Igneous rocks form from the cooling and hardening of molten magma in many different environments. These rocks are identified by their composition and texture. More than 700 different types of igneous rocks are known.
The rock beneath the Earth’s surface is sometimes heated to high enough temperatures that it melts to create magma. Different magmas have different composition and contain whatever elements were in the rock that melted. Magmas also contain gases. The main elements are the same as the elements found in the crust. Table 1 lists the abundance of elements found in the Earth’s crust and in magma. The remaining 1.5% is made up of many other elements that are present in tiny quantities.
|Table 1. Elements in Earth’s Crust and Magma|
Whether rock melts to create magma depends on several factors:
- Temperature: Temperature increases with depth, so melting is more likely to occur at greater depths.
- Pressure: Pressure increases with depth, but increased pressure raises the melting temperature, so melting is less likely to occur at higher pressures.
- Water: The addition of water changes the melting point of rock. As the amount of water increases, the melting point decreases.
- Rock composition: Minerals melt at different temperatures, so the temperature must be high enough to melt at least some minerals in the rock. The first mineral to melt from a rock will be quartz (if present) and the last will be olivine (if present).
The different geologic settings that produce varying conditions under which rocks melt will be discussed in the “Plate Tectonics” chapter.
As a rock heats up, the minerals that melt at the lowest temperatures will melt first.Partial melting occurs when the temperature on a rock is high enough to melt only some of the minerals in the rock. The minerals that will melt will be those that melt at lower temperatures. Fractional crystallization is the opposite of partial melting. This process describes the crystallization of different minerals as magma cools.
Bowen’s Reaction Series indicates the temperatures at which minerals melt or crystallize (figure 1). An understanding of the way atoms join together to form minerals leads to an understanding of how different igneous rocks form. Bowen’s Reaction Series also explains why some minerals are always found together and some are never found together.
Follow this link to see a diagram illustrating Bowen’s Reaction Series.
This excellent video that explains Bowen’s Reaction Series in detail.
If the liquid separates from the solids at any time in partial melting or fractional crystallization, the chemical composition of the liquid and solid will be different. When that liquid crystallizes, the resulting igneous rock will have a different composition from the parent rock.
Intrusive and Extrusive Igneous Rocks
Igneous rocks are called intrusive when they cool and solidify beneath the surface. Intrusive rocks form plutons and so are also called plutonic. A pluton is an igneous intrusive rock body that has cooled in the crust. When magma cools within the Earth, the cooling proceeds slowly. Slow cooling allows time for large crystals to form, so intrusive igneous rocks have visible crystals. Granite is the most common intrusive igneous rock (see figure 2 for an example).
Igneous rocks make up most of the rocks on Earth. Most igneous rocks are buried below the surface and covered with sedimentary rock, or are buried beneath the ocean water. In some places, geological processes have brought igneous rocks to the surface. Figure 3 below shows a landscape in California’s Sierra Nevada made of granite that has been raised to create mountains.
Igneous rocks are called extrusive when they cool and solidify above the surface. These rocks usually form from a volcano, so they are also called volcanic rocks (figure 4).
Extrusive igneous rocks cool much more rapidly than intrusive rocks. There is little time for crystals to form, so extrusive igneous rocks have tiny crystals (figure 5).
Some volcanic rocks have a mixed texture. A rock such as an andesite may have large crystals set within a matrix of tiny crystals. In this case, the magma cooled enough to form some crystals before erupting. Once erupted, the rest of the lava cooled rapidly. This is called porphyritic texture.
Cooling rate and gas content create other textures (see figure 6 for examples of different textures). Lavas that cool extremely rapidly may have a glassy texture. Those with many holes from gas bubbles have a vesicular texture.
Igneous Rock Classification
Igneous rocks are classified by their composition, from felsic to ultramafic. The characteristics and example minerals in each type are included in table 2.
|Table 2. Properties of Igneous Rock Compositions|
|Felsic||Light||Low||Quartz, orthoclase feldspar|
|Intermediate||Intermediate||Intermediate||Plagioclase feldspar, biotite, amphibole|
|Ultramafic||Very dark||Very high||Olivine|
|Table 3. Silica Composition and Texture of Major Igneous Rocks|
|Type||Amount of Silica||Extrusive||Intrusive|
Some of the rocks in the table 3 were pictured earlier in this chapter. Look back at them and, using what you know about the size of crystals in extrusive and intrusive rocks and the composition of felsic and mafic rocks, identify the rocks in the following photos in figure 7:
Uses of Igneous Rocks
Igneous rocks have a wide variety of uses. One important use is as stone for buildings and statues. Granite is used for both of these purposes and is popular for kitchen countertops (figure 8).
Pumice is commonly used as an abrasive. Pumice is used to smooth skin or scrape up grime around the house. When pumice is placed into giant washing machines with newly manufactured jeans and tumbled, the result is “stone-washed” jeans. Ground up pumice stone is sometimes added to toothpaste to act as an abrasive material to scrub teeth.
Peridotite is sometimes mined for peridot, a type of olivine that is used in jewelry. Diorite was used extensively by ancient civilizations for vases and other decorative artwork and is still used for art today (Figure 9).
The White House (shown in the figure 10) is the official home and workplace of the President of the United States of America. Why do you think the White House is white? If you answered, “Because it is made of white rock,” you would be only partially correct. Construction for the White House began in 1792. Its outside walls are made of the sedimentary rock sandstone. This sandstone is very porous and is easily penetrated by rainwater. Water damage was common in the early days of construction for the building. To stop the water damage, workers covered the sandstone in a mixture of salt, rice, and glue, which help to give the White House its distinctive white color.
Sandstone is one of the common types of sedimentary rocks that form from sediments. There are many other types. Sediments may include:
- fragments of other rocks that often have been worn down into small pieces, such as sand, silt, or clay.
- organic materials, or the remains of once-living organisms.
- chemical precipitates, which are materials that get left behind after the water evaporates from a solution.
Rocks at the surface undergo mechanical and chemical weathering. These physical and chemical processes break rock into smaller pieces. Physical weathering simply breaks the rocks apart. Chemical weathering dissolves the less stable minerals. These original elements of the minerals end up in solution and new minerals may form. Sediments are removed and transported by water, wind, ice, or gravity in a process called erosion (figure 11). Much more information about weathering can be found in the “Weathering and Formation of Soil” chapter. Erosion is described in detail in the “Erosion and Deposition” chapter.
Streams carry huge amounts of sediment (figure 12). The more energy the water has, the larger the particle it can carry. A rushing river on a steep slope might be able to carry boulders. As this stream slows down, it no longer has the energy to carry large sediments and will drop them. A slower moving stream will only carry smaller particles.
Sediments are deposited on beaches and deserts, at the bottom of oceans, and in lakes, ponds, rivers, marshes, and swamps. Avalanches drop large piles of sediment. Glaciers leave large piles of sediments, too. Wind can only transport sand and smaller particles. The type of sediment that is deposited will determine the type of sedimentary rock that can form. Different colors of sedimentary rock are determined by the environment where they are deposited. Red rocks form where oxygen is present. Darker sediments form when the environment is oxygen poor.
Sedimentary Rock Formation
Accumulated sediments harden into rock by lithification, as illustrated in figure 13. Two important steps are needed for sediments to lithify.
- Sediments are squeezed together by the weight of overlying sediments on top of them. This is called compaction. Cemented, non-organic sediments become clastic rocks. If organic material is included, they are bioclastic rocks.
- Fluids fill in the spaces between the loose particles of sediment and crystallize to create a rock by cementation.
The sediment size in clastic sedimentary rocks varies greatly (see table 4).
|Table 4. Sedimentary rock sizes and features|
|Rock||Sediment Size||Other Features|
|Siltstone||Silt-sized, smaller than sand|
When sediments settle out of calmer water, they form horizontal layers. One layer is deposited first, and another layer is deposited on top of it. So each layer is younger than the layer beneath it. When the sediments harden, the layers are preserved. Sedimentary rocks formed by the crystallization of chemical precipitates are called chemical sedimentary rocks. As discussed in the “Earth’s Minerals” chapter, dissolved ions in fluids precipitate out of the fluid and settle out, just like the halite in figure 14.
Biochemical sedimentary rocks form in the ocean or a salt lake. Living creatures remove ions, such as calcium, magnesium, and potassium, from the water to make shells or soft tissue. When the organism dies, it sinks to the ocean floor to become a biochemical sediment, which may then become compacted and cemented into solid rock (figure 15).
Table 5 shows some common types of sedimentary rocks.
Uses of Sedimentary Rocks
Sedimentary rocks are used as building stones, although they are not as hard as igneous or metamorphic rocks. Sedimentary rocks are used in construction. Sand and gravel are used to make concrete; they are also used in asphalt. Many economically valuable resources come from sedimentary rocks. Iron ore and aluminum are two examples.
In the large outcrop of metamorphic rocks in figure 16, the rocks’ platy appearance is a result of the process metamorphism. Metamorphism is the addition of heat and/or pressure to existing rocks, which causes them to change physically and/or chemically so that they become a new rock. Metamorphic rocks may change so much that they may not resemble the original rock.
Any type of rock—igneous, sedimentary, or metamorphic—can become a metamorphic rock. All that is needed is enough heat and/or pressure to alter the existing rock’s physical or chemical makeup without melting the rock entirely. Rocks change during metamorphism because the minerals need to be stable under the new temperature and pressure conditions. The need for stability may cause the structure of minerals to rearrange and form new minerals. Ions may move between minerals to create minerals of different chemical composition. Hornfels, with its alternating bands of dark and light crystals, is a good example of how minerals rearrange themselves during metamorphism. Hornfels is shown in table 6.
Extreme pressure may also lead to foliation, the flat layers that form in rocks as the rocks are squeezed by pressure (figure 17). Foliation normally forms when pressure is exerted in only one direction. Metamorphic rocks may also be non-foliated. Quartzite and limestone, shown in table 6, are nonfoliated.
The two main types of metamorphism are both related to heat within Earth:
- Regional metamorphism: Changes in enormous quantities of rock over a wide area caused by the extreme pressure from overlying rock or from compression caused by geologic processes. Deep burial exposes the rock to high temperatures.
- Contact metamorphism: Changes in a rock that is in contact with magma because of the magma’s extreme heat.
Table 6 shows some common metamorphic rocks and their original parent rock.
Uses of Metamorphic Rocks
Quartzite and marble are commonly used for building materials and artwork. Marble is beautiful for statues and decorative items such as vases (see an example in figure 18). Ground up marble is also a component of toothpaste, plastics, and paper.
Quartzite is very hard and is often crushed and used in building railroad tracks (see figure 19). Schist and slate are sometimes used as building and landscape materials. Graphite, the “lead” in pencils, is a mineral commonly found in metamorphic rocks.
- Igneous rocks form either when they cool very slowly deep within the Earth (intrusive) or when magma cools rapidly at the Earth’s surface (extrusive).
- Rock may melt to create magma if temperature increases, pressure decreases, or water is added. Different minerals melt at different temperatures.
- Igneous rocks are classified on their composition and grain size, which indicates whether they are intrusive or extrusive.
- Weathering and erosion produce sediments. Sediments are transported by water, wind, ice, or gravity.
- After sediments are deposited, they undergo compaction and/or cementation to become sedimentary rocks.
- Biochemical sedimentary rocks form when living creatures using ions in water to create shells, bones, or soft tissue die and fall to the bottom as sediments.
- Metamorphic rocks form when heat and pressure transform an existing rock into a new rock.
- Contact metamorphism occurs when hot magma transforms the rock that it contacts.
- Regional metamorphism transforms large areas of existing rocks under the tremendous heat and pressure created by geologic processes.
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