3.2 Igneous Rocks Igneous comes from the Latin word which means “fire” Igneous rocks form by...

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