Geology is the study of Earth's structure, which includes the liquid and solid Earth, the rocks that the Earth is composed of, and the ways that those rocks change over time. As you can probably imagine, geologists spend a LOT of time looking at rocks, and microscopes help geologists see things in those rocks that are otherwise invisible to the naked eye. Some of the specific geological applications that involve the use of microscopes include lithology, petrology, and structural geology. Let's take a look at each discipline's definition and discover the unique role that microscopy plays in the world of geology.
Lithology: The Study of the Physical Characteristics of Rocks
The lithology of a rock is the summation of its physical characteristics. In geology classrooms, students use a low-magnification microscope to observe a specimen's traits – or lithology – to identify it by name and class. While many geologists identify rocks in the lab, the identification of rocks in the field is more often associated with the term lithology.
Some of the physical characteristics of rocks that usually get recorded include rock type, grain/clast size, mineralogy, color, fabric, and texture. Lithology is especially important in the field of mud logging, which involves the creation of a detailed log describing the physical traits of the cuttings that come from the borehole that is being drilled. Mud loggers use a microscope to examine cuttings, which are small, solid pieces of material that are released from the borehole and then brought to the surface with the drilling mud. Using microscopy with only a 10x magnification, the mudlogger can determine rock type, pore type, whether traces of oil are present, and whether the cuttings can hold oil or gas. These observations are drawn into a graph of information, and that graph is included as part of the mud log.
There was a time when lithology and petrography were synonymous, and people still conflate the two. Lithology focuses on describing the macroscopic characteristics of rocks, which are traits that are visible even to the naked eye, but petrography, which is a branch of petrology (see below), takes note of the details so small that you NEED a microscope to see them. Thorough microscopic examination is difficult to do in the field, which is why petrology is more concretely associated with laboratory settings.
Petrology: The Study of the Conditions under which Rocks Form
While lithology focuses on listing a specimen's physical characteristics, petrology focuses on identifying the specimen AND discovering how that specimen was formed. With a high-magnification petrographic microscope, geologists can study specimens in thin sections to determine their pleochroism, birefringence, and interference properties. Electron microprobes allow scientists to see the exact chemical makeup of a specimen, and further studies into the stable and radioactive isotopes will provide understanding about the specimen's geochemical evolution.
Electron microprobes, also known as electron probe microanalyzers, utilize two different techniques to study a sample thoroughly. These techniques include X-ray spectroscopy and electron microscopy: X-ray spectroscopy uses X-ray excitation to analyze a sample, while electron microscopes use a beam of accelerated electrons as an illumination source. Electron microprobes work similarly to scanning electron microscopes, enabling scientists to discover and analyze the sample's elements. Petrologists use this data to determine whether their samples are igneous, metamorphic, or sedimentary, which tells them how the rocks formed in the first place.
All the information that petrologists collect using microscopes allows them to determine the origins of the rocks they're studying. On the other hand, structural geology deals with those histories on a wider scope, revealing the history not only of the sample but also its immediate environment and even the planet itself.
Structural Geology: The Study of the History Rock Geometries to Uncover their Histories
Structural geologists use microscopes to examine thin sections of rock, paying special attention to the fabric of their samples. In geology, “fabric” refers to the geometric and spatial arrangement of the sample's elements. These patterns help geologists understand geologic structures, which sheds light on weaknesses within the rock, the likelihood that an earthquake will occur, how geologic sites affect groundwater flow, and even how the tectonic plates move and shift.
Petrographic microscopes are especially helpful when studying rock textures, also referred to as rock microstructures. Metamorphic and sedimentary rocks are the most common rock types studied when it comes to structural geology, though the texture of igneous rocks is also very valuable. Information gathered with petrographic microscopes can give insight into the conditions under which the sample was formed and the ways it underwent deformation, folding, and other events that altered it.
The aim of microstructure analysis mostly depends on the type of rock being analyzed. Scientists who study sedimentary microstructures want to understand the conditions under which the sediment was deposited, the environments of the sample's past, and the sample's earliest place of origin. Meanwhile, the texture of metamorphic rocks helps scientists determine when, how, and why deformations occurred within the sample. It all comes down to the sample's histories, which in turn helps geologists understand the geological structure of the planet we live on.
Small Picture, Big World
Isn't it amazing how a tiny sample of rock can tell us so much about the structure of our planet? Geologists play an incredibly important role in helping us understand Earth's history, and they couldn't do it without the help of microscopes. Microscopes allow scientists to see things within rocks that no one would know was there otherwise, and the study of geology tells us as much about the planet's future as it does about its past. We hope that this article taught you something about one of the most important scientific disciplines ever developed. Still, more importantly, we hope that it helps you appreciate the incredible amount of time and effort that geologists put into their work.