You don’t know what a coulee is…admit it. You have no idea at all! Well, if you’re like I was, you don’t. I had been intensively studying volcanology for over a year and had completely ignored this subject. There was a similarly titled article in a text I was reading, which I had skipped repeatedly for the better part of eight months. After all, lava domes are kind of boring and who the hell knows what a coulee is! After a while, however, I decided to bite my lip and go for it. Imagine my surprise when I discovered how dangerous and important these features are to communities all across the world. In fact, lava domes and coulees are hazards that linger for months and years. This makes them hard to ignore…especially when you actually know what they are. Maybe we should talk about that next.
Lava domes are just what the name suggests: domes of lava that form atop a volcano. Lava domes are usually the end-product of an eruption; they are composed of extremely viscous and partially degassed magma. This magma oozes slowly from the top as already-nearly-solid rock. Think of it as toothpaste being squeezed from the bottle. Since the lava is so viscous, it is unable to flow as we typically think of it. Instead, it piles up and slowly spreads out from the vent. As it builds up, the dome can become extremely unstable; moreover, though the lava is mostly degassed, it is not entirely so. A collapse can trigger an explosion from the dome. Coulees differ only slightly from domes, in that the coulee flows a little more like lava as we picture it. Coulees can be worse than domes as they begin to travel down valleys, which may cause them to collapse more frequently in some cases. On the other hand, this usually makes their impact area more predictable.
Lava domes, as mentioned earlier, are usually the end product of an eruption, although many of our largest eruptions have been briefly preceded by lava domes. After (or sometimes before) all of the volatile-rich magma at the top of the magma reservoir has been erupted, the degassed magma makes its way upward. This magma rises slowly, without the explosive gases pushing it up faster or breaking it apart. Like toothpaste squeezed from the tube, it oozes out of the top. The longer ascent through the volcano may mean that the lava is cooler, but this is relatively unimportant when compared to the gas content. As the lava makes its way outward, it spreads out at the surface. The first act of most lava domes is to begin filling the crater it is being ejected into. The growth is usually slow, with the dome acting more as solid rock than as a fluid. The surface is usually blocky and highly fractured, as the semi-solid rock breaks and spreads under the tremendous forces pushing it. Domes frequently take the shape of cones or hills. Some domes are very smooth, but contain large rock spines protruding near the top. These spines can reach tens of meters in height and present numerous stability problems. These spines are pushed up by the underlying magma…you could think of a pinnacle as a weak-spot in the dome that is being shoved upward faster than the rest. It is, as you would probably picture it, tall and thin (small diameter). Domes are usually composed of rhyolite, dacite, or obsidian (and usually a combination of these).
Virtually all danger associated with lava domes and coulees are associated with pyroclastic flows (for more information of pyroclastic flows, consult the similarly titled article). The danger arises, as mentioned earlier, from the inherent instability of the domes themselves. Since they are being “squeezed” from the vent in a toothpaste-like fashion, domes sometimes take on shapes that defy gravity.
At basaltic volcanoes, the erupted lava is extremely fluid and so flows outward. This means that, when cooled, it is extremely stable. Domes, on the other hand, stack up on themselves. Pinnacles, or spines, may rise from the vent region upward. The coulee may move into a valley where gravity wants to take it faster than it can move. The dome may outgrow its crater, and begin to expand precariously over the edge. Inside the dome, due to the high resistance to flow, some areas move faster than others; some regions endure greater forces and pressures than others. All of these factors cause the dome to be unstable.
But if this was all there was to worry about, we wouldn’t care much for the few minor rock falls associated. The problem lies with the over pressurization of the dome. The magma erupted is degassed enough to keep from exploding right away, but the situation changes as the eruption continues. The material that is expelled first can be thought of as the dome’s skin, which will always be on the edge of the dome moving outward. As this lava continues to cool, it hardens and acts as a dam for the magma rising behind it. This means that the pressure continues to build as the dome grows. Disaster comes when a section breaks away.
It may be part of a coulee that broke into a valley; the dome may have out-grown its crater and an overhanging section collapsed; even a pinnacle collapsing from the top can trigger an event. One thing these events all have in common is that something broke away and the interior of the dome was suddenly exposed to virtually zero pressure. This causes an explosive expansion of the gas bubbles and the formation of a pyroclastic flow. As the gas bubbles explode, they cause the dome to fracture further and induce shockwaves that may shatter more bubble walls. In any case, the collapse of a dome, even a small section, can result in devastation nearby.
There are a number of features lava domes display that help us to understand how they work. Large lava blocks help us to understand how quickly a lava dome is being extruded: rapid effusion means the blocks break up more rapidly, while larger blocks indicate the extrusion is more gentle. Most of the blocks form near the vent, but some still form near the edges. Lose blocks are known to cause rock falls and occasionally induce pyroclastic flows.
Some sizeable domes display explosion pits on their surface. These likely occur due to collecting gases that may have risen through cracks, but were trapped by the colder “skin” of the dome. A rapid depressurization may result in an entire section exploding.
Ridges and creases help to reveal the stresses that the dome is undergoing. Ridges indicate that the dome is compressing, and form 90 degrees to the direction of the stress. For example, if the ridge was in an E-W direction, that would indicate that stresses were acting in a N-S direction. Take a piece of paper, put your fingers on either side, and slowly move them together. Ridge formation also indicates that the flow interior is more fluid than the exterior, but to a limited extent. Creases appear just as the name suggests, and indicate locations at which the dome is spreading. Due to the amount of compression, it might be hard to imagine a situation in which the opposite is happening. There is one important place where we find this, however; where the lava is exiting the vent! Remember that the lava is rising in our “toothpaste” model and does two things at the surface: builds up and spreads outward. Thus, the creases indicate where the lava is rising to the surface. Some domes are even composed entirely of a single crease! Other creases may occur where one section travels faster than the section behind it; this may be due to gravity pulling it through a valley or over a crater rim.
Despite everything we’ve talked about, there is one kind of dome that I’ve skipped over entirely. This is the cryptodome (crypto meaning hidden or mysterious). Cryptodomes are so-named because they never reach the surface. It can be easy to think that lava has to reach the surface, but why is that? If the lava finds a zone of weakness below the volcano’s summit, it will begin to collect and expand there…as a dome. As the dome expands, the walls push against the overlying rock, causing it to expand. The result is a dramatic bulging, usually on one side of the volcano. The most dramatic example of a cryptodome comes from the eruption of Mt. St. Helens, in 1980. Prior to the eruption, a small bulge began to form on the mountain’s north flank. This bulge continued to grow until it became incredibly large…and unstable. It was recognized to be a cryptodome and evacuation orders were given accordingly. On May 18, the bulge became too unstable and a landslide occurred. This landslide was enormous and exposed the cryptodome to the air. The massive release of pressure from the overlying rocks resulted in an enormous lateral blast and pyroclastic flows that covered miles. Compared to “normal” lava domes, cryptodomes are relatively rare; they do still periodically occur, however, in places like Unzen, Japan.
Since we began this article, we’ve discussed the types of domes and the manners in which they form. We’ve also talked about the dangers presented and just what influences how dangerous a dome is. Hopefully you now have a better understanding of this continual process. For more information on lava domes and coulees, I would suggest consulting textbooks and science journals.