Scientific Method / Science & Exploration

Want to study craters? Geologists grab explosives and make some

We have video of researchers blowing things up to understand volcanic craters.

Enlarge / View from the edge of Kilbourne Hole in New Mexico—an example of a maar crater.
Scott K. Johnson

While craters cover many other bodies in the solar system, plate tectonics and weathering continually renew the Earth’s surface, preserving its youthful beauty. Still, that process doesn't happen overnight, and there are many craters to be found on our planet. Some record violent impacts with meteorites, and others formed during a variety of volcanic eruptions.

Maar craters, like the one pictured above, are created when fingers of magma beneath the surface of the Earth interact with groundwater, causing a violent explosion. Measuring the size of a meteorite impact crater can provide a lot of information about the size and impact angle of the meteorite. But when it comes to maar craters, geologists have been unsure just how much information about the eruption can be gleaned from the remnant crater.

Part of the problem results from the explosion being able to occur at a range of depths. An explosion of the same size could create a very different crater at the surface depending on how deep it occurs. To complicate matters further, there can sometimes be multiple eruptions beneath the same crater.

While this has been studied in detail using numerical simulations, a group of researchers decided a lot could be gained from examining these things with actual experiments. That is to say, they opted to go outside and blow stuff up.

Obviously, they had to work on a much smaller scale—as a general rule, astronauts shouldn't be able to notice your experiment from the space station—so they built piles of layered sand, gravel, and crushed asphalt about a meter deep and four meters across. Within the sediment, they buried charges of TNT and plastic explosive (for good measure, one assumes) in several arrangements. This seems like a good point to mention that absolutely no one should be trying this at home.

It was a pretty straight-forward setup, as Prof. Greg Valentine, a geologist at the University of Buffalo, explained to Ars. “The configurations of explosives were also very simple, and really this was intended to be a scoping exercise in preparation for more detailed and costly experimental runs in the future.  However, I would say that the setup worked well beyond our expectations—despite the simplicity, we learned an incredible amount.”

A single charge goes off 50cm deep.

In the first experiment (which can be seen in the video above), one large charge was buried 50 cm below the surface. After detonation, measurements were made of the crater diameter and volume, how much material (called “ejecta”) was blasted out of the crater, and the high-speed video was used to track the process in slow motion.

In subsequent experiments, the charge was broken into three parts and detonated one at a time, to simulate multiple eruptions. The first thing the experimenters learned was that the final dimensions of the craters were pretty similar to ones caused by a single explosion. What this says is that it's easy to overestimate the size of an eruption if you assume a crater was the result of a single eruption when it was actually several. And that’s important if the area is still volcanically active, and you’re trying to figure out how big a future eruption might be.

A charge goes off within the crater generated by an earlier "eruption."

The explosion patterns also varied considerably depending on the depth of the charge. Those that were buried around 50 centimeters below the surface launched debris more than 16 meters from the crater. In the deeper blasts, on the other hand, most of the sediment was launched upward and collapsed back into the crater, partially filling it in.

The video of those different explosion types also shed light on an interesting feature seen around some maar craters—deposits resembling wind-blown dunes. While the sand and larger bits in the ejecta fell to the ground rather promptly, the finer-grained dust hung in the air, drifting more slowly. The rapid collapse of material into the crater following the deeper explosions forced the dusty air rapidly outward along the surface. (This can be seen in the second video.)

Prof. Valentine told Ars that the research group also examined cross-sections of the sediment in an around the craters, picking up a few more insights that will be reported in a separate paper. And they have plans to do more with further experiments.

“One question is, what are the effects of different configurations of explosions on crater size?” Valentine wrote. “For example, in natural maar eruptions, we know that the explosions do not always take place beneath the center of a newly-formed crater, but sometimes are off to one side, and at varying depths. A second question is, how does the composition of the ejecta deposits (in terms of the proportions of materials from different depths) reflect the processes that go on in the subsurface explosions?”

Prof. Valentine was quick to point out that these experiments have their shortcomings, such as an inability to see what’s going on beneath the surface during the explosion, and that some aspects of maar eruptions can’t be understood by playing around with a miniature crater. “But” he added, “I have to admit, while the numerical simulations are fun and important, I really had a blast doing the experiments and micro-field work!”

Videos from Greg A. Valentine, et. al. (linked below), courtesy of the American Geophysical Union.

Geophysical Research Letters, 2012. DOI: 10.1029/2012GL053716  (About DOIs).

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