(a) IGNEOUS ROCKS
Igneous Rocks and the Bowen Reaction Series
(b) SEDIMENTARY ROCKS
Earlier it was suggested that there were two types of non-clastic sedimentary rocks. One group forms through the chemical precipitation and crystallization of elements and compounds from solution. Elements such as calcium, sodium, potassium, and magnesium are commonly released into the environment through a variety of chemical weathering processes. These elements can then become dissolved into aqueous solutions that are often transported via runoff, stream flow, or groundwater flow. If this solution enters a basin environment where evaporation exceeds precipitation and in-flow, sedimentary evaporites can form because of the loss of water from the solution.
Quartzite forms from the recrystallization of silica found in sandstone.
Igneous rocks are produced by the crystallization and solidification of molten magma. Magma forms when rock is heated to high temperatures (between 625 and 1200° Celsius) beneath the Earth's surface. The exact temperature needed to melt rock is controlled by several factors. Chemistry of the rock material, pressure, presence of gases (like water vapor) all influence when melting occurs. Most of the heat required to melt rock into magma comes from the Earth's central internal region known as the core.
Scientists estimate that the temperature of the Earth's core is about 5000° Celsius. Heat moves from the Earth's core towards the solid outer crust by convection and conduction. Convection moves hot plumes of magma vertically from the lower mantle to the upper mantle. Some of these plumes melt through the Earth's solid lithosphere and can produce intrusive igneous features and extrusive igneous features on the surface. Heat can also be generated in the lower lithosphere through friction. The tectonic movement of subducted crustal plates can generate enough heat (and pressure) to melt rock. This fact explains the presence of volcanoes along the margin of some continental plates.
TYPES OF IGNEOUS ROCKS
The type of igneous rocks that form from magma is a function of three factors: the chemical composition of the magma; temperature of solidification; and the rate of cooling which influences the crystallization process. Magma can vary chemically in its composition. For example, the amount of silica (SiO 2 ) found in magma can vary from 75% to less than 45%. The temperature of cooling determines which types of minerals are found dominating the rock's composition. Rocks that begin their cooling at low temperatures tend to be rich in minerals composed of silicon, potassium, and aluminum.
High temperature igneous rocks are dominated by minerals with higher quantities of calcium, sodium, iron, and magnesium. The rate of cooling is important in crystal development. Igneous rocks that form through a gradual cooling process tend to have large crystals. Relatively fast cooling of magma produces small crystals. Volcanic magma that cools very quickly on the Earth's surface can produce obsidian glass which contains no crystalline structures.
High temperature igneous rocks are dominated by minerals with higher quantities of calcium, sodium, iron, and magnesium. The rate of cooling is important in crystal development. Igneous rocks that form through a gradual cooling process tend to have large crystals. Relatively fast cooling of magma produces small crystals. Volcanic magma that cools very quickly on the Earth's surface can produce obsidian glass which contains no crystalline structures.
Geologists have classified the chemistry of igneous rocks into four basic types: felsic, intermediate, mafic, and ultramafic. Igneous rocks derived from felsic magma contain relatively high quantities of sodium, aluminum, and potassium and are composed of more than 65% silica. Rocks formed from felsic magma include granite , granodiorite, dacite, and rhyolite. All of these rock types are light in color because of the dominance of quartz, potassium and sodium feldspars, and plagioclase feldspar minerals (Figure 10e-1). Dacite and granodiorite contain slightly more biotite and amphibole minerals than granite and rhyolite.
- Rhyolite and dacite are produced from continental lava flows that solidify quickly. The quick solidification causes the mineral crystals in these rocks to be fine grained. Granite and granodiorite are common intrusive igneous rocks that are restricted to the Earth's continents. Large expanses these rocks were formed during episodes of mountain building on the Earth. Because granite and granodiorite form beneath the Earth's surface their solidification is a relatively slow process. This slow solidification produces a rock with a coarse mineral grain.
- Mafic magma produces igneous rocks rich in calcium, iron, and magnesium and are relatively poor in silica (silica amounts from 45 to 52%). Some common mafic igneous rocks include fine grained basalt and coarse grained gabbro. Mafic igneous rocks tend to be dark in color because they contain a large proportion of minerals rich in iron and magnesium (pyroxene, amphiboles, and olivine). Basalt is much more common than gabbro. It is found in the upper portion of the oceanic crust and also in vast continental lava flows that cover parts of Washington, Oregon, Idaho, and California. Gabbro is normally found in the lower parts of oceanic crust and sometimes in relatively small intrusive features in continental crust.
- Andesite and diorite are intermediate igneous rocks that have a chemistry between mafic and felsic (silica amounts between 53 to 65%). These rocks are composed predominantly of the minerals plagioclase feldspar, amphibole, and pyroxene. Andesite is a common fine grained extrusive igneous rock that forms from lavas erupted by volcanoes located along continental margins. Coarse grained diorite is found in intrusive igneous bodies associated with continental crust.
- Ultramafic igneous rocks contain relative low amounts of silica (< 45%) and are dominated by the minerals olivine, calcium-rich plagioclase feldspars, and pyroxene. Peridotite is the most common ultramafic rock found in the Earth's crust. These rocks are extremely rare at the Earth's surface.
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Figure 10e-1: The classification of igneous rocks. This graphic model describes the difference between nine common igneous rocks based on texture of mineral grains, temperature of crystallization, relative amounts of typical rock forming elements, and relative proportions of silica and some common minerals.
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Igneous Rocks and the Bowen Reaction Series
In the 1920s, N.L. Bowen created the following model to explain the origin of the various types of igneous rocks (Figure 10e-2). This model, known as the Bowen reaction series, suggests that the type of igneous rocks that form from magma solidification depends on the temperature of crystallization and the chemical composition of the originating magma. Bowen theorized that the formation of minerals, which make up igneous rocks, begins with two different chemical sequences at high temperatures that eventually merge into a single series at cooler temperatures.
One sequence, the discontinuous series, involves the formation of chemically unique minerals at discrete temperature intervals from iron and magnesium rich mafic magma. In the other sequence, known as the continuous series, temperature reduction causes a gradual change in the chemistry of the minerals that form calcium and sodium rich felsic magma. The discontinuous series starts with the formation of rocks that are primarily composed of the mineral olivine. Continued temperature decreases change the minerals dominating the composition of the rock from pyroxene, to amphibole, and then biotite.
The continuous series produces light colored rocks rich in plagioclase feldspar minerals. At high temperatures, the plagioclase feldspar minerals are dominated with the element calcium. With continued cooling, the calcium in these minerals is gradually replaced with sodium. The convergence of both series occurs with a continued drop in magma temperature. In the merged series, the minerals within the crystallizing rock become richer in potassium and silica and we get the formation of first potassium feldspars and then the mineral muscovite. The last mineral to crystallize in the Bowen reaction series is quartz. Quartz is a silicate mineral composed of just silicon and oxygen (SiO2).
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| Figure 10e-2: Bowen reaction series. |
Sedimentary rocks can be categorized into three groups based on sediment type. Most sedimentary rocks are formed by the lithification of weathered rock debris that has been physically transported and deposited. During the transport process, the particles that make up these rocks often become rounded due to abrasion or can become highly sorted. Examples of this type of sedimentary rock include conglomerate and sandstone. Scientists sometimes call this general group of sedimentary rocks clastic. The remaining types of sedimentary rocks are created either from chemical precipitation and crystallization, or by the lithification of once living organic matter. We identify these sedimentary rocks as being non-clastic.
Figure 10f-1: Conglomerate.
Figure 10f-2: Sandstone.
All sedimentary rocks are lithified into some collective mass. Lithification is any process that turns raw rock sediment into consolidated sedimentary rock. The process of lithification usually produces identifiable layering in these type of rocks (Figure 10f-3). Lithification can occur by way of:
- Drying and compaction.
- Oxidation of iron and aluminum.
- Precipitation of calcium and silica.
Figure 10f-3: Dipping sedimentary layers of rock, Rocky Mountains, Canada.
The classification of clastic sedimentary rocks is based on the particle types found in the rock. Some types of clastic sedimentary rocks are composed of weathered rock material like gravel, sand, silt, and clay. Others can be constructed from the break up and deposition of shells, coral and other marine organisms by wave-action and ocean currents. Table 10f-1 describes some of the main types of clastic sedimentary rocks.
Table 10f-1: Clastic sedimentary rocks.
| Name of Rock | Fragment Type |
| Breccia | Coarse Fragments of Angular Gravel and Rocks |
| Conglomerate | Coarse Fragments of Rounded Gravel and Rocks |
| Sandstone | Sand Sized Particles that are 90% Quartz |
| Arkose | Sandstone composed of 25% Feldspar Grains |
| Shale | Clay Particles |
| Siltstone | Silt Particles |
| Mudstone | Mixture of Clay and Silt |
| Limestone |
Mixture of Shells, Coral, and Other Marine Skeletons
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Figure 10f-4: Arkose.
Figure 10f-5: Shale.
Figure 10f-6: Siltstone.
Figure 10f-7: Limestone.
The oceans are almost saturated with dissolved calcium carbonate. This compound originates from the shells of a variety of marine organisms that use it for the construction of shells and other hard body parts. Because these organisms are surrounded in a solution, some of the calcium carbonate dissolves into the ocean waters. Under the right circumstances the dissolved calcium carbonate can precipitate out forming chemically created limestone deposits. The formation of dolomite involves the chemical modification of limestone deposits by a magnesium rich solution.
Figure 10f-8: Dolomite.
The following table describes some of the common forms of chemical precipitated sedimentary rocks.
Table 10f-2: Sedimentary rocks formed as chemical precipitates.
| Name of Rock | Precipitate Type |
| Halite | Sodium and Chlorine |
| Gypsum | Calcium, Sulfur, and Oxygen |
| Silcretes | Silica |
| Ferricretes | Iron |
| Limestone | Calcium Carbonate |
| Dolomite | Calcium Magnesium Carbonate |
Figure 10f-9: Halite.
Figure 10f-10: Gypsum.
Several types of sedimentary rocks are formed from the lithification of once living organisms. Limestone deposits can be formed by the direct lithification of coral reefs, marine organism shells, or marine organism skeletons. Chalk is a particular variety of limestone that is composed of the skeletons of marine microorganisms like forminifera. Coal and lignite are the lithified remains of plants.
Figure 10f-11: Chalk.
Figure 10f-12: Coal.
(c) METAMORPHIC ROCK
Metamorphism involves the alteration of existing rocks by either excessive heat and pressure, or through the chemical action of fluids. This alteration can cause chemical changes or structural modification to the minerals making up the rock. Structural modification may involve the simple reorganization of minerals into layers or the aggregation of minerals into specific areas within the rock.
Much of the Earth's continental crust is composed of metamorphic and igneous rocks. Together, these two rock types form the base material at the core of the Earth's major continental masses. Overlying this core are often thick layers of sedimentary rocks. In some regions, this base rock is exposed to the atmosphere and is known as shields. On the Canadian Shield we can find some of the oldest rocks found on the planet (3.96 billion years old). These very old rocks are primarily metamorphic. Metamorphic rocks also are the rock type found at the core of the world's various mountain ranges.
Heat and Metamorphism
Heat is an important agent in the metamorphic modification of rock. Rocks begin to change chemically at temperatures above 200° Celsius. At these temperatures, the crystalline structure of the minerals in the rock are broken down and transformed using different combinations of the available elements and compounds. As a result, new minerals are created. The metamorphic process stops when the temperatures become high enough (600 to 1200° Celsius) to cause complete melting of the rock. If rocks are heated to the point where they become magma, the magma when cooled creates new igneous rocks. Thus, metamorphism only refers to the alteration of rock that takes place before complete melting occurs.
Heat can be applied to rock through two processes: tectonic subduction and the intrusion of magma. Some rocks that are formed at the surface are subsequently transported deep into the crust and the upper mantle at tectonic subduction zones. Temperatures beneath the Earth's surface increase with depth at a rate of about 25° Celsius per kilometer. Scientists estimate that the temperature at the base of the crust is about 800 to 1200° Celsius. This heat is generated from the decay of radioactive materials, mainly in the crust, and heat released from the Earth's core.
Magma can sometimes migrate up through the crust forming an igneous intrusion. This is especially true along continental boundaries, like the western side of North America, where subduction is taking place. Metamorphism takes place in the rock surrounding the magma body because of heat dissipation. Because of the nature of the dissipation process, the level of metamorphic alteration in the influenced rock decreases with distance from the igneous intrusion.
Pressure and Metamorphism
Rocks that buried are subjected to pressure because of the weight of overlying materials. Pressure can also be exerted on rocks due the forces involved in a variety of tectonic processes. The most obvious effect of pressure on rocks is the reorientation of mineral crystals. Under extreme levels of pressure rocks become plastic creating flow structures in their crystalline structure. Pressure almost never acts in isolation as temperatures do get higher with increasing depth below the Earth's surface.
Chemical Action of Fluids
Water and carbon dioxide are often found in small amounts in the perimeter between mineral crystals or in the pore spaces of rocks. When mixed, the resulting fluid enhances metamorphism by dissolving ions and by causing chemical reactions. Usually, the end product of this process is the creation of new minerals by the substitution, removal, or addition of chemical ions. Sometimes fluids can also permeate into rock from adjacent magma.
Types of Metamorphism
Geologists suggest that metamorphism can occur by way of the following three processes.
- Thermal metamorphism involves the heating and structural and chemical alteration of rocks through processes associated with plate tectonics. This type of metamorphism has two sub-categories:
- Regional metamorphism is the large scale heating and modification of existing rock through the creation of plutons at tectonic zones of subduction. It involves large areas and large volumes of rock.
- Contact metamorphism is the small scale heating and alteration of rock by way of a localized igneous intrusion (for example, volcanic dykes or sills).
Dynamic metamorphism causes only the structural alteration of rock through pressure. The minerals in the altered rocks do not change chemically. The extreme pressures associated with mountain building can cause this type of metamorphism.
Metasomatic metamorphism involves the chemical replacement of elements in rock minerals when gases and liquids permeate into bedrock.
Common Metamorphic Rocks
Examples of metamorphic rock types include: Slate is a fine grained metamorphic rock. It is created by minor metamorphism of shale or mudstone. This rock is characterized by the foliation (Figure 10g-6) of its mineral grains which causes it to have cleavage that is parallel.
Figure 10g-1: Slate.
Schist is a medium to coarse grained foliated rock. Foliation is the result of the rearrangement of mica, chlorite, talc, and hematite mineral grains into parallel structures. When compared to slate, schists result from more intense metamorphism.
Figure 10g-2: Schist.
Gneiss is a coarse grained metamorphized igneous rock. In this rock, you get the recrystallization and foliation of quartz, feldspars, micas, and amphiboles into alternating light and dark colored bands.
Figure 10g-3: Gneiss.
Marble is a nonfoliated metamorphized limestone or dolomite).
Figure 10g-4: Marble.
Quartzite forms from the recrystallization of silica found in sandstone.
Figure 10g-5: Quartzite.
| Figure 10g-6: The mineral grains in rocks subjected to extreme pressure often rearrange themselves in a parallel fashion, creating a foliated texture (Image A - before metamorphism; Image B - after metamorphism). |
CITATION
Pidwirny, M. (2006). Fundamentals of Physical Geography, 2nd Edition. 01/01/2012. http://www.physicalgeography.net/fundamentals/10g.html
