Igneous Rocks: Texture, Mineralogy and Classification
Igneous Rocks and How They Form
Igneous rocks form when magma cools and solidifies. They can form deep underground, where slow cooling produces coarse-grained rocks, or at the surface where rapid cooling gives fine-grained or glassy textures. Your specification focuses on four igneous rock types: peridotite, basalt, andesite and granite. You must be able to classify these from their colour, crystal size and mineral content — the three properties tested most frequently in hand-specimen identification questions.
Texture: Grain Size and Its Meaning
Texture is the single most important initial observation when classifying an igneous rock. Coarse-grained rocks have crystals larger than 3 mm, easily seen with the naked eye. Fine-grained rocks have crystals smaller than 1 mm, too small to see individually without a hand lens. Glassy rocks cooled so rapidly that no crystals formed at all.
Coarse grain size means slow cooling deep underground — the rock is intrusive. Fine grain size means rapid cooling, typically at the surface — the rock is extrusive. A porphyritic texture contains two populations of crystal size: large crystals called phenocrysts set in a fine-grained groundmass. Porphyritic texture tells you the magma experienced two stages of cooling: slow cooling at depth (growing phenocrysts), then rapid ascent to the surface (fine groundmass).
An equicrystalline texture (all crystals roughly the same size) indicates uniform cooling conditions. A porphyritic texture (phenocrysts in a finer groundmass) indicates two-stage cooling — slow at depth then rapid near the surface. These texture names appear in exam questions on igneous rocks.
Colour and Mineralogy
Igneous rocks range from very dark (ultramafic, rich in iron and magnesium) to pale (felsic, rich in silica, aluminium, sodium and potassium). The colour is a guide to mineral content. Dark minerals — olivine, pyroxene (augite), amphibole — are magnesium- and iron-rich. Pale minerals — quartz, orthoclase feldspar, plagioclase feldspar — are silica- and aluminium-rich. A rock’s colour therefore reflects its mineral make-up, and its mineral make-up reflects the chemistry of the original magma.
Peridotite — coarse-grained, very dark green to black, equicrystalline. Composed almost entirely of olivine with lesser pyroxene. No quartz, no feldspar. This is an ultramafic rock and forms the bulk of the Earth’s upper mantle. It is intrusive but comes from great depth.
Basalt — fine-grained, dark grey to black, often vesicular (containing gas bubble holes called vesicles) or porphyritic. Minerals include plagioclase feldspar, pyroxene (augite) and olivine. The most common igneous rock at the Earth’s surface; forms ocean floors and many volcanic islands. Extrusive.
Andesite — fine-grained, grey to brownish-grey, often porphyritic with visible phenocrysts of plagioclase feldspar. Named after the Andes mountains, where it is typical of subduction-zone volcanoes. More silica-rich than basalt; commonly contains hornblende as well as pyroxene. Extrusive.
Granite — coarse-grained, pale pink to white-grey, equicrystalline. Contains quartz (grey, glassy), orthoclase feldspar (pink or white), plagioclase feldspar (white), and mica (biotite = black flakes; muscovite = silver flakes). A felsic rock; forms large deep intrusive bodies called batholiths. Intrusive.
Summary: Peridotite (coarse, very dark, olivine + pyroxene) — Basalt (fine, dark, plagioclase + augite) — Andesite (fine, grey, plagioclase phenocrysts + pyroxene) — Granite (coarse, pale, quartz + orthoclase + mica). Only these four names are spec-listed for igneous rocks. Do not write “gabbro” or “dolerite” in your answers.
Reading a Photomicrograph
Your exam may ask you to interpret a photomicrograph — a photograph taken through a microscope of a thin slice of rock ground to 30 micrometres, allowing light to pass through. Under the microscope, different minerals show different colours, shapes and cleavage patterns. In exam questions you are not expected to use advanced polarisation theory; instead, look for crystal shape (well-formed crystals grew early with space around them; irregular shapes grew last and filled the gaps), relative crystal sizes (equicrystalline vs porphyritic), and colour contrast between pale and dark minerals. These observations are enough to classify the rock into one of the four spec types.
Students often lose marks by using rock names that are not on the specification. The most common offenders are gabbro (coarse dark equivalent of basalt), dolerite (medium-grained basic intrusive) and rhyolite (fine pale volcanic rock). None of these earn any credit. Check every answer against the four spec-listed igneous rock names: peridotite, basalt, andesite, granite. If your hand specimen is coarse-grained and dark, it is peridotite. If it is fine-grained and dark, it is basalt. If it is pale and fine-grained, check whether andesite fits rather than inventing a new name.
Similarly, avoid using “igneous rock” as your final answer in a question asking for the specific rock name. Broad categories earn partial credit at best; the exam is testing whether you can distinguish between the four types.
Classification of igneous rocks connects directly to the geological settings in which they form. The textures and mineralogy you have studied here become the foundation for the next lesson, which explores how magma viscosity and different eruptive settings produce the volcanic landforms and intrusive bodies you will encounter on geological maps.