Studying Rock

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)
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The Basis Of Rock Classification

Beginning in the 18th century, geologists struggled to develop a sensible way to classify rocks, for they realized, as did miners from centuries past, that not all rocks are the same. Classification schemes help us organize information and remember significant details about materials or objects, and they help us recognize similarities and differences among them. By the end of the 18th century, most geologists had accepted the genetic scheme for classifying rocks that we continue to use today. This scheme focuses on the origin (genesis) of rocks. Using this approach, geologists recognize three basic groups: (1) igneous rocks, which form by the freezing (solidification) of molten rock (figure above a); (2) sedimentary rocks, which form either by the cementing together of fragments (grains) broken off preexisting rocks or by the precipitation of mineral crystals out of water solutions at or near the Earth’s surface (figure above b); and (3) metamorphic rocks, which form when pre-existing rocks change character in response to a change in pressure and temperature conditions (figure above c). Metamorphic change occurs in the solid state, which means that it does not require melting. In the context of terbaru plate tectonics theory, different rock types form in different geologic settings (figure below). A cross section illustrating various geologic settings in which rocks form. Each of the three groups contains many different perorangan rock types, distinguished from one another by physical characteristics. Describing grains in rock. Grain size: The dimensions of perorangan “grains” (here used in a general sense to mean fragments or crystals) in a rock may be measured in millimetres or centimetres. Some grains are so small that they can’t be seen without a microscope, whereas others are as big as a fist or larger. Some grains are equant, meaning that they have the same dimensions in all directions; some are inequant, meaning that the dimensions are not the same in all directions (figure above a, b). In some rocks, all the grains are the same size, whereas other rocks contain a variety of grain sizes. Composition: A rock is a mass of chemicals. The term rock composition refers to the proportions of different chemicals making up the rock. The proportion of chemicals, in turn, affects the proportion of different minerals  constituting the rock.  Texture: This term refers to the arrangement of grains in a rock, that is, the way grains connect to one another and whether or not inequant grains are aligned parallel to each other.  Layering: Some rock bodies appear to contain distinct layering, defined either by bands of different compositions or textures, or by the alignment of inequant grains so that they trend parallel to each other. Different types of  layering occur in different kinds of rocks. For example, the layering in sedimentary rocks is called bedding, whereas the layering in metamorphic rocks is called metamorphic foliation (figure below a, b). Each distinct rock type has a name. Names come from a variety of sources. Some come from the dominant component making up the rock, some from the region where the rock was first discovered or is particularly abundant, some from a root word of Latin origin, and some from a traditional name used by people in an area where the rock is found. Credits: Stephen Marshak (Essentials of Geology)
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