The Beware of Movies! series is meant to point out some of the scientific inaccuracies of popular movies, specifically in points related to the geological sciences.
This post will present some basic information about volcanoes and how they work, and point out the major inaccuracies portrayed in movies about volcanoes.
What is magma? How is that different from lava?
It’s a good question. We’ve all heard the terms before, magma and lava, and probably you’ve got a general sense of what they mean, but do you know the specific definitions.
Magma and lava are both words for molten rock. Liquid rock. Very hot. This distinction is that magma is below the surface of the Earth and lava is what you call it when it’s on (or above) the Earth’s surface.
Types of magma
Molten rock is not all created equal. When you look at rocks, you realize that they’re of all different kinds. So if you melt that rock, it’s going to be of different compositions reflecting the rock that was melted.
To discuss the composition (or types) of magma, we refer back to Bowen’s Reaction Series. Magma can be felsic (high silica, high calcium, aluminum, and potassium) or mafic (low silica, high iron and magnesium), or anything in between. These compositions affect the behavior of the molten rock. Silica tends to polymerize (make chains) in molten rock, which means that a more felsic magma tends to be more viscous (or that it doesn’t flow very well). The high levels of iron and magnesium in mafic magma makes it more dense. Also felsic rocks melt (to make a felsic magma) at much lower temperatures than mafic rocks.
From this discussion, one might gather (and rightfully so), that the composition of a magma is determined by the type of rock that melted to make the magma. There are other ways to change the composition of a magma (which will serve to explain why we have different compositions of rocks in the world)
Changing the composition of magma
1) The composition of the parent rock (the rock that melted to form the magma) – If you melt a mafic rock, you get a mafic magma. This is the most straightforward means to change or determine the composition of a magma.
2) Assilimation – If a magma body incorporates another rock, which then melts, unless the incorporated rock is of the exact same composition of the magma, the composition of the magma will change. As magma rises through the crust, it ‘eats’ through the rock above it (country rock), melting and incorporating the country rock, changing the composition of the magma body.
3) Magma mixing – Two separate magma bodies while moving together may wind up coming in contact and mixing. Unless the two magma bodies have the exact same composition, a new magma will result with a composition in between the original two magmas.
4) Partial melting of rock – As noted earlier, felsic rock melts at a lower temperature than mafic rock. When any rock melts, the first minerals to melt are the most felsic minerals. Over time, as the rock heats up, the more mafic minerals begin to melt. But a rock doesn’t always completely melt over time. If a rock only partially melts, and the melted rock (magma) moves away, the magma is more felsic than the original rock and the remaining rock is more mafic than the original rock. Thus, through partial melting, a magma may become more felsic.
5) Fractional crystallization – As a magma cools, the first minerals to crystallize are the most mafic ones, causing the remaining magma to become more felsic. These newly-formed mafic minerals might settle out, leaving behind a more felsic magma to continue on its way toward the Earth’s surface.
Intrusive or extrusive
Not all magma makes it to the surface. If it does, you get an eruption and a volcano. Lava erupts onto the surface to cool and form what we call extrusive igneous rocks. The magma that doesn’t make it to the surface will crystallize below the surface, and is called intrusive igneous rock, or just an intrusion.
Since we’re talking about volcanoes, we’re going to talk only about extrusive igneous rocks formed from lava.
What comes out of volcanoes?
Lava is the molten rock that comes out of volcanoes, but there is much more that comes out. The opening out of which the lava flows is called a vent. But it isn’t always lava flows that come from vents. There is also ash and other pyroclastic debris. So what does that mean?
The stuff that flies out of volcanoes is called volcaniclastics or pyroclastics. The root ‘clastic’ refers to little bits and pieces that get deposited together. The volcani- part refers to volcanoes (obviously), and pyro- refers to fire. So the words mean bits and pieces of rock coming from volcanoes or coming from fire.
Those bits and pieces have specific names depending upon their size and shape:
Ash – tiny glass shards that form when the lava crystallizes almost instantaneously.
Beware of movies: It seems that in almost every movie (for example, Dante’s Peak and Volcano), there’s always a ton of ash snowing down upon the main characters of the story. This ash is glass shards! If you inhale this, it will cut your lungs to bits and you will die a miserable death. Yet, somehow, in movies this is never an issue. In paleontology, some of the best fossil assemblages came to be when an eruption occurred and the ash killed the animals in just this way. Then the ash buried the animals and perfectly preserved the animals for geoscientists to later discover.
Lapilli – pea to plum sized fragments that can be streamlined from flying through the air.
Blocks and bombs – apple to refrigerator sized fragments that are shot from a volcano. Blocks are bits of rock that were already cool when they were shot, whereas bombs were molten when they were shot and often are streamlined in flight.
The deposits left behind are also given special names, depending on how they formed:
Ignimbrites (or pyroclastic flows) – avalanches composed of ash or ash plus lapilli. These things can go very fast, riding down the slope on a cushion of air, just like a snow avalanche.
Lahars – a rapid slurry of volcaniclastic material in water (like a mudflow), most often caused when snow caps suddenly melt off of volcano peaks during an eruption.
Beware of movies: In Dante’s Peak, there was ample opportunity for good pyroclasic flows and lahars, but there weren’t many. There should have been. And the ones that they did show were too slow and did not flow far enough. It was pretty weak, honestly.
Lava flows and types of eruptions
Different compositions of magma (lava) behave differently as they flow out of a volcano. Because of the polymerization of silica, felsic lavas tend to flow more slowly and in a more ‘chunky’ form than does mafic lava. Mafic flows can move much greater distances than can felsic flows.
Eruptions of fast-moving, low-viscosity mafic lavas are often what we call ‘effusive,’ or characterized by huge flows and lava lakes. Other eruptions like that of Mount Saint Helens in May of 1980 (or of Dante’s Peak), are ‘explosive’, which characterizes eruptions of intermediate to felsic lava.
Magma (and lava) often contains dissolved gasses, which must escape when the lava erupts. If the gas can escape easily, an eruption will be more effusive, but if the gas cannot escape, eruptions may be very explosive.
The interaction of eruptions with water also effects whether and eruption will be explosive or effusive. If a volcano erupts under water (or lava flows into water), the results can be explosive.
What kinds of volcanoes are there?
The types of flows coming from a volcano (and therefore, the composition of the erupting lava) determines the shape of the volcano.
Shield volcanoes are huge, flat (shield-shaped) volcanoes that result from dominantly mafic flows that are effusive in character. The Hawaiian Islands are all enormous shield volcanoes.
Stratovolcanoes (also called composite volcanoes) are tall, pointed volcanoes, often associated with explosive eruptions and felsic to intermediate compositions of lava. Such volcanoes tend to be composed of repeated layers of flows and pyroclastic depositis. Mount Saint Helens and Mount Rainier are two prominent examples of stratovolcanoes.
Where do we find volcanoes?
Volcanoes tend to be in specific places throughout the world, not just stuck willy-nilly where ever they seem needed. For example, you don’t find volcanoes in the middle of continents.
The positions of volcanoes on the Earth is almost entirely dictated by plate tectonics. Volcanoes arise do to the interactions of tectonic plates, and thus tend to be at or very near plate boundaries. Additionally, plate tectonics dictates what kind of volcano might be found where.
To review, there are three important types of plate tectonic boundaries: divergent (spreading centers, or mid-ocean ridges), convergent (including subduction zones and collisions), and transform (where plates slide past each other).
Volcanoes are not common along transform faults, and usually only on those that have a slight component of spreading across them. In those cases, some mafic eruptions might occur, but they tend to be very small.
Beware of movies: In the movie Volcano, a volcano forms from the La Brea Tar Pits in the middle of Los Angeles, California. This is along a transform boundary, but not one that would be a candidate for a volcano. This transform boundary has a slight component of convergence, so if anything there should be mountain building, not volcanoes occuring.
Spreading centers, being the place where new crustal material is being formed from eruptions linked right to the mantle, tend to have large, mafic volcanoes. There has been little opportunity for the magma that forms these volcanoes to undergo any of the processes that would make them more felsic.
Convergent boundaries, especially subduction zones, usually have large stratovolcanoes associated with them. Much of the margin of the Pacific Ocean is characterized by stratovolcanoes and subduction zones, and is called the “Ring of Fire” because of it. The magma formed at subduction zones forms as the subducted plate melts. The magma slowly rises through the crust, changing its composition as it goes due to assimilation and fractional crystallization, so that when it erupts it is generally intermediate or felsic in composition.
Beware of movies: in the movie Dante’s Peak, the subject volcano is a stratovolcano in the Cascades. This is perfectly reasonable, as the Cascade Mountains are the result of the subduction of the Juan de Fuca plate under the North American plate. The one mistake that is made in the film is that it depicts rapidly-flowing, presumably mafic flows coming from the volcano, which do not make sense given the type of volcano they’re dealing with.
Not all volcanoes are associated with plate boundaries, however. There are a few exceptions that form due to what are called “hot spots,” or heated plumes of magma that (so far as we know at this time) form at the core-mantle boundary and get all the way to the Earth’s surface. These volcanoes can appear anywhere on the Earth’s surface, two good examples being the Hawaiian Islands and the Yellowstone hotspot. Because the Hawaiian hotspot is in the middle of oceanic crust, the eruptions tend to be mafic, resulting in shield volcanoes. The magma for the Yellowstone hotspot passes through continental crust, resulting in highly explosive, intermediate or felsic eruptions.
Can we predict volcanic eruptions?
I’ve already discussed at great length that earthquakes cannot be predicted. Is the same true for volcanic eruptions? Actually, the story isn’t as dismal for volcanoes. We do often get some manner of advanced warning of an impending eruption, though the warnings might be months or only hours ahead of an eruption, or they might be false alarms.
The movement of magma below the Earth’s surface, toward vents for example, are detectable by seismometers. In fact, the direction in which the magma flows below the Earth’s surface is even measurable if an appropriate seismometer network is in place. Outgassings, and temperature and pH changes of water bodies can also provide evidence of a possible eruption, similar to what was seen in Dante’s Peak. And, since we tend to know where volcanoes should exist and we know how they are formed, we have some hints about what to look for if an eruption may be immanent.
There’s hope for predicting volcanic eruptions. But to know exactly when or how intense an eruption might be, is elusive.