Rocks
There are three types of rocks. The major difference among them is how they form. We'll address the modes of origin, the individual rock types, and features of each one.
Igneous
The word "igneous" basically means "from fire" or "on fire." This implies that these rocks come from fire...and they do. Igneous rocks were the first rocks to form on Earth. As the molten surface cooled, it formed rocks that we call igneous. Over time, different types formed as well, but igneous rocks are formed from scratch, so to speak. Igneous rocks are classified based on two characteristics: composition and rate of cooling. Each rock can be classified two different ways.
The rocks form from cooling molten (melted) material. Where this molten material is when it cools will help determine how fast or slow it cools. If it stays inside the Earth, then it will cool slowly because the heat of the Earth is still present and causes the magma (molten rock under the surface of Earth) to stay warm longer. The result is a rock with medium to large (phaneritic) or really large crystals (pegmatitic). The minerals forming the rocks have more time to grow when they cool slowly. We call these rocks intrusive. If the molten material is exposed to the air or water on the surface of the Earth, we call it lava and it cools quickly. These rocks are called extrusive. The minerals do not have much time to grow, so the rocks will have no crystals (glass) or tiny to small crystals (aphanitic). We consider crystals that are too small to be seen without magnification to be aphanitic. Phaneritic crystals can be seen easily without magnification. And pegmatitic crystals are so big that they can be seen across a room.
Igneous rocks are also divided up by composition. This is where silica is so important. Rocks that do not contain a lot of silica (silica-poor) tend to be dark in color and dense. These rocks are mafic. Rocks that contain a significant amount of silica (silica-rich) tend to be light in color and not as dense. These rocks are felsic.
Very important igneous rocks
Granite: felsic; intrusive -- light-colored, phaneritic crystals; common rock on continents
Basalt: mafic; extrusive -- dark-colored, aphanitic crystals; common rock in ocean crust
Pumice: mafic/felsic; extrusive -- light or dark-colored, no crystals (called glass), low density (floats)
Obsidian: mafic/felsic; extrusive -- typically black or dark red, no crystals, often called "volcanic glass"
The word "igneous" basically means "from fire" or "on fire." This implies that these rocks come from fire...and they do. Igneous rocks were the first rocks to form on Earth. As the molten surface cooled, it formed rocks that we call igneous. Over time, different types formed as well, but igneous rocks are formed from scratch, so to speak. Igneous rocks are classified based on two characteristics: composition and rate of cooling. Each rock can be classified two different ways.
The rocks form from cooling molten (melted) material. Where this molten material is when it cools will help determine how fast or slow it cools. If it stays inside the Earth, then it will cool slowly because the heat of the Earth is still present and causes the magma (molten rock under the surface of Earth) to stay warm longer. The result is a rock with medium to large (phaneritic) or really large crystals (pegmatitic). The minerals forming the rocks have more time to grow when they cool slowly. We call these rocks intrusive. If the molten material is exposed to the air or water on the surface of the Earth, we call it lava and it cools quickly. These rocks are called extrusive. The minerals do not have much time to grow, so the rocks will have no crystals (glass) or tiny to small crystals (aphanitic). We consider crystals that are too small to be seen without magnification to be aphanitic. Phaneritic crystals can be seen easily without magnification. And pegmatitic crystals are so big that they can be seen across a room.
Igneous rocks are also divided up by composition. This is where silica is so important. Rocks that do not contain a lot of silica (silica-poor) tend to be dark in color and dense. These rocks are mafic. Rocks that contain a significant amount of silica (silica-rich) tend to be light in color and not as dense. These rocks are felsic.
Very important igneous rocks
Granite: felsic; intrusive -- light-colored, phaneritic crystals; common rock on continents
Basalt: mafic; extrusive -- dark-colored, aphanitic crystals; common rock in ocean crust
Pumice: mafic/felsic; extrusive -- light or dark-colored, no crystals (called glass), low density (floats)
Obsidian: mafic/felsic; extrusive -- typically black or dark red, no crystals, often called "volcanic glass"
Sedimentary
Sedimentary rocks are made from other materials. This material comes from other rocks, living things, or from water. We can divide sedimentary rocks by, again, mode of origin: clastic, chemical, and organic. All sedimentary rocks are formed through compaction and cementation. Compaction is the process of squeezing the material together until it becomes a rock. Cementation is the process of water depositing particles that act as a "glue" that holds everything together to become a rock.
Clastic sedimentary rocks are made from bits and pieces of other rocks (sediment). We can further divide this group by the size of the sediment found in the rock. Conglomerate and breccia are made with pebbles (large sediment; > 2 mm). Sandstone is made from sand (medium sediment; 2 mm - 1/16 mm). Shale is made from mud (small sediment; < 1/16 mm).
Chemical sedimentary rocks are made from either chemicals precipitating (falling out of) from water or from water evaporating and leaving the chemicals behind. Limestone is the most important of these and forms from calcite (calcium carbonate = CaCO3) precipitating out of the ocean then getting compacted over time. Rock salt is an example of water evaporating and leaving behind chemicals that compact together.
Organic* sedimentary rocks are made from previously living things. Coal is probably the most important (as a resource in Virginia) organic sedimentary rock for us to look at. Coal comes from ancient plants that fell into a swampy environment. The water in swamps typically do not have much oxygen due to the decay of many plants and animals. This lack of decay allows the plant material to collect over time and the weight of overlying sediment helps change the organics into a rock. First, the material is called peat which is a gooey black/gray muddy substance. Then, it turns into lignite which is woody and brown. Then, it becomes coal. There are two types of coal, but we'll satisfy ourselves with coal being a sedimentary rock.
* - remember: organic means that it is or was living.
Most fossils are found in sedimentary rocks. Not all fossils, but most. The reason is that when creatures die, they can become fossils when sediment buries it and hardens to rock.
Sedimentary rocks are made from other materials. This material comes from other rocks, living things, or from water. We can divide sedimentary rocks by, again, mode of origin: clastic, chemical, and organic. All sedimentary rocks are formed through compaction and cementation. Compaction is the process of squeezing the material together until it becomes a rock. Cementation is the process of water depositing particles that act as a "glue" that holds everything together to become a rock.
Clastic sedimentary rocks are made from bits and pieces of other rocks (sediment). We can further divide this group by the size of the sediment found in the rock. Conglomerate and breccia are made with pebbles (large sediment; > 2 mm). Sandstone is made from sand (medium sediment; 2 mm - 1/16 mm). Shale is made from mud (small sediment; < 1/16 mm).
Chemical sedimentary rocks are made from either chemicals precipitating (falling out of) from water or from water evaporating and leaving the chemicals behind. Limestone is the most important of these and forms from calcite (calcium carbonate = CaCO3) precipitating out of the ocean then getting compacted over time. Rock salt is an example of water evaporating and leaving behind chemicals that compact together.
Organic* sedimentary rocks are made from previously living things. Coal is probably the most important (as a resource in Virginia) organic sedimentary rock for us to look at. Coal comes from ancient plants that fell into a swampy environment. The water in swamps typically do not have much oxygen due to the decay of many plants and animals. This lack of decay allows the plant material to collect over time and the weight of overlying sediment helps change the organics into a rock. First, the material is called peat which is a gooey black/gray muddy substance. Then, it turns into lignite which is woody and brown. Then, it becomes coal. There are two types of coal, but we'll satisfy ourselves with coal being a sedimentary rock.
* - remember: organic means that it is or was living.
Most fossils are found in sedimentary rocks. Not all fossils, but most. The reason is that when creatures die, they can become fossils when sediment buries it and hardens to rock.
Metamorphic
Heat and pressure. These words are tied to the last rock type: metamorphic. Metamorphic rocks are rocks that change form...in fact, that's what metamorphic means: change form. Rocks change form by being subjected to immense heat and pressure. Not enough heat to melt the rocks because then we'd have an igneous rock. There are only two types of metamorphic rocks and two ways in which rocks are exposed to the heat and pressure needed to change them. Metamorphism occurs either regionally or by contact. Regional metamorphism occurs in a mountain-building event when two plates collide. The stresses and heat involved in the collision is more than enough to change the rocks. And, because it is where two plates collide, the metamorphism happens over a very large region. When a magma chamber (a large blob of magma that forms volcanoes) comes into contact with rocks around it, contact metamorphism happens. As far as the types of rocks formed, there are two: foliated and nonfoliated.
Foliated metamorphic rocks show distinct patterns of bands or layers. The bands typically alternate between black and white or light and dark. The major foliated rocks are: gneiss, schist, and slate.
Nonfoliated metamorphic rocks do not show bands or layers. The major nonfoliated rocks are: marble, and quartzite.
Heat and pressure. These words are tied to the last rock type: metamorphic. Metamorphic rocks are rocks that change form...in fact, that's what metamorphic means: change form. Rocks change form by being subjected to immense heat and pressure. Not enough heat to melt the rocks because then we'd have an igneous rock. There are only two types of metamorphic rocks and two ways in which rocks are exposed to the heat and pressure needed to change them. Metamorphism occurs either regionally or by contact. Regional metamorphism occurs in a mountain-building event when two plates collide. The stresses and heat involved in the collision is more than enough to change the rocks. And, because it is where two plates collide, the metamorphism happens over a very large region. When a magma chamber (a large blob of magma that forms volcanoes) comes into contact with rocks around it, contact metamorphism happens. As far as the types of rocks formed, there are two: foliated and nonfoliated.
Foliated metamorphic rocks show distinct patterns of bands or layers. The bands typically alternate between black and white or light and dark. The major foliated rocks are: gneiss, schist, and slate.
Nonfoliated metamorphic rocks do not show bands or layers. The major nonfoliated rocks are: marble, and quartzite.
The Rock Cycle
The rock cycle is simply a graphic representation of how one rock type turns into another by being subjected to specific conditions unique to each rock type. Igneous rocks are made by any rock melting and cooling to form crystals. Sedimentary rocks are made by any rock first undergoing weathering and erosion to form sediment. Then, the sediment is compacted and/or cemented together. Metamorphic rocks are made by any rock being subjected to heat and/or pressure making it change form. Any rock type can become the same or another rock type as long as it undergoes the process of formation for that rock type. Igneous rocks can become sedimentary, metamorphic, or another igneous rock. Sedimentary rocks can become igneous, metamorphic, or another sedimentary rock. Metamorphic rocks can become igneous, sedimentary, or another metamorphic rocks. As long as you understand the process of formation for each rock type, you can easily create the rock cycle.
The rock cycle is simply a graphic representation of how one rock type turns into another by being subjected to specific conditions unique to each rock type. Igneous rocks are made by any rock melting and cooling to form crystals. Sedimentary rocks are made by any rock first undergoing weathering and erosion to form sediment. Then, the sediment is compacted and/or cemented together. Metamorphic rocks are made by any rock being subjected to heat and/or pressure making it change form. Any rock type can become the same or another rock type as long as it undergoes the process of formation for that rock type. Igneous rocks can become sedimentary, metamorphic, or another igneous rock. Sedimentary rocks can become igneous, metamorphic, or another sedimentary rock. Metamorphic rocks can become igneous, sedimentary, or another metamorphic rocks. As long as you understand the process of formation for each rock type, you can easily create the rock cycle.
Concept Map
This is a concept map that I make with my students to illustrate the relationships among the terms and rock types. I'm including an image that can be viewed in the browser as well as a PDF file that can be printed if you want to do so.
This is a concept map that I make with my students to illustrate the relationships among the terms and rock types. I'm including an image that can be viewed in the browser as well as a PDF file that can be printed if you want to do so.
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