Historical Geology/Igneous rocks
In this article we shall look at igneous rocks: what they are and how they are classified.
Intrusive and extrusive igneous rocks
Igneous rocks are rocks formed by the cooling and solidification of molten rock. They fall into two main categories:
- Intrusive rocks are those which are caused by the cooling of molten rock underground. Subterranean molten rock is known as magma.
- Extrusive rocks are those formed from molten rock on the surface, which is known as lava.
Igneous rocks can be further be identified and classified by their texture and their chemistry, as will be described in the following two sections of this article.
It is a universal law that fast crystallization makes small crystals and slow crystallization makes large crystals. This is because crystallization is a kinetic process: for a molecule to join onto a crystal it must bump into it and then align with it.
The thermal properties of rock are such that magma cooling underground will cool slowly as compared to lava cooling above ground. Hence, by looking at the texture of the rock, we can find out how it cooled: an intrusive rock will be coarse-grained; an extrusive rock will be fine-grained.
Sometimes lava is ejected from a volcano with such force that it goes shooting high up into the air, causing it to solidify so quickly that it doesn't have time for crystals to form at all, making an amorphous solid known as a glass. The glass in windows is an artificial glass produced by the rapid cooling of molten silica; examples of natural glasses are obsidian and pumice.
Occasionally magma will begin to cool below the surface and than be ejected on to the surface; in this case it will have a porphyritic texture, with a few larger crystals (phenocrysts) embedded in a finer-grained ground mass.
The simplest way to classify the chemistry of igneous rocks is by the amount of silica they contain.
An igneous rock with a high silica content is said to be felsic, and an igneous rock which is low in silica is said to be mafic. You will recall that these are the same terms used for high-silica and low-silica minerals; and in fact it is the case that felsic rocks will contain felsic minerals and mafic rocks will contain mafic minerals.
Classifying rocks by their silica content is convenient because typically the chemistry of igneous rocks lies on a continuum such that if you know the proportion of silica in an igneous rock, you can say what minerals it contains. The rules for doing so can be represented by the diagram to the right. Note that this applies only to igneous rocks, and not to sedimentary or metamorphic rocks.
To read the diagram, look along the bottom of the graph for the silica content of the rock: then a line drawn directly upwards from that point cuts through the minerals it will contain in their relative proportions. So, for example, if we tell you that a certain rock contains 50% silica, then you can see from the chart that it contains about 5% olivine, 75% pyroxene, and the remaining 20% will be calcium-rich plagioclase feldspar.
This diagram divides the rock types into fairly coarse divisions. It is possible to make finer distinctions: we could, for example, have put granodiorite between granite and diorite, as a rock type having a silica content lying between granite and diorite; or we could have placed dunnite to the right of peridotite, to denote those rocks which consist of pure olivine. The divisions we have proposed are, however, sufficient for our present purposes. It is more important that the reader realizes that whatever divisions we impose on the diagram, they are arbitrary: there is a continuum between felsic and ultramafic rocks.
Also, as we look along the continuum from felsic to ultramafic, the rocks are progressively denser; they have a higher melting point; and they have a less viscous flow when molten. This is the same progression as we see as we pass from felsic to ultramafic minerals, and is a natural consequence of the fact that felsic rocks consist of felsic minerals and mafic rocks of mafic minerals.
We should perhaps add a note on the presence of komatiite (extrusive ultramafic rock) in our diagram, as some textbooks omit it entirely from such diagrams. Komatiite is never observed forming today: as ultramafic magma rises from the hot interior of the Earth to its cool surface, it will fall below its melting point before it gets near to the surface, forming peridotite, komatiite's intrusive counterpart. Consequently komatiite is found only in rocks dated to over 2.5 billion years ago, consistent with geologists' belief that the Earth was hotter at that time.
The diagram to the right shows some of the structures formed by igneous rocks. The black represents igneous rock; the other colors represent sedimentary rocks.
As this is a cutaway diagram, it may be slightly misleading. The reader should bear in mind that a fissure is a crack in the surface; we have shown it end-on. Similarly, the lava flow which emerges from a fissure will be a sheet of lava; and a dike is not a spike of rock, but a vertical or near-vertical sheet of rock. And a sill, again, is a horizontal sheet of rock.
That last statement needs a little qualification. In the diagram, we have shown the layers of rock lying flat, except around the lacolith (item (6) on the diagram) and so we have shown the sills as horizontal structures. However, layers of rock can be folded by tectonic activity. When a sill intrudes into rocks like this, it intrudes between the layers of rock (this is the definition of a sill) and so will itself be contorted.
We shall have more to say about igneous structures when we consider stratigraphy and cross-cutting relationships, but for now this brief introduction is sufficient.
How do we know?
How do we know that igneous rocks are igneous? Like everything else in geology, this had to be proved at some point: indeed, there was once a body of thought known as "Neptunism" which asserted (amongst other things) that granite was sedimentary.
In the case of extrusive rocks, the answer is obvious: we can see basalt (for example) forming when lava flows cool: so it certainly can form as an extrusive rock. But it could not also form as an intrusive igneous rock, because under such circumstances, being thermally insulated, it could not cool quickly enough to produce a fine-grained structure, and the physics of the situation would dictate the formation of gabbro instead.
Since we can actually watch the formation of basalt, we can make further deductions about it. When basalt cools underwater (as observed by divers), it forms the distinctive shapes known as pillow basalt, which is not the case when it is observed forming on dry land. This criterion allows us to distinguish between basalt formed on land and on the sea floor; a deduction confirmed by the association of pillow basalt with marine sedimentary rocks.
But what about intrusive rocks? Take granite, for example, since it is the commonest intrusive igneous rock. If we are absolutely right about how it forms, we should never see it forming. So how do we know how it forms?
As a matter of fact, the fact that we never see it forming is one of the predictions of the theory that it is an intrusive igneous rock, and so tends to confirm the theory. We do not see granite or granite-like sediment forming by surface processes; what else can we conclude but that it is formed underground?
In the second place, as we have observed, granite has the same chemical composition as rhyolite, differing from it only in its texture. Now, as we know that larger crystals form when cooling is slower, and as the thermal properties of rock as opposed to air or water will lead to slower cooling underground, we must conclude that granite is exactly what we should expect to see if the magma that forms rhyolite when extruded onto the surface was to cool below the surface instead.
A close look at its texture through a microscope confirms the igneous nature of its formation. The picture to the right is a photomicrograph of granite. Note how the crystals, however bizarre their shape, fit together perfectly. We may compare this with the texture of sedimentary rocks such as sandstone, which are clearly made of non-interlocking particles cemented together.
Then we may consider the structures formed by intrusive rocks. It is difficult to see how something such as a dike, which, as explained above, is a vertical or near-vertical sheet of rock, could form by any process except the intrusion of magma into a crack in pre-existing rocks.
Finally, we may note that the rocks into which granite intrudes are typically changed in ways we would expect if they had been subjected to great heat; for example, when granite intrudes through a layer of limestone, the limestone immediately adjacent to the granite will be turned to marble. This suggests that the granite was itself once at a high temperature and has subsequently cooled, consistent with the theory that it is an intrusive igneous rock.
For these reasons, we may conclude that granite is an intrusive igneous rock; similar remarks might be made about the other rocks classified as igneous intrusive.
Note on vocabulary
Igneous rocks are sometimes called primary rocks, extrusive rocks are sometimes called volcanic rocks, and intrusive rocks are sometimes called plutonic rocks. We shall not use these terms in this text, and mention this only for the benefit of those readers who wish to pursue a course of further reading.
The rocks that we have described as fine-grained and coarse-grained are also known by the terms aphanitic and phaneritic respectively. These terms are rather commonly used by geologists, but I shall stick to the more self-explanatory terms.
Finally, just as silicate minerals are sometimes referred to (erroneously) as "acidic", "basic" and "ultrabasic" rather than felsic, mafic, and ultramafic, the same is true of igneous rocks; as in the case of minerals, I do not intend to use these terms, as they are obsolete and misleading.