moving-magma-and-faults
We checked it on 31 October 2026
This page tells us the rock under our feet is solid, with only small pockets of melted rock.
The U.S. Geological Survey, the government's Earth scientists
usgs.gov/faqs/are-tectonic-plates-floating-magma
We checked it on 31 October 2026
This page tells us how many volcanoes on land could erupt.
How many active volcanoes are there on Earth? — The U.S. Geological Survey, the government's Earth scientists
usgs.gov/faqs/how-many-active-volcanoes-are-there-earth
We checked it on 31 October 2026
This page tells us most of Earth's volcanoes hide on the ocean floor.
Do volcanoes occur in the ocean? — NOAA's Ocean Service, U.S. ocean scientists
oceanservice.noaa.gov/facts/volcanoes.html
We checked it on 31 October 2026
This page tells us most volcano eruptions happen deep underwater.
Do volcanic eruptions happen underwater? — NOAA Ocean Exploration, U.S. deep-sea explorers
oceanexplorer.noaa.gov/facts/volcanoes.html
We checked it on 31 October 2026
Has this changed?
Volcanoes work the same way they always have, so this answer is not going to change.
Which volcanoes are active keeps changing, so scientists keep watching and counting.
Words from this page
- magma
- Hot, melted rock underground.
- lava
- Magma that has reached the surface.
- vent
- The opening that magma comes out of.
Try it!
Shake a soda to see how bubbles push liquid out.
1. Outside with a grown-up, get two small bottles of soda. 2. Shake just one, 10 times. 3. Point them away from faces. Open both slowly.
What happens: the shaken one gushes! Its tiny bubbles grow fast, like bubbly magma.
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