Tephra is any type and size of rock fragment that is forcibly ejected from a volcano and travels an airborne path during an eruption. Examples include ash, bombs, scoria, and shards of volcanic glass such as Pele’s hair and Pele’s tears).
The tephra lab will help HVO geologists streamline quantitative measurements of physical characteristics such as density, size, and shape of individual tephra particles along with types of tephra. Using this information, HVO geologists can address a range of topics — from magma ascent and eruption processes to broader ash fall deposition from past explosive eruptions. It is important to get these measurements as accurately and quickly as possible during an eruption.
HVO’s new lab is unique in its ability to analyze a wide size range of samples, from 3 feet to one micron (0.00004 inches). The sample processing is non-destructive and analysis is fast with each type of measurement taking only minutes, and all measurements are estimated to take one to two hours total. The non-destructive nature of these new instruments and methods is revolutionary and allows us to perform a full suite of analyses on the same sample (instead of different samples of the same material) for a more integrated understanding of eruptions, and allows samples to be fully preserved.
So, what are the new machines? How does a sample travel through our new lab and what can we learn from it?
First, we need to know what the sample is made of — its components. Componentry helps us understand what type of eruption we are dealing with.
For tephra samples straight from the field, HVO has two new stereoscopes that use reflected light. Looking through them, geologists can manually separate the different components that might make up the sample, such as fresh glassy lava, crystals, and small pieces of the crater wall.
Next, we measure density. For pieces of lava, measuring density helps us understand how frothy (or buoyant) the magma was when it erupted, which tells us about eruption dynamics.
We determine sample density by measuring its mass and volume. For pieces of tephra larger than 2 inches, the volume is calculated using a 3D scanner, and then the sample is weighed. Smaller grains from gravel to powdery ash sizes will be placed in a pycnometer — a machine that calculates density directly using Archemides principle of volume displacement with nitrogen gas. The pycnometers work with foams (scoria and pumice) as well as ash and helps us understand eruption dynamics.
Then, the samples will be measured for size, which informs us about how magma gets ripped apart to produce tephra from lava fountains and explosions.
Fragments larger than 1.2 inches are sieved in the traditional (manual) way, while smaller grains will run through one of the new Camsizers—a machine that photographs each fragment and converts the image to a size measurement. The Camsizers can measure tens of thousands of fragments in as little as 5 minutes! Additionally, they use the images to measure the 2D shape of fragments using established mathematical parameters. Size information is essential for models of lava fountaining and ash fall.
A final, longer-term step (weeks to months) might occur for fragments from any sample. Those pieces will be turned into a thin section (rock epoxied to a glass plate and cut thin enough to see through it) for final analysis on a petrographic microscope. HVO has two new petrographic microscopes with different sets of lenses: one can assess bubble sizes, bubble textures, and magma-mixing textures, while the other can focus on crystals and melt inclusions within them.
HVO’s new tephra lab instruments are here, installed, and currently being calibrated. Samples from the Dec. 20, 2020, eruption in Halema‘uma‘u at Kilauea’s summit will be the first analyzed. The HVO tephra lab brings physical volcanology monitoring of eruptions to near-real time analysis.
Note: Volcano Awareness Month 2021 programs are temporarily postponed. HVO will be posting a revised Volcano Awareness Month schedule of recorded programs to the HVO website (www.usgs.gov/hvo) later this month.
Visit https://www.usgs.gov/observatories/hawaiian-volcano-observatory for past Volcano Watch articles, Kilauea and Mauna Loa updates, volcano photos, maps, recent earthquake info, and more. Email questions to askHVO@usgs.gov.
Volcano Watch is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates.