1. Igneous Rocks

2. Igneous rock textures
Igneous rocks are formed by the solidification of magma.
They have various textures.
Texture is determined by the rate of cooling

3. Observe these textures of igneous rocks p. 110
05.01.a

4. Texture of Igneous Rocks
Intrusive rocks – Form below the surface of the earth.
Slow cooling causes formation of large, well-defined crystals.
Extrusive rocks – Form above the surface of the earth.
Fast cooling causes formation of small, poorly-defined crystals.

5. Cooling Rate and Texture
Slow cooling
Moderate cooling
Fast cooling
Slow then fast
05.10.b

6. Formation of igneous rocks
Igneous composition and texture depend upon
material that was melted
The way the magma solidified
Whether magma solidified above or below the earth’s surface

7. Formation of igneous rocks
Step 1: Melting of rock ( km)
The area where it melts is known as the source area
Step 2: Magma rises, due to decreased density and accumulates in pockets.

8. Formation of igneous rocks
Step 3: Some magma may accumulate to form a magma chamber
Solidification far below the surface forms plutons
Step 4: Some magma continues up and cools below the surface to form intrusions.

9. Formation of igneous rocks
Step 5: Magma reaches the surface and erupts as lava or ash (extrusive rock)

10. Forming Igneous Rocks
Eruption as lava or ash
Forms magma chamber (solidifies or rises)
Accumulates into rising magma body
Partial melting of source

11. Composition of Igneous rocks
Igneous rocks organized by 2 criteria
Size of crystal
Composition of minerals

12. Felsic
Rocks light in color and contain abundant quartz and feldspar are Felsic
Fel meaning feldspar and SIC which refers to silica.

13. Mafic
Rocks dark in color and contain minerals rich in magnesium and iron are Mafic.
MA comes from magnesium and fic comes from the latin prefix meaning iron.
Usually forms in the lower mantle

14. Size of crystals Coarse crystals – large, defined crystals.
Fine or glassy crystals – small, undefined crystals

15. Melting Rocks Solid Pressure tends to hold lattice together Liquid
Increasing temperate vibrates atoms and can break bonds
How could changing temperature or pressure cause melting?
05.05.a1-3

16. 3 ways rocks melt 1. Increasing the temperature
2. Decreasing the pressure
3. Adding water

17. Observe conditions where rocks are liquid and solid
Line slopes because increased pressure acts to keep rocks solid
05.05.b1
How could changing conditions cause a rock starting at point A to melt?
How could changing conditions cause a rock starting at point C to melt?

18. Observe what happens when a rock at point A is heated, with or without an increase in pressure

19. Observe what happens when a hot rock at point C is uplifted so rapidly it does not have time to cool off
05.05.c2

20. Adding water lowers melting temperature of rocks
Consider what happens to a rock at point E if a small amount of water is added to hot, dry rocks?

21. Decompression melting
How could melting occur along a divergent boundary (mid-ocean ridge)?
05.06.c2
05.06.a1
Solid asthenosphere rises to fill gap
Decompression melting

22. Summary of melting!!

23. Common igneous rocks Granite – made of mostly feldspar and quartz.
Rhyolite – the fine-grained equivalent of granite. Can Contain pieces of glass or pumice.

24. Common igneous rocks
Diorite- Contains more mafic minerals than granite.
Andesite – fine-grained version of diorite.

25. Common igneous rocks
Gabbro – made of dark-colored pyroxene and calcium-rich feldspar.
Basalt – dark lava rock

26. Common igneous rock
Peridotite- more magnesium and iron minerals such as olivine and dark pyroxene.
Ultramafic lavas – Very long crystals that are unusual for lava flow.