3.2 Igneous Rocks

Igneous comes from the Latin word which means “fire”

Igneous rocks form by “crystallization” when hot molten rock solidifies.

Classifications:A. Intrusive Rocks (Plutonic Rock)

1. Rocks that form when magma hardens beneath Earth’s surface.

2. Cools slowly, forms large crystals. Example: granite

A. Extrusive Rocks (Volcanic Rock)1. Rocks that form when lava hardens at

Earth’s surface2. Cools rapidly, forms small crystals.

Example: rhyolite

3.2 Igneous Rocks

When Magma/Lava composition is mainly: quartz & feldspar (Si and O) then they are considered “Felsic.” They tend to be light in color and with a low specific gravity. (sometimes called granitic composition)

3.2 Igneous Rocks

When Magma/Lava composition is mainly olivine or pyroxine (Mg, Fe) then they are considered “Mafic.” They tend to be dark in color and with a high specific gravity. (sometimes called basaltic composition)

Formation of Igneous Rocks

1. F fire

2. C intrusive igneous (plutonic)

3. G extrusive igneous (volcanic)

4. A magma

5. D lava

6. B granite

7. E rhyolite

Composition is the same, mostly quartz, feldspar and hornblend. Texture is very different

Compare/Contrast Igneous Rocks

Granite Rhyolite

Compare Igneous, composition-quartz, feldspar, hornblend (felsic)

Igneous, made of quartz, feldspar, (felsic)

Contrast Slow cooling from magmaIntrusive/Plutonic Large crystals

Rapid cooling from lavaExtrusive/Volcanic Small crystals

Igneous Rock Classification

9. Texture and composition Texture – size, shape and

arrangement of interlocking crystals Composition – proportions of light and

dark minerals in the rock

Classification Terminology

Large Crystals/ Coarse Grained

Small Crystals/ Fine Grained

Glassy Porphyritic

Very slow coolingLarge crystals

Rapid coolingSmall interconnected crystals

Very fast coolingNo crystals

Different sized crystalsCrystals did not form at same rate

Composition TerminologyFelsic/Granitic

Mafic/ Basaltic

Other

Light colored silicate minerals

Quartz/ feldspar

Dark colored silicate minerals

Fe, Mg, plagioclase feldspar

Andesitic – Ultramafic-

andesite lava peridotite

Rock olivine &

25% dark pyroxene

Silicate and not found

plagioclase at Earth’s

Feldspar surface

Classification of Igneous Rocks10. True11. Mafic composition (basaltic)

12. a, b, c, d

13. Ultramafic

14. False

15. b