Magma is hot molten mobile rock. Igneous rocks form when magma cools and solidifies. Magmas come out of active volcanoes as lavas. The most abundant magma is a melt of silicate composition and this can carry suspended crystals and gases which bubble out in air.
Magmas consolidate at the surface as lava flows (volcanic), or fall as lithic (rock), crystal and/or glass fragments (pyroclastic). Some solidify within the earth (plutonic or subvolcanic rocks).
The temperature of a magma is hard to measure. It can be done at a distance using an optical pyrometer (glowing filament) but this needs many corrections (e.g. for air temperature, distance, and elevation). The temperature of some lavas can be measured in the field using a thermocouple providing the eruption is not violent (as in lava flows or lakes).
Less silica-rich lavas generally come out at 1000° C – 1230° C while silica-rich lavas are cooler at 750° C – 900° C. Komatiitic lavas (very rich in magnesium and low in silica) probably erupted at 1300° C – 1400° C and mostly date from the Archean Era when the earth was hotter.
The density of chilled magma (volcanic glass) is usually measured. It ranges from 2.4 g/cm3 (for silica-rich lavas) to 2.9 g/cm3 (for silica-poor lavas).
Viscosity is the resistance of a fluid to motion. For example, syrup is more viscous than water. The viscosity of a silicate melt (liquid magma) is controlled by its degree of structure, with increased structure in magmas correlating strongly with increased viscosity. Although silica-rich magmas have lower temperatures than silica-poor magmas, a high degree structure gives them higher viscosity. Rises in temperature and in water content break down the structure of magmas and decrease their viscosity. Therefore, a water-rich magma is more fluid than a dry magma of the same composition.
Many melts are rich in basic oxides such as MgO and FeO and relatively low in silica. The silicate structures that develop favour the crystallisation of minerals of the olivine and pyroxene groups (i.e. simple silicate structures). Conversely, silica-rich melts favour the crystallisation of silicates such as feldspars, micas and quartz (i.e. more complex silicate structures).
Many lavas are vesicular (have cavities), indicating former gas bubbles which escaped from within the magma as it erupted at the surface. Some 99% of all gases in lavas are water vapour. Other gases that bubble out at the surface include nitrogen, oxygen, argon, carbon dioxide, boron, carbon monoxide, methane, hydrogen, nitric acid, hydrochloric acid, hydrofluoric acid, sulfuric acid, sulfur dioxide and sulfur trioxide. In some lava lakes, gases come out as invisible, colourless and odourless gas bursts and may kill many people and animals.
The most common igneous rocks by far are granites and basalts. Granites (containing around >15% quartz, and more alkali feldspar than plagioclase feldspar) form >95 % of all igneous plutonic rocks, while basalts form >98 % of all volcanic rocks.
Major elements: Oxides that account for >98% of the chemical composition of all igneous rocks, including: SiO2 (35 – 75%), Al2O3 (12 – 18%), FeO+Fe2O3 +MgO+CaO (20 – 30%). Also, Na2O (2.5 – 4%) and K2O (0.5 – 5%) tend to increase with SiO2 content.
Minor elements: TiO2, MnO, H2O and P2O5 usually make up between 0.1 and 1%.
Trace elements: The remaining elements (<0.1%), which are usually reported as parts per million (ppm) (1 ppm = 1 gram per ton).
Molten magma can invade the Earth’s upper layers and then solidify. Gravity influences the emplacement of igneous rocks because it acts on the density differences between the magma and surrounding wallrocks (country rocks). In general, silica-rich magmas are less dense than wallrocks, while silica-poor magmas are similar in density to wallrocks. Because of this, lower density intrusions take different shapes to higher density intrusions.
Low-density magmas (such as granitic magmas) are more buoyant in their invasions and cause subsidence of the surrounding wallrocks. High-density magmas (such as basaltic magmas) are closer in hydrostatic equilibrium with the surrounding wallrocks.
Dykes are sheet-like bodies of igneous rock that cut across sedimentary bedding or foliations in rocks. They may be single or multiple in nature. Dykes often have a fine-grained margin where they were chilled rapidly against the wallrocks. Small dykes tend to be fine-grained while large dykes tend to be coarser grained.
Matic xenolith in granite, Duckmaloi, New South Wales. Photo: I Graham © Australian Museum.
Stoping happens when a rising magma breaks off jointed blocks from the overlying country rock. The magma forces its way into the cracked roof and fragments of the wallrock sink into the magma. These fragments within the magmatic rock are known as xenoliths and can range in size from less than a millimetre up to tens of kilometres. The xenolith-rich Corralillo granodiorite of Peru is a good example, where xenoliths are concentrated near the roof of the intrusions (roof pendants) but become smaller and more rounded when digested further within the intrusion.
In the Thorr granodioritic pluton of Donegal, Ireland many of the xenoliths are large (several hundred metres) and their origin (e.g. limestone, schist) is still recognisable and can be correlated with the surrounding rock structures. This means there was little movement in the xenoliths after they were incorporated from their parent rocks.
Ring dykes form a cylinder around a subsided block of country rock, fill ing the ring-shaped fracture with magma. Excellent examples of ring dykes are seen on the island of Mull, Scotland. Large subsidence of a core of country rock can also form a circular pluton in the shape of a bell-jar.
In composite intrusions, some are arranged in concentric rings. These often formed as rings of intrusion spread outwards from a focus. These are called centred complexes.
Some intrusions form relatively flat-lying sheets, often with an undulating surface.
Diapirs are circular-shaped intrusions with vertical walls that force their way into place, strongly deforming the surrounding country rocks. The diapirs probably rose as buoyant magma, forcing their way up and through the denser country rocks. Diapirs are generally unfoliated (unbanded) in their centre. They become more foliated (banded) towards the contact with the country rocks, where numerous xenoliths have become more flattened. The foliation and flattening of the xenoliths runs parallel to this contact. The adjacent surrounding country rocks are intensely deformed and stretched out parallel to the intrusion.
Granite country near Pyalong, Victoria. Photo: I Graham © Australian Museum.
In many parts of the world, granites extend for many hundreds of kilometres in large masses called batholiths. These are not single intrusions but usually composite intrusions of similar magmas. The individual plutons in batholiths vary in size but the largest are about 30 km across. The individual plutons can show quite different forms and ages of emplacement (sometimes separated by millions of years). At the surface, granites in batholiths often weather and erode into rounded masses called tors.
The massive batholiths represent repeated and voluminous production of magmas during a period of plate tectonic activity. An excellent example is the Berridale Batholith of the Snowy Mountains, New South Wales.
Flat-lying sheets (sills) range in size from less than a metre thick up to huge intrusions underlying thousands of square kilometres. The Whin Sill of northern England is up to 100 m thick and intrudes an area of over 5000 square kilometres. Although sills typically intrude along the surrounding rock layers, almost all large sills vary in thickness and transgress into higher layers when mapped over a large area. These transgressions can occur as abrupt steps. The dolerite sills of the Karoo South Africa, the Transantarctic mountains and Tasmania occur as undulating discordant sheets.
Laccoliths are lens-shaped intrusions where magmas were emplaced like a sill between sedimentary layers but then bulged up into a dome. This commonly happens in dioritic intrusions. An excellent example of a laccolith is the Prospect intrusion of Sydney, New South Wales.
Cones are thin intrusive sheets that expand upwards and outwards in cones. Individual sheets are generally a few metres thick, but arranged so that the outer cones dip at lower angles to the inner ones so that they all converge towards a common source at depth.
These have a much thicker top that plunges down to a very narrow neck. A classic example is the Skaergaard intrusion of Greenland.
Funnel dykes are elongated in outcrop like a dyke but with a V-shaped cross-section that narrows downwards. All known examples are very large (over 100 km in length). The Great Dyke of Zimbabwe is over 700 km long and the Jimberlana dyke of Western Australia is over 180 km long.
Lopoliths are the largest known intrusions of dense magma and form a thick saucer shape within the surrounding country rocks. A famous example is the Bushveld Complex of South Africa, which is over 550 km across and up to 8 km in thickness. Lopoliths contain many important economic deposits of nickel, copper, platinum, palladium and chromium. The Sudbury intrusion of Ontario, Canada formed in an oval-shaped depression probably caused by a large meteorite impact.
Dykes reach their highest numbers in dyke swarms, which may form lines or radial patterns. Radial dykes usually converge on volcanic centres or igneous intrusions. Linear dyke swarms are more extensive than radial dyke swarms but can also be concentrated around large intrusions or volcanic centres. In south-eastern Iceland, the Cenozoic lava succession is cut by thousands of aligned near-vertical basaltic dykes averaging less than a metre thick.
Diatremes are steep pipe-like bodies filled with fragments of both igneous rocks and wallrocks. They form by explosion which results from the release of contained carbon dioxide gas (CO2) and water vapour (H2O) near the surface. Some very silica-poor magma types are economically important as the host rocks for diamonds.
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