1. Classification of Igneous Rocks

2. Mineral Composition of Igneous Rocks
Intrusive
Magma that cools and crystalizes below the Earth’s crust.
Crystals are large enough to see with just the eye.
Extrusive
Form on the Earth’s surface.
Crystal are too small to be seen with just the eye.

3. Mineral Content Classification
Basaltic
AKA mafic
Dark colour
Lower levels of silica
Mostly plagioclase and pyroxene

4. Mineral Content Classification
Granitic
AKA felsic
Light colour
High levels of silica
Mostly quartz and feldspar
Intermediate
Between basaltic and granitic rock
Mostly plagioclase feldspar and hornblende

5. Texture Crystal or grain Crystal size Size Shape Distribution
Extrusive (coarse)- large – forms from slow cooling beneath the Earth’s crust
Intrusive (fine) – small – forms from quick cooling on the Earth’s surface

6. Porphyritic Rock These rocks have 2 different crystal sizes
The reason this happens is that the magma cooled at different locations
The first cooling was below the Earth’s crust and the second cooling was either on the Earth’s surface or inside the crust.

7. Vesicular Rocks The rocks appear like a sponge (vesicular texture)
The reason that happens is that the trapped gasses were not able to escape due to the thickness of the magma (very thick)
Trapped gasses produce holes

8. Thin Sections
Rocks are cut into thin sheets to make it possible to observe the size of mineral grains
Slice dimensions: 2 cm x 4 cm and 0.03 mm thick (that is thinner than paper)
Petrographic microscope: used to observe the thin sections and determine mineral composition

9. Igneous Rocks as Resources
Veins
Elements that were not incorporated into igneous rocks can deposit in the gaps in the minerals (veins)
Some of these important elements are:
Gold
Silver
Lead
Copper

10. Igneous Rocks as Resources
Pegmatites
Minerals deposited in fractures of rock
The minerals retain their shape
Pegmatites include:
Beryllium (Be)
Lithium (Li)

11. Igneous Rocks as Resources
Kimberlites
Initially found in Kimberly, South Africa
They contain deposits of Diamonds
Diamonds can only form under EXTREME PRESSURE
The exact formation of Kimberlite is not clear but it is predicted to be produced when magma is intruded rapidly upward towards the Earth’s surface in tubes.
Diamond tubes are only found in certain locations on the Earth. Several are in South Africa.

12. Igneous rocks in construction
Granite is a popular construction materials for the following reasons
Strong
Resistant to weathering
Durable
Uses
Counter tops
Floors
Statues

13. Section 5.2 Questions and Answers
Infer why obsidian, which is black or red in colour, usually has a granitic composition.
The reason this is the case is because obsidian contains high levels of silica which is typical of granitic rocks.
Describe the three major compositional groups of igneous rocks
Basaltic – dark in colour and tend to have low levels of silica.
Intermediate – contain both light and dark colored minerals.
Granitic – light in colour and tend to have high levels of silica.

14. Section 5.2 Questions and Answers
Apply what you know about cooling relates to explain differences in crystal sizes.
Slower cooling enables well formed crystals to form. The slower the cooling the larger the crystals. If the cooling is very quick small incomplete crystals form.
Distinguish between andesite and diorite using two physical properties of igneous rocks.
The crystals in andesite are smaller than the crystals in diorite. Their mineral composition is very similar.