Lava Fountains vs Lava Flows: Hazards Compared

Lava fountains and lava flows are two different ways molten rock reaches the surface, and they pose very different hazards. A lava fountain is a jet of molten rock and gas blasted into the air, while a lava flow is a stream of molten rock moving across the ground. This article explains how each forms, what makes them dangerous, and how scientists and residents assess the risks.

What Is a Lava Fountain?

A lava fountain (also called a fire fountain) occurs when gas-rich magma rises rapidly and expands violently as pressure drops near the vent. The expanding gas fragments the magma into clots and droplets that are thrown tens to hundreds of meters into the air. Fountains can be continuous, lasting hours or days, or episodic, pulsing every few minutes.

Fountain heights vary widely. Small fountains may reach only a few meters, while the 1986 Izu-Oshima eruption in Japan produced fountains up to 1,600 meters. The 2018 Kīlauea eruption in Hawaii had fountains up to 80 meters in the lower East Rift Zone. The tallest documented fountain in Hawaii reached about 580 meters during the 1959 Kīlauea Iki eruption.

Fountains are most common in basaltic eruptions, where low viscosity allows gas to escape easily. They also occur at Strombolian volcanoes (like Stromboli in Italy) where larger gas slugs burst at the surface, producing short-lived fountains of coarse material.

What Is a Lava Flow?

A lava flow is a body of molten rock that moves across the land surface. It forms when magma reaches the surface and continues to flow due to gravity, often fed by a vent or a fissure. Lava flows are typically associated with effusive eruptions, where the magma is relatively low in gas and viscosity.

Two main types of basaltic lava flows exist:

• Pāhoehoe – smooth, ropy, or billowy surfaces formed by highly fluid lava that develops a thin crust. It often advances in lobes or toes.
• ʻAʻā – rough, jagged, blocky surfaces formed when lava loses gas and becomes more viscous. It advances as a thick, crumbling front.

Lava flows can also be blocky (common in andesitic and dacitic lavas) or pillow-shaped when erupted underwater. Flow speeds vary from a few meters per hour to tens of kilometers per hour on steep slopes, but most are slow enough to outwalk.

Hazards of Lava Fountains

Lava fountains are spectacular but dangerous for several reasons:

• Ballistic projectiles – large clots of molten rock can be thrown hundreds of meters, causing injury or death if they strike people. The 1993 eruption at Galeras volcano in Colombia killed nine people, partly from ballistic impacts.
• Volcanic gases – fountains release large amounts of sulfur dioxide, carbon dioxide, and hydrogen halides. These can cause respiratory problems and, in high concentrations, be lethal. The 1783 Laki eruption in Iceland released gases that contributed to a famine and high mortality.
• Lava spatter and tephra fall – fine ash and larger fragments can blanket areas downwind, damaging crops, buildings, and infrastructure. Heavy tephra can collapse roofs.
• Lava fountains can feed lava flows – the material that falls back to the ground often accumulates into a cone or feeds a flow that moves away from the vent.
• Fires – hot spatter can ignite vegetation and structures.

Fountain hazards are generally localized within a few kilometers of the vent, but gas plumes can travel far downwind.

Hazards of Lava Flows

Lava flows are less explosive but can be more destructive over time because they cover and burn everything in their path:

• Burning and burial – lava flows destroy buildings, roads, crops, and natural habitats. The 2018 Kīlauea eruption destroyed over 700 homes.
• Slow but unstoppable – most flows move slowly enough to evacuate, but they cannot be stopped or diverted easily. The 1973 Eldfell eruption in Iceland attempted to cool the flow with seawater, with limited success.
• Fire and smoke – as lava burns vegetation and structures, it produces toxic smoke and gases.
• Collapse and explosions – when lava enters the ocean, it can cause steam explosions and send molten fragments flying. The interaction also produces hydrochloric acid and volcanic glass particles in the plume.
• Lava tube formation – insulated tubes can transport lava far from the vent, allowing flows to advance rapidly and unexpectedly.

Lava flows are a major hazard in Hawaii, Iceland, and other volcanic regions. The main risk is to property and infrastructure rather than human life, as most people can evacuate in time.

Comparing the Two: Which Is More Dangerous?

It depends on the context. Lava fountains are more immediately lethal because of projectiles and gas, but their reach is limited. Lava flows are slower but can cover vast areas and destroy everything in their path over weeks or months.

In terms of human casualties, fountains and associated explosive activity cause more deaths. The 1902 Mount Pelée eruption killed about 30,000 people, but that was a pyroclastic flow, not a lava fountain. Lava flows rarely kill people directly because they are slow; the main danger is being trapped by a fast-moving flow on a slope or entering the ocean.

For property damage, lava flows are far more destructive. The 2018 Kīlauea eruption destroyed hundreds of homes, while fountains alone typically damage only the immediate vent area.

Scientists assess risk by monitoring gas emissions, ground deformation, and seismic activity. If a fountain is likely, they warn of projectiles and gas; if a flow is likely, they map potential paths and evacuation routes.

How to Stay Safe Around Lava

If you live near a volcano or are visiting one, follow these general guidelines:

• Stay out of restricted zones. Authorities close areas based on volcanic activity.
• If you see a lava fountain, move away from the vent and seek shelter from falling debris.
• Wear a mask or cover your mouth if you are downwind of gas plumes.
• If a lava flow is approaching, evacuate early. Do not try to outrun a fast flow on foot.
• Never approach lava entering the ocean; the steam plume is acidic and explosive.
• Follow official alerts from volcano observatories.

Remember that volcanic hazards can change quickly. A fountain can start without warning, and a flow can speed up if the eruption rate increases.

Frequently Asked Questions

Can lava fountains occur without lava flows?

Yes. Some eruptions produce only fountains of tephra that build a cinder cone, with no significant lava flow. The 1943 Parícutin eruption in Mexico began with a fissure and built a cinder cone, but later produced lava flows. In some cases, fountains are short-lived and the material falls back into the vent, creating a spatter cone.

What is the difference between a lava fountain and a lava lake?

A lava fountain is a jet of molten rock and gas that rises into the air. A lava lake is a pool of molten lava that sits in a crater or vent, often with a crust that breaks and reforms. Lava lakes can have fountains within them, but the lake itself is a persistent body of lava.

Why do some lava flows move faster than others?

Flow speed depends on lava viscosity, slope, eruption rate, and channel geometry. Basaltic lava is less viscous and can flow faster than more silicic lava. Steeper slopes and higher eruption rates also increase speed. In 1977, a lava flow at Nyiragongo in the Democratic Republic of Congo moved up to 60 km/h, the fastest recorded, because of a steep slope and low viscosity.

Can lava flows be stopped or diverted?

In rare cases, humans have attempted to divert lava flows using barriers or explosives, but success is limited. The 1983 eruption of Mount Etna saw attempts to divert flows, with mixed results. The 1973 Eldfell eruption in Iceland used seawater to cool the flow, which slowed it but did not stop it. Generally, lava flows are too powerful to stop.

What is the most common type of lava flow?

Basaltic lava flows are the most common on Earth, found at mid-ocean ridges, shield volcanoes like Hawaiʻi, and flood basalt provinces. Pāhoehoe and ʻaʻā are the two main forms, with pāhoehoe often transitioning to ʻaʻā as it cools and loses gas.

How do scientists monitor lava fountains and flows?

Volcanologists use seismometers to detect magma movement, tiltmeters and GPS to measure ground deformation, gas sensors to measure emissions, and thermal cameras to track flow advancement. Satellite imagery helps map flow extent and temperature. This data is used to issue warnings and hazard maps.

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