Contents under pressure

-Alison

How does it make you feel when a headline reads "molten magma beneath your feet"? It sure sounds exciting, and when it’s followed immediately by some line about disaster or cataclysm it can be quite stressful. Before we tackle the details, let’s start off with-- there is very little magma down there. Most of the Earth’s mantle is quite solid, it is hot, but not much of it is liquid. Some headlines are just click bait, or they've taken some grain of truth and stretched it until it causes that right amount of drama. We can, with a bit of detective work, weed out what real information in the news from the soap opera.
The lovely glow from molten rock. This was made in my lab from remelting natural lava.


One way is to first wrap our heads around the scale of things "beneath our feet".

Just what do we mean when we say the Earth is hot inside? How far must we go to get to hot stuff? More importantly, how much is liquid down there? If you head to a geology text book or USGS website you'll find lots of numbers that describe depths and pressures. I want to break these down for you a bit, like we did with viscosity, size of eruptions, and the speed of volcanic processes.

Let's start with distances (depth) in the Earth and ‘where is the hot stuff’. The Earth's crust is quite thin (35-70 km thick), relative to the diameter of a planet (12,700 km or 7,900 mi). That is 0.005% of the total distance you’d have to travel to come out the other side of the Earth. The example I've always liked is comparing Earth's crust to the skin of an apple. The crust is the coldest part of our planet and it's where the rocks you know and love, hang out.

 
 
Molten rock at the surface of the Earth during the 2014/2015 Holuhraun eruption in Iceland.

Beneath that, focusing on the biggest changes, is the mantle. Which is hot (2,000-3,000 C). It's a huge chunk of the planet and roughly 2,900 km (1,800 mi) thick (45% of the distance you need to travel to get through the Earth). Importantly the mantle, though hot, is solid. That is because of the pressure. Remember the volume of the majority of liquids1 is greater than that of the same material as a solid (and gas takes up more room than the same material as a liquid), so under pressure it is more stable to be solid. Think of the CO2 in your soda, it stays part of the liquid until the pressure is low enough - when you take the cap off - for there to be room for it as gas and then you get bubbles. The mantle does melt, but only in specific conditions. And when it does, it is only small (relative to the whole) and then not all of it goes on to feed volcanoes.

At the very center of the Earth is the core, a hot blob of nickel and iron. The core is important for lots planetary awesome, but we'll need another blog to begin to break that down. Part of the core is liquid and that is why we have a magnetosphere, but it is made of nickel and iron, not magma (magma contains a lot of other elements, particularly silica). This is because composition is important to the stability of a material, and thus whether it can be solid, liquid or gas at various pressures. Water freezes at different temperatures depending on how much salt is in it, as an example.

To help wrap our heads around the ‘molten rock under our feet’ concept, here we are going to focus on the place where magma is born, the mantle, and its trip to the surface through the crust.

Earth, contents under pressure.

But even though the crust is thin, it is heavy (I'm mean, it's made of rock!). All that rock, plus the weight of the mantle itself means pressure. Pressure means things can be really hot, but not melt (same way a pressure cooker keeps things from boiling). The pressure is less the closer you get to the surface, but let's calibrate ourselves to what these pressures mean.



Pressure Cookers work by increasing the pressure inside the vessel so the temperature of the food can rise without boiling so it cooks faster (image from Wikinaut user Wikimedia Commons). Inside the Earth rocks are under pressure and will only melt if the temperature is higher or pressure decreases. 

We measure pressures in the Earth using a bunch of SI (Système Internationale or International System) units: bars and pascals. We've talked pascals before in the viscosity blog, but then it was a function of time. Here we are using it on its own as a static measure of pressure. Let’s us look at a pressure measurement we know. In the UK and Europe they use bars and in North America we use PSI (pounds per square inch).

I like to ask my students what pressure they like to see in a standard car tire, and enthusiastic pedantic debates erupt fighting over the 30-35 psi (2-2.5 bars) range. While that is a 15% difference, you will see it is a small amount relative to where we are headed. 30 psi is about 206,843 pascals. That is a lot of digits. So as we get into big pressure measurements we use kilopascals for thousands (207 KPa for that standard car tire), mega pascals for millions (0.2 MPa) and gigapascals for billions of pascals (0.0002 GPa). While these bigger numbers seem ridiculous in tires, you will see why they are needed as we get going.

For more context, 1 bar (100,000 Pa) is about the same as the pressure we feel every day near sea level from our atmosphere. So a car tire is up to 2.5 times your daily atmospheric pressure. Anyone who has been diving may have experienced this sort of pressure. You get 2.5 bars at 20 m under water, which is the very very high end of most standard SCUBA training. That’s because the human body isn’t that fond of being under that much pressure. To get that same pressure with rock, because rock is more dense than water, we only need < 10 m of rock to create the same pressures (not recommended at home).


A comparison of pressures we think about on a human time scale and those under water or under rock.

Ok, so now we get pascals or bars, what does this mean for magma? Magma is produced from that hot solid mantle material in three way: 1) decrease pressure, 2) add water, or 3) add heat. This is where plate tectonics come in. At mid ocean ridges the crust is pulling apart, decreasing the pressure above the mantle allowing melting of the mantle and voila: magma. This is how new ocean crust gets formed. At subduction zones, the subducting slab is adding water from the wet sediments on the sinking crust and minerals that have water in their crystal structure. Finally, the mantle, while solid, is not still. There is lots of heat, and it is unevenly distributed, and while I won’t get into all the awesome about them here, Hot Spots, like the one that forms Hawaii, means hotter material rises through the mantle and helps magma form. These processes happen at 50-250 km beneath the surface, in that solid mantle.

To oversimplify (because rocks have slight variations in density and so pressure is a bit more variable than this) melting happens at pressures of 15-70 bars (1.5-7 MPa). So that is where magma is made, but if it is going to erupt at a volcano, it has to get up to the surface. Because the liquid magma is less dense than the rock around it (by the nature of being a liquid) it will rise.

The more we study magma and its solidified products, we are learning that the melting starts along crystal boundaries as small amounts of liquid in a solid network. As the liquid rises it starts to accumulate into larger blebs2. These blebs then keep rising. However, as the magma blebs rise, they also are cooling as they contact colder surrounding rock. This means there is still solid material within the magma, new crystals. So it is a balance between how much liquid there is, the composition of that magma (and therefore its density relative to the surrounding rock), and the cooling process. There are like 8 future blog posts in that one sentence, so let’s just focus here on the fact that the magma wants to rise, but only as long as it is less dense than the rock around it. The closer the magma gets to the surface, the difference in density (how much less dense the magma is) is less, because crustal rocks are not under all the pressure of the mantle where the magma started and are 
Lots of magma cools in the crust instead of erupting. This image of the dikes around Ship Rock New Mexico are just one of innumerable examples of eroded intrusions (magma that cooled before reaching the surface).

Going back to our original dramatic headline: just because there is molten rock in the crust, this does not mean it will erupt! So where does this magma hang out? Depending on the pathways and relative densities of the magma and the crust we’ve detected magma bodies hanging out from 2-20 km beneath volcanoes. So they are under 280-2,800 bars or 2.8-28 MPa of pressure. That is still a lot of material between the magma and the surface. The magma will also only erupt when conditions are right, like enough melt, and enough pressure to push past that overlying rock. So to ruin all of the fantasies of 6 year olds, you won’t be digging down to any molten magma in your backyard. Which means that those headlines about molten magma are not usually giving you all the details you need to know before worrying. If you want to know more about one of the most discussed magma chambers, you can check very well written blogs and updates on the Caldera Chronicles and other information pages from the USGS.

What might be more comforting, is when magma gets to these shallower depths, we can start to see changes caused by the moving magma like long period earthquakes, deformation of the overlying rock, and disruptions in springs or hydrothermal systems. This means we can look for those signals and detect changes (see all the links at this USGS page explaining different types of volcano monitoring). Magma sitting there in the crust is fine, magma moving requires more attention, but does not always mean an eruption is just around the corner.

Footnote:
1 Water is the wonderful weird exception to this rule. Ice cubes take up less space than the same mass of liquid water, which is why ice cubes float. This is not true of most substances.
2 Blebs is a word we do use, but is not the technical term, it just works well. 

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