Ahh, lava flows; one of the prettiest aspects of a volcano. Too bad it's not important.
Wait a second, it's not important?! How can that be?? I've seen the movies with rivers of lava burying entire towns and out-running SUVs! Besides, shouldn't you be trying to get me pumped up about the subject before I read this?
Well, I figure you've seen all the Hollywood hype and, quite frankly, lava flowing out of a volcano isn't really that dangerous; it's usually slow, highly visible, and not all that common. The simple fact is that lava has been blown far out of proportion in its role, simply because it's dramatic and everyone has come to expect it in any "good" volcano movie. Now I want you to understand something: lava flows are very important for the people living around them- they cause property loss and in rare instances even loss of life, but there are just not that many dramatic examples in the world. Moreover, I'm not saying that they aren't interesting- they're one of the more entriguing aspects of the field in my opinion. Their lack of importance simply stems from their scarcity. Still, this is hardly a reason to ignore them and we're going to delve into how they work below.
For this article I am going to assume that the reader has a rudimentary understanding of how lava flows, so if you don't, you should read the "basics of volcanology" section first. Now most lava flows are basalt because of the whole issue with viscosity, but you do have andesitic, dacitic, and rhyolitic lava flows. This may confuse you because you thought that the silicic magmas produced explosive eruptions, but you don't get explosions without gas. The initial eruption is usually the gaseous one, which is often followed by the slow effusion of degassed magma. Because it is highly viscous it usually doesn't travel far and can build up to become unstable (lava domes and coulees are an entirely different section). For this reason, the silicic lava flows themselves are not that dangerous at all (though when unstable, are deadly).
Now, let's get into the basaltic flows. These are the ones we are familiar with, where lava runs down the flanks of Kilauea and toward the coast of Hawaii. The basics of pahoehoe and a'a lava have been discussed elsewhere, and so we'll move beyond that. There are a few things left to talk about here, however. First of all, nothing in science is simple and so you have to look at the way the lava flows in order to understand it and predict how it will behave. We can start with pahoehoe, which generally flows flat and through toes. The problem is that, due to the viscosity and temperature, we can't just assume it flows like water...because it doesn't. As the lava flows, the exposed surface rapidly cools and forms a crust over it. Meanwhile the lava inside is still flowing, pushing the head out in front of it. Remember that the head is exposed lava and so has thus cooled into hard rock. The internal pressure of the moving lava keeps the crust above it supported while it pushes the head out in front of it. The annoying thing about pahoehoe is that it tends to wander. For example, it is heavily influenced by topography and generally flows slow enough not to jump any embankments. As the toes move forward, the crust at the head is moved upward as fluid lava moves forward to harden and then move up onto the top of the flow. Despite this method for relieving the internal pressure, the crust in the middle section of the flow (outside, but back from the head) is being pulled and stretched. If it cracks, lava will begin pouring out of the crack and the head will stop moving. The new toe will move in it's own direction, away from the topographically high old toe, which has since stopped moving. The new toe moves on as the last one did until the process repeats itself. While the overall flow keeps moving, the front of it meanders greatly in its forward motion. The outer crust of the flow serves another important role: it insulates the flowing lava. This is bad news for people who own property in the path of the flow. Because the lava is being insulated by that shell of crust, it can now flow longer and further than before, resulting in a generally more extensive flow. For a'a flows we have a formula that can be used to predict about how far a flow will travel before it solidifies and stops moving. We have no such formula for pahoehoe because its internal processes are poorly understood, both scientifically and mathematically.
A'a flows are a little bit odd when it comes to characteristics of flow, however. See, you would figure that the steeper the slope is that the lava is traveling down, the further it will go. Not so for a'a, if you have a flat field and a separate 45 degree slope, the a'a flow will travel much further on the field than it will down the slope. This seems completely counter-intuitive: if the flow moves faster down the slope, it can get further before it cools and so it should go further than the flat field. You forget one thing though, the a'a is covered by a thick crust which insulates it from the cold air. A'a flows can go weeks and even months before cooling so the air really isn't the biggest factor in how far the flow will go. The problem is this: a'a flows can't handle going very fast. When they do move fast, the crust at the top stretches and breaks revealing the hot interior to the cold air; you just broke the insulating layer. The faster the flow moves, the more the crust stretches and breaks meaning that the interior lava cools much quicker. This will stop the lava early, limiting its reach. The slow lava, however, maintains its insulating layer and slowly trudges along, bulldozing everything in its path. It's not as fast as the steep-slope flow, but it's got all day to get where it's going and it certainly takes it's time in getting there. It's a classic example of the tortoise and the hare!
Another type of lava flow, and the most common, is the submarine flow. The irony is that, though they are the most common, they are the most rarely seen due to where they occur. Submarine flows typically act like pahoehoe lava in that the top rapidly crusts over due to the high heat capacity and low temperature of the surrounding water. Small flows meander about in toes while more voluminous flows can travel in sheets. An interesting formation associated with submarine flows occurs when the lava is traveling down an extremely steep slope or off of a cliff. While on land, the crust would break and the lava would flow right over the edge, but in the ocean, the water is cold enough that the lava flowing over the cliff crusts over even as it falls to form column-like structures. That is not to say that the crust on the submarine flows never breaks; it breaks just as much as the surface pahoehoe does, but the crust forms in response much more rapidly. It also appears that, even more common than flows traveling over the seafloor, are flows that travel through sills. A sill is a sheet of magma that travels parallel to the surface. The flows may actually expand through the soft seafloor muds, cooking the water out and moving laterally beneath the surface. These often have telltale signs such as sedimentary grains showing up in the hardened lava flow. The sills have a crust between the flow and the saturated sediments, but particles will still end up becoming entrained in the flow as it pushes it's way through. This is similar to the way sheet flows operate as well- the lava flows up through the earth and exits the seafloor in a spot that already has a magmatic roof of crust over it. The lava may pause there or move out toward the edge of the flow, all the while being encased in a lava "sill." On land it can be very easy to identify a submarine flow, though not always immediately. First of all, one big signal is the physical way in which it lies- it should be in tubes unless it was an extremely voluminous flow. For sheet flows, the presence of meta-sedimentary minerals can be a give-away. The sedimentary grains are entrained with the flow (which is rare in terrestrial flows) and are "cooked" by the heat. The mineralogy of the particle will change as a result into a specific metamorphic mineral, depending on the temperature of the flow and the composition of the grain.
You have one last type of lava flow called the Flood Basalt. Many scientists classify this as a type of volcano, but I have trouble classifying volcanoes merely by size. Eruption style and magma composition should be the only determining factors as far as I'm concerned. Flood Basalts all arise from fissure volcanism. The reasons for which they occur are not completely known as we've never witnessed a flood basalt eruption before. These eruptions are enormous and put out hundreds of thousands of cubic kilometers of basaltic lava over the course of weeks to years. The eruptions occur on a grand scale and although there is a great deal of debate as to how long the eruptions last, there is little debate as to the enormous size of the flows and their impact on regional and global environmental conditions. Sources ranging from mantle plumes to meteorites have all been proposed, each with it's supporting evidence and weaknesses. The truth is that until we witness an eruption, we will never truly know just how these things form; all we have to work with are lava flows that are meters thick and which cover thousands of square miles. A single flood basalt actually contains a sequence of lava flows, all of which are interpreted to have erupted within a very short time of each other (due to the lack of sedimentation and erosion between flow contacts). Flows, meters high, stack on top of one another to create a series of flows collectively referred to as a flood basalt and are absolutely enormous when mapped. If you're thinking that it would be nice to see a flood basalt erupt so that we could understand it, think again. God help us if one erupted, the entire planet would be in trouble. First of all, the local region would be devastated and that local region could include several moderately large nations. Depending on the location, anywhere from millions to hundreds of millions of people would be displaced in a matter of weeks. You would have an immediate economic and political crisis on your hands. The huge amount of heat rising from the flow's surface would cause major regional weather problems and gasses escaping from the lava could create a massive global greenhouse or refrigerator effect. The eruption would cause famine on an unimaginable scale as global temperatures caused local crops to fail in nearly every part of the world. Millions permanently displaced, climate change, famine, and the ensuing political chaos could potentially knock our civilization right back down to levels analogous to 12 and 1300's. While not trying to turn this into a piece of sensational journalism, it is important for you to understand the devastating impact an eruption like this would have on our society. Thank God they only occur about once over tens of millions of years.
At this point you have a fairly good understanding of lava flows, but there is one more style of flow that is very rare. If it's so rare, you might wonder why I bother mentioning it. Well, it's because in 1977 it killed 70 people around Nyiragongo and in 2002 killed another 45. The event is the catastrophic emptying of a lava lake. Nyiragongo, located within the Democratic Republic of the Congo near its eastern border with Rwanda, is a basaltic volcano which contains a large lava lake. Twice, once in '77 and again in 2002, fissures appeared in the side of the volcano which caused the lava within the crater to rapidly empty out in the direction of Goma. The lava traveled at speeds in excess of 40 mph and resulted in the deaths of a number of residents and animals that happened to be in the way. In 1977 it took only an hour for the lava lake to completely empty. Nyiragongo is the only place where this event has been observed to occur, but for local residents it is a very real and pressing danger.
People aren't left without options, however. Take the case of Heimaey Island, of Iceland, in 1977. When the eruption of Eldfell Fissure began to overtake the town of Vestmannaeyjar the people were evacuated, but when the flows threatened the economically vital harbor, fire crews began attacking the lava with fire hoses. The eruption lasted seven months, but the residents were able to save the majority of the city and the harbor by slowing down the flow enough to endure the eruption.
Well, you should now have a general idea of how lava flows behave and how they affect the people around them. We've looked at several different styles of flows and what characteristics determine their behavior. There is plenty of more in-depth information available on the topic, but this article was only meant to give you a relatively quick look at the subject. For a more intensive look at lava flow behavior, you might look to textbooks and scientific journals.