Measuring Lava Fountains & Eruption Rates

This guide explains the practical methods volcanologists use to measure lava fountain height and estimate eruption rates, both in the field and remotely. You will learn the key techniques, the instruments involved, and how the data are turned into numbers that help forecast hazards. No specialized equipment is required to understand the process—this is a conceptual walkthrough of the methodology.

What You’ll Need

• Access to a safe viewing location (or remote sensing data)
• Camera with known focal length (for photogrammetry)
• Reference object of known size in the image (e.g., a building, a tree, or a known distance between two points)
• GPS or map tool to measure distances
• Thermal infrared camera (optional, for temperature-based estimates)
• Doppler radar or infrasound sensors (for remote, continuous monitoring)
• Basic trigonometry calculator or spreadsheet

Step-by-Step

1. Establish a safe observation point. Choose a location with a clear, unobstructed view of the fountain. For ground-based methods, you need to know the exact distance from your viewpoint to the vent. Use GPS or a laser rangefinder to measure this distance accurately.
2. Set up a reference scale. In your field of view, identify an object of known size—a building, a tower, or a marked distance on the ground. This will serve as a scale for converting pixels in your image to real-world meters.
3. Record video or a rapid sequence of photos. Use a camera with a known focal length. Keep the camera level and steady. If possible, record at a high frame rate to capture the fountain’s pulsations.
4. Measure the fountain height from the image. Using image analysis software (or even a ruler on a printed photo), measure the height of the fountain in pixels. Then, using the known size of your reference object, convert that pixel height to meters. The formula is: Fountain height (m) = (pixels of fountain / pixels of reference) × reference size (m).
5. Correct for viewing angle (if needed). If your line of sight is not perpendicular to the fountain, apply a trigonometric correction. If you know the angle of elevation from your viewpoint to the top of the fountain, the true height is distance × tan(angle).
6. Estimate the fountain’s exit velocity. Using the measured maximum height (H) and the acceleration due to gravity (g ≈ 9.81 m/s²), calculate the initial velocity (v) of the lava as it leaves the vent: v = √(2 × g × H). This assumes a ballistic trajectory with no air resistance, which is a reasonable approximation for coarse pyroclasts.
7. Determine the vent area. Measure the diameter of the erupting vent. This can be done from the same imagery or from prior geological mapping. The vent area (A) is π × (diameter/2)².
8. Calculate the volumetric eruption rate. Multiply the exit velocity by the vent area: Q = v × A. This gives the volume of lava emitted per second (m³/s). For example, if v = 50 m/s and A = 10 m², then Q = 500 m³/s.
9. Convert to mass eruption rate (optional). Multiply the volumetric rate by the density of the lava (typically 2600–2800 kg/m³ for basaltic magma). This gives kilograms per second, which is often used in hazard models.
10. Use complementary remote methods. For continuous monitoring, scientists deploy Doppler radar systems that measure the velocity of particles in the fountain, and infrasound sensors that detect pressure waves. These are calibrated against the visual measurements to provide real-time eruption rates.

Common Mistakes

• Ignoring the angle of view. If you photograph the fountain from an angle, the apparent height is shorter than the true height. Always correct for the viewing angle.
• Using a reference object that is not perpendicular to your view. The reference object must be at the same distance and orientation as the fountain; otherwise, the scale is wrong.
• Assuming a constant vent area. The vent can widen or narrow during an eruption, changing the rate. Use an average or time-series of vent size.
• Forgetting air resistance. For very fine ash, air resistance slows particles, so the ballistic formula overestimates velocity. Use it only for coarse lapilli and bombs.
• Mixing units. Ensure all distances are in meters and times in seconds before calculating.

Tips

• Take multiple measurements over time to capture the pulsating nature of lava fountains—they often have a period of a few seconds to minutes.
• Use a thermal camera to measure the temperature of the fountain; this can help estimate the density and gas content, which affect the eruption rate.
• Combine visual, radar, and infrasound data to cross-check your numbers and reduce uncertainty.
• Always record the exact time and duration of each measurement, as eruption rates can change rapidly.
• If you are using photogrammetry, take images from multiple angles to triangulate the height more accurately.

Troubleshooting

• Fountain height is too large or too small. Recheck your reference object’s actual size and ensure it is in the same plane as the fountain. Also verify that your camera’s focal length is correct.
• No clear reference object in view. Use a known distance between two fixed points (e.g., crater rim features) or deploy a temporary marker if safe.
• Wind is bending the fountain. Measure the height along the curved path, not the vertical. If using the ballistic formula, use the vertical component of the velocity, which can be obtained from the initial angle of ejection.
• Vent diameter is unclear. Use thermal imagery, where the hot vent is often visible, or measure from a drone survey if permitted.
• Radar or infrasound data are noisy. Apply signal filtering and compare with visual observations to identify the correct signal.

Frequently Asked Questions

How accurate are these measurements?

Accuracy varies with method. Visual photogrammetry can be within ±10–20% if done carefully. Radar and infrasound are calibrated to within similar ranges when compared to visual data.

Why do scientists measure lava fountains?

To understand eruption dynamics, estimate magma supply rates, and forecast lava flow paths and ash dispersion, which is critical for hazard assessment.

Can I do this at home?

You can practice the photogrammetry method with videos of eruptions, but only if you have a known reference object in the frame. Never approach an active volcano.

What is a typical eruption rate for a basaltic fountain?

Rates range from a few cubic meters per second for small fountains to over 1000 m³/s for major events like the 2014–2015 Holuhraun eruption in Iceland.

How do scientists measure the temperature of a lava fountain?

Using thermal infrared cameras, which detect heat radiation. The temperature helps estimate the lava’s density and viscosity.

What is the difference between volumetric and mass eruption rate?

Volumetric rate is volume per second (m³/s), while mass rate is mass per second (kg/s). Mass rate is more useful for atmospheric dispersion models because it accounts for the density of the material.

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