A Rock is A group of minerals that have been put together in several different ways.
Depending on how they are put together, rocks are classified as: 1. Sedimentary 2. Igneous 3. Metamorphic
Sedimentary Rocks are formed by several different processes.
1. Deposition, Compaction and Cementation (Lithification) of Sediments.
2. Crystallization or Chemical Reaction of Minerals in Solution in water.
Igneous Rocks are formed by the Crystallization, by Cooling and Hardening, of molten Magma (molten rock below ground) or Lava (molten rock above ground).
Metamorphic rocks are formed by the Recrystallization or Chemical Change of minerals due to Heat and/or Pressure.
Let s Focus on
How are Igneous Rocks Classified?
Igneous Rocks are classified based upon several different qualities or characteristics: 1. Mineral Composition 2. Location of Crystallization 3. Texture of Grain Size
1. Igneous rocks are identified based upon Mineral Composition (high or low Silica content)
High Silica rocks are called Felsic Igneous Rocks and are light in color.
Felsic Rocks and Minerals
Silica-Rich, or Felsic Rocks Igneous rocks that have a Silicate composition of 65% or more are considered to be Felsic.
Low Silica rocks are called Mafic Igneous Rocks, and are dark in color.
Mafic Rocks
Silica-Poor, or Mafic Rocks Igneous rocks that have a Silicate composition of 45-50% or less are considered to be Mafic.
Igneous rocks that have a Silicate composition of 50-65% are considered to be Intermediate or Andesitic (named after a rock made from a mixture of Mafic magma and minerals from Felsic country rock ).
Igneous rock can be classified by Silica and Oxide content
Pure Silica
2. Igneous Rocks are identified based upon where they are formed (above or below ground).
Igneous rocks formed underground from Magma are called Plutonic or Intrusive rocks. Notice the large crystals
Igneous rocks formed above ground from lava are called Extrusive or Volcanic rocks. Felsite Obsidian Rhyolite
3. Igneous Rocks are identified based upon Texture (size of mineral crystals) Glassy Large-Grained
Texture of Grain Size is evidence of rate of cooling; The faster magma or lava cools, the smaller the size of the crystals in the solidified material will be. Pumice Obsidian
When Magma cools slowly underground, large crystals have time to form. Therefore, Igneous rocks with large crystals are most likely Plutonic.
When Lava cools rapidly above ground, crystals have a limited time to form. Therefore, Igneous rocks with small or no crystals are most likely Volcanic.
Igneous Rock Textures
Obsidian-Glassy Felsic
Basalt- Fine-grained Mafic
Andesite- Fine-grained Intermediate
Rhyolite- Fine-grained Felsic
Granite- Coarse-grained Felsic
Diorite- Coarse-grained Intermediate
Gabbro- Coarse-grained Mafic
Pumice- Vesicular Felsic
Vesicular Basalt- Mafic
Scoria-Vesicular Mafic
Basalt Porphyry- Mafic
Rhyolite Porphyry-Felsic
Andesite Porphyry- Intermediate
How does Mineral Composition of magma or lava affect an Igneous Rock or the molten material that it forms from?
High Silica content causes Felsic Magma to be thick, or viscous, and slow-flowing.
Felsic Dome
Felsic Dome
Low Silica content causes Mafic Magma to be thin or low-viscosity, and fast-flowing.
Flood Basalt Plateau
Igneous rock can be classified based on where they were formed. (location of Crystallization)
Intrusive Igneous Rock forms structures such as
Dikes Dike: Sheet-like Igneous rock that cuts across the structure of pre-existing rock.
Dike
Volcanic Dike Volcanic Neck
This would form a Dike.
Sills Sills: Sheet-like Igneous rock that lies parallel to the layering of pre-existing rock.
Sill
Sill
Laccoliths Laccoliths: A Laccolith is a vertically developed sill.
Exposed/Eroded Laccoliths
Batholiths Batholiths: A Batholith is a large Plutonic mass of Igneous Rock (old Magma bubble ).
Halfdome- Yosemite, California
Enchanted Rock-Texas
Extrusive Igneous Rock forms structures such as
Felsic Lava Flows
Mafic Lava Flows Pahoehoe Lava
Domes
Flood Basalt Plateaus- Mafic Lava
Tephra or Ash Deposits
Volcanic Cones
Composite Cone (lava and Tephra)- Felsic
Cinder Cone- Mafic Lava
Shield Cone-Mafic Lava