ERB222 Chap.1 Igneous Classification and Rock Description
Igneous Classification and Rock Description
Start with texture and setting, not the name
The Week 1 lecture treats composition and texture as the two properties that carry most of the information in an igneous rock. Texture is set mainly by cooling rate, so a granite and a rhyolite can share a chemistry and still look unrelated.
The unit's description flow chart therefore asks first whether a rock is coherent or clastic, then whether a coherent rock is plutonic, subvolcanic or volcanic, and only then turns to mineralogy and composition.
Two routes to a classification
One route combines mineralogy and texture: estimate a mode by eye, normalise quartz, alkali feldspar and plagioclase to 100%, and plot the result on the IUGS QAP triangle.
The other route uses whole-rock chemistry, such as the total alkalis versus silica diagram for volcanic rocks or a CIPW norm to judge silica saturation.
The lecture warns that names can change between field, thin section and geochemistry, so a good answer always states which scheme it used.
Why this chapter carries exam weight
The practical exam gives you unknown samples to describe and interpret, and the coordinator's tips list terminology and classification diagrams among the most common sources of lost marks.
What this chapter covers
- 01
Coherent versus clastic rocks
- 02
Plutonic, subvolcanic and volcanic settings
- 03
Phenocrysts and groundmass
- 04
Grain size and crystallinity terms
- 05
QAP modal classification
- 06
CIPW norm and silica saturation
- 07
Tholeiitic versus alkali basalt petrography
Worked example · free
Normalise a visual mode and name a plutonic rock
- 1Add the felsic minerals: Q + A + P = 24 + 22 + 30 = 76%.
- 1Normalise each to 100%: Q = 31.6%, A = 28.9%, P = 39.5%.
- 1Locate the field: Q lies between 20% and 60%, and A/(A+P) = 22/52 = 42.3% lies between 35% and 65%.
- 1Name the rock monzogranite and report the colour index of 24 from the dark minerals.
Key terms
- Mode
- The volume percentage of each mineral seen in a rock, estimated by eye with abundance charts or measured by point counting.
- CIPW Norm
- A calculation that recasts oxide weight percentages into an idealised set of normative minerals, used to judge silica saturation.
- Phenocryst
- A crystal distinctly larger than the surrounding groundmass, typically 1 to 5 mm, recording growth before the final rapid cooling.
- Colour Index
- The area percentage of dark minerals in a rock, a semi-quantitative guide to composition that can be estimated by eye.
- Holocrystalline
- Describes a rock made entirely of crystals with no glass, such as a granite.
- Silica Saturation
- The balance between silica and the other constituents that decides whether quartz, olivine or feldspathoids can coexist with feldspar.
Igneous Classification and Rock Description FAQ
What should come first when describing an unknown igneous sample?
Decide whether the rock is coherent or clastic, because that choice changes the whole description template. Coherent rocks need phenocrysts, groundmass and crystallinity; clastic rocks need clast types, sorting, matrix and fabric before any mineral list.
Why can QAP fail for a basalt or rhyolite?
Fine or glassy groundmass hides the feldspars, so the modal proportions cannot be measured reliably. The practical describes QAP as very useful for plutonic rocks but limited for volcanic and hypabyssal ones, where chemistry or a norm does better.
How does a CIPW norm show silica saturation?
Read the normative pair. Quartz with hypersthene means oversaturated, olivine with hypersthene means saturated, and olivine with nepheline means undersaturated. Nepheline and quartz, or forsterite and quartz, never appear together in one norm.
Exam move
Practise the flow chart aloud on every sample image you can find: coherent or clastic, then setting, then minerals, then name. Time yourself writing descriptive names in template order, and drill QAP normalisation until you can do it without a calculator. Keep a card of the textures that separate tholeiitic from alkali basalt, and test it against the reference basalt from the practical.
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