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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.

Thanks for watching!