Unit introduction that briefly summarizes the three rock types and their characteristics.
Rocks on earth are classified according to the way they were formed. Igneous rocks come from magma or lava. Sedimentary rocks are made from sediments. Metamorphic rocks are the result of great heat and pressure that have changed existing rocks into new rocks. Together, these three rock types account for all the rocks on earth!
Igneous rocks can be classified into two main categories: intrusive and extrusive. Intrusive rocks form from lava and contain large crystals, which indicate slow cooling that allows time for these crystals to grow. Intrusive rocks cool slowly because they solidify under the surface of the earth, where the rocks are insulated from cool surface temperatures. Extrusive rocks are made of small, often microscopic crystals, which indicate rapid cooling and little time for crystal growth. Extrusive rocks cool rapidly because they are exposed to the air at the earth's surface.
Within the two main categories igneous rock can be classified even further using texture and chemical composition. Texture describes different characteristics of how rocks look. Examples of texture are: the size of the mineral grains (crystals), holes or vesicles in the rock, or a mass of mineral grains or a bunch of smaller rocks melted or welded together. The chemical composition of igneous rock can be broken into four main types: felsic, intermediate, mafic and ultramafic. The mineral proportions of the rock are what allows geologists to classify the rocks chemically. The following list gives more information about igneous chemical categories. This information is provided for you as the teacher to better understand igneous rocks.
- Felsic rocks are high in silica (65% +). They are usually light-colored. Some examples are: Rhyolite (extrusive) and granite (intrusive).
- Intermediate rocks have lower silica content (55-65%). They are darker than felsic rocks but lighter than mafic rocks. Some examples are: Andesite/dacite (extrusive) and diorite/granodiorite (intrusive).
- Mafic rocks have low silica content (45-55%). They are usually dark-colored and contain more iron and magnesium than more felsic rocks. Some examples are: Basalt (extrusive) and gabbro (intrusive).
- Ultramafic rocks have extremely low silica content (less than 45%) and contain large amounts of iron and magnesium. They are usually dark-colored, but high olivine content can lend green shades to the rock. Ultramafic rocks also occur in other colors, although rarely. An example of ultramafic rock is Peridotite (intrusive).
- Yosemite National Park
- Crater Lake National Park
- Devils Tower National Monument
- Mount St. Helens National Volcanic Monument
Sedimentary rocks can be broken into two major classifications: clastic and chemical/biochemical. Clastic rocks are formed from solid particles of previous rocks in the form of clay, silt, sand, pebbles, or boulders. Some examples of clastic rocks are sandstone, shale, and siltstones. Chemical/Biochemical rocks are made of sediments that precipitated out of water either chemically or biochemically. Some examples of chemical/biochemical rocks are limestone, chert, and rock salt. These classifications can be simplified into clastic and chemical.
Sedimentary rocks are formed when the clastic or chemical particles stick together by means of a chemical cement or by particles sticking together on their own. Through various physical and chemical means, sediments are hardened and turned into sedimentary rocks, a process known as lithification. A few familiar examples of sedimentary rocks found in National Parks are:
- Claron Limestone, Bryce Canyon National Park
- Navajo Sandstone, Zion National Park
- Morrison Formation, Dinosaur National Monument
Metamorphic rocks are formed when the chemistry, mineralogy, and/or texture of previously-existing rocks are changed without actually melting the rock. The primary forces of change are:
There are many different kinds of metamorphism. Two of the common types are: regional metamorphism and contact metamorphism. Regional metamorphism is caused by moderate-to-high pressures and/or temperatures over a large region. Examples of places where regional metamorphism is found would be continent/continent collision zones and subduction zones. Contact metamorphism is caused high temperature/low pressure environment found when a hot igneous body intrudes cold, preexisting rocks. These types of metamorphic processes occur in different pressure/temperature environments and over different amounts of time. A few examples of metamorphic rocks in National Parks are:
Included National Parks and other sites:
Black Canyon of the Gunnison
Bryce Canyon National Park
Crater Lake National Park
Devils Tower National Monument
Dinosaur National Monument
Grand Canyon National Park
Mount St. Helens National Volcanic Monument
Yosemite National Park
Zion National Park
Utah Science Core:
2nd Grade Standard 6 Objective 1, 2, 3
4th Grade Standard 3 Objective 1, 2