Igneous Rocks

Lava (Hawaii, Wikimedia)
Igneous rocks have solidified from a molten state that we call magma. There are quite a few types of magma, made of different proportions of chemicals and with different minerals. These different magmas have different properties we will look at later. When magma comes to the surface and is erupted, it is called lava.

A bit about magma

What is magma? Magma is liquid (molten, or melted) rock. Some magmas are completely liquid, but most contain some mineral crystals in a suspension.
Where do you find magma? Many people think that the whole of the inside of the earth consists of liquid magma, but in fact the crust and mantle are both more or less solid -(the mantle can flow slowly liky silly-putty, but is not liquid or molten). However, the lower crust and parts of the mantle are often very close to their melting point and small changes in temperature, pressure or chemical composition can cause them to partly melt. The liquid produced then moves to become a body of magma. If it is less dense than the surrounding rock it will rise towards the surface; any that is denser than the surrounding rock would sink into the mantle. Such a body of magma is called a pluton (or alternatively, an intrusion), or, if it is feeding a volcano, a magma chamber.
How does magma form? as suggested above, rock is often close to its melting point deep inside the earth. Melting can be caused by several things. A major cause is chemical change - usually the addition of water. Alternative causes of melting can be heating (e.g. by a mantle plume), or a drop in pressure caused by faults.
Where does magma form?: Magma is often formed at a convergent plate boundary, probably due melting triggered by water released from subduction. At mid-ocean ridges, magma is formed by some combination of water introduced through deep faults and pressure release because of mantle convection. In 'hot spots' like Auckland or Hawaii, melting is probably triggered by heat from a rising 'mantle plume', although it is hightly likely that pressure release also plays a role. In the exam, you are most likely to be asked about magma formed at a convergent plate boundary. This is how the volcanoes from Mt Ruapehu to White Island are formed. For a more detailed discussion, go to the level 3 page.

Volcanic and plutonic

Only a small proportion of this magma actually reaches the surface to be erupted as volcanoes and volcanic rocks. Much more remains below the ground, hundreds or thousands of metres, slowly solidifying as it gives up its heat to the surrounding rocks. These underground bodies of magma are called plutons, or intrusions, and the resulting rocks are termed plutonic (or intrusive). They are fairly easy to tell apart:
  • volcanic rocks have mostly small crystals with sometimes a few larger ones, and sometimes have bubbles. They could also be glassy, such as obsidian or pumice.
  • plutonic rocks have larger and much more evenly sized crystals and never have bubbles.
Andesite (volcanic), Taupo Bay, Northland
Diorite (plutonic), Paritu, Coromandel

The pictures above show andesite, a volcanic rock and diorite, a plutonic rock of similar chemical makeup. The andesite has some large crystals, formed before eruption, of the mineral hornblende in a matrix of smaller crystals. The diorite has fairly evenly sized crystals of hornblende and pyroxene, (dark) and quartz and feldspar (light). This particular plutonic rock was formed not very deep down so the crystals are not as large as plutonic rocks from deeper.

Plutonic rocks

These are rocks that never make it to the surface while they are still liquid. They come from deep down (2-20 km).
So how is it that we see them at the surface? The answer has to do with uplift. As the rocks are lifted up by faults and folds, erosion wears away the overlying rocks until the deep rocks are visible at the surface (see:rock cycle).
The photo above comes from a place called Paritu, near the end of the Coromandel Peninsula. Here, there is a 900m high mountain block - known as Mt Moehau, which is a 'fault block' - a mountain is surrounded by an almost rectangular arrangement of faults which have lifted it up (though the edges of the mountain, near the fault are worn away more than the middle). The diorite is found near the fault, and was brought to the surface from maybe 2km down by it. Interestingly, not far away - on the other side of one fault - are volcanic rocks that were erupted as part of the same activity, which happened about 10 million years ago when this area was a set of island arc volcanoes (read more about this rock). The steps at the front of the Auckland War Memorial Museum are made of this rock from Paritu, which is known locally as "Coromandel Granite" (although it is really a diorite, becuase it has a larger proportion of darker crystals than granite).
Note: plutonic and intrusive mean basically the same things; similarly we can talk about a pluton or an intrusion as the same thing. Which word is used depends mostly on the author. I use plutonic to contrast with volcanic; the equivalent opposite of intrusive is extrusive (for volcanic).

Reasons for the differences between plutonic and volcanic rocks

Plutonic and volcanic rocks which have been formed from exactly the same magma can show some quite profound differences, even if they are made of the same chemicals and minerals.. This is because plutonic rocks have cooled down much more slowly, deep underground, and with no opportunity for gas to escape.
The slow cooling in plutonic rocks means that crystals form much more slowly, and therefore will have time to grow much larger. Plutonic rocks also never have the gas bubbles (called vesicles) which are common in volcanic rocks, because the high pressure means that gases such as CO2 and H2O remain dissolved in the magma and get incorporated into the minerals (not
Basaltic scoria on the top of Tarawera
e that not all volcanic rocks have vesicles).
Most volcanic magmas have undergone some crystallization underground before they are erupted, resulting in a few large crystals called phenocrysts mixed in with the smaller crystals, called groundmass. Sometimes you get a fine grained volcanic rock with no phenocrysts at all (geologists use the word aphyric to describe this). These rocks are fairly rare and usually indicate very freshly formed magma. An example would be the basalt erupted from Mt Tarawera in 1886.
Some volcanic rocks cool so quickly they have no time to form any crystals at all after they are erupted. Instead of making crystals, these cool into volcanic glass. Obsidian and pumice (which is just obsidian with lots of bubbles in it, like shaving foam made of lava) are the commonest examples of volcanic glass, and the only crystals you will find in pumice will be phenocrysts. The term for this rapid cooling is quenching. This is most common in silica-rich lavas, which form glass more easily (note: silica poor magmas do form glass, called tachylite, but these are relatively rare and not required in this AS). Scoria tends to be quite glassy.

Magma composition and rock type

The other major variable in igneous rocks is the chemical makeup, or composition, of the magma that formed it. There are many ways in which this can vary, but the most important is the amount of silica (SiO2, or molten quartz) dissolved in the magma. The way that this shows up in the rock is the proportion of light coloured (white, cream, pink) minerals, which are compounds of silica and aluminium, calcium, sodium and potassium oxides.
Dark coloured (black, brown or green) looking minerals are compounds of silica and iron or magnesium oxide. These contain much less silica.
This means that the more light coloured minerals there are in a rock, the more silica-rich it is. Rocks with a high proportion of dark minerals are silica-poor. Rocks in between are termed intermediate. Volcanic and plutonic rocks can be roughly named according to the proportion of silica present in the magma:

Note: Pumice, obsidian and scoria
I have included obsidian with rhyolite, unlike the table in the AS. Obsidian is rhyolite lava that has cooled very quickly, to form glass, and pumice is obsidian full of bubbles. Scoria is basalt full of bubbles. Silica poor lava does not form glass as easily as silica rich lava.
Intermediate lava can vary; if it is less than about 63% silica it will form scoria and if it is more than 63% silica it will form pumice.

Minerals in igneous rocks
Below is a chart showing the proportion of different minerals in igneous rocks of different compositions


The minerals coloured white, cream and pink on the chart are light coloured minerals (quartz and feldspar). Those coloured brown and green are dark minerals (biotite, pyroxene, olivine and amphibole).
A rock with few light coloured minerals present is silica poor – if it s volcanic it will be a basalt, and if plutonic (with large crystals) it will be a gabbro. A rock with a fairly equal distribution of dark and light coloured minerals is intermediate. A rock with mostly light coloured minerals is silica rich.
The diagram below shows magnified views of thin sections of basalt and gabbro. A thin section is a slice of rock about the thickness of a human hair, stuck onto a microscope slide. A geologist seeing these pictures in colour would fairly easily be able to identify most of the minerals present. The minerals appear in some bright colours because the pictures are taken with polarized light; this causes the light passing through certain crystals to change colour.
Both pictures are the same scale. Notice the larger and more uniform crystal size in the gabbro. The basalt contains some large crystals – phenocrysts – that were formed before the magma was erupted. The same minerals are present in both rocks, but in the basalt some are so small they are difficult to identify.

Note: Ultramafics
Ultramafics are extremely silica-poor igneous rocks,containing no quartz crystals and little or no feldspar. An ultramafic rock called dunite (made almost entirely of olivine) was named after Mt. Dun, near Nelson, and is well known to geologists all over the world. It is thought that the mantle is made up of ultramafic material. This is important, because the mantle is thought to be the source of basalt magma (see below).

How different magmas form:

How these different compositions of magma are formed is still the subject of ongoing research.
Scientists are reasonably sure that the mantle is composed of ultramafic material (see above).
Melting without water of this mantle material produces basaltic magma (basalt, gabbro). This can happen at a mid-ocean ridge, where melting is caused by the pressure drop of the plates being pulled apart. It can also happen in a mantle plume where It is caused by the hotter temperature.
Andesitic magma (andesite, diorite) seems to be the result of 'wet melting' of the mantle, as a result of water released by subducting oceanic crust. There is also some crustal mixing involved, and some andesites may result from mixing of basalt with crustal rocks.
Rhyolitic/granitic magmas seem to result from melting of the crust. If you completely melted your 'average' crust, such as NZ greywacke, you would get andesite. But the melting is only partial, and the more silica-rich components melt preferentially.
To complicate matters, the composition of a magma can evolve over time because, as the magma begins to crystallize, dark minerals will settle towards the bottom of the magma chamber because they are denser than the magma. The light minerals can sometimes float towards the top for similar reasons, so the composition of the magma that erupts can depend on where it comes from in the magma chamber and how long it has had to cool down, and whether or not it has melted other rocks and mixed with them. Many volcanoes erupt a range of different rocks. Developing a detailed understanding of the processes involved is an advanced paper at University and requires a very good working knowledge of chemistry.

Magma type and volcanoes

The type of magma has a profound effect on the style of eruption. This is examined in the Level 3 course, and you can read more about it here. Scroll to the bottom of the page to read about volcanoes and eruptions.

Distinguishing features of igneous rocks
Basalt (Wikimedia commons)
Plaque made from gabbro

Basalt: fine grained, dark grey rock with no or few visible light crystals. Sometimes with large green crystals of olivine. May contain gas bubbles (veiscles).
Gabbro: Dark grey to black with large dark crystals and few or no visible light crystals (no quartz). Often used for plaques, but can be mis-labelled as granite as above. Sometimes ccalled 'black granite'.

Andesite (Wikimedia)
Diorite (Wikimedia)

Andesite: fine grained, may contain gas bubbles. A mix of dark and light crystal. Crystals of dark, angular hornblende or pyroxene may be visible and white feldspar and clear quartz.
Diorite: Similar to granite, but a much more even mix of dark and light crystals. Large crystals, no gas bubbles.

Rhyolite quarry, Ngogataha
Rhyolite, USGS

Rhyolite: Light coloured, fine grained. May contain dark, glassy bands (obsidian) or may consist of fine dark and light bands (banded rhyolite, due to layers which cool more quickly and form more glass; these get stretched out as the toffee-like rhyolite lava slowly flows). Often pinkish. Welded ignimbrite is almost identical to rhyolite but is formed by pyroclastic flow rather than lava flow.
Granite (Wikimedia)
Granite with pink feldspar (Wikimedia)

Granite: Less than 15% black minerals but may appear quite dark due to pink feldspars. Large crystals. No gas bubbles. Often polished by grinding with carborundum dust to produce ornamental stones, benchtops, building facings and headstones. "Black granite" is really gabbro, but granite may be white with black specks through to quite a dark pink-red (this is stil the 'light coloured' mineral feldspar, it just appears this colour because of tiny inclusions of rust; this happens when there is a lot of water in the magma).

Another way of organising igneous rocks is shown in the chart below: