Geology

Rocks that go Boom

This should have gone out Saturday, but I’m still getting the hang of this, so thank you for your patience. I’m going to start the rock articles with a bang, as it were. I had someone ask me about volcano’s so here we go. Before we get into the nitty-gritty exploding details, let me cover two things 1) how the rock articles will be formulated, by and large, and 2) Plate tectonics in a nutshell.

I will start of, in most cases with a quick overview of the topic. Introductions like this will be kept brief if they’re present at all. Today’s is longer because it’s the first. The next section of the post will be the geology. For general overviews there will be fewer citations. Much of this is off the top of my head. I do not have internet at home and most of my books are in boxes. As that changes, citations will increase in number. As much as possible, I will try and provide citations that you can look up without a handy university library, professional society membership, or paying out the ears for the article. I will do my best to provide background information, though sometimes that will be a link to a previous article. Since most of the people asking me questions are writers, I’ll try and include notes on what parts are likely to only be useful as background knowledge, not likely to impinge on the written world. (Most fantasy worlds don’t need to know their mountain range is an ancient suture zone. It is enough that ‘here there be mountains, but they don’t explode. Oh look! Mineral deposits!’)

When it comes to volcanoes, Plate Tectonics is the driving factor. Tectonic plates are just giant (from the human perspective, they range in size from roughly Nebraska to the majority of the Pacific Ocean.) chunks of the very surface of the Earth called the crust. Below the crust is the Mantle. The mantle is more or less rock that is the consistency of silly putty; however, parts of it are more liquid than others and it flows and moves and the plates move with it. (Note: vast oversimplification, the actual motion isn’t quite that simple, and we haven’t found all the driving factors, nor figured out all the ways that the plates themselves influence the process, but it is enough for this particular article.)

The plates have boundaries. These boundaries come in three types: Convergent, divergent, and transverse. Convergent means two plates smack into each other. Divergent means two or more plates move away from each other, and transverse means the plates are sliding past each other. Any can produce earthquakes. Convergent produce the majority of volcanoes. (We’re going to ignore Iceland today. Iceland has it’s own issues geologically and it would take at least one article to even get into them.) Transverse boundaries do not produce volcanoes. These are boundaries like the San Andreas fault. Convergent boundaries are like the one off the coast of Japan.

A volcano is where magma from beneath the earth’s surface comes to the surface and erupts either pyroclastically or as lava. In school they teach about three basic kinds: shield volcanoes, composite or Strata volcanoes, and cinder cones. While these are very useful, they do not encompass historical or modern flood basalt (Iceland for modern examples, and the entire Snake River Valley flood basalt in Oregon and Washington) nor do they cover spreading centers, nor complex systems like Yellowstone. For today’s article we will not cover these topics either.  Today is for the basics. The mountains we typically think of as volcanoes.

The mildest form of volcano is the Shield volcano. Kilauea is a good example of this kind of volcano. They are usually caused by hot spots and have lavas that are rich in metals and low in silicates (which means they flow very well, don’t tend to explode, and dry very dark.) They create very gently sloped mountains, and tend not to form mid-continent. (Continental crust adds silica which makes the magma more viscous and more likely to be ejected violently rather than flowing as the lava we’re used to seeing in pictures of the Hawaiian volcano.) These lava flows are so gentle there are people in Hawaii who keep them back with a garden hose. The main danger of this form is the heat, and if you walk across a lavafield, falling through and discovering that there is an underground tunnel (called a Lava tube) full of the stuff ready to incinerate you. If it is hot enough to melt rock, it is hot enough to melt you. This kind of lava can flow very quickly under the right circumstances, playing with it is ill advised. Side note: A hot spot is a plume of magma that seems to rise continually from a place other than a plate boundary. Beyond that would be an article for another time.

The next form of volcano is the Cinder Cone. These tend to be relatively short lived things Staying with Hawaii, there are cinder cones on that shield volcano as well (on Muana Kea, though I do not recall if the peak itself is a cinder cone or not). If there is a high gas content in the lava (less common in low silica volcanoes as the magma is less viscous so doesn’t trap it as well), it may erupt as fine particles rather than lava. The geological term for this is ‘scoria’. The pretty fire fountains that make up so many of the volcanic pictures usually form scoria. The eruption fountains out, the molten rock droplets cool, and the particles fall to the ground. The volcano Paricutin in Mexico is another example of a Cinder cone. Eruptions from Cinder Cones tend to be short lived, though Paricutin is an exception erupting for 9 years. Most eruptions are measured in mere days. Paricutin is also rare in that it is one of the few volcanoes (I am not willing to say the only volcano without more research to check my memory) born in modern times, having first erupted in 1943.

The most violent of the basic volcano types is the Strata Volcano.  This particular class encompasses such volcanoes as Vesuvius, Krakatoa, and Mt. St. Helens.  These volcanoes are found along collision boundaries,  Specifically where one tectonic plate is forced under another.  The more dense plate subducts sliding under the less dense plate.  The rock heats as it goes further down towards the Mantle.  The heated rock melts and starts melting it’s way up through the upper plate.  This is what formed islands such as Japan, the Aleutians, and Indonesia.  When the rock melts through continental crust it picks up silicon as it goes (it melts the continental crust which tends to be very high in Quartz and other silica rich minerals.) The silicates thicken the magma, trapping any gasses in the magma more firmly.  The Magma collects until pressure and temperature are high enough that something gives and boom.  Vesuvius takes out Pompey and Herculaneum.  Mt. St. Helens takes out a chunk of the Washington forest and the guy who wouldn’t leave his house on the mountain.  These are the mountains that tend to blow their tops and eject huge amounts of ash, dust, rock, and gasses into the atmosphere all in one go.  Note: I’m not going to talk about Krakatoa here as there was a lot more going on with that one than just a magma chamber that over pressurized.

Strata volcanoes tend to be very long lived, and can be dormant (not actively erupting) for centuries.  This leads to some issues, as ice packs build up and then are flash-melted in an explosion causing volcanic mud flows called Lahars.  They also cause massive pyroclastic fronts called Nuee Ardentes.  These get very hot and move at high rates of speed.  On the up side they burn out relatively quickly as the gas and shock wave disperse.

For a writer, the big things to remember are that volcanoes do not pop out of transform boundaries (the pressure is relieved laterally, and unless there is a hot spot or another plate near by subducting, there is no material migrating upward to fuel the volcano.  Erupting volcanoes are dangerous, even shield volcanoes can cause issues if you treat them casually.  Do not use them to cover attacks on near by settlements.   I will get more into depth on this as we get deeper into the topic.

On Wednesday I intend to go into more detail on Shield volcanoes with more detail for the Strata volcanoes on Thursday.    Tomorrow is for things Russian, and Friday for things writing related.  There will be more on Nuee Ardentes (even if I can’t spell it consistently!) and other volcanic hazards in weeks to come.

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