Source: Laboratory of Alan Lester - University of Colorado Boulder
Igneous rocks are products of the cooling and crystallization of high temperature liquid rock, called magma. Magmatic temperatures typically range from approximately 800 °C to 1,200 °C. Molten rock is, perhaps luckily for humans, an anomaly on planet Earth. If a random and imaginary drill hole were made in the Earth, it would most likely not reach a region of truly and totally molten material until the outer core, at nearly 2,900 km beneath the surface (Earth's radius is 6,370 km). Even there, this molten material would predominantly consist of liquid iron, not true silicate rock, and be incapable of ever reaching Earth's surface.
Volcanic eruptions and igneous rocks do occur though, and they are evidence that there are indeed isolated regions of melting and magma generation within the Earth.
There are three primary mechanisms for rock melting within the Earth:
1) Addition of heat
Melting can occur when rocks in the Earth's mantle or crust experience an increase in ambient temperature. This is a result of high temperature magma coming into contact with rocks that have a lower melting temperature.
2) Addition of volatiles
Melting occurs in the Earth's mantle when volatile components (generally H2O, but other components, such as CO2, are possible) diffuse into a zone of rocks that are near but not quite at their melting temperature. This is called flux melting and is analogous to a welder using a flux to lower the temperature of melting for the metals that they are working with. This is the primary mechanism for melting above a down-going slab at a subduction zone, where volatiles escaping from the subducting oceanic lithosphere enter the overlying mantle and bring about flux melting. Above subduction zones, we often see a chain of volcanoes, e.g. the Cascade and Andes mountains.
Melting occurs in the Earth's mantle when the plastic and mobile asthenospheric mantle rises and undergoes decompression. This rising mantle experiences relatively minimal heat loss (as rocks are poor conductors of heat), and since melting is pressure dependent, the loss of pressure can cause the rising asthenospheric mantle to melt.
Cooling and Crystallization of Magma
Magmatic cooling and crystallization can occur in a variety of environments. However, we distinguish between the two key circumstances of surface (rapid) cooling and Earth interior (slow) cooling. These generate rocks with different crystal size, shape, and arrangement - the combination of factors that geologists refer to as texture. Surface (rapid) cooling generates rocks that are collectively called extrusive. Extrusive igneous rocks are characterized by very small crystals (invisible to the naked eye), a kind of texture referred to as aphanitic.
In contrast, cooling that takes place as a result of magma bodies solidifying in the Earth's interior (i.e. subsurface cooling) is much slower, and this leads to rocks with relatively large crystals, visible to the naked eye, and are collectively called intrusive igneous rocks. The coarser and larger grain sizes generate a texture referred to as phaneritic (Figure 1).
Composition of Magma
Ultimately, as described above, igneous rocks are classified on the basis of two features - texture (which is generally a consequence of the environment of cooling, i.e. surface or subsurface) and their composition. Compositionally, igneous rocks span a range of felsic to intermediate to mafic. Felsic rocks are rich in aluminum and silica (silicon and oxygen), whereas mafic refers to rocks that contain less silica and more iron and magnesium. Magmas compositions can range the entire spectrum between felsic and mafic. Those that are neither highly felsic nor highly mafic are referred to as intermediate. In a quantitative sense, felsic rocks contain approximately 60-75% (by weight) SiO2, and are broadly called granitic. Mafic rocks contain approximately 45-60% (by weight) SiO2, and are broadly basaltic in composition. Intermediate compositions are in the 55-63% SiO2 range, and are "andesitic" in composition.
Two experiments are commonly performed that relate to the principles of igneous rock formation. The first experiment demonstrates a key principle of melting in the Earth, and the second relates to the process of crystallization.
1) A key aspect of magma generation (whether it occurs via heat addition, volatile addition, or decompression) is that the composition of the initial melt is typically different from the composition of the mantle or crustal rock that undergoes melting. This is called partial melting and it simply means that when melting occurs in the Earth, the initial liquid (melt fraction) will be more silica-rich (more felsic) compared to the parent rock that is being melted.
A demonstration of partial melting is the squeezing of frozen grape juice. When squeezed, the liquid that oozes out is generally more purple- or grape-colored than the remaining frozen material. In other words, there is a difference in composition between the liquid (melt fraction) and the remaining frozen (solid) parent material.
2) A key aspect of igneous rock crystallization, as discussed above, relates to cooling rate, and its associated control on grain size. Although rocks can be melted in the lab, it requires highly specialized equipment and temperatures in excess of 800 °C. However, the relationship between cooling rate and crystal size can be demonstrated with a low melting point (and non-toxic) organic compound, thymol (oil of thyme), C10H14O.
Figure 1. Granite is a common type of intrusive, felsic, igneous rock, which is granular and phaneritic in texture.
1. Grape Juice experiment
- Open a canister of store-bought artificial grape juice.
- Empty some of the contents into hands and squeeze.
- Note that the liquid is a deep purple color, and the remaining solid has lost some of its purple coloration and is now more like clear ice.
2. Cooling Rate and Crystal Size
- Sprinkle a layer of thymol crystals in the bottom of a Petri dish, just covering the bottom of the dish.
- Set Petri dish on hot plate, in well-ventilated area.
- Set heat of the plate on a very low setting, just enough to begin melting. Low heat is important, otherwise the crystals will volatilize.
- Once melted, take the dish and set on a table to watch cool.
- Repeat the above steps (2.1-2.3) with a second Petri dish, but once melted, take dish and set on top of an ice water bath.
- Compare the crystal size between the Petri dish that underwent slow cooling on a table to the Petri dish that underwent rapid cooling atop the ice water bath.
Determining the composition of igneous rocks can inform scientists about the past volcanic activity of a location.
Igneous rocks are formed by the cooling and crystallization of high temperature liquid rock, known as magma. Magma is a relatively rare occurrence on the surface and upper layers of the Earth. However, magma can sometimes reach the surface through volcanic eruption or a similar event, forming extrusive igneous rocks. Alternatively, magma that cools and crystallizes under the Earth's surface is referred to as intrusive igneous rock.
This video will illustrate how intrusive igneous rocks are formed, and demonstrate how to simulate their formation with two simple experiments.
Magma cooling and crystallization can occur in a variety of environments, in a variety of ways. The speed of cooling, rapid or slow, can have large effects on the resultant rock formed. Different cooling rates generate rocks with various crystal size, shape, and arrangement, factors which define the overall rock texture. Surface, or rapid cooling, generates rocks that are characterized by very small crystals, in a texture referred to as aphanitic.
In contrast, cooling that happens in the subsurface as magma bodies solidify in the Earth's interior happens much more slowly. Magma may exist in a stage known as partial melt. This cooling and solidification generates rocks with relatively large crystals, visible to the naked eye. Rock of this type is referred to as intrusive igneous rock, and the coarser and larger grain sizes generate a texture referred to as phaneritic.
Both texture and composition define the specific types of igneous rock. Compositionally, igneous rocks span a range of felsic, to intermediate, to mafic. Felsic rocks are rich in aluminum and silica, whereas mafic rocks contain less silica, but more iron and magnesium. Magma compositions can fall anywhere on the spectrum between felsic and mafic.
Quantitatively, felsic rocks contain approximately 60-75% silicon dioxide by weight, and are more broadly called granitic. Mafic rocks contain around 45-60% silicon dioxide, and are broadly basaltic in composition. Intermediate compositions, at roughly 55-63% silicon dioxide, are referred to as andesitic.
Using two laboratory demonstrations, we can illustrate the processes of intrusive igneous rock formation and crystal formation at different cooling temperatures.
The first stage in partial melt demonstration is to select an appropriate lava substitute. Colored liquids like fruit juices can work well for this. To start the experiment, open a canister of frozen store-bought grape juice.
Next, empty a quarter of the container into gloved hands. Squeeze the frozen juice, making sure to provide constant and firm pressure. Note that the liquid draining off the frozen juice is a deep purple color. In contrast, the remaining solid has lost some of its coloration and appears paler than before.
The melting of grape juice demonstrates the concept of partial melting, as seen in magma. An initial melt, which will be liquid, is typically of different composition than the parent rock that undergoes melting.
The pigmented portion of the grape juice melts fastest, meaning that much of the pigment will run into the container early in the experiment, leaving less color behind. This simulates partial melting, and highlights differences in magma composition. The first liquid formed during partial melting of a rock, simulated by the dyed portion of the grape juice, is enriched in felsic components. When this liquid is removed from the system, as typically happens, then the remaining rock, represented by the clearer ice, will be of a more mafic composition.
Thymol, a naturally occurring organic compound, is used to simulate rock crystallization. Sprinkle a layer of thymol crystals into a Petri dish, enough to cover the bottom. Set the Petri dish on a hot plate on a very low setting in a well-ventilated area. Low heat is important to prevent the crystals volatizing. Once the crystals have melted, remove the Petri dish from the heat. Set the dish on a table at room temperature and observe the cooling. Repeat the above heating steps with a second Petri dish and thymol crystals, but once melted, take the dish and place on top of an ice water bath to cool.
The thymol crystal experiment demonstrates what happens to igneous rock grain size at different cooling rates. Rapid cooling generates smaller crystals than slow cooling, and this difference is easily observed in the re-formed thymol crystals. The mixed crystals formed under slower cooling conditions resemble those seen in intrusive igneous rocks, which are formed during a slower process of cooling in the Earth's subsurface. In contrast, the smaller crystals formed under rapid cooling resemble extrusive igneous rocks, also known as aphanitic rocks, which form after magma breaches the surface via an eruption.
Identifying and understanding the properties and formation of intrusive igneous rock has vast applications for geologists and human populations as a whole.
Intrusive igneous rocks can be markers for certain types of ore deposit. For example, felsic to intermediate intrusive magma bodies are often associated with the formation of copper, molybdenum, gold, or silver ores. In contrast, mafic intrusions may be associated with chromium, platinum, and nickel deposits. The ability to identify potential deposits easily allows targeted drilling or mining, and has cost and environmental implications for the industry.
If magmas breach the surface, volcanic eruptions occur. Intrusive igneous rocks present in an area act as a marker for field geologists to check for any evidence of volcanic rocks, and determination of the area as potentially volcanically active, or previously volcanically active. This information can be used to predict the likelihood of areas still being volcanically active, or having the potential to become so in the future. This is important for land-use planning or management, or assessing potential risks to existing settlements or structures.
Intrusive igneous rocks are also useful markers for deciphering Earth history. Igneous rocks are relatively easy to date. This can be achieved by measuring the relative abundance of radiogenic parent to daughter, or "decay product" isotopes. Qualitatively, rocks that have higher ratios of radiogenic daughter to parent abundances are older, because there has been more time for parent isotopes to decay into daughter isotopes. The type of igneous rocks present in an area can also indicate past regions of melting within the continental crust, subduction zone activity, and continental or mid-ocean rift zones. This gives geologists the ability to infer what sort of tectonic settings were present during the time of the rock formation.
You've just watched JoVE's introduction to intrusive igneous rocks. You should now understand the differences between intrusive and extrusive igneous rock, how intrusive rocks are formed, and how to simulate partial melting and intrusive rock formation in a laboratory.
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1) The grape juice experiment demonstrates the concept of partial melting. Where an initial liquid (melt) is typically of a different composition than the parent rock that undergoes melting.
2) The thymol experiment demonstrates the concept of igneous rock grain size as being related to cooling rate. Rapid cooling generates smaller crystals than slow cooling.
Applications and Summary
Igneous rocks are of substantial importance.Geologists identify and map intrusive igneous rocks for a variety of reasons.
Intrusive igneous rocks can be markers of certain kinds of ore deposits. For example, felsic to intermediate composition intrusive magma bodies can act as the heat sources that drive hydrothermal circulation systems, and concomitant precipitation within fractures (veins) of ore minerals including Cu, Mo, Au, Ag, and others. In contrast, mafic to ultramafic intrusions are associated with Cr, Pt, and Ni deposits.
Intrusive igneous rocks can also be markers of past magmatic activity. If magmas breach the surface, then volcanic eruptions occur. Therefore the recognition of intrusive igneous rocks will lead a field geologist to assess whether or not any associated volcanic rocks are present.
Intrusive igneous rocks are part of deciphering Earth's history. This is partly because intrusive igneous rocks are relatively easy to date using isotopic techniques, and because the type of igneous rock can be a marker of a past plate tectonic setting. For example, felsic rocks are characteristic of melting within the continental crust (i.e. intraplate magmatism). Intermediate rocks are characteristic of subduction zone settings. Mafic rocks are characteristic of mid-ocean ridges and continental rift zones.