1. Igneous rocks form from molten rock
3.2 Notes
Igneous rocks form from molten rock

2. Think About…
Why do two rocks made of the same minerals look very different?

3. Magma and Lava Form Different Types of Igneous Rocks
Deep within Earth, temperatures are hot enough-750 to 1250 degrees C-to melt rock.

4. Magma and Lava Form Different Types of Igneous Rocks
Molten rock deep in Earth is called magma.
Molten rock that reaches the surface is called lava.

5. Magma and Lava Form Different Types of Igneous Rocks
An igneous rock is classified on the basis of its mineral composition and the size of its mineral crystals.

6. Magma and Lava Form Different Types of Igneous Rocks
A rock formed from magma can have the same composition as a rock formed from lava, but they will have different crystal structures and different names.

7. Origin of Igneous Rocks
Intrusive igneous rock is formed when magma cools underground.
Example: Granite

8. Origin of Igneous Rocks
Extrusive igneous rock is formed when lava cools on Earth’s surface.
Example: Rhyolite

9. Origin of Igneous Rocks
Granite and rhyolite have the same composition, but were formed in different ways.

10. Textures of Igneous Rocks
The texture of an igneous rock refers to the size of its mineral crystals.

11. Textures of Igneous Rocks
Large crystals can form in intrusive igneous rock because:

12. Textures of Igneous Rock
The interior of the Earth is very hot
The high temperatures allow magma to cool slowly
Slow cooling allows time for large mineral crystals to form

13. Textures of Igneous Rocks
Very small crystals form in extrusive rocks because…

14. Textures of Igneous Rocks
The surface of the Earth is cooler than Earth’s interior.
The lower temperatures cause the lava to cool quickly.
There is no time for large mineral crystals to form.

15. Composition of Igneous Rocks
To identify igneous rocks, you must also consider their compositions.

16. Composition of Igneous Rocks
Most igneous rocks are made up of silicate minerals.

17. Composition of Igneous Rock
Silicate minerals contain different amounts of silica, a mixture of silicon and oxygen.

18. Composition of Igneous Rock
Geologists classify igneous rocks on how much silica they have in them.

19. Composition of Igneous Rock
You can estimate the level of silica in an igneous rock by looking at its color.

20. Composition of Igneous Rock
Igneous rocks with high levels of silica are light in color.
Examples: granite and rhyolite

21. Composition of Igneous Rock
Igneous rocks with low levels of silica are dark in color.
Examples: Gabbro and basalt

22. Igneous Rocks Make Long-Lasting Landforms
A landform is a natural feature on Earth’s land surface.

23. Igneous Rocks Make Long-Lasting Landforms
Ship Rock in New Mexico is an example of the kinds of landforms that are made of igneous rocks.

24. Intrusive Rock Formations
As magma pushes toward Earth’s surface, it makes channels and other formations underground.

25. Intrusive Rock Formations
When the rock above these formations wears away, we have intrusive rock formations on the surface.

26. Extrusive Rock Formations
When magma makes its way to Earth’s surface, the lava can erupt in different ways.

27. Extrusive Rock Formations
Lava that is low in silica flows easily and spreads out in thin sheets.

28. Extrusive Rock Formations
Lava that is high in silica does not flow easily, and usually results in violent and explosive eruptions.

29. Review
Geologists classify an igneous rock on the basis of its crystal size and the amount of ______ its minerals contain.
Carbon
Silica
Sediment
Foliation