The study of rocks begins by examining a rock in an outcrop. If the outcrop is big enough, such an examination will reveal relationships between the rock you’re interested in and the rocks around it, and will allow you to detect layering. Geologists carefully record observations about an outcrop, then break off a hand specimen, a ﬁst-sized piece, that they can examine more closely with a hand lens (magnifying glass). Observation with a hand lens enables geologists to identify sand-sized or larger grains, and may enable them to describe the texture of the rock.
Geologists often must examine rock composition and texture in minute detail in order to identify a rock and develop a hypothesis for how it formed. To do this, they take a specimen back to the lab, make a very thin slice (about 0.03 mm thick, the thickness of a human hair) and mount it on a glass slide (figure above a–c). They study the resulting thin section with a petrographic microscope (petro comes from the Greek word for rock). A petrographic microscope differs from an ordinary microscope in that it illuminates the thin section with transmitted polarized light. This means that the illuminating light beam ﬁrst passes through a special polarity ﬁlter that makes all the light waves in the beam vibrate in the same plane, and then the light passes up through the thin section and then up through another polarizing ﬁlter. An observer looks through the thin section as if it were a window. When illuminated with transmitted polarized light, and viewed through two polarizing ﬁlters, each type of mineral grain displays a unique suite of colours (figure above d). The speciﬁc colour the observer sees depends on both the identity of the grain and its orientation with respect to the waves of polarized light.
The brilliant colours and strange shapes in a thin section viewed in polarized light rival the beauty of an abstract painting or stained glass. By examining a thin section with a petrographic microscope, geologists can identify most of the minerals constituting the rock and can describe the way in which the grains connect to each other. They can make a record of the image by using a camera. A photograph taken through a petrographic microscope is called a photomicrograph.
High-Tech Analytical Equipment
|An electron micro-probe uses a beam of electrons to analyse the chemical composition of minerals.|
Beginning in the 1950s, high-tech electronic instruments became available that enabled geologists to examine rocks on an even ﬁner scale than is possible with a petrographic microscope. Modern research laboratories typically boast instruments such as electron micro-probes, which can focus a beam of electrons on a small part of a grain to create a signal that deﬁnes the chemical composition of the mineral (figure above); mass spectrometers, which analyse the proportions of atoms with different atomic weights contained in a rock; and X-ray diffractometers, which identify minerals by measuring how X-ray beams interact with crystals. Such instruments, in conjunction with optical examination, can provide geologists with highly detailed characterizations of rocks, which in turn help them understand how the rocks formed and where the rocks came from. This information enables geologists to use the study of rocks as a basis for deciphering Earth history.
Credits: Stephen Marshak (Essentials of Geology)