Beneath The Volcano: The Magma Chamber
    By Gareth Fabbro | November 5th 2011 02:04 PM | 9 comments | Print | E-mail | Track Comments
    About Gareth

    For those of you who are not geologists, a tuff is a volcanic rock, made up of solidified ash. Hence the pun as my blog title. Actually, my research...

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    What goes on underneath the most deadly volcanoes?  As my PhD investigates the processes that occur below the ground on Santorini, I thought I would try to explain some of what goes on under every active volcano.

    We have long known that many of the Earth's rocks are formed by the solidification of molten rock.  Since James Hutton wrote his 1788 book 'The Theory of Earth', granite has been acknowledged to be formed by the solidification of molten rock.  Since then, large granitic bodies, along with their andesitic and gabbroic equivalents, have been recognised as 'fossil magma chambers'.  Formed deep underground and then uplifted and eroded, they have provided valuable information on their current-day relatives beneath active volcanoes.  Along with the study of volcanic products and more recently geophysical methods they have lead to an understanding of the processes that go on beneath volcanoes, although that understanding is far from complete.

    Reproduction of a Watercolor print done by geologist James Hutton entitled, Detailed East-West Section, Northern Granite, Isle of Arran, Strathclyde or Theory of the Earth.
    "Upon the whole, therefore, whether we shall consider granite as a stratum or as an irregular mass, whether as a collection of several materials, or as the separation of substances which had been mixed, there is sufficient evidence of this body having been consolidated by means of fusion, and in no other manner."  James Hutton.  Picture: Detailed East-West Section, Northern Granite, Isle of Arran, Strathclyde USGS Multimedia Gallery.

    Discussions of magmatic processes often seem a little academic, especially when viewed in isolation.  The relevance to eruptions of many of the questions geologists have asked about the subsurface may not be immediately apparent.  If you want to predict eruptions, however, it is essential to know what goes on at depth.  This post will contain a lot of jargon, but please bear with me.  None of the concepts I will discuss are particularly difficult, and I hope by peeling away at the jargon that obscures them I can set the scene before discussing my own research in a future post.

    Torres del Paine, Chile | Fitzgabbro | Flickr
    The Torres del Paine fossil magma chamber.  The boundary between the granitic magma chamber (the light coloured rock) and the surrounding 'country' rock (dark coloured). Note the fractures, where granitic material has been injected into the country rock.

    Solidified magma chambers, or plutons as they are often referred to by geologists, have provided much of what we know about deep volcanic processes.  One of the classic sites, one that much of the early work on magma chambers to date has been done on, is Skaergaard in Greenland.  This gabbroic intrusion has been rotated slightly, allowing geologist to walk over a slice from what was originally the roof to the floor.  What makes Skaergaard particularly useful for studying is its simplicity; the magma was was emplaced and then left to cool.  None made it to the surface, and no new magma was injected.  Its slow, tranquil construction allowed the processes taking place to be recorded.  In the case of Skaergaard, the dominant process was fractional crystallisation.

    Magma, like its solid counterpart (rock) is chemically very complex.  Its biggest chemical constituent (usually) is silica, or SiO2.  Other important elements include iron, magnesium, aluminium, calcium, sodium and potassium.  There are also dozens of others, and current technology can measure traces of them of only a few parts per million (ppm).  These make up various minerals (such as quartz, olivine and feldspars), which in turn make up the rock.

    Certain minerals, such as olivine, have a higher melting point, so as the magma cools these crystallise first.  The same thing can happen if you leave a bottle of wine in the freezer for too long.  The water will freeze at 0ºC, but the alcohol will stay liquid much longer.  This is slightly complicated in magmas, as each mineral is a mixture of chemicals.  However just as the wine becomes more alcoholic as more water freezes, the melt in a magma chamber becomes more silicic as more crystals form.  The first crystal to solidify are usually low in silica and high in iron and magnesium.

    This is just the first part of the story.  If the crystals just stayed floating in the magma, its overall composition wouldn't change.  If, however, the crystals are separated from the melt, we would see it 'evolve'*.  Separating the melt from the crystals is known as fractionation, hence 'fractional' crystallisation.  This can be done in many ways.  The crystals themselves can be removed, either by sinking to the bottom or floating to the top of the chamber (depending on their density).  On Skye, in Scotland, an old, cold magma chamber has been exposed at the surface.  Here many sedimentary structures
    such as ripples are seen in the crystals along the bottom of the chamber.  These show that not only did the crystals settle out of the magma, but that they formed flows and currents along the bottom.

    The other way of fractionating magma is fix the crystals and then remove the melt.  Crystals often form on the edges of the chamber, where it is cooler and the crystals have a surface to grow on.  Sometimes the growing crystals form a rigid framework.  The evolved melt can then be extracted, leaving the crystals behind.  In Skaergaard the melt was never removed, but we can still see the effect of fractional crystallisation.  Crystallisation started at the outside and worked its way in, so now as we do the same we can recreate its chemical evolution.

    The Coulins on the Isle of Skye, a fossil magma chamber
    The Coulins, on the Isle of Skye, Scotland.  Like Skaergaard they are gabbros formed by the slow solidification of a magma chamber.

    When the mantle melts it usually produces a basalt, a type if rock/melt low in silica.  Mantle melting is the most common source for the magma at volcanoes (although there are others).  Through fractionation, these basalts become first an intermediate (intermediate silica content) andesite, then an evolved, high silica rhyolite.  While there are many, many names for rocks of slightly different compositions, generally most differences in behaviour of magma come from how much silica it contains.  There are also many names for describing how evolved a magma is, which although they have subtly different meanings.  Basalts are variously described as basic, primitive or mafic, while rhyolites can be acidic, evolved or silicic.  Another confusion can arise because gabbro is chemically the same as basalt, just with larger crystals.  Granite is the large crystal (coarse-grained) version of rhyolite.

    Crystal fractionation is not the only process to affect the composition of the melt.  The heat will melt the surrounding (or country) rock, which will contaminate the magma.  This assimilation has much the same effect as fractional crystallisation.  The first minerals in the crust to melt, those with the lowest melting point, will be the same type as the last ones to crystallise from the magma.  These added melts (crustal contaminants) will drive the chemistry of the magma towards more silica-rich compositions.

    Because of the similarity in the effects of these two processes there was for a long time arguments over which process produced the most evolved magmas, and most destructive eruptions.  While we have seen that partial melting and fractional crystallisation can effect the chemical composition of the melts produced, different isotopes of the same element should behave the same during these processes.  The ratio of isotopes like 16O/18O (that is oxygen with atomic weights of 16 and 18 respectively), or 86Sr/87Sr (strontium) should stay the same.  By measuring the isotopic composition of both the crust near the volcano and primitive, unfractionated magmas, we can calculate how much of the erupted rhyolites were produced by fractionation/crustal melting.  For almost all volcanoes fractionation is found to be much more important.  There are, however, many large granite bodies around the world formed from crustal melts, but these melts rarely seem to make it to the surface.

    So why does all this matter?  The most obvious reason is that the composition, and mainly the silica content, has a large impact on the eruptive style.  Basaltic eruptions tend to be rather gentle, think Hawaii or Stromboli.  Silicic magmas are more sticky or viscous, and trap volcanic gases.  This leads to explosive eruptions, like Mt St Helens and Pinatubo.  Understanding how these explosive magmas are created, especially in the huge volumes found in the largest eruptions (Yellowstone, Fish Canyon, Taupo) will help us predict where and when the next one will blow.  The processes I've mentioned here have been pretty well studied, and now make up the starting point for any studies on a volcano.  In a future post I will write about some of the more recent developments and open questions about magma chambers, mainly how fast are they constructed and how long do they last.  Traditionally it has been assumed that the largest chambest must have taken hundreds of thousands of years to build, but recent results have begun to change that view.  My supervisor has a paper in press about this, so as soon as it is published I will write something about it here.

    *Note that in this context the word evolution has very little to do with Darwin's theory of natural selection.


    Dammit. want to go to Skaergaard. And the Orkneys. And the Isle of Skye. And other cool places like that

    Hi Gareth,

    There has been a lot of buzz around Uturuncu in the past several weeks. Unfortunately, an awful lot of that buzz is generated by fringe and pseudoscience sites and it is hard and at times frustrating when trying to learn about the science and natural processes that are at work. This is one of the rarest and most interesting learning opportunities of a lifetime, and it is occurring on the planet now and I want to take this opportunity to learn more about it.

    I'm glad that you provided the explanation above and, if you wouldn't mind, using the template of the measurable characteristics that you describe above, can you describe the magmatic processes that are occurring at Uturuncu?

    Here are some questions that I have about it:
    What are its chemical characteristics? Since it is in the Andes, is it a result of the plate subduction magmatic processes that formed that range of mountains? Why is it so big (40+ miles in diameter, if I read it right)?, Is the magma chamber new or is it an old magma chamber that is being refilled? Why is its fill-rate (a cubic meter per second) so different from others? (I read that it is filling several times faster than what is expected under other volcanoes). I suppose I can throw a in really broad question and ask, why is Uturuncu of so much interest to the geological community?

    Thanks in advance.

    Because of its remote location (and lack of recent eruptions), not a huge amount is known about Uturuncu.  Also, I have only read a little of what has been done, but I will try and answer your questions.  The Andes did form due to plate subduction, but it is more the compression that has pushed them so high rather than being built up of volcanic rocks.  Chemically, the lavas that currently make up the volcano are andesites and dacites (dacites are more silicic than andesites, but not as silicic as rhyolites).  These would have needed a magma chamber to form, and they did so by a mixture of fractional crystallisation and assimilation.  Whether this new magma (if it is new magma, there are other possibilities but new magma is the most likely in this case) is being intruded into this chamber or a new one we can't tell.  The last eruption was 271,000 years ago, meaning a lot of time for producing a lot of high-silica magma.  I can't find right now how big, but 40 miles diameter doesn't sound unreasonable.  Why some volcanoes grow such large magma chambers, rather than erupt more often, is one of the big questions in vulcanology at the moment.

    The reason there is a fair bit of geological interest is that all this suggests that it might be preparing for something big.  Nothing world-ending, but perhaps the formation of a caldera and some large ignimbrites.  These sort of eruptions don't happen very often, but when they do they can have a large impact.  The remoteness of Uturuncu should limit that impact while still giving us an insight into the processes that go on before such an eruption.  That is if something does happen, it may end up doing nothing...

    If you want to read more, Erik Klemetti over on Eruptions Blog is running a Q&A with Dr. Shanaka de Silva, who has done work on Uturuncu.  Unfortunately he has currently closed submission of questions, but has yet to post the answers, but when he does it should be informative.

    I am grateful to you for taking a little time to answer my questions. I get tired quickly by the seemingly endless stream of prophecy/doomsday/alarmist sites that purport to have some special knowledge of preordained catastrophes. Feh.

    The Uturuncu problem does sound exciting and, I'd be lying if I said that it wasn't just a little frightening. However, I live in Georgia, where three of our large granite outcroppings are major tourist attractions. If I remember correctly, our Stone Mountain is a large granite pluton that happens to be exposed, and, like an iceberg, extends for several miles underground. Would it be appropriate to say that the magma chamber under Uturuncu is something like a rather large pluton, just unexposed (and, of course, molten)?


    A little.  When magma chambers under volcanoes cool and solidify they do leave behind plutons, but many plutons (especially granite ones) form without ever reaching the surface and so have no volcanoes associated with them.  I don't know which category the plutons in Georgia fall into, but there isn't a huge difference between the two especially on the large scale.  Something that is worth bearing in mind though is that most plutons were not completely molten all at once.  The magma chamber under Uturuncu (if one still exists) that the new magma is intruding into is likely to be more solid than liquid, and a sort of crystal "mush".  I am currently in the middle of writing a new post on this sort of thing in more detail (part two of the post above) that I hope to finish before the end of the week, so do come back if you're interested.
    Indeed. I look forward to reading more about how "the world" literally works.

    (The mechanics of how volcanoes work just keeps getting more interesting).

    Thanks again,

    Thanks for a very understandable introduction to our outer inner Earth. :-)

    You wouldn't happen to be going here next week, will you?

    I am right now writing about what I call the Fantastic Four - earthquakes, tsunamis, volcanoes and landslides. The extreme versions of these geohazards to be more precise. Your 'baby', the Santorini volcanic eruption belong to one of the most dramatic events.

    I will link to some of your stuff in my coming article on the subject of geohazards. :-)

    Bente Lilja Bye is the author of Lilja - A bouquet of stories about the Earth
    I read your introduction post, that conference does sound interesting.  I won't be there, unfortunately, I don't really work on volcanic hazards directly.

    I look forward to reading some more of your posts on the conference.
    Very much enjoy your article and the posts here...thank you!!!