Igneous Rocks
Formation
Igneous rocks form when magma cools and solidifies.Since minerals typically crystalize as igneous rock is created from magma, most igneous rock can be called crystalline, which means that it is made of crystals.
The Families of Igneous Rock
Felstic Rock- When rocks form from magma that contains large proportions of of silica, it is a felistic rock. They generally have the light coloring of quartz and potassium feldspar. This family includes granite, rhyolite, obsidian, and pumice.
Mafic Rocks-It forms from magma that contains less silica than felstic rock and has lots of magnesium and iron. Mafic rock has a dark color due to the mineral components of feldspar and pyroxene minerals. This family includes basalt and gabbro.
Intermediate Rocks- This type of rock contains more silica than mafic rocks, but less than felstic rocks. The rocks in this family include diorite and andesite.
The Textures of Igneous Rock
Course-Grained Texture- These are igneous rocks that has large mineral grains due to a slow loss of heat. An example of this type of rock is granite.
Fine-Grained Texture- When magma cools rapidly, it creates small crystals that cannot be seen by the bare eye,. These crystals make up a fined-grain texture rock. An example of this type of rock is basalt.
Other Textures:
Porphyritic Texture- This type of texture is created when magma starts to cool slowly, but then begins to cool quickly. It produces large crystals that are embedded swith small crystals. An example of this rock is granite.
Glassy Texture- This type of texture is created when magma cools quickly.An exampe of this texture is obsidian.
Vesicular Texture- Magma that contains a large amount of gas that cools rapidly forms rocks, such as pumice, that have a vesicular texture.
Fine-Grained Texture- When magma cools rapidly, it creates small crystals that cannot be seen by the bare eye,. These crystals make up a fined-grain texture rock. An example of this type of rock is basalt.
Other Textures:
Porphyritic Texture- This type of texture is created when magma starts to cool slowly, but then begins to cool quickly. It produces large crystals that are embedded swith small crystals. An example of this rock is granite.
Glassy Texture- This type of texture is created when magma cools quickly.An exampe of this texture is obsidian.
Vesicular Texture- Magma that contains a large amount of gas that cools rapidly forms rocks, such as pumice, that have a vesicular texture.
Intrusion
It is an igneous rock mass that forms underground. They form when magma enters into other rock masses and cools in the Earth's crust. The largest intrusion is called a batholiths, which extend several thousand meters below earth's surface. Another type is Stocks, they cover less than 100 km2 at the surface. Laccoliths form when magma flows between rock layers and spreads upwards. They occur in groups and look like dome shaped mountains.Sills form when magma flows between the layers of rock and harden. When magma forces itself through rock layers by following vertical fractures and then solidifies, a dike forms. They are common in areas of volcanic activity.
Extrusions
Extrusions are igneous rock masses that form on earths surface, many of which are flat masses of rock calledlava flows.Volcanic rock, called tuff, forrms when a volcano releases ash and other solid particles.