1. Don’t you wish you were here?

2. The Rock Cycle
Igneous Rocks

3. THE ROCK CYCLE
Rocks are naturally occurring combinations of minerals, fossils or other hard materials. They are classified by the way in which they form. The three rock types are: igneous, sedimentary and metamorphic.
All rocks on Earth are locked into a system of cycling and re-cycling known as the ROCK CYCLE. (*)

4. Earth as a system: The rock cycle

5. Earth as a system: The rock cycle
Full cycle does not always take place due to "shortcuts" or interruptions
e.g., sedimentary rock melts
e.g., igneous rock is metamorphosed
e.g., sedimentary rock is weathered
e.g., metamorphic rock weathers

6. Igneous Rocks Form as magma cools and crystallizes
Rocks formed inside Earth
(intrusive) rocks
Rocks formed on the
surface
(extrusive) rocks

7. Magma The nature of magma Consists of three components:
A liquid portion, called melt, that is composed of mobile ions
Solids, if any, are silicate minerals that have already crystallized from the melt
Volatiles, which are gases dissolved in the melt, including water vapor

8. Magma Crystallization of magma
Crystal size is determined by the rate of cooling
Slow cooling = large crystals
Fast cooling = microscopic crystals
Very fast cooling = glass

9. Magma Crystallization of magma
Texture in igneous rocks is determined by the size and arrangement of mineral grains
Igneous rocks are typically classified by
Texture
Mineral composition

11. Igneous textures Fine-Grained or (Aphanitic) Rapid rate of cooling
Microscopic crystals
May contain holes from gas bubbles
Typically occurs in extrusive / volcanic rocks
Example is Rhyolite

12. Igneous textures Course-Grained or (Phaneritic) Slow cooling
Crystals can be identified without a microscope
Typically occurs in intrusive / plutonic rocks
Example is granite

13. Igneous textures Large Crystals or (Porphyritic)
Minerals form at different temperatures as well as differing rates
Large crystals, called phenocrysts, are embedded in a matrix of smaller crystals, called the groundmass

14. Igneous textures Glassy texture Very rapid cooling of molten rock
Resulting rock is called obsidian

15. Igneous textures Vesicular
Cellular; full of holes; often light weight.
Example is Pumice.

16. Igneous Textures Fragmental
Composed of fragments ejected during a volcanic eruption
Varieties
Tuff – ash-sized fragments
Volcanic breccia – particles larger than ash

17. Igneous Compositions Composed mainly of silicate minerals
Dark silicates = rich in iron and/or magnesium, creates dark rocks
Olivine
Pyroxene
Amphibole
Biotite mica
Light silicates = greater amounts of potassium, sodium, and calcium, creates light colored rocks
Quartz
Muscovite mica
Feldspars

18. Igneous Compositions
Igneous rocks are often classified by mineral (chemical) composition
Mafic (or basaltic))= Dark colored
Felsic (or granitic)= Light colored

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