Is there anything more spectacular than a towering lava fountain? Think about what it is: glowing red molten rock being shot into the sky, potentially hundreds or thousands of feet. As ubiquitous and dramatic as lava fountains might be, they are also not well understood to the point where we’re not even sure where they fit in the types of volcanic eruptions.Lava Fountains at Volcanoes"Pele's hair" on a black reflective surface. This curious form of volcanic glass forms when liquid lava is thrown into the air during eruptions. These fibres are up to 5 cm long. (Image Credit: MarcelClemens/Shutterstock) Over the last year, we’ve seen lava fountains at volcanoes like Kīlauea in Hawai’i, Etna in Italy, and Svartsengi near Reykjavík in Iceland. These fountains have sometimes reached heights over 1,000 feet and fed lava flows that travelled many miles. These volcanoes share a trait that allows for lava fountains: basalt lava. This is the kind of lava that has the lowest silica (silicon dioxide) content, and that means it has the lowest viscosity of any modern lava. It is runny! Most eruptions in Hawai’i and Iceland start with lava fountains before settling down to erupt lava flows.Etna is famous for producing lava fountains that have reached over 3,000 feet above the volcano. That isn’t even the record for lava fountains. The 1986 eruption of Izu-Oshima in southern Japan generated a lava fountain that was measured at just short of a mile tall (about 5,280 feet). The 1888 eruption of Tarawera in New Zealand may have produced lava fountains that reached about 32,000 feet, although that eruption may have been more than just lava fountaining. Instead, it might have transitioned into an explosive eruption (more on that in a bit).Lava fountains create some distinct volcanic products thanks to the runny nature of the lava erupting. In places like Hawai’i, Pele’s tears and hair are common – they are droplets and strands of lava, respectively. Pele’s hair can be so thin that you can think of it as natural fiberglass. You can also get extremely bubble-rich, glassy material called reticulite.There are even some lava fountains that can create lots of volcanic ash (volcanic glass fragments). This is what happens at volcanoes like Etna, so the airports on Sicily frequently have to close due to the hazard of the volcanic ash in the air.When the lava doesn’t get stretched out as much or is stickier, you get volcanic bombs. They are blobs of molten rock that fly through the air and then splat back onto the ground. Sometimes they cool enough to keep their shape, but close to the fountain, they can pile up while remaining hot and start to move like a lava flow.Two Types of Volcano EruptionsVolcanologists divide eruptions into two main styles: explosive and effusive. Explosive eruptions are the type that produce giant ash clouds that rise from a volcano. Their explosive force comes from bubbles that form in the high-silica magma. These bubbles get trapped in the sticky magma, grow, and then cause the liquid rock to fragment.More or less, the bubbles pop, breaking the magma up into pieces while it is still inside the volcano. That popping in a confined space generates enough force to send these fragments, now called ash, potentially over 100,000 feet into the atmosphere like champagne shooting out of a bottle. Effusive eruptions are the lava flows that snake across the landscape. There are still bubbles, but usually those bubbles can escape the more fluid magma, preventing that molten rock from fragmenting. That means it can ooze across the landscape, sometimes like a red-hot river and sometimes like a slow-moving pile of scorching rubble.Lava fountains have been classified as explosive eruptions, but this is an uncomfortable categorization. Lava fountains don’t fragment the same way as an explosive eruption, like what happens at places like Mount St. Helens. Yet, they can still throw volcanic material thousands of feet from the vent or create ash that drifts hundreds of miles from the volcano. They also feed lava flows as the lava in the fountain falls back to the ground near its source. They don’t really fit neatly into either effusive or explosive eruptions.What About Lava Fountains?In a 2021 paper in Earth & Planetary Science Letters, Giuseppe La Spina and his colleagues tried to solve this categorization conundrum. They mathematically modeled the dynamics of lava fountains to better understand which type of eruption lava fountains actually fit. It turns out that lava fountains may actually not fit into either, but instead could be an eruption style all their own.When it comes to lava fountains, there are quite a few variables that might control how they behave. The obvious one is magma viscosity – how sticky it is – and that is normally a function of how much silica is in the magma and how hot it is. Less silica and hotter temperatures mean a runnier magma.That’s not all, though. Like water coming out of a hose, the size of the vent might play a role, as does how quickly the magma is rising. That last factor is connected to how much gas is dissolved in the magma. So, there are a lot of variables that can change and might impact how lava fountains form. La Spina and his colleagues found that it was the rate at which magma rises that plays the biggest role in the vigor and style of lava fountaining. That rate of rise is dependent on the viscosity of the magma, so runnier (hotter, lower silica) magma will rise faster, like water will flow faster than honey.In fact, the lava at Kīlauea will rise so quickly that it never has a chance to fragment beneath the surface. Only when it erupts does it fragment when the magma feels the rapid drop in pressure shooting out of the vent, forming the ubiquitous Pele’s tears and hair in the process.This all means that you might expect that lava erupted from volcanoes like Kīlauea is much more likely to produce tall lava fountains than stickier lava from volcanoes like Etna. This is true – to an extent. The curveball is that stickier basalt at Etna is more likely to transition from lava fountain to explosive eruption, so the overall eruption could end up throwing ash and debris much higher than even the most vigorous lava fountains at Kīlauea.La Spina and his colleagues conclude that lava fountains really don’t behave like either true explosive or effusive eruptions. Unlike effusive eruptions, the magma is being thrown into the air. On top of that, unlike explosive eruptions, the magma isn’t fragmenting while it is still underground. Instead, it breaks up into fragments like Pele’s tears, hair, and volcanic bombs once the magma is freed from the subsurface. “Lava fountaining” becomes its own, special style of volcanic activity.Sticky Path to Volcano ExplosionsWhat is the upshot of understanding this? In their modeling, La Spina and his colleagues found that tweaking the viscosity of magma erupting in a lava fountain can generate big changes. When the magma is runny, like at Kīlauea, the fountains might go from a thousand feet tall to a few tens of feet as the viscosity gets higher, usually due to cooling.However, the stickier magma at Etna will go from lava fountaining to truly explosive eruptions with an increase in viscosity, and likely without much warning. This increases the risk posed by eruptions at Etna, even if they start with relatively low-hazard lava fountains.Rarely does nature fit neatly into our anthropocentric categories. If this research into the behavior of magma during lava fountaining is robust, then we can add a third category to our models of eruptions. In any case, lava fountains will remain one of the most spectacular events that happen at many of the planet’s volcanoes.Article SourcesOur writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:NASA Earth Observatory. A Glowing Plume Over Mount EtnaIzu Oshima Geopark. 1986 Fissure Eruption B CratersGeology.com. Pele's Hair and Pele's Tears National Park Service. Pyroclasts and Pyroclastic RocksEarth & Planetary Science Letters. Explosivity of basaltic lava fountains is controlled by magma rheology, ascent rate and outgassing