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Geology (from the
Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek (, ), refers collectively to the diale ...
γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is a branch of
Earth science Earth science or geoscience includes all fields of natural science Natural science is a branch of science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific method, builds and Ta ...
concerned with both the liquid and
solid EarthSolid earth refers to "the earth beneath our feet" or '' terra firma'', the planet's solid surface and its interior. It contrasts with the Earth's fluid envelopes, the atmosphere and hydrosphere (but includes the ocean basin), as well as the bio ...
, the
rocks In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is composed, and the proc ...
of which it is composed, and the processes by which they change over time. Geology can also include the study of the solid features of any
terrestrial planet A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate Rock (geology), rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Su ...
or
natural satellite A natural satellite is in the most common usage, an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science Natural science is a branch ...

natural satellite
such as
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, being larger than only Mercury (planet), Mercury. In English, Mars carries the name of the Mars (mythology), Roman god of war and is often referred to ...
or
the Moon The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...
. Modern geology significantly overlaps all other Earth sciences, including
hydrology Hydrology (from Ancient Greek, Greek wikt:ὕδωρ, ὕδωρ, ''hýdōr'' meaning "water" and wikt:λόγος, λόγος, ''lógos'' meaning "study") is the scientific study of the movement, distribution, and management of water on Earth and ...
and the
atmospheric sciences Atmospheric science is the study of the Atmosphere of Earth, Earth's atmosphere and its various inner-working physical processes. Meteorology includes atmospheric chemistry and atmospheric physics with a major focus on weather forecasting. Climat ...
, and so is treated as one major aspect of integrated
Earth system science Earth system science (ESS) is the application of systems science Systems science is an interdisciplinary Interdisciplinarity or interdisciplinary studies involves the combination of two or more academic disciplines into one activity (e.g. ...
and
planetary science Planetary science (or more rarely, planetology) is the scientific study of planets (including Earth), Astronomical object, celestial bodies (such as Natural satellite, moons, Asteroid, asteroids, Comets on Fire, comets) and planetary systems (in p ...
. Geology describes the
structure of the Earth The internal structure of Earth, structure of the solid Earth, or simply structure of Earth refers to concentric spherical layers subdividing the Solid earthSolid earth refers to "the earth beneath our feet" or '' terra firma'', the planet's sol ...
on and beneath its surface, and the processes that have shaped that structure. It also provides tools to determine the
relative Relative may refer to: General use *Kinship and family, the principle binding the most basic social units society. If two people are connected by circumstances of birth, they are said to be ''relatives'' Philosophy *Relativism, the concept that p ...
and
absolute agesAbsolute dating is the process of determining an age on a specified chronology 222px, Joseph Scaliger's ''De emendatione temporum'' (1583) began the modern science of chronology Chronology (from Latin Latin (, or , ) is a classical language ...
of rocks found in a given location, and also to describe the histories of those rocks. By combining these tools,
geologist A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes Earth and other terrestrial planets, as well as the processes that shape them. Geologists usually study geology, although backgrounds in physics, chem ...

geologist
s are able to chronicle the geological
history of the Earth The history of Earth concerns the development of planet Earth from its formation to the present day. Nearly all branches of natural science have contributed to understanding of the main events of Earth's past, characterized by constant geolo ...
as a whole, and also to demonstrate the
age of the Earth The age of Earth is estimated to be 4.54 ± 0.05 1,000,000,000, billion years This age may represent the age of the Earth's accretion (astrophysics), accretion, or core formation, or of the material from which the Earth formed. This dating is b ...
. Geology provides the primary evidence for
plate tectonics upright=1.35, Diagram of the internal layering of Earth showing the lithosphere above the asthenosphere (not to scale) Plate tectonics (from the la, label=Late Latin Late Latin ( la, Latinitas serior) is the scholarly name for the written L ...
, the
evolutionary history of life The history of life on Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water ...
, and the Earth's past climates. Geologists use a wide variety of methods to understand the Earth's structure and evolution, including
field work Field research, field studies, or fieldwork is the empirical research, collection of raw data outside a laboratory, library, or workplace setting. The approaches and methods used in field research vary across branches of science, disciplines. ...

field work
, rock description, geophysical techniques,
chemical analysis Analytical chemistry studies and uses instruments and methods used to separate, identify, and quantify matter. In practice, separation, identification or quantification may constitute the entire analysis or be combined with another method. Separ ...
,
physical experiment An experiment is a procedure carried out to support, refute, or validate a hypothesis A hypothesis (plural hypotheses) is a proposed explanation for a phenomenon. For a hypothesis to be a scientific hypothesis, the scientific method ...
s, and numerical modelling. In practical terms, geology is important for
mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

mineral
and
hydrocarbon In organic chemistry Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, prop ...
exploration and exploitation, evaluating
water resources Water resources are natural resources of water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main con ...
, understanding of
natural hazard A natural hazard is a natural phenomenon Types of natural phenomena include: Weather, fog, thunder, tornadoes; biological processes, decomposition, germination seedlings, three days after germination. Germination is the process by which ...
s, the remediation of
environmental A biophysical environment is a life, biotic and Abiotic component, abiotic surrounding of an organism or population, and consequently includes the factors that have an influence in their survival, development, and evolution. A biophysical environ ...
problems, and providing insights into past
climate change Contemporary climate change includes both the global warming caused by humans, and its impacts on Earth's weather patterns. There have been previous periods of climate change, but the current changes are more rapid than any known even ...
. Geology is a major
academic discipline An academic discipline or academic field is a subdivision of knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in the real world. The usual test for a statement of fact is ...
, and it is central to
geological engineering Geoprofessions is a term coined by the Geoprofessional Business Association to connote various technical disciplines that involve engineering, earth and environmental services applied to below-ground (“subsurface”), ground-surface, and ground-su ...
and plays an important role in
geotechnical engineering Geotechnical engineering, also known as geotechnics, is the branch of civil engineering concerned with the engineering behavior of earth materials. It uses the principles of soil mechanics and rock mechanics for the solution of its respective e ...
.


Geological materials

The majority of geological data comes from research on solid Earth materials. These typically fall into one of two categories: rock and unlithified material. Meteorites and other extra-terrestrial natural materials are also studied by geological methods.


Mineral

Minerals are natural occurring elements and compounds with a definite homogeneous chemical composition and ordered atomic composition.


Rock

A rock is any naturally occurring solid mass or aggregate of minerals or
mineraloid A mineraloid is a naturally occurring mineral-like substance that does not demonstrate crystallinity. Mineraloids possess chemical compositions that vary beyond the generally accepted ranges for specific minerals. For example, obsidian is an amorp ...
s. 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 Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others being Sedimentary rock, sedimentary and metamorphic rock, metamorphic. Igneous rock i ...
,
sedimentary Sedimentary rocks are types of rock (geology), rock that are formed by the accumulation or deposition of mineral or organic matter, organic particles at Earth#Surface, Earth's surface, followed by cementation (geology), cementation. Sedimentatio ...

sedimentary
, and
metamorphic Metamorphic rocks arise from the transformation of existing rock Rock most often refers to: * Rock (geology) A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the mineral ...
. The
rock cycle The rock cycle is a basic concept in geology that describes transitions through geologic time among the three main rock (geology), rock types: sedimentary, metamorphic rock, metamorphic, and igneous. Each rock type is altered when it is forced ...

rock cycle
illustrates the relationships among them (see diagram). When a rock solidifies or
crystallizes Crystallization or crystallisation is the process by which a solid forms, where the Atom, atoms or molecules are highly organized into a Crystal structure, structure known as a crystal. Some of the ways by which crystals form are Precipitation ( ...

crystallizes
from melt (
magma Magma () is the molten or semi-molten natural material from which all igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others ...

magma
or
lava Lava is magma Magma () is the molten or semi-molten natural material from which all s are formed. Magma is found beneath the surface of the , and evidence of has also been discovered on other and some s. Besides molten rock, magma may al ...

lava
), it is an igneous rock. This rock can be
weathered ''Weathered'' is the third studio album by American Rock music, rock band Creed (band), Creed, released on November 20, 2001. It was the last Creed album to be released until ''Full Circle (Creed album), Full Circle'' came out in October 2009, wit ...
and
eroded In earth science Earth science or geoscience includes all fields of natural science Natural science is a branch of science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific ...
, then redeposited and
lithified Lithification (from the Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following per ...
into a sedimentary rock. It can then be turned into a
metamorphic rock Metamorphic rocks arise from the transformation of existing rock Rock most often refers to: * Rock (geology) A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the mineral ...

metamorphic rock
by heat and pressure that change its
mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

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.


Tests

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: 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 Porcelain () is a ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant Corrosion is a Erosion, natural process that converts a refined metal into a more chemically stable form such as oxide, ...

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 Hydrochloric acid +(aq) Cl−(aq) or H3O+ Cl− also known as muriatic acid, is an aqueous solution An aqueous solution is a solution Solution may refer to: * Solution (chemistry) Image:SaltInWaterSolutionLiquid.jpg, upMaking a salin ...

hydrochloric acid
on the mineral to test for fizzing. * Magnetism: Involves using a magnet to test for
magnetism Magnetism is a class of physical attributes that are mediated by magnetic field A magnetic field is a vector field In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. For in ...

magnetism
. * Taste: Minerals can have a distinctive taste, such as
halite Halite (), commonly known as rock salt, is a type of salt Salt is a mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition a ...
(which tastes like
table salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of Salt (chemistry), salts; salt in the form of a natural crystallinity, crystalline mineral is known as rock salt or halite. ...
). * Smell: Minerals can have a distinctive odor. For example,
sulfur Sulfur (in nontechnical British English: sulphur) is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: th ...

sulfur
smells like rotten eggs.


Unlithified material

Geologists also study unlithified materials (referred to as '' drift''), which typically come from more recent deposits. These materials are
superficial deposits Superficial deposits refer to geological deposits typically of Quaternary Quaternary ( ) is the current and most recent of the three Period (geology), periods of the Cenozoic Era (geology), Era in the geologic time scale of the International Comm ...
that lie above the
bedrock Bedrock in geology Geology (from the γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is a branch of concerned with both the liquid and , the of which it is composed, and the processes by which they cha ...

bedrock
. This study is often known as
Quaternary geology Quaternary geology is the branch of geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which i ...
, after the
Quaternary period The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic The Cenozoic Era ( ) meaning "new life" is the current and most recent of the three geological eras of the Phanerozoic Eon. The Cretaceous–Paleogene ext ...
of geologic history.


Magma

Unlithified material does not only include
sediments Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently sediment transport, transported by the action of wind, water, or ice or by the force of gravity acting on the particles. ...
. Magma is the original unlithified source of all
igneous rocks Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the p ...

igneous rocks
. The active flow of molten rock is closely studied in
volcanology Volcanology (also spelled vulcanology) is the study of volcano A volcano is a rupture in the crust of a planetary-mass object A planet is an astronomical body orbit In physics, an orbit is the gravitationally curved trajecto ...

volcanology
, and
igneous petrologyIgneous petrology is the study of igneous rock Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the ar ...
aims to determine the history of igneous rocks from their final crystallization to their original molten source.


Whole-Earth structure


Plate tectonics

In the 1960s, it was discovered that the Earth's
lithosphere A lithosphere ( grc, λίθος [] for "rocky", and [] for "sphere") is the rigid, outermost shell of a terrestrial planet, terrestrial-type planet or natural satellite. On Earth, it is composed of the crust (geology), crust and the portion o ...
, which includes the crust and rigid uppermost portion of the
upper mantle The upper mantle of Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth's surface is land consisting of continent A continent is one of several large landmasses. ...
, is separated into
tectonic plate This is a list of tectonic plates on Earth's surface Earth is the third planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibri ...
s that move across the plastically deforming, solid, upper mantle, which is called the
asthenosphere The asthenosphere ( grc, ἀσθενός 'asthenos''meaning "without strength", and thus "weak", and 'sphaira''meaning "sphere") is the highly viscous The viscosity of a fluid In physics Physics is the that studies , its ...
. 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 bulk movement of molecules within fluids). Thus, oceanic plates and the adjoining mantle
convection currents Convection is single or Multiphase flow, multiphase fluid flow that occurs Spontaneous process, spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see bu ...

convection currents
always move in the same direction – because the oceanic lithosphere is actually the rigid upper thermal
boundary layer In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular su ...
of the convecting mantle. This coupling between rigid plates moving on the surface of the Earth and the convecting
mantle Mantle may refer to: *Mantle (geology) A mantle is a layer inside a planetary body A planet is an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a n ...
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. For example: *
Mid-ocean ridge A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics upright=1.35, Diagram of the internal layering of Earth showing the lithosphere above the asthenosphere (not to scale) Plate tectonics (from the la, label=Late ...
s, high regions on the seafloor where
hydrothermal vent A hydrothermal vent is a fissure A fissure is a long, narrow crack opening along the surface of the Earth. It is derived from the Latin word , which means 'cleft' or 'crack'. Fissures emerge in the Earth's crust, on ice sheets and glaciers, an ...
s 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
subducts
, or moves, under another.
Transform boundaries A transform fault or transform boundary is a fault (geology), fault along a plate boundary where the motion (physics), motion is predominantly Horizontal plane, horizontal. It ends abruptly where it connects to another plate boundary, either anot ...
, such as the
San Andreas Fault The San Andreas Fault is a continental transform fault A transform fault or transform boundary, sometimes called a strike-slip boundary, is a fault Fault commonly refers to: *Fault (geology), planar rock fractures showing evidence of relative ...
system, resulted in widespread powerful earthquakes. Plate tectonics also has provided a mechanism for
Alfred Wegener Alfred Lothar Wegener (; ; 1 November 1880 – November 1930) was a German polar researcher, geophysicist and meteorologist. During his lifetime he was primarily known for his achievements in meteorology and as a pioneer of polar researc ...

Alfred Wegener
's theory of
continental drift Continental drift is the hypothesis that the Earth's continent A continent is one of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven regions are commonly reg ...
, in which the
continents A continent is one of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven regions are commonly regarded as continents. Ordered from largest in area to smallest, these ...

continents
move across the surface of the Earth over geological time. They also 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.


Earth structure

Advances in
seismology Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "Earthquake, earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of Linear elasticity#Elastic wave, elast ...
,
computer modeling Computer simulation is the process of mathematical modelling, performed on a computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern computers can perform ...
, and
mineralogy Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical mineralogy, optical) properties of minerals and mineralized artifact (archaeology), artifacts. Specific st ...

mineralogy
and
crystallography Crystallography is the experimental science of determining the arrangement of atoms in crystalline solids (see crystal structure). The word "crystallography" is derived from the Greek language, Greek words ''crystallon'' "cold drop, frozen drop" ...

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 wave Seismic waves are waves The United States Naval Reserve (Women's Reserve), better known as the WAVES (for Women Accepted for Volunteer Emergency Service), was the women's branch of the United States Naval Reserve The United States Navy ...
s in reverse to image the interior of the Earth. Early advances in this field showed the existence of a liquid
outer core Earth's outer core is a fluid layer about thick and composed of mostly iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transi ...
(where
shear waves In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular ...
were not able to propagate) and a dense solid
inner core Earth's inner core is the innermost structure of Earth, geologic layer of planet Earth. It is primarily a solid ball (mathematics), ball with a radius of about , which is about 20% of Earth radius, Earth's radius or 70% of the Moon's radius. Th ...
. These advances led to the development of a layered model of the Earth, with a crust and
lithosphere A lithosphere ( grc, λίθος [] for "rocky", and [] for "sphere") is the rigid, outermost shell of a terrestrial planet, terrestrial-type planet or natural satellite. On Earth, it is composed of the crust (geology), crust and the portion o ...
on top, the
mantle Mantle may refer to: *Mantle (geology) A mantle is a layer inside a planetary body A planet is an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a n ...
below (separated within itself by seismic discontinuities at 410 and 660 kilometers), and the outer core and inner core below that. More recently, seismologists have been able 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 in 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.


Geological time

The geological time scale encompasses the history of the Earth. It is bracketed at the earliest by the dates of the first
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

Solar System
material at 4.567 Ga (or 4.567 billion years ago) and the formation of the Earth at 4.54 Ga (4.54 billion years), which is the beginning of the informally recognized
Hadean eon The Hadean ( ) is a geologic eon The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontology, paleontologists, and other earth scien ...
a division of geological time. At the later end of the scale, it is marked by the present day (in the
Holocene epoch The Holocene ( ) is the current geological epoch In chronology 222px, Joseph Scaliger's ''De emendatione temporum'' (1583) began the modern science of chronology Chronology (from Latin Latin (, or , ) is a classical language belongi ...
).


Timescale of the Earth


Important milestones on Earth

* 4.567 Ga (gigaannum: billion years ago): Solar system formation * 4.54 Ga: Accretion, or formation, of Earth * c. 4 Ga: End of
Late Heavy Bombardment The Late Heavy Bombardment (LHB), or lunar cataclysm, is a hypothesized event thought to have occurred approximately 4.1 to 3.8 billion year A year is the orbital period The orbital period is the time a given astronomical object takes ...
, the first life * c. 3.5 Ga: Start of photosynthesis * c. 2.3 Ga: Oxygenated atmosphere, first snowball Earth * 730–635 Ma (unit), Ma (megaannum: million years ago): second snowball Earth * 541 ± 0.3 Ma: Cambrian explosion – vast multiplication of hard-bodied life; first abundant fossils; start of the Paleozoic * c. 380 Ma: First vertebrate land animals * 250 Ma: Permian-Triassic extinction – 90% of all land animals die; end of Paleozoic and beginning of Mesozoic * 66 Ma: Cretaceous–Paleogene extinction event, Cretaceous–Paleogene extinction – Dinosaurs die; end of Mesozoic and beginning of Cenozoic * c. 7 Ma: First hominins appear * 3.9 Ma: First Australopithecus, direct ancestor to modern Homo sapiens, appear * 200 Ma (unit), ka (kiloannum: thousand years ago): First modern Homo sapiens appear in East Africa


Timescale of the Moon


Timescale of Mars


Dating methods


Relative dating

Methods for relative dating were developed when geology first emerged as a natural science. Geologists still use the following principles today as a means to provide information about geological history and the timing of geological events. The principle of uniformitarianism states that the geological processes observed in operation that modify the Earth's crust at present have worked in much the same way over geological time. A fundamental principle of geology advanced by the 18th-century Scottish physician and geologist James Hutton is that "the present is the key to the past." In Hutton's words: "the past history of our globe must be explained by what can be seen to be happening now." Intrusion (geology), The principle of intrusive relationships concerns crosscutting intrusions. In geology, when an igneous rocks, igneous intrusion cuts across a formation of sedimentary rock, it can be determined that the igneous intrusion is younger than the sedimentary rock. Different types of intrusions include stocks, laccoliths, batholiths, Sill (geology), sills and Dike (geology), dikes. The principle of cross-cutting relationships pertains to the formation of Fault (geology), faults and the age of the sequences through which they cut. Faults are younger than the rocks they cut; accordingly, if a fault is found that penetrates some formations but not those on top of it, then the formations that were cut are older than the fault, and the ones that are not cut must be younger than the fault. Finding the key bed in these situations may help determine whether the fault is a normal fault or a thrust fault. The principle of inclusions and components states that, with sedimentary rocks, if inclusions (or ''Clastic rocks, clasts'') are found in a formation, then the inclusions must be older than the formation that contains them. For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer. A similar situation with igneous rocks occurs when xenoliths are found. These foreign bodies are picked up as
magma Magma () is the molten or semi-molten natural material from which all igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others ...

magma
or lava flows, and are incorporated, later to cool in the matrix. As a result, xenoliths are older than the rock that contains them. The principle of original horizontality states that the deposition of sediments occurs as essentially horizontal beds. Observation of modern marine and non-marine sediments in a wide variety of environments supports this generalization (although cross-bedding is inclined, the overall orientation of cross-bedded units is horizontal). The Law of superposition, principle of superposition states that a sedimentary rock layer in a tectonics, tectonically undisturbed sequence is younger than the one beneath it and older than the one above it. Logically a younger layer cannot slip beneath a layer previously deposited. This principle allows sedimentary layers to be viewed as a form of the vertical timeline, a partial or complete record of the time elapsed from deposition of the lowest layer to deposition of the highest bed. The principle of faunal succession is based on the appearance of fossils in sedimentary rocks. As organisms exist during the same period throughout the world, their presence or (sometimes) absence provides a relative age of the formations where they appear. Based on principles that William Smith laid out almost a hundred years before the publication of Charles Darwin's theory of evolution, the principles of succession developed independently of evolutionary thought. The principle becomes quite complex, however, given the uncertainties of fossilization, localization of fossil types due to lateral changes in habitat (facies change in sedimentary strata), and that not all fossils formed globally at the same time.


Absolute dating

Geologists also use methods to determine the absolute age of rock samples and geological events. These dates are useful on their own and may also be used in conjunction with relative dating methods or to calibrate relative methods. At the beginning of the 20th century, advancement in geological science was facilitated by the ability to obtain accurate absolute dates to geological events using radioactive isotopes and other methods. This changed the understanding of geological time. Previously, geologists could only use fossils and stratigraphic correlation to date sections of rock relative to one another. With isotopic dates, it became possible to assign Absolute dating, absolute ages to rock units, and these absolute dates could be applied to fossil sequences in which there was datable material, converting the old relative ages into new absolute ages. For many geological applications, isotope ratios of radioactive elements are measured in minerals that give the amount of time that has passed since a rock passed through its particular closure temperature, the point at which different radiometric isotopes stop diffusing into and out of the Crystal structure, crystal lattice. These are used in geochronology, geochronologic and thermochronology, thermochronologic studies. Common methods include uranium–lead dating, K–Ar dating, potassium–argon dating, argon–argon dating and uranium–thorium dating. These methods are used for a variety of applications. Dating of
lava Lava is magma Magma () is the molten or semi-molten natural material from which all s are formed. Magma is found beneath the surface of the , and evidence of has also been discovered on other and some s. Besides molten rock, magma may al ...

lava
and volcanic ash layers found within a stratigraphic sequence can provide absolute age data for sedimentary rock units that do not contain radioactive isotopes and calibrate relative dating techniques. These methods can also be used to determine ages of pluton emplacement. Thermochemical techniques can be used to determine temperature profiles within the crust, the uplift of mountain ranges, and paleo-topography. Fractionation of the lanthanide series elements is used to compute ages since rocks were removed from the mantle. Other methods are used for more recent events. Optically stimulated luminescence and Cosmogenic isotope#Natural, cosmogenic radionuclide dating are used to date surfaces and/or erosion rates. Dendrochronology can also be used for the dating of landscapes. Radiocarbon dating is used for geologically young materials containing Organic matter, organic carbon.


Geological development of an area

The geology of an area changes through time as rock units are deposited and inserted, and deformational processes change their shapes and locations. Rock units are first emplaced either by deposition onto the surface or intrusion into the Country rock (geology), overlying rock. Deposition can occur when sediments settle onto the surface of the Earth and later lithification, lithify into sedimentary rock, or when as volcanic rock, volcanic material such as volcanic ash or lava flows blanket the surface. Igneous intrusions such as batholiths, laccoliths, dike (geology), dikes, and sill (geology), sills, push upwards into the overlying rock, and crystallize as they intrude. After the initial sequence of rocks has been deposited, the rock units can be deformation (mechanics), deformed and/or metamorphism, metamorphosed. Deformation typically occurs as a result of horizontal shortening, extension (geology), horizontal extension, or side-to-side (strike-slip) motion. These structural regimes broadly relate to convergent boundaries, divergent boundaries, and transform boundaries, respectively, between tectonic plates. When rock units are placed under horizontal compression (geology), compression, they shorten and become thicker. Because rock units, other than muds, Incompressible surface, do not significantly change in volume, this is accomplished in two primary ways: through fault (geology), faulting and fold (geology), folding. In the shallow crust, where brittle deformation can occur, thrust faults form, which causes the deeper rock to move on top of the shallower rock. Because deeper rock is often older, as noted by the law of superposition, principle of superposition, this can result in older rocks moving on top of younger ones. Movement along faults can result in folding, either because the faults are not planar or because rock layers are dragged along, forming drag folds as slip occurs along the fault. Deeper in the Earth, rocks behave plastically and fold instead of faulting. These folds can either be those where the material in the center of the fold buckles upwards, creating "antiforms", or where it buckles downwards, creating "synforms". If the tops of the rock units within the folds remain pointing upwards, they are called anticlines and synclines, respectively. If some of the units in the fold are facing downward, the structure is called an overturned anticline or syncline, and if all of the rock units are overturned or the correct up-direction is unknown, they are simply called by the most general terms, antiforms, and synforms. Even higher pressures and temperatures during horizontal shortening can cause both folding and metamorphism of the rocks. This metamorphism causes changes in the mineral, mineral composition of the rocks; creates a foliation (geology), foliation, or planar surface, that is related to mineral growth under stress. This can remove signs of the original textures of the rocks, such as bed (geology), bedding in sedimentary rocks, flow features of
lava Lava is magma Magma () is the molten or semi-molten natural material from which all s are formed. Magma is found beneath the surface of the , and evidence of has also been discovered on other and some s. Besides molten rock, magma may al ...

lava
s, and crystal patterns in crystalline rocks. Extension causes the rock units as a whole to become longer and thinner. This is primarily accomplished through normal faulting and through the ductile stretching and thinning. Normal faults drop rock units that are higher below those that are lower. This typically results in younger units ending up below older units. Stretching of units can result in their thinning. In fact, at one location within the Maria Fold and Thrust Belt, the entire sedimentary sequence of the Grand Canyon appears over a length of less than a meter. Rocks at the depth to be ductilely stretched are often also metamorphosed. These stretched rocks can also pinch into lenses, known as ''boudinage, boudins'', after the French word for "sausage" because of their visual similarity. Where rock units slide past one another, strike-slip faults develop in shallow regions, and become shear zones at deeper depths where the rocks deform ductilely. The addition of new rock units, both depositionally and intrusively, often occurs during deformation. Faulting and other deformational processes result in the creation of topographic gradients, causing material on the rock unit that is increasing in elevation to be eroded by hillslopes and channels. These sediments are deposited on the rock unit that is going down. Continual motion along the fault maintains the topographic gradient in spite of the movement of sediment and continues to create accommodation space for the material to deposit. Deformational events are often also associated with volcanism and igneous activity. Volcanic ashes and lavas accumulate on the surface, and igneous intrusions enter from below. Dike (geology), Dikes, long, planar igneous intrusions, enter along cracks, and therefore often form in large numbers in areas that are being actively deformed. This can result in the emplacement of dike swarms, such as those that are observable across the Canadian shield, or rings of dikes around the lava tube of a volcano. All of these processes do not necessarily occur in a single environment and do not necessarily occur in a single order. The Hawaiian Islands, for example, consist almost entirely of layered basaltic lava flows. The sedimentary sequences of the mid-continental United States and the Geology of the Grand Canyon area, Grand Canyon in the southwestern United States contain almost-undeformed stacks of sedimentary rocks that have remained in place since Cambrian time. Other areas are much more geologically complex. In the southwestern United States, sedimentary, volcanic, and intrusive rocks have been metamorphosed, faulted, foliated, and folded. Even older rocks, such as the Acasta gneiss of the Slave craton in northwestern Canada, the Oldest rock, oldest known rock in the world have been metamorphosed to the point where their origin is indiscernible without laboratory analysis. In addition, these processes can occur in stages. In many places, the Grand Canyon in the southwestern United States being a very visible example, the lower rock units were metamorphosed and deformed, and then deformation ended and the upper, undeformed units were deposited. Although any amount of rock emplacement and rock deformation can occur, and they can occur any number of times, these concepts provide a guide to understanding the Historical geology, geological history of an area.


Methods of geology

Geologists use a number of fields, laboratory, and numerical modeling methods to decipher Earth history and to understand the processes that occur on and inside the Earth. In typical geological investigations, geologists use primary information related to petrology (the study of rocks), stratigraphy (the study of sedimentary layers), and structural geology (the study of positions of rock units and their deformation). In many cases, geologists also study modern soils, rivers, landscapes, and glaciers; investigate past and current life and biogeochemistry, biogeochemical pathways, and use geophysics, geophysical methods to investigate the subsurface. Sub-specialities of geology may distinguish endogenous and exogenous geology.


Field methods

Geological
field work Field research, field studies, or fieldwork is the empirical research, collection of raw data outside a laboratory, library, or workplace setting. The approaches and methods used in field research vary across branches of science, disciplines. ...

field work
varies depending on the task at hand. Typical fieldwork could consist of: * Geological mapping ** Structural mapping: identifying the locations of major rock units and the faults and folds that led to their placement there. ** Stratigraphic mapping: pinpointing the locations of sedimentary facies (Lithology, lithofacies and biofacies) or the mapping of isopachs of equal thickness of sedimentary rock ** Surficial mapping: recording the locations of soils and surficial deposits * Surveying of topographic features ** compilation of topographic maps ** Work to understand change across landscapes, including: *** Patterns of erosion and deposition (geology), deposition *** River-channel change through meander, migration and avulsion (river), avulsion *** Hillslope processes * Subsurface mapping through Geophysical survey, geophysical methods ** These methods include: *** Shallow seismology, seismic surveys *** Ground-penetrating radar *** Aeromagnetic surveys *** Electrical resistivity tomography ** They aid in: *** Exploration geophysics, Hydrocarbon exploration *** Finding groundwater *** Archaeological geophysics, Locating buried archaeological artifacts * High-resolution stratigraphy ** Measuring and describing stratigraphic sections on the surface ** Well drilling and well logging, logging * Biogeochemistry and geomicrobiology ** Collecting samples to: *** determine biochemical pathways *** identify new species (biology), species of organisms *** identify new chemical compounds ** and to use these discoveries to: *** understand early life on Earth and how it functioned and metabolized *** find important compounds for use in pharmaceuticals * Paleontology: excavation of fossil material ** For research into past life and evolution ** For museums and education * Collection of samples for geochronology and thermochronology * Glaciology: measurement of characteristics of glaciers and their motion


Petrology

In addition to identifying rocks in the field (lithology), petrologists identify rock samples in the laboratory. Two of the primary methods for identifying rocks in the laboratory are through optical microscopy and by using an electron microprobe. In an optical mineralogy analysis, petrologists analyze thin sections of rock samples using a petrographic microscope, where the minerals can be identified through their different properties in plane-polarized and cross-polarized light, including their birefringence, pleochroism, Crystal twinning, twinning, and interference properties with a Conoscopy, conoscopic lens. In the electron microprobe, individual locations are analyzed for their exact chemical compositions and variation in composition within individual crystals. Stable isotope, Stable and radioactive isotope studies provide insight into the Geochemistry, geochemical evolution of rock units. Petrologists can also use Fluid inclusions, fluid inclusion data and perform high temperature and pressure physical experiments to understand the temperatures and pressures at which different mineral phases appear, and how they change through igneous and metamorphic processes. This research can be extrapolated to the field to understand metamorphic processes and the conditions of crystallization of igneous rocks. This work can also help to explain processes that occur within the Earth, such as subduction and magma chamber evolution.


Structural geology

Structural geologists use microscopic analysis of oriented thin sections of geological samples to observe the fabric (geology), fabric within the rocks, which gives information about strain within the crystalline structure of the rocks. They also plot and combine measurements of geological structures to better understand the orientations of faults and folds to reconstruct the history of rock deformation in the area. In addition, they perform analogue modelling (geology), analog and numerical experiments of rock deformation in large and small settings. The analysis of structures is often accomplished by plotting the orientations of various features onto stereographic projection, stereonets. A stereonet is a stereographic projection of a sphere onto a plane, in which planes are projected as lines and lines are projected as points. These can be used to find the locations of fold axes, relationships between faults, and relationships between other geological structures. Among the most well-known experiments in structural geology are those involving Accretionary wedge, orogenic wedges, which are zones in which mountains are built along convergent boundary, convergent tectonic plate boundaries. In the analog versions of these experiments, horizontal layers of sand are pulled along a lower surface into a back stop, which results in realistic-looking patterns of faulting and the growth of a critical taper, critically tapered (all angles remain the same) orogenic wedge. Numerical models work in the same way as these analog models, though they are often more sophisticated and can include patterns of erosion and uplift in the mountain belt. This helps to show the relationship between erosion and the shape of a mountain range. These studies can also give useful information about pathways for metamorphism through pressure, temperature, space, and time.


Stratigraphy

In the laboratory, stratigraphers analyze samples of stratigraphic sections that can be returned from the field, such as those from drill cores. Stratigraphers also analyze data from geophysical surveys that show the locations of stratigraphic units in the subsurface. Geophysical data and well logs can be combined to produce a better view of the subsurface, and stratigraphers often use computer programs to do this in three dimensions. Stratigraphers can then use these data to reconstruct ancient processes occurring on the surface of the Earth, interpret past environments, and locate areas for water, coal, and hydrocarbon extraction. In the laboratory, biostratigraphy, biostratigraphers analyze rock samples from outcrop and drill cores for the fossils found in them. These fossils help scientists to date the core and to understand the Sedimentary depositional environment, depositional environment in which the rock units formed. Geochronologists precisely date rocks within the stratigraphic section to provide better absolute bounds on the timing and rates of deposition. Magnetic stratigraphers look for signs of magnetic reversals in igneous rock units within the drill cores. Other scientists perform stable-isotope studies on the rocks to gain information about past climate.


Planetary geology

With the advent of space exploration in the twentieth century, geologists have begun to look at other planetary bodies in the same ways that have been developed to study the Earth. This new field of study is called planetary geology (sometimes known as astrogeology) and relies on known geological principles to study other bodies of the solar system. Although the Greek-language-origin prefix ''wikt:geo-, geo'' refers to Earth, "geology" is often used in conjunction with the names of other planetary bodies when describing their composition and internal processes: examples are "the geology of Mars" and "Lunar geology". Specialized terms such as ''selenology'' (studies of the Moon), ''areology'' (of Mars), etc., are also in use. Although planetary geologists are interested in studying all aspects of other planets, a significant focus is to search for evidence of past or present life on other worlds. This has led to many missions whose primary or ancillary purpose is to examine planetary bodies for evidence of life. One of these is the Phoenix lander, which analyzed Mars, Martian polar soil for water, chemical, and mineralogical constituents related to biological processes.


Applied geology


Economic geology

Economic geology is a branch of geology that deals with aspects of economic minerals that humankind uses to fulfill various needs. Economic minerals are those extracted profitably for various practical uses. Economic geologists help locate and manage the Earth's natural resources, such as petroleum and coal, as well as mineral resources, which include metals such as iron, copper, and uranium.


Mining geology

Mining geology consists of the extractions of mineral resources from the Earth. Some resources of economic interests include gemstones, metals such as gold and copper, and many minerals such as asbestos, perlite, mica, phosphates, zeolites, clay, pumice, quartz, and silica, as well as elements such as
sulfur Sulfur (in nontechnical British English: sulphur) is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: th ...

sulfur
, chlorine, and helium.


Petroleum geology

Petroleum geologists study the locations of the subsurface of the Earth that can contain extractable hydrocarbons, especially petroleum and natural gas. Because many of these reservoirs are found in sedimentary basins, they study the formation of these basins, as well as their sedimentary and tectonic evolution and the present-day positions of the rock units.


Engineering geology

Engineering geology is the application of geological principles to engineering practice for the purpose of assuring that the geological factors affecting the location, design, construction, operation, and maintenance of engineering works are properly addressed. Engineering geology is distinct from
geological engineering Geoprofessions is a term coined by the Geoprofessional Business Association to connote various technical disciplines that involve engineering, earth and environmental services applied to below-ground (“subsurface”), ground-surface, and ground-su ...
, particularly in North America. In the field of civil engineering, geological principles and analyses are used in order to ascertain the mechanical principles of the material on which structures are built. This allows tunnels to be built without collapsing, bridges and skyscrapers to be built with sturdy foundations, and buildings to be built that will not settle in clay and mud.


Hydrology

Geology and geological principles can be applied to various environmental problems such as stream restoration, the restoration of brownfields, and the understanding of the interaction between Habitat (ecology), natural habitat and the geological environment. Groundwater hydrology, or hydrogeology, is used to locate groundwater, which can often provide a ready supply of uncontaminated water and is especially important in arid regions, and to monitor the spread of contaminants in groundwater wells.


Paleoclimatology

Geologists also obtain data through stratigraphy, boreholes, core samples, and ice cores. Ice cores and sediment cores are used for paleoclimate reconstructions, which tell geologists about past and present temperature, precipitation, and sea level across the globe. These datasets are our primary source of information on global climate change outside of instrumental data.


Natural hazards

Geologists and geophysicists study natural hazards in order to enact safe building codes and warning systems that are used to prevent loss of property and life. Examples of important natural hazards that are pertinent to geology (as opposed those that are mainly or only pertinent to meteorology) are:


History

The study of the physical material of the Earth dates back at least to ancient Greece when Theophrastus (372–287 BCE) wrote the work ''Peri Lithon'' (''On Stones''). During the Roman Empire, Roman period, Pliny the Elder wrote in detail of the many minerals and metals, then in practical use – even correctly noting the origin of amber. Additionally, in the 4th century BCE Aristotle made critical observations of the slow rate of geological change. He observed the composition of the land and formulated a theory where the Earth changes at a slow rate and that these changes cannot be observed during one person's lifetime. Aristotle developed one of the first evidence-based concepts connected to the geological realm regarding the rate at which the Earth physically changes. Some modern scholars, such as Fielding H. Garrison, are of the opinion that the origin of the science of geology can be traced to Persia after the Muslim conquests had come to an end. Abū al-Rayhān al-Bīrūnī, Abu al-Rayhan al-Biruni (973–1048 CE) was one of the earliest Persian people, Persian geologists, whose works included the earliest writings on the geology of India, hypothesizing that the Indian subcontinent was once a sea. Drawing from Greek and Indian scientific literature that were not destroyed by the Muslim conquests, the Persian scholar Avicenna, Ibn Sina (Avicenna, 981–1037) proposed detailed explanations for the formation of mountains, the origin of earthquakes, and other topics central to modern geology, which provided an essential foundation for the later development of the science. In China, the polymath Shen Kuo (1031–1095) formulated a hypothesis for the process of land formation: based on his observation of fossil animal shells in a geological stratum in a mountain hundreds of miles from the ocean, he inferred that the land was formed by the erosion of the mountains and by Deposition (sediment), deposition of silt. Nicolas Steno (1638–1686) is credited with the law of superposition, the principle of original horizontality, and the principle of lateral continuity: three defining principles of stratigraphy. The word ''geology'' was first used by Ulisse Aldrovandi in 1603, then by Jean-André Deluc in 1778 and introduced as a fixed term by Horace-Bénédict de Saussure in 1779. The word is derived from the Ancient Greek, Greek γῆ, ''gê'', meaning "earth" and λόγος, ''logos'', meaning "speech". But according to another source, the word "geology" comes from a Norwegian, Mikkel Pedersøn Escholt (1600–1699), who was a priest and scholar. Escholt first used the definition in his book titled, ''Geologia Norvegica'' (1657). William Smith (geologist), William Smith (1769–1839) drew some of the first geological maps and began the process of ordering rock strata (layers) by examining the fossils contained in them. In 1763, Mikhail Lomonosov published his treatise ''On the Strata of Earth''. His work was the first narrative of modern geology, based on the unity of processes in time and explanation of the Earth's past from the present. James Hutton (1726-1797) is often viewed as the first modern geologist. In 1785 he presented a paper entitled ''Theory of the Earth'' to the Royal Society of Edinburgh. In his paper, he explained his theory that the Earth must be much older than had previously been supposed to allow enough time for mountains to be eroded and for sediments to form new rocks at the bottom of the sea, which in turn were raised up to become dry land. Hutton published a two-volume version of his ideas in 1795. Followers of Hutton were known as ''Plutonism, Plutonists'' because they believed that some rocks were formed by ''vulcanism'', which is the deposition of lava from volcanoes, as opposed to the ''Neptunism, Neptunists'', led by Abraham Gottlob Werner, Abraham Werner, who believed that all rocks had settled out of a large ocean whose level gradually dropped over time. The first Geologic map of Georgia, geological map of the U.S. was produced in 1809 by William Maclure. In 1807, Maclure commenced the self-imposed task of making a geological survey of the United States. Almost every state in the Union was traversed and mapped by him, the Allegheny Mountains being crossed and recrossed some 50 times. The results of his unaided labours were submitted to the American Philosophical Society in a memoir entitled ''Observations on the Geology of the United States explanatory of a Geological Map'', and published in the ''Society's Transactions'', together with the nation's first geological map. This antedates William Smith (geologist), William Smith's geological map of England by six years, although it was constructed using a different classification of rocks. Sir Charles Lyell (1797-1875) first published his famous book, ''Principles of Geology'', in 1830. This book, which influenced the thought of Charles Darwin, successfully promoted the doctrine of uniformitarianism. This theory states that slow geological processes have occurred throughout the History of Earth, Earth's history and are still occurring today. In contrast, catastrophism is the theory that Earth's features formed in single, catastrophic events and remained unchanged thereafter. Though Hutton believed in uniformitarianism, the idea was not widely accepted at the time. Much of 19th-century geology revolved around the question of the Age of the Earth, Earth's exact age. Estimates varied from a few hundred thousand to billions of years. By the early 20th century, radiometric dating allowed the Earth's age to be estimated at two billion years. The awareness of this vast amount of time opened the door to new theories about the processes that shaped the planet. Some of the most significant advances in 20th-century geology have been the development of the theory of
plate tectonics upright=1.35, Diagram of the internal layering of Earth showing the lithosphere above the asthenosphere (not to scale) Plate tectonics (from the la, label=Late Latin Late Latin ( la, Latinitas serior) is the scholarly name for the written L ...
in the 1960s and the refinement of estimates of the planet's age. Plate tectonics theory arose from two separate geological observations: seafloor spreading and
continental drift Continental drift is the hypothesis that the Earth's continent A continent is one of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven regions are commonly reg ...
. The theory revolutionized the Earth sciences. Today the Earth is known to be approximately 4.5 billion years old. File:Hutton James portrait Raeburn.jpg, James Hutton, Scottish
geologist A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes Earth and other terrestrial planets, as well as the processes that shape them. Geologists usually study geology, although backgrounds in physics, chem ...

geologist
and father of modern geology File:John Tuzo Wilson in 1992.jpg, John Tuzo Wilson, Canadian geophysicist and father of
plate tectonics upright=1.35, Diagram of the internal layering of Earth showing the lithosphere above the asthenosphere (not to scale) Plate tectonics (from the la, label=Late Latin Late Latin ( la, Latinitas serior) is the scholarly name for the written L ...
File:MSH80 david johnston at camp 05-17-80 med (cropped).jpg, The volcanologist David A. Johnston 13 hours before his death at the
1980 eruption of Mount St. Helens


Fields or related disciplines

*
Earth system science Earth system science (ESS) is the application of systems science Systems science is an interdisciplinary Interdisciplinarity or interdisciplinary studies involves the combination of two or more academic disciplines into one activity (e.g. ...
* Economic geology ** Mining, Mining geology ** Petroleum geology * Engineering geology * Environmental geology * Environmental science * Geoarchaeology * Geochemistry ** Biogeochemistry ** Isotope geochemistry * Geochronology * Geodetics * Geography * Geological engineering * Geological modelling * Geometallurgy * Geomicrobiology * Geomorphology * Geomythology * Geophysics * Glaciology * Historical geology * Hydrogeology * Meteorology * Mineralogy * Oceanography ** Marine geology * Paleoclimatology * Paleontology ** Micropaleontology ** Palynology * Petrology * Petrophysics * Physical geography * Plate tectonics * Regional geology * Sedimentology * Seismology * Soil science ** Pedology (soil study) * Speleology * Stratigraphy ** Biostratigraphy ** Chronostratigraphy ** Lithostratigraphy * Structural geology * Systems geology * Tectonics * Volcanology


See also

* Glossary of geology * Geoprofessions * Geotourism * Glossary of geology terms * Index of geology articles * International Union of Geological Sciences (IUGS) * Outline of geology * Timeline of geology


References


External links


One Geology: This interactive geological map of the world is an international initiative of the geological surveys around the globe. This groundbreaking project was launched in 2007 and contributed to the 'International Year of Planet Earth', becoming one of their flagship projects.

''Earth Science News, Maps, Dictionary, Articles, Jobs''

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