Igneous Rocks & Rock Structures

These notes go on pp. 7 & 9 of your Igneous rock INB!!!

Igneous rocks are formed by the crystallization of molten rock (lava or

magma).

Classification

1. Mineral Composition2. Texture of Grain Size

Mineral Composition

Felsic: high amounts of silica & light in color

Felsic Rocks and Minerals

Mafic: low amounts of silica & dark in color

Mafic Rocks

Silica Composition

Felsic rocks - more than 65% silica contentIntermediate rocks - 50-65% silica content (also called andesitic) Mafic rocks - less than 40-50% silica

Pure Silica

Texture of Grain Size

1. Igneous rocks formed above ground from lava are called “Extrusive” or “Volcanic” rocks.

Obsidian

Rhyolite

cools rapidly and crystals have a limited time to form; as a result there are small or no mineral crystals

Resulting Textures:1. Glassy

2. Porous/Vesicular

3. Fine-Grained

Obsidian-Glassy Felsic

Scoria-Vesicular Mafic

2. Igneous rocks formed underground from Magma are called Plutonic or

Intrusive rocks.

Notice the large crystals

cools slowly and large crystals have time to form.

Resulting Textures

1. Coarse Grained

Granite- Coarse-grained Felsic

Gabbro- Coarse-grained Mafic

Igneous rocks that begin to cool underground and then are erupted to the surface have both fine and coarse grains.

These rocks have undergone two stages of cooling, therefore they have two sizes of crystals. These rocks have porphyritic

texture.

Basalt Porphyry- Mafic

Rhyolite Porphyry-Felsic

These notes go on p. 9 of your INB!!!

Intrusive Igneous Rock formations:

DikesDike: Sheet-like Igneous rocks that cut across the structure of pre-existing rock.

Dike

SillsSills: Sheet-like Igneous rock that lies parallel

to the layering of pre-existing rock.

Sill

LaccolithsLaccoliths: vertically developed sills.

BatholithsBatholiths: a large Plutonic mass of Igneous Rock.Usually makes up the core of volcanic mountains

Halfdome- Yosemite, California

Enchanted Rock-Texas