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 % silica content (also called andesitic) Mafic rocks - less than 40-50% silica
Pure Silica
Texture of Grain Size
Texture of grains in igneous rocks depends on where the rock forms. Where it forms, will change the rate of cooling.
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