Geology

Geology is a branch of natural sciences concerned with the Earth and other astronomical bodies, the rocks of which they are composed, and the processes by which they change over time. The name comes from Ancient Greek γῆ (gê) 'earth' and λoγία (-logía) 'study of, discourse'. 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. 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, remediating 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.

Geological material

The majority of geological data comes from research on solid Earth materials. Meteorites and other extraterrestrial natural materials are also studied by geological methods.

Minerals

Minerals are naturally occurring elements and compounds with a definite homogeneous chemical composition and an ordered atomic arrangement. Amorphous substances that resemble a mineral are sometimes referred to as mineraloids, although there are exceptions such as georgeite and autunite. Some amorphous substances formed by geological processes are considered minerals if the original substance was a mineral before metamictisation.

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:

• 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 identify the mineral.
• Hardness: The resistance of a mineral to scratching or indentation.
• 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.
• Luster: Quality of light reflected from the surface of a mineral. Examples are metallic, pearly, waxy, dull.
• 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).

Rock

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. The active flow of molten rock is closely studied in volcanology, and igneous petrology aims to determine the history of igneous rocks from their original molten source to their final crystallization.

Rocks can be weathered and eroded, then redeposited and lithified into a sedimentary rock. Sedimentary rocks are mainly divided into four categories: sandstone, shale, carbonate, and evaporite. This group of classifications focuses partly on the size of sedimentary particles (sandstone and shale), and partly on mineralogy and formation processes (carbonation and evaporation). Igneous and sedimentary rocks can then be turned into metamorphic rocks 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 again solidify. Organic matter, such as coal, bitumen, oil, and natural gas, is linked mainly to organic-rich sedimentary rocks.

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.

Unlithified material

Geologists study unlithified materials (referred to as superficial deposits) that lie above the bedrock. This study is often known as Quaternary geology, after the Quaternary period of geologic history, which is the most recent period of geologic time.

Whole-Earth structure

Plate tectonics

In the 1960s, it was discovered that the Earth's lithosphere, which includes the crust and rigid uppermost portion of the upper mantle, is separated into tectonic plates that move across the plastically deforming, solid, upper mantle, which is called the asthenosphere. This theory is supported by several types of observations, including seafloor spreading and the global distribution of mountain terrain and seismicity.

There is an intimate coupling between the movement of the plates on the surface and the convection of the mantle (that is, the heat transfer caused by the slow movement of ductile mantle rock). Thus, oceanic parts of plates and the adjoining mantle convection currents always move in the same direction – because the oceanic lithosphere is actually the rigid upper thermal boundary layer of the convecting mantle. This coupling between rigid plates moving on the surface of the Earth and the convecting mantle is called plate tectonics.

The development of plate tectonics has provided a physical basis for many observations of the solid Earth. Long linear regions of geological features are explained as plate boundaries:

• Mid-ocean ridges, high regions on the seafloor where hydrothermal vents and volcanoes exist, are seen as divergent boundaries, where two plates move apart.
• Arcs of volcanoes and earthquakes are theorized as convergent boundaries, where one plate subducts, or moves, under another.
• Transform boundaries, such as the San Andreas Fault system, are where plates slide horizontally past each other.

Plate tectonics has provided a mechanism for Alfred Wegener's theory of continental drift, in which the continents move across the surface of the Earth over geological time. They provided a driving force for crustal deformation, and a new setting for the observations of structural geology. The power of the theory of plate tectonics lies in its ability to combine all of these observations into a single theory of how the lithosphere moves over the convecting mantle, forming a "grand unifying theory of geology".

Earth structure

Advances in seismology, computer modeling, and mineralogy and crystallography at high temperatures and pressures give insights into the internal composition and structure of the Earth.

Seismologists can use the arrival times of seismic waves to image the interior of the Earth. Early advances in this field showed the existence of a liquid outer core (where shear waves were not able to propagate) and a dense solid inner core. These advances led to the development of a layered model of the Earth, with a lithosphere (including crust) on top, the mantle below (separated within itself by seismic discontinuities at 410 and 660 kilometers), and the outer core and inner core below that. Starting in the 1970s, seismologists have been able to use new techniques such as seismic full-waveform inversion to create detailed images of wave speeds inside the earth in the same way a doctor images a body in a CT scan. These images have led to a much more detailed view of the interior of the Earth, and have replaced the simplified layered model with a much more dynamic model.

Mineralogists have been able to use the pressure and temperature data from the seismic and modeling studies alongside knowledge of the elemental composition of the Earth to reproduce these conditions in experimental settings and measure changes within the crystal structure. These studies explain the chemical changes associated with the major seismic discontinuities in the mantle and show the crystallographic structures expected in the inner core of the Earth.