Geology (from the Ancient Greek
γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse"))
is an Earth science
concerned with the solid Earth
, the rocks
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
or natural satellite
such as Mars
or the Moon
. Modern geology significantly overlaps all other Earth sciences, including hydrology
and the atmospheric sciences
, and so is treated as one major aspect of integrated 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. It also provides tools to determine the relative
and absolute ages
of rocks found in a given location, and also to describe the histories of those rocks. By combining these tools, geologist
s are able to chronicle the geological history of the Earth
as a whole, and also to demonstrate the age of the Earth
. Geology provides the primary evidence for plate tectonics
, the evolutionary history of life
, and the Earth's past climates
Geologists use a wide variety of methods to understand the Earth's structure and evolution, including field work
, rock description
, geophysical techniques
, chemical analysis
, physical experiment
s, and numerical modelling
. In practical terms, geology is important for mineral
exploration and exploitation, evaluating water resources
, understanding of natural hazard
s, the remediation of environmental
problems, and providing insights into past climate change
. Geology is a major academic discipline
, and it plays an important role in geotechnical engineering
The majority of geological data comes from research on solid Earth materials. These typically fall into one of two categories: rock and unlithified material.
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
, and metamorphic
. The rock cycle
illustrates the relationships among them (see diagram).
When a rock solidifies
from melt (magma
), it is an igneous rock. This rock can be weathered
, then redeposited
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.
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
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
* 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
that lie above the bedrock
. This study is often known as Quaternary geology
, after the Quaternary period
of geologic history.
However, unlithified material does not only include sediments
. Magma is the original unlithified source of all igneous rocks
. The active flow of molten rock is closely studied in volcanology
, and igneous petrology
aims to determine the history of igneous rocks from their final crystallization to their original molten source.
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 plate
s 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 bulk movement of molecules within fluids). Thus, oceanic 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 geologic features are explained as plate boundaries.
* Mid-ocean ridge
s, high regions on the seafloor where hydrothermal vent
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
, or moves, under another.
, such as the San Andreas Fault
system, resulted in widespread powerful earthquakes. Plate tectonics also has provided a mechanism for Alfred Wegener
's theory of continental drift
, in which the continents
move across the surface of the Earth over geologic 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.
Advances in seismology
, computer modeling
, and mineralogy
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
s in reverse 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 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. 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.
The geologic time scale encompasses the history of the Earth. It is bracketed at the earliest by the dates of the first Solar System
material at 4.567 Ga
(or 4.567 billion years ago) and the formation of the Earth at
(4.54 billion years), which is the beginning of the informally recognized Hadean eon
a division of geologic time. At the later end of the scale, it is marked by the present day (in the Holocene epoch
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 first life
* c. 3.5 Ga: Start of photosynthesis
* c. 2.3 Ga: Oxygenated atmosphere
, first snowball Earth
* 730–635 Ma
(megaannum: million years ago): second snowball Earth
* 541 ± 0.3 Ma: Cambrian explosion
– vast multiplication of hard-bodied life; first abundant fossil
s; start of the Paleozoic
* c. 380 Ma: First vertebrate
* 250 Ma: Permian-Triassic extinction
– 90% of all land animals die; end of Paleozoic and beginning of Mesozoic
* 66 Ma: Cretaceous–Paleogene extinction
s die; end of Mesozoic and beginning of Cenozoic
* c. 7 Ma: First hominin
* 3.9 Ma: First Australopithecus
, direct ancestor to modern Homo sapiens
* 200 ka
(kiloannum: thousand years ago): First modern Homo sapiens appear in East Africa
Timescale of the Moon
Timescale of Mars
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 geologic history and the timing of geologic events.
The principle of uniformitarianism
states that the geologic processes observed in operation that modify the Earth's crust at present have worked in much the same way over geologic 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."
The principle of intrusive relationships
concerns crosscutting intrusions. In geology, when an 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, laccolith
The principle of cross-cutting relationships
pertains to the formation of 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 ''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 xenolith
s are found. These foreign bodies are picked up as 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 principle of superposition
states that a sedimentary rock layer in a 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.
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 geologic events using radioactive isotope
s and other methods. This changed the understanding of geologic 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 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 geologic applications, isotope ratio
s 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 lattice
. These are used in geochronologic
studies. Common methods include uranium–lead dating
, potassium–argon dating
, argon–argon dating
and uranium–thorium dating
. These methods are used for a variety of applications. Dating of 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
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 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 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 lithify
into sedimentary rock, or when as volcanic material
such as volcanic ash
or lava flow
s blanket the surface. Igneous intrusion
s such as batholith
, and 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 deformed
. Deformation typically occurs as a result of horizontal shortening, 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
, they shorten and become thicker. Because rock units, other than muds, do not significantly change in volume
, this is accomplished in two primary ways: through faulting
. 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 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 "antiform
s", or where it buckles downwards, creating "synform
s". If the tops of the rock units within the folds remain pointing upwards, they are called anticline
s and syncline
s, 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 composition
of the rocks; creates a 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 bedding
in sedimentary rocks, flow features of lava
s, and crystal patterns in crystalline rock
Extension causes the rock units as a whole to become longer and thinner. This is primarily accomplished through normal fault
ing 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 ''boudins
'', after the French word for "sausage" because of their visual similarity.
Where rock units slide past one another, strike-slip fault
s develop in shallow regions, and become shear zone
s 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. 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 swarm
s, 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 basalt
ic lava flows. The sedimentary sequences of the mid-continental United States and the 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 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 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, river
s, and glacier
s; investigate past and current life and biogeochemical
pathways, and use geophysical methods
to investigate the subsurface. Sub-specialities of geology may distinguish endogenous and exogenous geology.
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Geological [[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
) or the mapping of isopach
s of equal thickness of sedimentary rock
** Surficial mapping: recording the locations of soils and surficial deposits
* Surveying of topographic features
** compilation of topographic map
** Work to understand change across landscapes, including:
*** Patterns of erosion
*** River-channel change through migration
*** Hillslope processes
* Subsurface mapping through geophysical methods
** These methods include:
*** Shallow seismic
*** Ground-penetrating radar
*** Aeromagnetic survey
*** Electrical resistivity tomography
** They aid in:
*** Hydrocarbon exploration
*** Finding groundwater
*** Locating buried archaeological artifacts
* High-resolution stratigraphy
** Measuring and describing stratigraphic sections on the surface
** Well drilling
** Collecting samples to:
*** determine biochemical pathway
*** identify new species
*** identify new chemical compound
** and to use these discoveries to:
*** understand early life on Earth and how it functioned and metabolized
*** find important compounds for use in pharmaceuticals
: excavation of fossil
** For research into past life and evolution
** For museum
s and education
* Collection of samples for geochronology
: measurement of characteristics of glaciers and their motion
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 section
s 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
, and interference properties with a conoscopic lens
. In the electron microprobe, individual locations are analyzed for their exact chemical compositions and variation in composition within individual crystals. Stable
and radioactive isotope
studies provide insight into the geochemical
evolution of rock units.
Petrologists can also use 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
Structural geologists use microscopic analysis of oriented thin sections of geologic samples to observe the 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 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 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 geologic structures.
Among the most well-known experiments in structural geology are those involving orogenic wedges
, which are zones in which mountain
s are built along 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 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.
In the laboratory, stratigraphers analyze samples of stratigraphic sections that can be returned from the field, such as those from drill core
Stratigraphers also analyze data from geophysical surveys that show the locations of stratigraphic units in the subsurface. Geophysical data and well log
s 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, 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 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.
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 geologic principles to study other bodies of the solar system.
Although the Greek-language-origin prefix ''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 Martian
polar soil for water, chemical, and mineralogical constituents related to biological processes.
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 resource
s, such as petroleum and coal, as well as mineral resources, which include metals such as iron, copper, and uranium.
consists of the extractions of mineral resources from the Earth. Some resources of economic interests include gemstone
s such as gold
, and many minerals such as asbestos
, and silica
, as well as elements such as sulfur
, and helium
s 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 basin
s, 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 is the application of geologic principles to engineering practice for the purpose of assuring that the geologic factors affecting the location, design, construction, operation, and maintenance of engineering works are properly addressed.
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 and environmental issues
Geology and geologic principles can be applied to various environmental problems such as stream restoration
, the restoration of brownfields
, and the understanding of the interaction between natural habitat
and the geologic 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.
Geologists also obtain data through stratigraphy, boreholes
, core sample
s, and ice core
s. 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.
Geologists and geophysicists study natural hazards in order to enact safe building code
s 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:
The study of the physical material of the Earth dates back at least to ancient Greece
(372–287 BCE) wrote the work ''Peri Lithon'' (''On Stones''). During the 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 BC 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
File:Hutton James portrait Raeburn.jpg|James Hutton, Scottish geologist and father of modern geology
File:John Tuzo Wilson in 1992.jpg|John Tuzo Wilson, Canadian geophysicist and father of plate tectonics
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
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. Abu al-Rayhan al-Biruni
(973–1048 CE) was one of the earliest 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 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
(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 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).
(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.
(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 sediment
s 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 1795Vol. 1Vol. 2
Followers of Hutton were known as ''Plutonists
'' because they believed that some rocks were formed by ''vulcanism'', which is the deposition of lava from volcanoes, as opposed to the ''Neptunists
'', led by Abraham Werner
, who believed that all rocks had settled out of a large ocean whose level gradually dropped over time.
The first 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
'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 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 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
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
. The theory revolutionized the Earth sciences
. Today the Earth is known to be approximately 4.5 billion years old.
Fields or related disciplines
* Outline of geology
* Glossary of geology
* Index of geology articles
* Geologic modeling
* Glossary of geology terms
* International Union of Geological Sciences
* Timeline of geology
One Geology: This interactive geologic 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''American Geophysical UnionAmerican Geosciences InstituteEuropean Geosciences UnionGeological Society of AmericaGeological Society of LondonVideo-interviews with famous geologistsGeology OpenTextbookChronostratigraphy benchmarks