DEFINITION and VARIABILITY

Igneous Rocks - defined, variations


IGNEOUS ROCKS

Solidified from molten material or a MAGMA

MAGMA - naturally occurring liquid (mobile rock material) generated within the earth, capable of intrusion and/or extrusion from which igneous rocks have been derived by solidification and other processes.

Magma may or may not contain:

Solids - crystals, xenocrysts, xenoliths or rock fragments

Gases - dissolved gaseous phases in the liquid

LIQUID PHASE:

Chemical species in liquid phase consist of metallic ions, e.g. Fe2+ (ferrous), Fe3+ (ferric), Mg2+, Na+ etc. held within a discontinuous, fluctuating matrix of variously linked Si, Al and O atoms.

The relative abundance of the ions present is dependant on the composition of the magma and the physical conditions, T and P, under which the magma exists.

GAS PHASE:

Most melts are capable of containing gases dissolved in solution in the liquid phase

Gas species include H2O, CO2, H2S, CH4 (Methane), NH3 (Ammonia)

With a decrease in Pressure, gases come out of solution and form discrete gas bubbles in the liquid - produces vesicles in volcanic rocks, responsible for explosive nature of volcanic eruptions.

The composition of the gas phase varies widely in magmas reflecting the composition, T and P of formation, source material etc.

SOLID PHASE:

May be crystals which form early as the result of crystallization from the liquid or rock fragments, xenoliths, or inclusions of country rocks.


VARIABILITY

Igneous rocks are not everywhere abundant on and/or within the earth. They generally lack large scale mineral deposits, so they have very minor economic significance. Igneous activity, volcanism, provides the only samples available with which we may directly study the composition of the Earth's interior. Material samples the lower crust and upper mantle, the outer 100 km of the earth.

Igneous rocks are extremely variable, mineralogically and chemically, yet a link between the original and final liquid composition can often be inferred.

Two possibilities exist to explain the variety of igneous rock types observed:

  1. all rocks were originally created differently, or
  2. some process(s) exist which has/have the ability to generate a variety of rock types from a universal, original starting composition.

The latter possibility is only applicable to a restricted or particular locality.

Source compositions are sufficiently different with each composition undergoing similar process(s) after derivation. These processes which can effectively change the composition and character of the magma are termed fractionation or differentiation mechanisms. Which are defined as:

formation of variety of liquids of variable composition from an initially homogeneous, single parental material.

Will look at fractionation latter in the term.