Watch real lava feed a channel. Take apart a gas-volume calculation, compare Mount St. Helens through time, inspect volcanic rock under a microscope and explore an actual surveyed crater.
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Gas can occupy a great deal of space while contributing little mass. How gas separates, expands and escapes matters, alongside the material’s resistance to flow and its changing history.
At fixed pressure, you remove half of the gas from a melt-and-gas sample. Must the gas volume fraction become half?
The fraction has a changing numerator and denominator: fR/(1+fR).
The USGS May 21, 2018 overflight records a fountain feeding flows at the Fissure 20 complex in Kīlauea’s lower East Rift Zone. Airborne fragments and flowing channels are parts of the same activity. These observations start our questions; they do not measure the conditions underground.
Magma is hot rock material beneath the surface, including a liquid component called melt. Some volatile ingredients are dissolved in that melt. Under changing conditions, some separate into a gas phase. That separation is exsolution. Existing gas can also expand; expansion and exsolution are different processes.
In one published 0.2 MPa source state, gas is 0.2671% of the mass but about 95.4% of the volume. Gas and melt have different densities. A percentage without its measured quantity can be misleading.
Our removal thought experiment holds the pressure and one normalized melt volume fixed. Keeping half the already-exsolved gas changes both the gas volume and the total remaining volume. A half-sized numerator does not halve the fraction when the denominator also changes.
Connected pores and fractures can provide pathways. Permeability describes how readily a material transmits fluid under driving conditions. Viscosity describes resistance to flow. Neither a photo’s color nor the number of visible holes alone gives a complete gas-escape rate.
An eruption column and a growing dome can belong to the same volcano at different times. Gas pathways, material properties, pressure, cooling, deformation and timing can interact. Our bounded calculation helps explain a process; it does not decide a volcano’s next behavior.
The sixteen discrete states come from the upper Table 2 in Mastin’s 1995 USGS Open-File Report 95-756, printed page 38. Its reservoir-equilibrated Kīlauean basalt example begins with 0.27 wt% water, 0.0195 wt% carbon dioxide and 0.07 wt% sulfur. The original report uses steady homogeneous flow and equilibrium volatile partitioning. We read its reference table; we do not reimplement or validate its entire conduit solver.
Let R be gas volume divided by melt volume. Normalizing melt volume to 1 gives gas fraction R/(1+R). If the retained share is f at fixed pressure and melt volume, the remaining fraction is fR/(1+fR). At 0.5 MPa, R is 6.3952. Retaining half gives 3.1976/(1+3.1976), approximately 76.2%, rather than half of 86.5%.
The selected equilibrium source table includes gas separating from the melt as conditions change. Between 0.5 and 0.2 MPa, the source gas/melt volume ratio increases by about 3.263 times. A separate fixed-amount isothermal ideal-gas comparison would expand by 2.5 times. Those are different models, not contradictory measurements.
Gaunt and colleagues studied crystalline dome rock from Mount St. Helens. The intact section’s image porosity is 6.7%, while the separately reported connected bulk porosity is 6.0%. The pictured plane is perpendicular to the experiment’s fluid-flow direction. Cyclically heated and once-heated images show 2.1% and 5.5% porosity; those discrete samples do not establish a universal continuous temperature curve.
At 900 °C, flow could not be resolved over the study’s observation period. The authors describe an approximate permeability upper bound of 10⁻¹⁹ m². An instrument’s lack of detected flow is different from proving that all flow is impossible.
Gonnermann and colleagues’ experiments on Glass Mountain pumice showed that expansion and compaction can produce different relationships between porosity and permeability. This supports keeping history and sample context attached to a claim. It does not supply a universal pore percentage that predicts every magma’s fragmentation.
Our summit surface samples an actual USGS HVO October 5, 2022 elevation raster. The source reports no formal accuracy tests; stated horizontal and vertical targets are not measured errors. We preserve absent samples and source pixel-center coordinates. A 1-meter source pixel does not establish 1-meter accuracy or a certified sea-level datum.
Describe what you can actually see before assigning an underground cause. The 2018 fountain and channels are observations; the 1995 numerical example is a model from another context.
Microscopy reveals small features, while separate experiments test bulk transport. The same distinction matters in filters, rocks, soils and engineered porous materials.
Survey dates, coordinate systems, missing values and uncertainty belong with a terrain model. Display exaggeration can help you inspect relief while leaving the measurements unchanged.
Fold a sheet. Label its outside “surface we could photograph.” Inside, draw a simple passage and sample window labeled “imagined teaching cross-section.” Label magma below ground and lava above.
Draw two equal rectangles with six equal circles each. Leave one set isolated. Connect the other set to a top exit using pencil lines. Keep the circle sizes and counts unchanged.
Trace from each circle to an exit without crossing solid boundaries. Count the routes available in your drawing. Do not assign a speed; the drawing does not specify pressure, material properties or time.
Would one slice show every connection through a real rock? Fold the paper and explain what lies beyond the visible plane. Compare the actual microscope image and caption.
Write 1 melt volume + 6.3952 gas volumes. Then write the half-gas case: 1 + 3.1976. Predict whether the gas fraction halves; calculate or compare the on-screen strips.
Rewatch the credited source clips. Write one observation, one result from the model, and one question this activity cannot answer.
Can two samples have the same drawn spaces but different routes—and can removing half the gas leave more than half the volume as gas?
An original paper representation. No heat, powders, chemicals, pressurized containers, ash handling or volcano visits. It does not reproduce magma chemistry or measure a real rock’s permeability.
A small mass can occupy a large volume. Gas and melt have different densities; the denominators are different quantities.
Some dissolved material separates into a gas phase. Exsolution redistributes existing material between phases.
About 76.2%. 3.1976 gas volumes divided by 4.1976 total volumes is about 0.7618.
A fountain and flowing channels can occur together. The same source footage records fountaining and flows.
Permeability, driving conditions and time. Connected pathways and driving conditions affect transport.
The features deserve investigation; bulk flow needs other evidence. The pictured plane is perpendicular to the source experiment’s flow direction.
Different conditions and histories can produce different behavior. Both records identify Mount St. Helens at different dates.
No; this is a bounded calculation, not current monitoring. The lesson does not establish a universal fragmentation threshold or a present hazard forecast.
USGS OFR 95-756, upper Table 2, printed page 38 / PDF page 43. Historical equilibrium calculation. The displayed derivative CSV is transcribed from the original table.
Mastin (1995) · original USGS conduit reportUSGS-created footage, public domain in the US. Self-hosted full-duration silent 960 × 540 derivative and credited stills at source times 10 and 35 seconds.
USGS HVO · May 21, 2018 overflightAuthors report dominant fluidal fragmentation and interpret a smaller vesicular population in terms of rapid gas separation and cooling. Our video is not a particle-temperature measurement.
Namiki et al. (2021) · Hawaiian fountain researchBulletin of Volcanology 78:30. Figures 1 and 6, CC BY 4.0, preserved in full with 500 µm scale bars. Resized/WebP-encoded images do not add detail. Bulk porosity is distinct from image porosity.
Gaunt et al. (2016) · dome-rock permeabilityJournal of Geophysical Research: Solid Earth. Narrow qualitative interpretation of the original experiments; no restricted source artwork is copied.
Gonnermann et al. (2017) · expansion and compactionAustin Post / USGS. Public domain in the US; full framing retained, resized and WebP encoded.
USGS · Mount St. Helens 1980 column photographUSGS-created footage, public domain in the US. Full framing, embedded dates and duration retained in a silent 960 × 540 derivative.
USGS CVO · dome growth, 2004–2008Kristi L. Wallace / USGS AVO; sample collected by Sherry Harmes at Larsen Bay, Kodiak Island. November 13, 2015 SEM image; 50 µm scale bar retained.
USGS AVO · original volcanic ash micrographInstitutional ash definition; ash is not defined as burned wood residue. No individual grain’s chemistry is inferred from appearance.
USGS · what volcanic ash containsOriginal USGS FOIA release DOI-USGS-2023-000279, preserved in Ben Welsh’s pinned public-records distribution. Full source hash, 257 × 257 sample mapping, missing values and derivative hashes accompany the lesson.
USGS HVO · original survey notes in public-records mirrorIndependent subject review is pending.
Read the sources and model assumptions
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