Note · Earth and the oceans
How do volcanoes form? Magma, plates and hotspots
Volcanoes form where rock deep underground partly melts and the magma rises to the surface: at subduction zones, at rifts and ridges, and over hotspots. Why some ooze lava and others explode.
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Contents · 6 sections
Volcanoes form where rock in the upper mantle or deep crust partly melts, and the melt, called magma, rises because it is lighter than the rock around it. Where it breaks through, it erupts as lava, ash and gas, and each eruption piles more rock on the vent until a mountain grows. Rock melts in three kinds of place: where one plate sinks under another, where plates pull apart, and over unusually hot spots in the mantle such as the one under Hawaii.
In short
- The mantle is solid. Rock melts only when the pressure on it drops, when water is added, or when it is heated, and each kind of volcano uses one of those.
- Whether a volcano oozes or explodes depends mostly on its magma's silica and gas: runny magma lets gas out, sticky magma traps it until it bursts.
- The Volcanic Explosivity Index rates eruptions from 0 to 8, and from VEI 2 up each step is about ten times more material thrown out.
How does solid rock melt to make magma?
The rock under the crust is hot, well over 1,000 °C in places, but the weight of rock above keeps it solid, as the note on what the Earth is made of explains. High pressure raises the temperature rock needs to melt. So magma forms in three ways, each of which tips that balance:
- Lower the pressure. Where hot mantle rises, the load on it drops faster than it cools, and part of it melts. This is decompression melting, and it feeds mid-ocean ridges and hotspots.
- Add water. Water lowers the melting point of rock, the way salt lowers the freezing point of ice on a road. A sinking ocean plate carries water down with it, and that water rises into the hot mantle above, which partly melts. This is flux melting, and it feeds subduction volcanoes.
- Add heat. Hot magma rising into the crust can melt crustal rock around it, making new magma richer in silica.
Rock rarely melts completely. Usually only a few percent to a few tens of percent of it becomes liquid, and that melt collects, rises through cracks and gathers in chambers a few kilometers under the volcano, where it can sit for years to millennia before erupting.
Where do volcanoes form?
Almost all volcanoes sit along plate boundaries or over hotspots, and each setting makes a different kind of volcano. The mechanics of the boundaries are in the note on plate tectonics.
| Setting | How the rock melts | Typical magma | Typical eruption | Examples |
|---|---|---|---|---|
| Subduction zones, where one plate sinks under another | Water from the sinking plate | Andesite and dacite, more silica | Explosive, tall steep cones | Mount St. Helens, Mount Fuji, Pinatubo, the Andes |
| Rifts and mid-ocean ridges, where plates pull apart | Pressure drops as mantle rises | Basalt, less silica | Gentle, lava flows and fissures | Iceland, the East African Rift, the whole mid-ocean ridge |
| Hotspots, inside plates | Pressure drops in an unusually hot rising plume | Basalt in oceans; on continents, often explosive silica-rich magma too | Gentle in oceans, sometimes huge on land | Hawaii, Galápagos, Yellowstone |
Most of Earth's volcanism happens out of sight. According to the U.S. Geological Survey, about three quarters of the lava erupted on Earth comes from the spreading ridges under the oceans. Most of the volcanoes people see and fear, though, are subduction volcanoes, and the biggest belt of them is the Pacific's Ring of Fire.
How a hotspot builds a chain of islands
In 1963 the Canadian geophysicist J. Tuzo Wilson explained Hawaii: a hot plume rises from deep in the mantle and stays roughly in place while the Pacific Plate moves over it, so it punches through to make one volcano after another, like a conveyor belt passing under a candle. The islands should get older to the northwest, and they do. The USGS gives the oldest rocks on Kauai as about 5.5 million years old, while the Big Island's exposed rocks are all less than 0.7 million years old.
That gives a rough speed for the plate, worked out in three steps:
- Kauai lies roughly 530 km northwest of Kilauea, the Big Island's most active volcano.
- Kauai's rock has had about 5.5 million years to travel that far from the hotspot.
- 530 km ÷ 5.5 million years ≈ 0.0001 km a year, which is about 10 cm a year.
That agrees with the plate's speed measured today by satellite, within a few centimeters a year: two very different methods telling the same story.
Why do some volcanoes explode while others ooze lava?
The answer is in the magma's chemistry, mainly its silica, and how much gas it holds. Silica, the material of quartz and glass, links up into chains in the melt and makes it stiff. Gas, mostly water vapor and carbon dioxide, is dissolved in magma under pressure, like carbon dioxide in a sealed bottle of soda. As magma rises and the pressure drops, the gas comes out as bubbles. In runny magma the bubbles escape easily. In stiff magma they are trapped, grow, and finally shatter the magma into ash in an explosion.
Two American volcanoes show the two ends of the range:
| Kilauea, Hawaii | Mount St. Helens, Washington | |
|---|---|---|
| Setting | Hotspot, in the middle of the Pacific Plate | Subduction zone, the Cascades |
| Magma | Basalt, about 50% silica | Dacite, about 63% silica |
| Temperature of lava | About 1,150 °C | Roughly 900 °C |
| Stiffness | Runny, flows like warm syrup | Can be a million times more viscous |
| Gas | Escapes easily as fountains and flows | Trapped until it blasts free |
| Shape | Broad, gently sloping shield volcano | Steep cone of lava and ash layers |
| Typical eruption | Lava flows lasting months or years, such as the 1983–2018 eruption | 18 May 1980: a landslide uncorked the volcano and a sideways blast followed |
The 18 May 1980 eruption of Mount St. Helens is the classic explosive case. After two months of earthquakes and a bulge growing on its north side, a magnitude 5.1 earthquake set off the largest landslide in recorded history, according to the USGS. With the lid gone, the gas-charged magma blew out sideways. The summit lost 1,314 feet, about 400 m, and 57 people died.
Kilauea is the gentle case, but gentle doesn't mean harmless. Its slow lava flows destroy whatever lies in their path, and lava from its lower slopes destroyed hundreds of homes in 2018. Explosive volcanoes add dangers that travel farther: pyroclastic flows, which are avalanches of hot ash and gas moving faster than a car, mudflows of ash and melted snow, and ash clouds that can stop air traffic. Pyroclastic surges killed most of the people still in Pompeii in AD 79.
How is a volcanic eruption's size measured?
The Volcanic Explosivity Index, or VEI, was devised by Christopher Newhall of the USGS and Stephen Self in 1982. It rates an eruption from 0 to 8 by how much material it throws out and how high its plume rises. From VEI 2 upward, each step means about ten times more ejected material, so the scale is logarithmic, like the magnitude scale for earthquakes in the note on what causes earthquakes.
| VEI | Material thrown out | Plume height | Example |
|---|---|---|---|
| 0 | Less than 10,000 m³ | Under 100 m | Kilauea's lava flows |
| 1 | 10,000 m³ or more | 100 m to 1 km | The Sundhnúkur fissure eruptions, Iceland, 2023–2025 |
| 2 | 1 million m³ or more | 1–5 km | Mount Etna's frequent eruptions |
| 3 | 10 million m³ or more | 3–15 km | Nevado del Ruiz, Colombia, 1985 |
| 4 | 0.1 km³ or more | Over 10 km | Eyjafjallajökull, Iceland, 2010 |
| 5 | 1 km³ or more | Over 20 km | Vesuvius, AD 79; Mount St. Helens, 1980 |
| 6 | 10 km³ or more | Over 30 km | Krakatoa, 1883; Pinatubo, 1991 |
| 7 | 100 km³ or more | Over 40 km | Tambora, Indonesia, 1815 |
| 8 | 1,000 km³ or more | Over 50 km | Yellowstone's largest eruption, about 2.1 million years ago |
Size isn't the same as deadliness. Nevado del Ruiz was only a VEI 3, but its heat melted the summit's ice, and the mudflow that followed buried the town of Armero, killing more than 20,000 people. And a big eruption can reach the whole planet: Tambora's ash and sulfur gas cooled the Northern Hemisphere enough that 1816 was called the year without a summer.
Can scientists tell when a volcano will erupt?
Volcanoes give more warning than faults do, because magma has to move to erupt. Rising magma sets off swarms of small earthquakes, makes the ground swell, and lets out more sulfur dioxide and carbon dioxide, all of which can be measured. Mount St. Helens gave two months of signs, and access around it had been restricted before the blast; Pinatubo's evacuation in 1991 saved thousands of lives. But the signs can also fade without an eruption, and the exact day and size remain hard to call. One thing is clear: eruptions don't run on schedules. A volcano that has erupted every few hundred years on average isn't "overdue", because the average doesn't set a date.
Check yourself
- Without looking back: name the three ways solid rock can be made to melt, and the kind of volcano each one feeds.
- Why does stiff, silica-rich magma tend to explode while runny basalt flows?
- How do the ages of the Hawaiian islands show that the Pacific Plate is moving?
Sources
- Plate tectonics and people, in This Dynamic Earth — U.S. Geological Survey
- Hotspots, in This Dynamic Earth — U.S. Geological Survey
- Mount St. Helens, from the 1980 eruption to 2000 — U.S. Geological Survey, Fact Sheet 036-00, 2000
- Volcanic explosivity index — Wikipedia