Download Mafic rocks

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 Composition
2. 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 content
Intermediate 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:
Dikes
Dike: Sheet-like Igneous rocks that cut
across the structure of pre-existing rock.
Dike
Sills
Sills: Sheet-like Igneous rock that lies parallel
to the layering of pre-existing rock.
Sill
Laccoliths
Laccoliths: vertically developed sills.
Batholiths
Batholiths: a large Plutonic mass of Igneous Rock.
Usually makes up the core of volcanic mountains
Halfdome- Yosemite, California
Enchanted Rock-Texas