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Hawaiian lava fountains form when rising magma decompresses: dissolved gas separates into bubbles, expands, and helps propel a mixture of lava and gas into the air. Their height can rise or fall as magma supply, gas behavior, vent shape, lava properties, or conditions around the vent change. A taller fountain does not, by itself, prove that the magma contains more gas.
How a lava fountain forms
Magma rising toward the surface experiences lower pressure. Some gas that was dissolved in the magma then comes out of solution as bubbles. As those bubbles expand, they help push the surrounding molten rock upward. At the vent, the result can be a jet of gas and lava: a lava fountain.
The sequence is easier to follow as a chain of events: magma rises; pressure falls; bubbles form and expand; and the gas-bearing magma is propelled upward. The lava does not simply boil like water, and magma is not literally soda. A soda bottle can be a rough illustration of pressure release, but actual magma is a hot, complex mixture moving through a volcanic plumbing system.
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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11At Kīlauea’s summit, some eruptions occur in distinct episodes. The U.S. Geological Survey’s Hawaiian Volcano Observatory (HVO) describes a cycle in which incoming magma contributes to pressure buildup, followed by an eruptive episode and pressure drop. That is one setting for fountaining, not a universal description of every Hawaiian fountain.
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Summit episodes and flank fissures are not identical
The driving conditions can differ by location. HVO contrasts episodic summit activity with the 2018 lower East Rift Zone fissure 8 fountains. At fissure 8, the fountains were primarily driven by a pressure gradient as magma traveled from summit storage toward a lower-elevation vent. The same broad ingredients—magma, gas, pressure, and an outlet—can therefore produce fountains in different volcanic settings without following precisely the same pressure cycle.
Why a fountain gets taller or shorter
There is no single dial that sets fountain height. Height reflects how much material is moving, how gas expands and interacts with that material, the shape of the conduit and vent, and what happens to lava around the jet. The factors interact: a change in one can alter the fountain even if another, such as the magma’s water content, stays nearly constant.
Magma supply and volume flux
Volume flux is the amount of magma moving through the system per unit time. A change in flux can change how much material reaches the vent and the dynamics of the jet. In a 1995 study of Puʻu ʻŌʻō, E.A. Parfitt, L. Wilson, and C.A. Neal found that changes in volume flux, together with lava re-entrainment, could account for much of the observed fountain-height variability even when magma water content was nearly constant.
This matters when interpreting a changing fountain: a higher column need not mean a sudden increase in gas. The rate at which magma is supplied can change the height too, and height is an outcome of the whole eruptive system rather than a direct meter reading for one ingredient.
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Gas content, bubbles, and coalescence
Gas helps propel a fountain, but the amount of gas is not the only relevant detail. Bubbles can change as they rise, including by coalescing into larger bubbles. Parfitt, Wilson, and Neal identified magma gas content and gas-bubble coalescence among the controls on fountain height at Puʻu ʻŌʻō. Gas behavior is part of the explanation, but a height observation alone cannot isolate it from flux, vent conditions, or the other factors acting at the same time.
In their analysis, the authors estimated magma water content near 0.32 wt.% for the episodes considered. They also inferred a decline in gas content of about 0.05 wt.% across the last ten episodes they analyzed, attributing it to more extensive degassing before eruption as the feeder dike shallowed. Those are study-specific findings for Puʻu ʻŌʻō in 1983–1986, not values that can be applied to all Hawaiian fountains.
Conduit geometry and lava properties
The conduit is the pathway through which magma reaches the surface. Its geometry can influence how the erupting mixture moves and how a jet develops. HVO’s discussion of recent Kīlauea summit fountains also identifies lava viscosity and magma supply rate as likely influences on height. Viscosity describes resistance to flow; it is one property to consider alongside the vent geometry and supply, not a stand-alone explanation for every change.
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Re-entrainment and damping near the vent
Some erupted lava can fall back into or be drawn back toward the fountain system. This re-entrainment affects how much of the moving material contributes to the visible jet. A lava pool or lake around a vent can also damp a jet. Thus, the visible height depends not only on what leaves the vent but also on how the surrounding lava and the erupting column interact.
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Why height alone is not a gas meter
The 1995 Puʻu ʻŌʻō study’s central caution is that “variations in lava fountain height cannot be simply interpreted as variations in gas content.” Its analysis found that changing volume flux and re-entrainment could explain much of the observed height variation while water content remained near 0.32 wt.%. The authors also concluded that, when flux and entrainment data are available, fountain height can help constrain a minimum gas content. It does not, on its own, tell observers exactly how much gas is present.
For a reader watching an eruption, a useful distinction is between observation and interpretation. “The fountain became taller” describes what was measured. “The magma became more gas-rich” proposes a cause. The second statement requires other evidence because multiple processes can produce a change in height.
What recent Kīlauea episodes show
In an HVO article dated August 6, 2026, the observatory reported fountain heights ranging from 40 to 540 meters across summit episodes 1–51. It described three periods of growth and decline through episode 43. The figures show that the recent eruption’s fountain height has varied substantially, but the pattern should be read as a sequence of observations rather than a simple one-factor trend.
| Observation | What HVO reported | How to interpret it |
|---|---|---|
| Episodes 1–51 | Heights of 40–540 m; HVO described three growth-and-drop periods through episode 43. | A reported range and pattern for those summit episodes, not a universal range for Hawaiian fountains. |
| Episode 44 | Fountain height and MgO fell as a newly differentiated olivine population appeared. | A coincident change that HVO discusses as a possible chemistry clue, not proof of a single cause. |
| Episodes 46–48 | Both MgO and fountain heights rose again. | An observed association that contributes to the ongoing interpretation of the eruption. |
HVO reported a consistent positive correlation between MgO and fountain height from episode 29 in the third growth period. Episode 44 then brought an abrupt decline in both height and MgO, coinciding with the appearance of the differentiated olivine population; episodes 46–48 showed early renewed growth in both. These associations make chemistry relevant to the discussion, but they do not establish that MgO alone controls the height or settle the full cause of the pattern. Conduit geometry, viscosity, and magma supply rate remain among the factors HVO says are likely to matter.
Historical examples: height can change quickly
Hawaiian fountains do not only change over a sequence of eruptive episodes. Conditions at a vent can also change the visible jet rapidly. In its account of the 1959 Kīlauea Iki eruption, the USGS records a fountain falling from about 210 meters to a few gas bubbles in less than 40 seconds. The lake had risen over the vent, and lava drainback occurred as fountaining ended. This is a specific historical event, not a general timeline for fountains to stop.
The USGS gives 680 meters as the maximum historical Hawaiian fountain, during the 1959 Kīlauea Iki eruption. That maximum is a separate historical reference point from the 40–540-meter range HVO reported for recent summit episodes 1–51. The settings and time periods differ, so the two figures should not be treated as measurements from one comparable eruption sequence.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How scientists measure fountain height
A field estimate can be made from the observer’s distance to the fountain and the angles from eye level to the top and bottom of the main continuous column. Trigonometry is used to estimate the upper and lower portions relative to the observer’s eye level; those portions are combined to obtain total height. In a simplified geometry with horizontal distance D and sight angles measured from the horizontal, each vertical portion corresponds to D multiplied by the tangent of its angle. The actual field measurement depends on the viewing geometry and the observed fountain boundaries.
The boundary matters. A gas plume can carry sparse lava particles tens to hundreds of meters above the main continuous fountain. Those isolated particles are not necessarily counted as part of the fountain’s height. USGS descriptions of field measurement and HVO’s time-lapse dataset distinguish the continuous main body from sparse particles above it.
Recent summit measurements
For the ongoing summit eruption, HVO used time-lapse cameras on the south rim. Images were captured at intervals of 0.5–1.0 minute, and manual measurements were generally made each minute. Analysts measured the main body of the fountain and disregarded sparse particles above the continuous column.
The USGS data release dated September 22, 2026, and updated in August 2026, includes measured heights and associated time-lapse images for episodes 1–50. The release page reported 54 episodes by August 2026, while the included measured dataset covered episodes 1–50; later episode data were planned for a future update. The episode count and the dataset’s coverage are therefore different quantities, and a reader should check the release date when comparing figures.
How to read a reported fountain height
- Check the eruption and setting. An episodic summit fountain and a flank fissure fountain may have different pressure and supply conditions.
- Check what the number measures. A reported height may describe the main continuous column, not the highest particle lifted by the plume.
- Keep the time period attached. A range across many episodes is not the same as a single episode’s peak or a historical record.
- Separate correlation from cause. Height changing alongside chemistry can help researchers investigate the eruption, but it does not by itself prove which factor produced the change.
- Consider multiple controls. Flux, gas and bubble behavior, conduit geometry, viscosity, re-entrainment, and damping can all affect the visible height.
What the evidence can—and cannot—tell you
Fountain height is a useful observable: it can be measured, tracked over time, and interpreted together with other eruption data. The Puʻu ʻŌʻō analysis shows why a time series can be more informative than a single height, especially when paired with flux and entrainment information. HVO’s recent Kīlauea discussion adds chemistry and episode-to-episode changes to that broader picture.
But a height value does not reveal a unique internal state. It does not, by itself, quantify gas content, identify the magma supply rate, or establish that chemistry caused a change. Strong interpretation depends on the measurement definition and on evidence about the other parts of the eruptive system. For Hawaiian fountains, the most defensible explanation is usually a combination of processes, with the relative importance varying by eruption and episode.
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