Petrology: Rocks Origin Stories

By Simone Wright

Rocks are cool. They’re like small time capsules that describe the Earth’s history. The variety of characteristics impies a natural process unique to that rock; like a game of mystery to describe the events leading up to them. Geologists study rocks not only to understand/model Earth processes, but to access fundamental knowledge about the universe around us. Rocks are only a small portion of what we know about the solar system and early Earth. We know the composition of the Earth and the moon from studying the mineralogy of meteors. We’ve gathered that information from petrology, the study of small scale structures and the processes to create them. 

non- polarized sample from Sudbury, Canada

Petrology is a subdivision of geology that helps analyze the conditions of rocks and learn not only how the rock was made but why it connects to the universe at large. Rocks give us hints to the geological record of the Earth, specifically to the conditions during its formation. By comparing our sample rock composition to a carbonaceous chondrites (carbon based meteors) we can see the similarities in amounts of trace elements. These are elements used for radiometric age dating, a method of using the accumulation of daughter nuclides, the nuclei of different elements produced by the decay of a parent isotope. In petrology we use the radiometric age of a parent isotope, like Rubidium-87, to discover the age of the rock and rate of decay. The use of isotopes in age dating a rock is a very common practice in Geology ,however it depends on the trace elements found. We’d take a sample from the rock and measure several mineral phases at different heats and pressures. From there we can make an isochron, a linear plot of the ratios of radiometric parents to daughters.

Isochrons are helpful in analyzing the conditions rocks are made in, but we need to know what minerals are produced, what is the final composition of the melt (liquid rock), and how it connects to the solar system. This is where petrology is essential, we look at the small structures under a specialized microscope and with polarized lenses to further analyze the properties. Light polarizers are lenses in a microscope used to place over (or under) a thin section to study the crystal structures in the larger body. We use optical properties of crystals to further categorize our sample; isotropic minerals under crossed polarized light (using two polarizers above and below a sample) will deflect light at certain angles. This is typical of an isometric crystal structure, which is a cube, we’d see this as a salt grain.

Utilizing optical properties we can further comprehend minerals in the matrix (crystal structure) and based on our isochron we can look at how long those minerals took to form. But we want to know how this connects to the solar system. That’s why trace elements are important, depending on the element we’d have to use different methods of obtaining our age plot. Like the Rubidium-87, it is abundant in Earth’s crust as a replacement for potassium. In a crystal structure potassium is a large molecule requiring massive structural accommodation. So if Rubidium-87 replaced it then we’d have different mineral properties to uncover and new ages to find.

In a poetic way, petrology is similar to a DNA test but for rocks: using chemistry and thin sections of rocks we can link Earth to space. I got a chance to study the Sudbury, Canada crater impact site, and look at the geological and petrologic impacts the crater had. In the thin section, the feldspar grains show the effects of intense, pervasive, and small-scale deformation. Original single crystals can be seen, under crossed polarizers, to be composed of a mosaic of fine, irregular, and strongly disoriented crystalline domains which blur, distort, or even obliterate original features such as albite twinning or larger areas of optical continuity (French, 1967). Though with these mosaic clasts they do show isotropic minerals such as garnet or glass,  It’s understood that a garnet can occur in these conditions because the area had large bodies of water surrounding the sight. But this doesn’t mean there should be garnet scattered around the crater basin but it should show up in thin sections. 

polarized sample of Sudbury, Crater

Petrology is something I stumbled upon in my academic career after a mineralogy class and honestly pursuing it was the best decision I’ve made. The topic is so unique to geology but its applications are used everywhere in cosmochemistry, and planetary physics. Petrology is only a stepping stone into our understanding of the history of the solar system. With this knowledge we can find the age of the universe or even estimate an age of a planet. If we personified a rock, it literally came out of “hell” to just sit and live until it’s time to be swallowed and reproduced into a new rock of a different composition. It’s like a makeover for a rock, compositional changes give them new characteristics and properties that are a unique signature from Earth itself. Rock are fucking cool.

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