31/10/2021
Come misurare l’altezza di una fontana di lava 🤓
How high is that lava fountain? Scientists show you how fountains measure up, in this week’s , https://www.usgs.gov/observatories/hawaiian-volcano-observatory/volcano-watch
One of the things field geologists do is measure the height of lava fountains and other vent dimensions to help assess how energetic the eruption is. By knowing the height of a lava fountain and measuring its change overtime, geologists assess short-term and long-term eruption dynamics even if it is not obviously visible hour to hour or day to day.
Measuring the height of a lava fountain during an eruption can be accomplished with a few simple instruments and some basic trigonometry.
Here's how:
First, geologists measure the angles to the top and bottom of the fountain. The top of the lava fountain is defined as the upper boundary of the optically dense column (observer cannot see through it). This is where the vast majority of the lava stops rising and falls back to the ground.
The base is easy to determine at the start of an eruption - it is where lava is erupting from the ground. But over time, the base becomes hidden as lava, spatter, and cinder accumulates around the vent area. When this happens, geologists look for other unobstructed exposures of the fissure, which could be a glowing crack near the bottom of the fountain and use that as a base.
To make the angle measurements, you need either a hand-held inclinometer, compass, rangefinder (laser or optical), or even a handy app on your phone. To make the math easy, we always measure the angle from our eye-height to the top of the fountain and then a second angle from our eye-height to the bottom of the fountain. This way no matter where you are in relation to the fountain (above it on a cliff rim, at ground level, or below it, because the lava fountain is up on a cliff), the math will always be the same. It is important to hold the instrument at eye level and not move the instrument up or down between the two measurements (pretend you are a tripod).
Second, we can then take these measured angles and use trigonometry to calculate the vertical distances for each angle—partial heights for each segment. The final part of the calculation is to add these two heights together.
Astute readers are probably thinking: but don’t we need to know the distance to the lava fountain? Excellent question. In the old days, geologists would estimate the horizonal distance from a map during the eruption and use surveyor’s instruments to measure the actual distance after the fountaining stopped. Today, HVO scientists use an accurate laser range finder that not only measures the distance, but also the angle, does the math, and then reports back the vertical height.
Lava fountains are spectacular to observe at Kīlauea and provide insight into eruption dynamics. If you are lucky enough to see fountaining, you too can measure lava fountains! Or you can simply estimate the height knowing that the spatter cone is about 20–25 m (65–80 ft) high. Either way, enjoy the view.
Annotated image showing how lava fountain heights are measured using a trigonometric equation. Geologists measure two angles, from eye-level to the top and bottom of the lava fountain. These angles, and the distance between the geologist and the fountain, are used in a trigonometric equation to determine heights of the top and bottom portions of the fountain. The two values are added together to get the total lava fountain height. USGS photo of fissure 22 taken during the 2018 Kīlauea lower East Rift Zone eruption on May 20, 2018 by L. DeSmither.
Volcano Activity Updates
At Kīlauea, lava continues to erupt from a single vent in the western wall of Halemaʻumaʻu crater. All lava activity is confined within Halemaʻumaʻu crater in Hawai‘i Volcanoes National Park. Sulfur dioxide emission rates remain high and were estimated at around 2,600 tonnes per day on Oct. 25, 2021. Seismicity is elevated but stable, with few earthquakes and ongoing volcanic tremor. Summit tiltmeters have remained relatively flat over the past week.
Mauna Loa is not erupting. This past week, about 94 small-magnitude earthquakes were recorded below the summit and upper elevation flanks of Mauna Loa—the majority of these occurred at shallow depths less than 10 km (6 mi). GPS measurements show no major deformation over the past week. Gas concentrations and fumarole temperatures at both the summit and at Sulphur Cone on the Southwest Rift Zone remain stable. Webcams show no changes to the landscape.
