Magma

Properties of magmas

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 properties of magmas include temperature, density, viscosity, gas content and abundance.

Temperature

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.

Density

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

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).

Gas content

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.

Abundance

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.

Chemical composition of magmas

As magma erupted at the surface is difficult to sample, the solid rocks that form after cooling provide us with the most information on the chemistry of magmas. Most contain silicate minerals (in which Si, Al, Mg, Fe, Ca, Na and K combine with oxygen) and some minor oxide minerals. These rocks are made up of:

• 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).