An introduction to igneous rocks – Part 2

Picture of Mica from


Of which there are two predominant varieties in igneous rocks – Biotite and Muscovite – are a group of phyllosilicates; the name in allusion to the method and ease of perfect cleavage of the mineral into exceedingly thin laminae. They are characteristic of a wide range of rock type. Biotite, the mafic variety, is found in anything from Basalt to Rhyolite, whilst Muscovite is very unusual in volcanic rock, being more common in intrusive igneous and metamorphic rocks.

Olivine Group

Olivine is the general term applied to a group of minerals of another infinitely variable continuous substitution series, of magnesium and iron silicates, the end members being Forsterite (magnesium Mg2SiO4) and Fayalite (iron Fe2SiO4). Forsterite has a melting point of 1890degC, Fayalite 1205degC, thus the temperature of the melt can be roughly determined by the composition of the olivine in the sample. It may be found that phenocrysts – crystals much larger than the ground mass of the rock – have a higher magnesium content than those of the ground mass and indicates crysyallisation began much earlier than at the time of emplacement. Zonation, where crystals have a Mg rich ‘core’ and have increasing Fe content towards the rim, also occur and is a similar melt condition/change indicator. Olivines are usually found in Basic and Ultrabasic lavas; Peridotites (Peridot an alternative name of olivine), are rocks predominantly composed of olivine and are assumed to be Mantle derived, occurring as rare lavas and as xenoliths in basalts – ie a fragment of the deeper mantle torn off by and ejected with the rising magma.


Excess silica in the form of free Quartz (SiO2), primarily occurs in Rhyolite in the acid division and in Rhyodacite and Trachyte, intermediary between Rhyolite and Andesite (Intermediate).

Metal oxides occur mainly in the Basic/Ultra-basic divisions, with Magnetite the most important (the presence of Magnetite in Basic rocks commonly leads to magnetic anomalies affecting compass bearings); titaniferousmagnetite and Chromite are found as density segregations in some very basic and ultra-basic flows, but usually are more economically important in intrusive complexes.

Reaction Series

This brings us to Bowen’s Reaction Series whereby in a magma cooling from say 2000°C, different minerals fractionally crystallise at gradually lower temperatures – comparable in a way to the ‘cat crackers’ used in hydrocarbon refining vessels where different hydrocarbons are distilled off at different temperatures, To simplify, if the differing minerals crystallise they remove from the melt their components and the melt chemistry changes continually, but if these minerals stay available in the melt for resorption, the mineralogy of the melt changes to a different mineral suite with various component minerals coming out of the melt as their solidification temperature is met, until such time as total melt crystallisation occurs, ie the melt solidifies. Two separate convergent lines of melt alteration, one mafic the other felsic occur thus:

First crystallisation       Olivine                                                                                     Bytownite

Mg Pyroxene                                                                   Labradorite

CaMg Pyroxene                                                        Andesine

Amphibole                                                         Oligoclase

Biotite                                                          Albite

K Feldspar                                          Muscovite

Last crystallisation                                                           Quartz

It is obvious that in coarse grained rocks the individual minerals are relatively easy to identify, but in fine grain volcanic material a thin section is invariably required. A thin slice is polished on one side, mounted on a microscope slide with canada balsam – a natural resin with the same refractive index as glass – and the slice is lapped down to 30 microns (0.0030mm). The section is then studied under natural and plane polarised light to identify the minerals present by their optical properties.

Picture from Basalt thin section.

In conclusion

Image by Igneous classification.

Finally, one for the ladies, a 4.2 carat Peridot gemstone – the common olivine in a better guise!!



The proud Author doing something rather Scottish to a lot of mud.

233 thoughts on “An introduction to igneous rocks – Part 2

  1. 1 hour for Margalef arrives at sea of Las Calmas.

    Fecha: 7/2/2012 09:10:47
    Latitud: 27.686563
    Longitud: -17.722362
    Speed: 10.1
    Course: 237.9

  2. I had 3 proposals for volcanoes in this period can be broken out, and i prefer the medium-sized Link! All areas are not well researched and i think this is a part of the earthquakes in these regions evt. volcanic nature. Whether Korea in question is coming, that you can better assess. Perhaps there are in your countries more about these volcanoes.

    • The Korean volcano is a bit of a trouble, if we ever thought that getting info on Bob was trouble, then a north Korean volcano is like a naked singularity. Information will just fall into the volcano, probably together with a couple of thousand North Korean dissidents…

  3. Good morning everyone. It’s taken me an hour to read and digest the comments from yesterday. A lovely start to this cold sunny day. I have so many comments but I do worry about those who are worried and fearful. Life is short and too much time feeling sad does not help anyone. Here are my ruminations over my 2nd coffee of the morning.

    @ Judith and others who worry greatly about climate change. Where I live, 9000 years ago the climate was like the Mediterranean of today. Then by The iron age (about 3000 years ago ) The Great British Climate had arrived, very wet, Very much colder, and the locals became dour and depressed, painted themselves Blue with woad as they were so used to their skin going blue with cold. When it got warmer in the summer and their skin turned pink and tanned they worried that they had lost their colour!! 😀

    Nobody knows exactly WHY the climate changed. The mesolithic hunters, the bronze age farmers have all been blamed for land clearance and adding CO2 to the atmosphere. Personally I think these huge huge changes were from other triggers, Maybe the sun, maybe the the world’s movement, maybe even Volcanoes. My mind is open.
    What I believe is that our world’s climates change mostly due to things that are out of our control. We must live with it.
    The last 60 years (My lifetime) has produced information Technology so rapid we are all having to adjust. Most of us are suffering from information overload..
    We know so much more about the happenings of the world as it happens, that we are left with a feeling of helplessness. So many Disasters, droughts, floods, famines.. The lucky ones who are buffered from these horrors feel helpless and worry.
    My advice to you who worry, do what we did before TV, before instant phone systems, Care firstly for those who live with you or around your neighbourhood. Those people are in as much need for your care as those thousands of miles away.
    I care for my garden, the wildlife, I grow too many vegetables or apples, I see that some elderly neighbour could benefit from my produce, jams and berry picking.So the land is cared for and so are humans in need.
    BY looking to your little patch of Earth, you are educating others to care for their neighbours and environment. If there is then a time of need, your neighbours and their children will know what to do to help each other.
    I would like to see all governments caring for their people’s welfare. This is a dream of world peace. Unfortunately world peace is as complicated as Volcanoes. Each country physically different. Each country a different human culture. Each Country highly unpredictable.

    Good grief! Look how difficult it is to produce a United Europe with all our history, commerce, great inventions and at times world domination!

    Just be optimistic. That is the only way to stay sane!!!

    Sorry to bore people. I didn’t expect to write so much.

    @Tyler. Come on in, even when you feel very Low. Many others here, myself included, have suffered as you are doing right now. Trust me, that big Blackness can eventually go away, but it takes time.

    • Very sensible advice, Diana! Incidentally, even if we are experiencing global warming then it is not as fast as the end of the last Ice Age. This happened in the blink of an eye geologically and archaeologically speaking – probably over 100 years. Think of that: your grand-parents live as their ancestors had lived for thousands of years, probably by a coast with occasional forays inland for hunting. Your parents survived the the warming but you have had to flee Doggerland and literally “head for the hills”, you have no points of reference, half your family went the other way 50 years ago and now there’s a sea between you. What to do? Get on with it!

      • I’ve looked up the Anglo-Saxon Chronicle. There was ‘the severe winter’ in 761 but no other special mention of weather for the 10 years either side of 763. That winter a bishop was consecrated so no problems travelling at that time. Hard to be definite but it looks like it was not a north-west European problem.

      • I am actually more surprised about how many people are kind. There is after all no real genetic or moral imperative saying that we should be kind.
        So my take is that we are on general a rather friendly bunch that sometimes do bad things. Of course not counting the rare real crasies out there.

        So please go out and kick somebody in the butt to celebrate the International Foot Awareness Day 🙂

        • Good leads to good. Peace and love my swedish friend.

          It’s slippery outside – dangerous to oneself trying to kick some strangers butt…

        • As I understand it – being all nice and kind has it’s evolutionary benefits – especially if you’re only being kind to people who have the genes you like (which according to your genes are more likely to be those folk who have genes in common with you).

          Also in a population of doves and hawks it is suggested that game theory ‘tit for tat’ is generally a winning evolutionary strategy – in that you will on average waste less energy fighting than hawks, and concede your valuables less often than doves.

      • Of course humans are kind! They even call them humankind! 😉 😛

        Far better for your mental health and for others to consider everyone you meet is kind until proved otherwise. So many really are or is it just that Diana and I are a little older than some on here and either remember when people were kinder or else people are kinder to us because we are older. DOH! Not too sure I understand what I wrote myself but I do know what I mean.

  4. Usually the action at CHIE suddenly starts and then returns slowly within some hours to low or almost no tremor levels. But this time it is different, the bursts started an 3am and increased at least until 9:30. Is this a new kind of activity?
    There is nothing special to see on the cams, only a faint stain……

    • Yesterday, a scientist at avcan Facebook a bit of 3 phase of the outbreak and the written in the north in the sea evt. deep Ausbruchstellen are. That is why is probably also the research vessel Magelef again on the spot and Carlos already waiting for the next results of the Bathymetry. Today we are also stormy sea, since you can not see much on the surface.

      • Ok, now it returns to normal stats. The last signal i saw when i wrote that first comment was the burst on 9:20 and i thoght ‘here we go, thats new’! But actually it’s the normal fluctuation of Bob’s activity.


    A. G.
    “It is not acceptable that the Teneguía volcano was in 1971 much better studied at the time that this [of El Hierro] forty years later with all the technical advances, and even with all the impediments that has studied a submarine volcano, which is more complex”. Thus of resounding yesterday showed the Professor of Petrology and geochemistry of the Gran Canaria University, Francisco José Perez Torrado, questioned on the timeliness of the Hierro phenomenon. “As I am not a magician, I cannot say when the volcano in El Hierro will cease its activity.” “Day of today, with current scientific knowledge, or you can predict when begin or when just a rash”. Torrado, at the same time, underlined that the volcano “has surpassed the usual expectations of duration in the historical record of eruptions in the Islands”.

    “The volcano is over the account, but it will take what touch you last, obviously, but in the sense that is beyond our initial expectations”. He added. “The historical record [of volcanoes] Canary is five hundred years;” in that time we have had fourteen eruptions and, except that of Timanfaya, which was long, others have been shorter than that of iron. That is why, in principle all gave you a month or two, but track three is on. “Course that we are based on a reference merely statistics.”

    “Already know” added Perez torrado, “How was the process: on 19 July began the seismic crisis, that it was clear that was linked to a rise of magma, and three days if had asked us if it was going to start the eruption in rigor we should have said that we did not know, because the odds were the same as the of to magma stay inside”, all the seismicity is fifteen kilometers deep [at sea] “.” However, he added, “if we had done again this question 8 and 9 October, there was a magnitude 4.4 earthquake, we would have said that the chances of eruption were high but we could never have said”will start tomorrow”.”

    The tremor continues

    With regard to the fact that the stain of eruptive materials has returned to reach the port of La Restinga, that continues the expulsion of pyroclastic surface, Perez Torrado wished to recall that “activity never has been, only that the intensity varies.” The pity is that we have never had a direct image of what is happening beyond below, we are not direct witnessing what happened, everything what we have deduced morphometrics and seismicity, but in principle the tremor not removed, is fixed: as it is the unique data, because do not give us another, unfortunately have not lowered cameras to look at or anything”, the only thing that can be said is that the eruption takes from October 10 to date, with periods of greater or lesser intensity, greater or lesser expulsion of pyroclastic and lava”. In his opinion, “it is true that the tremor has been easing and seemed that it was going to end, but the tremor measured only the noise of the duct [which moves the magma] but also the noise may be gas so strictly until magma does not stop to emerge on the surface not can say that the volcano has concluded.” “However, in the Teneguía is the day released the magma and the day which ended [terrestrial volcano which erupted in La Palma in 1971], but it does not”.

    Limited data

    Questioned about the number and position of different mouths underwater volcano, above which there are variety of opinions, Perez Torrado complained of “the data” that “we have been offered” the research vessel Ramon Margalef, of the Spanish Institute of Oceanography (IEO), prominent in the area to study the seismic phenomenon. “The data has taken in dribs and drabs and in three months has only made four campaigns morphometric, having been until an evacuation of the island of means”, said. And with their results “almost one could say anything.” It taught reveals that there is a main volcano and one of the morfometrías noted he had small adventitious cones, i.e. an eruption fisural – as they are all in the Canary Islands – with several mouths, which sometimes have been ejecting magma, but for spots that are seen in surface in any case we can deduce their numbertherefore does not reflect. “Input currents inevitably displace and divide that goes 15 kilometers deep.”

    Torrado here he stressed that unlike what happens with the prediction, today in day impossible, start and end of a volcanic activity, Yes is possible to determine the number and location of the mouths of a submarine volcano. “It’s as simple as downloading a robot with a video camera.” If it has not done is something that should be preguntárserle to the IEO, which fell the first time a robot, and hung on your Web site with great advertising videos

    • Would be nice, if you could post the direct link, when you post something. It is for many of us certainly easier to read the original than the Bing or Giggle translations – or sometimes even possible to make a better translation (esp. Carlos, Judith, Kiese W..

    • I’m missing something here. Why would any scientist assume that El Hierro would behave the same way as other Canary Island volcanoes – when it is known that it is at a different stage in its development & the last known eruption was ~130,000 years ago? This is like saying that all volcanoes in Iceland behave in the same way.

      • KarenZ, there was most likely one in 1793 with a Bob outside of La Restinga and some activity on the northwestern end of the Island.
        Then you have two uncertain ones and then one radiocarbon dated eruption at 550BC. And then two more radiocarbon eruptions, one a central vent, and one a flank eruption of Tanganasoga.
        So, I got a bit curious about 130 000 years?

        And then we have the high likelyhood of a lot of Bob happening out on the La Restinga volcanic line.

  6. Thanks for the updates on Bob. Alan, great second part of igneous rock 101!

    I’ve been spreading my attention, must also closely watching some other web cams and keep up with the latest news for some articles I need to write. I admit, I have “Eleven Cities Fever”. There’s this chance the 200 kilometer Eleven Cities Tour may be held just after the weekend, if ‘Fryslân’ will keep freezing conditions – the big “if”… Complete frenzy in the Netherlands. So here’s another webcam on which you can see those typical “dutch winter landscape scenes with skaters” that you all know from the famous paintings by the Dutch Masters from the 17th century…

  7. Ursula, could you do an RSS feed out of this one.

    The owner asked if we could do an exchange, and I think it would be a nice thing to do.

  8. UPA!
    Ramon Margalef at horizon I suppose!!!
    And I just witnessed a nice Bob’s burp that creates a nice wave 😀

    • Thank you so much for posting that criseh, that surely is one of my most favourite songs with lovely lyrics. Set to such beautiful scens too makes it really wonderful. Again, many thanks.

Leave a Reply

Please log in using one of these methods to post your comment: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s