An Interview with Scott Hughes
Joan Cartan-Hansen: What is so geologically interesting about Craters of the Moon?
Craters of the Moon is on a rift system that is perpendicular to extension in the Snake River Plain, which means that all the fissures are lined up in the same direction—always 90 degrees from the direction of extension. Because of its extensional tectonic, this great rift lines up with the mountain ranges that are also oriented in the same direction, which is one of the important things about this area.
Reddish part of lava flow surface caused by the oxidation of iron
The Snake River Plain doesn’t have a single source of magma —it’s all over. The magmatism, or the volcanism, occurs along these somewhat discrete rift zones.
We have maybe 30 or 40 various cones in a line here at Craters of the Moon. It’s what's known as a polygenetic eruptive center. You might have an eruption along the fissure, and it contracts to a single vent and builds up a cone around it. The next eruption might occur farther north or south along the rift to form another crater, which covers up the first one.
Craters of the Moon started erupting 15 thousand years ago. The last eruption was around 2,000 years ago. The amount of time between eruptions was as long as 3,000 years. The eruptions might have stopped forever, but we don’t know. Another eruption could occur here tomorrow, or it could occur in another 1,000 years. But somewhere on the Snake River Plain, another eruption will probably happen in the next 1,000 or 2,000 years and produce one more of these lava fields.
We are in a region characterized by one of the only continental hot spots in the world. The continental hot spot is currently at Yellowstone National Park. Eight or 10 million years ago, that hot spot was out here on this part of the Snake River Plain [where Craters of the Moon is now located]. We have different types of eruptions, which range in composition from what we call basaltic all the way up to rhyolitic. So what this really means is that we’ve got black, dark rock that is rich in iron and magnesium, and light-colored rock that is less rich in iron and magnesium and more rich in silicon dioxide, aluminum oxide and alkalis sodium and potassium.
Here at Craters of the Moon you see two main types of volcanoes, plus a third type of eruption that is very similar to what you find in Hawaii. The two types of volcanoes are spatter cones and cinder cones. Spatter cones are more like Hawaiian-type eruptions. Cinder cones are more like volcanoes you find in, say, Italy. As a matter of fact, cinder cones represent what is called a Strombolian eruption—which is named after the mountain Stromboli, in Sicily.
Spatter cones are produced when lava sort of burps out in a molten state, and forms blobs in the air. These contorted blobs are still molten when they land and pile up in a semi-molten state. This expelled lava cools and kind of glues all together to form what is called an agglutinate or an agglutinated spatter. The material piles up on itself to make these nice, little, steep cones, which are more typical of Hawaii.
The third type forms from a fissure eruption. Many volcanic eruptions are fed by fissures, but some fissure eruptions don’t produce either a spatter cone or a cinder cone. The eruptions might produce a structure called a spatter rampart or just a little fissure-fed outflow of congealed spatter along the margins, but they’re not nice little cones. They are definitely eruptions where you can see that lava has poured out or been blown out and then congealed and flowed out along an eruptive fissure.
Highly porous lava called scoria
Joan: What should visitors look for to learn a little more about the landscape?
Another thing to look for are the bits and pieces lying on the ground. These are called lapilli. These are pieces that were blown out of a volcano and fragmented to a size that could be blown by the wind. And there are particles that are finer than this, maybe about the size of a grain of rice, which are called ash.
Joan: Why does this place interest you?
However, it’s not just heat. It’s pressure. You take something that is very close to melting—if it’s very close to melting, you add a little heat and it can melt a little bit. Or if you reduce the pressure you can melt a little bit. So we have these dynamic forces going on down deep that are leading to the generation of these magmas.
Stretched vesicles and bluish color produce an interesting lava flow surface
Joan: The Apollo astronauts trained here because the theory was that the monument's surface looks like the Moon.
Well, at Craters of the Moon, we have these holes in the ground and we see rubble that is all around, and built up rims. And we say, whoa—it looks the same. But we didn’t know a hundred years ago or even seventy years ago whether the craters on the Moon were volcanic or whether they were something else.
And we now know that the craters on the Moon are mostly impact craters. This is how planets grow. They grow by impacts. So rocks fall from space and make these holes in the ground and splash out these ejecta blankets.
Here [at Craters of the Moon], the craters are volcanic. So craters can be made by two entirely different formational processes, but, still, on the Moon we find blankets of broken-up pieces of rock as large blocks, lapilli, and ash that are strewn all over the place, and no vegetation. Here at Craters of the Moon, we find fields of blocks and fragments of lava that are all over the place, and we slip and slide on them and so, in terms of the physiography, they are a little bit similar.
But it’s a good training ground. Scientists that can go to the Moon and walk around—learn how to walk on this kind of loose material and rough ground, and when we go to Mars it will be the same thing. We have to teach each other on Earth what we think it’s going to be like and train ourselves to learn how to get around and actually work on the rocks we find, or soils, or whatever we find. Craters of the Moon is a good training ground for anybody, whether they are studying biological things or geological things.
Joan: Anything else you want to add?
As kids, we grow up thinking about things. Kids think about space, insects, dinosaurs, volcanoes and geosciences, things like that. That’s why I’m a geologist, because these sorts of things fascinate us, and I think that humans should develop that fascination and, more so, an appreciation for the natural things that are around us. These wonderful creations that exist all around us are here to be enjoyed by all and admired, and not to be just cast off as some other wasted part of the landscape.