The majority of geological data comes from research on solid Earth materials. These typically fall into one of two categories: rock and unconsolidated material.
This schematic diagram of the rock cycle shows the relationship between magma and sedimentary, metamorphic, and igneous rock
The majority of research in geology is associated with the study of rock, as rock provides the primary record of the majority of the geologic history of the Earth. There are three major types of rock: igneous, sedimentary, and metamorphic. The rock cycle
illustrates the relationships among them (see diagram).
When a rock crystallizes from melt (magma or lava), it is an igneous rock. This rock can be weathered and eroded, then redeposited and lithified into a sedimentary rock. It can then be turned into a metamorphic rock by heat and pressure that change its mineral content, resulting in a characteristic fabric. All three types may melt again, and when this happens, new magma is formed, from which an igneous rock may once more crystallize.
To study all three types of rock, geologists evaluate the minerals of which they are composed. Each mineral has distinct physical properties, and there are many tests to determine each of them. The specimens can be tested for:
- Luster: Measurement of the amount of light reflected from the surface. Luster is broken into metallic and nonmetallic.
- Color: Minerals are grouped by their color. Mostly diagnostic but impurities can change a mineral’s color.
- Streak: Performed by scratching the sample on a porcelain plate. The color of the streak can help name the mineral.
- Hardness: The resistance of a mineral to scratch.
- Breakage pattern: A mineral can either show fracture or cleavage, the former being breakage of uneven surfaces and the latter a breakage along closely spaced parallel planes.
- Specific gravity: the weight of a specific volume of a mineral.
- Effervescence: Involves dripping hydrochloric acid on the mineral to test for fizzing.
- Magnetism: Involves using a magnet to test for magnetism.
- Taste: Minerals can have a distinctive taste, like halite (which tastes like table salt).
- Smell: Minerals can have a distinctive odor. For example, sulfur smells like rotten eggs.
Geologists also study unlithified materials (referred to as drift), which typically come from more recent deposits. These materials are superficial deposits which lie above the bedrock. This study is often known as Quaternary geology, after the Quaternary period of geologic history.