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 rock can be classified based on where they were formed. (location of Crystallization) 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)
Large-Grained
Glassy 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 Intrusive Igneous Rock forms structures such as… Dikes Dike: Sheet-like Igneous rock that cuts across the structure of pre-existing rock. Dike Volcanic Neck
Volcanic Dike
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