El Hierro and the Physics of magma chambers

Image from Nature GeoScience. From Phillip A. Allens article Geodynamics: Surface impact of mantle processes.

Part 1

Not many people think about what is great with physics. People are normally more occupied with buying Prada hand-bags to carry their rat-sized yapp-dogs than physics. The great thing with physics is that the laws of nature are universal. And with that I mean that they can be transferred easily from the school books into real life, and from one part of science into another.

I am as most of you know not a volcanologist or a geologist, but I am a physicist. So every time I try to understand a volcano I do it from how it is behaving from the point of perspective of the laws of nature.

This time I would like to write about a few things regarding how magma chambers must be formed according to physics. I will mainly not talk about magma chambers because they are fairly hard to visualize since nobody has seen one in real life as it is forming. But most of us have for instance blown up a balloon.

In this case we will be talking about magma chambers that come from hotspot volcanism; the process will be slightly different in a subduction volcano. But first we need some background, this post will be about precisely that background.

Hotspots, weightlessness and Blobs

Let us start with what is required for a magma chamber to even start forming. And as a physicist I am always talking about basic forces. And there is only one basic force, and that is energy. There are of course many types of energy, and in this case we are talking about energy as mechanical pressure and heat.

Thankfully for the poor fledgling magma chamber there is one thing that causes both pressure and heat, and that is your basic magma. So, let us drop up a ball of nice hot juicy magma from the hotspot under El Hierro.

It is not entirely clear how magma travels upwards via a hotspot, but we know there are two types of hotspots. First we have the deep Icelandic type that brings up material from the depth, this magma is hot and arrives at high (relatively) speed and with great force. It brings with it an assortment of rare and heavy metals from deep down at the boundary between the core and the mantle. The other type is a colder and less deep hotspot. The magma here is either brought up from within the mantle, or created as the hotspot heats up material close to the MOHO boundary either through heat or pressure, perhaps even a mixture between them. This type creates magma that is low in precious metals, and gives a low Uranium-Thorium (UrTh) count which in turn is a dead giveaway that it comes from a shallow source. The Canarian hotspot seems to end up somewhere in the middle of these two types, it is definitely not melting crust as a part of the magma creation, the almost pure basic basalt tells us that, on the other hand it is not from the core/mantle boundary since the UrTh count is wrong for that option. Let it suffice to say that the Canarian hotspot is a bastard mongrel of a hotspot.

So, where does now the pressure to drive any hotspot come from? Well, once again the answer is not simple. We have at least two sources. The first is heat; the Earth is producing loads of juicy heat due to at least 3 different processes. The first one is UrTh and other atomic nuclear processes. Yepp, we live on an atomic reactor. The second one a form of pressure called overburden pressure. That is the combined weight of the planet pushing downwards, this creates compression heat. The third is through the dear old gravity slowly massaging the planet, this is by far the smallest of powers creating the heat. Here I have simplified a bit, there are more forces at play than this.

Image of nested magma.

So, how come then that magma travels upwards? The answer might surprise you a lot. If you are getting a headache from this it is normal. Let us imagine that you where hanging at the exact mid-spot of the planet. The pressure would be phenomenal from the overburden pressure; still you would notice something odd. For the first time in your life you would be completely weightless. This would be due to the entire planets gravitational pull would be affecting your entire body in every direction at the same time, effectively cancelling out any gravitational effect.

What does this now have to do with magma? Well, you have magma under tremendous pressure that does not weigh a lot. A cubic decimeter of magma at the mantle/core boundary is considerably more lightweight than the same volume of water. And at the same time it is squeezed by tremendous pressure.  Here we enter a nice little simple physics, when you squeeze a fluid it will try to run away, in this case it can’t go down, it is fairly buoyant and will try to float. Now we just need one small thing, a conduit. Enter the heat.

Energy will always go from a high state to a lower state; this is the nutty little physics law that also gives that order will always go towards disorder, in other words, entropy and enthalpy. So, the core will try to lose heat, and the heat will always be able to escape, and once a convective current of heat has started to run upwards it will jolly well keep on going. When magma finds a stream of heat going upwards it will follow that stream because the fluid will follow the point of least resistance. And that is why a mantle plume and a hotspot is the same thing (simple physics). The mantle plume cannot exist without a hotspot, and the hotspot will sooner or later create the mantle plume.

Now our blob of magma is finally moving upwards towards El Hierro, the trip started a long time ago, it takes a while to go through all that semi-permeable heated pipe that runs up through the mantle. One day, let us say on the 24^th of June 2012 our blob of magma arrives at the bottom of the crust under El Hierro.

The speed with which it arrives is very slow even compared to a human walking, but the weight is enormous, the same goes for the amount of heat energy and the buoyancy pressure. Let us just say that it is like a comet sized blow-torch hitting the almost melted MOHO boundary. It will cut through the first layers in a rather short time. As it goes on up through the bottom of the crust it decelerates fairly quickly, and that is the point where all the fun starts, the formation of the magma chamber.

Until the next time!

CARL

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