1. Summer School Geosciences
Geology
Lecture 6 Igneous Rocks

2. Igneous Rocks Ignis is the Latin word for fire
Igneous rocks are those formed by the consolidation, or crystallization, of magmas resulting in a mass of interlocking crystalline units or super-cooled glass

3. Igneous Rocks
Magma is defined as a molten fluid, which is formed within the Earth’s crust or mantle. Magma is comprised of a complex of molten silicates with water and gasses in solution

4. Igneous Rocks Classification
Igneous rocks are classified according to their:
Rock chemistry
Size of crystals forming the rock

5. Igneous Rocks Rock Chemistry
Silica (chemically SiO2) is the dominant constituent in virtually all igneous rocks. Therefore classification is on the basis of silica variation:
Ultrabasic <45% silica
Basic 45 – 55% silica
Intermediate 55 – 65% silica
Acidic 65-80% silica

6. Igneous Rocks Crystal Size
If the majority of crystals have average dimensions of:
>/= 5mm then the rocks are coarse-grained
1 – 5 mm rocks are medium-grained
</= 1mm rocks are fine-grained (includes non-crystalline/glassy rocks)

7. Coarse-grained pegmatite Granite
Fine-grained
Obsidian
Medium-grained Granite

8. Igneous Rocks
Many Igneous Rocks are PORPHYRITIC, i.e. they contain some crystals (Phenocrysts) which are much larger than the bulk of the rock. When classifying these rocks ignore the phenocrysts.

9. Igneous Rocks Igneous Rocks may be of two types:
Extrusive – those formed at the Earth’s surface
Intrusive – those formed within the Earth’s crust

10. Igneous Rocks Grain Size and rate of cooling
Lavas tend to be fine-grained
Hypabyssal rocks tend to be medium-grained
Plutonic rocks tend to be course-grained

11. Igneous Rocks Extrusive Rocks include:
Lavas – which are formed from the consolidation of erupted magma
Pyroclastic rocks – fragmented material and welded rocks produced by explosive igneous activity
Hyaloclastites – rocks formed by the interaction of lava and water, e.g. sea or lakes

12. Igneous Rocks Intrusive Rocks are divided into:
Hypabyssal rocks – formed from relatively small bodies of magma, occurring along planes of weakness or fractures at comparatively shallow depths within the Earth’s crust
Plutonic Rocks – formed from large intrusions, sometimes with areas of 1000s of kms and huge volumes of magma emplaced at considerable depth within the Earth’s crust

13. Igneous Rocks
The centre of a lava may be medium-grained because it has taken longer to cool than the surface
So the crystalline form of an igneous rock is controlled mainly by its rate of cooling

14. Igneous Rocks
Where magma is intruded deep within the Earth’s crust, SLOW cooling - formation of COARSE-grained rock
Where magmas are intruded into the Earth’s crust at shallow levels, MODERATE rates of cooling form MEDIUM-grained rock
Where magmas are extruded onto the Earth’s surface, or into water, RAPID cooling results in FINE-grained rock

15. Igneous Rocks
Using the classification diagram enables rock names to be assigned on the basis of % SiO2 and grain size
There are no sharp boundaries between rocks of different chemistry, e.g. granites grade into syenites without a break

17. Igneous Rocks
The chemistry of an Igneous Rock is reflected in its mineralogy
Acid rocks contain ‘free’ silica – not combined with other oxides – and occurs as the mineral Quartz
Basic rocks are relatively poor in silica, and no quartz is present, they have high content of Iron, Magnesium and Calcium oxides

18. Igneous Rocks Other important igneous terms:
Colour Index (CI) depends on the proportion of dark (Mafic or ferro-magnesian) minerals present
Texture – the shapes of individual mineral grains and the relationship between them

19. Igneous Rocks Texture Terms:
Holocrystalline – the rock is entirely crystalline
Vitreous – containing abundant glass

20. Igneous Rocks
Euhedral – grains showing perfect or near perfect crystal form
Anhedral – showing no crystal form
Sub-hedral – with a recognisable but imperfect crystal form
Porphyritic – with large phenocrysts
Equigranular – grains of approximately equal size

21. Igneous Rocks
Vesicular – has cavities formerly occupied by gas bubbles
Amygdaloidal – has cavities infilled with secondary mineral
Xenolithic – has fragment of a foreign rock type

22. Igneous Rocks
Order of crystallization – the order in which the crystals in an igneous rock grew e.g. in porphyritic rock, the phenocrysts grew first and were subsequently encased in the later crystallizing groundmass

23. Igneous Rocks Reading Press and Siever Ch. 14
Thompson and Turk chapter on igneous rocks