Geology (from Ancient Greek γῆ (gê) 'earth', and λoγία* (-logía)* 'study of, discourse')[1][2] is a branch of natural science concerned with Earth and other astronomical objects, the rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth sciences, including hydrology. It is integrated with Earth system science and planetary science.
Geology describes the structure of the Earth on and beneath its surface, and the processes that have shaped that structure. Geologists study the mineralogical composition of rocks in order to get insight into their history of formation. Geology determines the relative ages of rocks found at a given location; geochemistry (a branch of geology) determines their absolute ages.[3] By combining various petrological, crystallographic and paleontological tools, geologists are able to chronicle the geological history of the Earth as a whole. One aspect is to demonstrate the age of the Earth. Geology provides evidence for plate tectonics, the evolutionary history of life, and the Earth's past climates.
Geologists broadly study the properties and processes of Earth and other terrestrial planets. Geologists use a wide variety of methods to understand the Earth's structure and evolution, including fieldwork, rock description, geophysical techniques, chemical analysis, physical experiments, and numerical modelling. In practical terms, geology is important for mineral and hydrocarbon exploration and exploitation, evaluating water resources, understanding natural hazards, remediation of environmental problems, and providing insights into past climate change. Geology is a major academic discipline, and it is central to geological engineering and plays an important role in geotechnical engineering.
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The majority of geological data comes from research on solid Earth materials. Meteorites and other extraterrestrial natural materials are also studied by geological methods.
Main article: Mineral
Minerals are natural occurring elements and compounds with a definite homogeneous chemical composition and ordered atomic composition.
Each mineral has distinct physical properties, and there are many tests to determine each of them. Minerals are often identified through these tests. The specimens can be tested for:[4]
Luster: Quality of light reflected from the surface of a mineral. Examples are metallic, pearly, waxy, dull.
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 scratching.
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 such as halite (which tastes like table salt).
The rock cycle shows the relationship between igneous, sedimentary, and metamorphic rocks.
Main articles: Rock (geology) and Rock cycle
A rock is any naturally occurring solid mass or aggregate of minerals or mineraloids. Most research in geology is associated with the study of rocks, as they provide the primary record of the majority of the geological 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 solidifies or 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 solidify. Organic matter, such as coal, bitumen, oil and natural gas, is linked mainly to organic-rich sedimentary rocks.
Quartz from Tibet. Quartz makes up more than 10% of the Earth's crust by mass.
To study all three types of rock, geologists evaluate the minerals of which they are composed and their other physical properties, such as texture and fabric.