How Do Igneous Rocks Form?
What Are Igneous Rocks?
Ignis, the Latin word for fire, is the perfect root word for igneous rocks, which are rocks formed by the cooling and solidification of molten materials.
Even though all igneous rocks are formed by the same basic processes, they can have many different compositions and textures based on the type of material that was melted, the speed of solidification, the presence of water, and whether the magma cooled deep in the earth or erupted onto the surface.
How are igneous rocks created, and how can we use the composition and texture of a rock to figure out how it was formed? First, we must look at how rocks melt.
What Causes a Rock to Melt?
Rock melting is influenced by three main factors: temperature changes, pressure changes, and the addition of water.
When a rock is heated, some or all of the minerals in it can melt if the rock is heated to a temperature higher than their melting point. On the graph above, this is demonstrated by going from point A to point B. Different minerals may have different melting temperatures, so often a rock will only partially melt unless the temperature increases tremendously. If both temperature and pressure are increased, in a situation like heating during burial, you might go from point A to point C, because if there is enough pressure on the rocks they won't be able to melt.
Decompression as a rock rises from depth can relieve pressure on the rock and allow it to melt. This can be shown on the graph by going from point C to point B; the rock is already hot, but with less pressure on it there are fewer forces constraining it into shape and it is able to melt. For this process to work, the rock must be fairly hot and must be uplifted relatively quickly so that it cannot cool while it is being uplifted. A rock moving from point C to point A would be an example of a rock that cools down while being slowly uplifted, staying solid throughout its rise.
The addition of water into or next to a rock can lower the temperature at which a rock will melt. This works because water molecules wedge themselves in between the small spaces within and between the rock's crystals, making the chemical bonds easier to break apart with the increased atomic vibrations that happen when a rock is heated. Adding water can reduce melting temperatures by as much as 500 degrees Celsius. A hot rock can melt if water moves into its vicinity even if the temperature and pressure do not change. A rock at point C may melt if water is introduced and the solid/liquid boundary changes from the solid line to the dotted line, moving it from a solid to a liquid.
Melting typically takes place 40-150 km beneath the surface, in the lower regions of the crust or the upper mantle. The place where the melting occurs is called the source area. Complete melting is very rare, so most magmas result from partial melting, leaving at least some of the source area unmelted.
What Happens When Magma Rises?
Magma may form in small pockets as individual crystals melt, and these pockets of magma may accumulate together as more of the rock melts, forming bigger aggregations of molten magma. As magma accumulates together, it begins to rise because it is less dense than the rocks around it.
If enough magma accumulates, a magma chamber will be formed. Some magma might solidify in the chamber and never reach the surface if it cools down enough. In other cases, the magma will only stay in magma chambers temporarily and will continue rising towards the surface.
Magma may stop in or pass through several magma chambers on the way to the surface, forming intrusions as the magma invades the surrounding rocks and assimilates material into itself. For this reason, any igneous rock that cools and solidifies beneath the surface is called an intrusive rock.
Igneous rocks that cool deep in the ground (over several kilometers down) are called plutonic rocks, from the Roman god Pluto, god of the underworld. Granite is an example of a plutonic rock, often cooling slowly in magma chambers.
Eventually, some magma will reach the surface, erupting as lava (molten rock that flows on the surface) or as volcanic ash, which forms when dissolved gases in the magma expand and shatter the magma into tiny fragments of volcanic glass.
Any igneous rock that forms on the surface is called extrusive rock, or volcanic rock, because it was extruded from the inside of the earth volcanically.
When large crystals formed deep in a magma chamber are ejected in surface eruptions and blend in with lava or ash to create rock, this blended rock is called porphyritic rock.
What Processes Influence the Composition of a Magma?
Magma composition will depend on the kind of rock that was melted in the source area and how thorough the melting of the source rock was.
Once a source rock has melted to create magma, its composition can be further changed by the formation of crystals as the magma cools, melting of rocks that touch the magma chamber, and the mixing of two or more different types of magma.
Bowen's Reaction Series Describes Which Minerals Crystallize First
Bowen's reaction series was developed by a Canadian petrologist named Norman L. Bowen. According to Bowen's research, mafic magma (magma that is rich in magnesium and iron) typically undergoes fractional crystallization, where early-formed mafic crystals are removed from the mixture by settling, leaving behind a magma with a slightly different composition.
As magma is allowed to settle and cool, it transitions from a mafic composition to a felsic composition (a more silica, aluminum, potassium, and sodium-rich magma), and becomes higher in viscosity. Due to this settling, lower parts of a magma chamber may be more mafic while the upper portions may be more intermediate to felsic, containing the lighter felsic crystals that floated up.
There are two parts to Bowen's reaction series: the discontinuous series and the continuous series. The discontinuous series has early formed minerals reacting with the melt to produce different minerals with different structures. Early in the series the minerals have more of a simple structure, like olivine's single chain structure, but as the magma cools the minerals bond together to form more complex minerals such as mica and biotite, which form in sheets.
The continuous series shows plagioclase feldspars going from being more calcium rich to sodium rich as the magma cools and they react continuously with the melt.
Partial vs. Complete Melting of Magma
Complete melting of a source rock is not very common, due to how long it can take to completely melt it and magma's tendency to rise upwards, but when it does happen the magma that is produced has a composition identical to that of the source rock. These rocks, such as komatiite and peridotite, are very rare on the surface because of their deep source locations.
Partial melting produces a magma that is more felsic than the source rock, since felsic minerals will melt at lower temperatures than mafic minerals. For example, the overall composition of the mantle is ultramafic, but magmas created in the mantle are usually mafic because mantle rocks are only partially melted.
Partial melting of mafic source rocks may yield an intermediate magma. If a more felsic source such as continental crust is melted, the resulting magma will be felsic.
Assimilation and Magma Mixing
When mafic magma touches felsic rocks, they will be melted and assimilated into the magma because the melting temperature of felsic rocks is lower than the temperature of molten mafic magma.
If felsic rock surrounds a mafic magma chamber, that felsic rock will be incorporated into the chamber and the chamber will become larger and more intermediate in composition. If felsic magma and mafic magma come into contact and mix together, the new magma will also be intermediate in composition. Sometimes you can have felsic magma surrounding chunks of mafic magma if the magma mixes unevenly.
What are the Different Types of Igneous Rocks?
The igneous rocks that result from the melting and crystallization of magma or lava are classified by their composition (mafic, intermediate, or felsic) and whether they are intrusive (cooling inside the earth) or extrusive (erupting onto the surface, as well as their texture.
- “Climate, Weather, and Their Influences on Geology.” Exploring Geology, by Stephen J. Reynolds et al., McGraw-Hill Education, 2019, pp. 114–117
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© 2019 Melissa Clason