Geology

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This article is about the Earth science. For the scientific journal, see Geology (journal). Not to be confused with Geography.
Part of a series on
Geology
Science of the solid Earth
Key components
Laws, principles, theories
Topics
  • Composition
  • Landform structures
  • Geologic history
Research
Applications
Planetary geology
  • By planet and body
Solidified lava flow in Hawaii
Sedimentary layers in Badlands National Park, South Dakota
Metamorphic rock, Nunavut, Canada

Geology (from

Ancient Greek γῆ (gê) 'earth' and λoγία (-logía) 'study of, discourse')[1][2] is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time.[3] Modern geology significantly overlaps all other Earth sciences, including hydrology. It is integrated with Earth system science and planetary science
.

Geology describes the

past climates
.

environmental problems, and providing insights into past climate change. Geology is a major academic discipline, and it is central to geological engineering and plays an important role in geotechnical engineering
.

Geological material

Native gold from Venezuela
Quartz from Tibet. Quartz makes up more than 10% of the Earth's crust by mass.

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

Minerals

Main article: Mineral

Minerals are naturally occurring elements and compounds with a definite homogeneous chemical composition and an ordered atomic arrangement.

Each mineral has distinct physical properties, and there are many tests to determine each of them. Minerals are often identified through these tests. The specimens can be tested for:[5]

  • Color: Minerals are grouped by their color. Mostly diagnostic but impurities can change a mineral's color.
  • Streak: Performed by scratching the sample on a porcelain plate. The color of the streak can help identify the mineral.
  • Hardness: The resistance of a mineral to scratching or indentation.
  • Breakage pattern: A mineral can either show fracture or cleavage, the former being breakage of uneven surfaces, and the latter a breakage along closely spaced parallel planes.
  • Luster: Quality of light reflected from the surface of a mineral. Examples are metallic, pearly, waxy, dull.
  • Specific gravity
    : the weight of a specific volume of a mineral.
  • Effervescence: Involves dripping hydrochloric acid on the mineral to test for fizzing.
  • Magnetism: Involves using a magnet to test for magnetism.
  • Taste: Minerals can have a distinctive taste such as
    table salt
    ).

Rock

metamorphic rocks
.
Main articles: Rock (geology) and Rock cycle

A rock is any naturally occurring solid mass or aggregate of minerals or

metamorphic. The rock cycle
illustrates the relationships among them (see diagram).

When a rock

lithified into a sedimentary rock. Sedimentary rocks are mainly divided into four categories: sandstone, shale, carbonate, and evaporite. This group of classifications focuses partly on the size of sedimentary particles (sandstone and shale), and partly on mineralogy and formation processes (carbonation and evaporation).[6] Igneous and sedimentary rocks can then be turned into metamorphic rocks by heat and pressure that change its mineral content, resulting in a characteristic fabric
. All three types may melt again, and when this happens, new magma is formed, from which an igneous rock may once again solidify. Organic matter, such as coal, bitumen, oil, and natural gas, is linked mainly to organic-rich sedimentary rocks.

To study all three types of rock, geologists evaluate the minerals of which they are composed and their other physical properties, such as texture and fabric.

Unlithified material

Geologists also study unlithified materials (referred to as

Quaternary period
of geologic history, which is the most recent period of geologic time.

Magma

Main article: Magma

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 original molten source to their final crystallization.

Whole-Earth structure

Plate tectonics

Main article: Plate tectonics
tectonic plates of the Earth[8]

In the 1960s, it was discovered that the Earth's

tectonic plates that move across the plastically deforming, solid, upper mantle, which is called the asthenosphere. This theory is supported by several types of observations, including seafloor spreading[9][10]
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 currents always move in the same direction – because the oceanic lithosphere is actually the rigid upper thermal boundary layer of the convecting mantle. This coupling between rigid plates moving on the surface of the Earth and the convecting mantle is called plate tectonics
.

The development of plate tectonics has provided a physical basis for many observations of the solid Earth. Long linear regions of geological features are explained as plate boundaries:[11]

Oceanic-continental convergence resulting in subduction and volcanic arcs illustrates one effect of plate tectonics.

Plate tectonics has provided a mechanism for

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

Main article:
Structure of the Earth
The Earth's layered structure. (1) inner core; (2) outer core; (3) lower mantle; (4) upper mantle; (5) lithosphere; (6) crust (uppermost part of the lithosphere)
Earth layered structure. Typical wave paths from earthquakes like these gave early seismologists insights into the layered structure of the Earth.

Advances in

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

Seismologists can use the arrival times of

inner core. These advances led to the development of a layered model of the Earth, with a lithosphere (including crust) on top, the mantle below (separated within itself by seismic discontinuities at 410 and 660 kilometers), and the outer core and inner core below that. 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 within the crystal structure. These studies explain the chemical changes associated with the major seismic discontinuities in the mantle and show the crystallographic structures expected in the inner core of the Earth.

Geological time

Main articles: Geological history of Earth and Geologic time scale

The geological time scale encompasses the history of the Earth.

Ga[14]
(or 4.567 billion years ago) and the formation of the Earth at 4.54 Ga[15][16] (4.54 billion years), which is the beginning of the
Holocene epoch
).

Timescale of the Earth

The following five timelines show the geologic time scale to scale. The first shows the entire time from the formation of Earth to the present, but this gives little space for the most recent eon. The second timeline shows an expanded view of the most recent eon. In a similar way, the most recent era is expanded in the third timeline, the most recent period is expanded in the fourth timeline, and the most recent epoch is expanded in the fifth timeline.

SiderianRhyacianOrosirianStatherianCalymmianEctasianStenianTonianCryogenianEdiacaranCambrianOrdovicianDevonianCarboniferousPermianTriassicJurassicCretaceousPaleogeneEoarcheanPaleoarcheanMesoarcheanNeoarcheanPaleoproterozoicMesoproterozoicNeoproterozoicPaleozoicMesozoicCenozoicHadeanArcheanProterozoicPhanerozoicPrecambrian
CambrianOrdovicianSilurianDevonianCarboniferousPermianTriassicJurassicCretaceousPaleogeneNeogeneQuaternaryPaleozoicMesozoicCenozoicPhanerozoic
PaleoceneEoceneOligoceneMiocenePliocenePleistoceneHolocenePaleogeneNeogeneQuaternaryCenozoic
GelasianCalabrian (stage)ChibanianLate PleistocenePleistoceneHoloceneQuaternary

(Horizontal scale is millions of years for the above timelines; thousands of years for the timeline below)

GreenlandianNorthgrippianMeghalayanHolocene

Important milestones on Earth

eras
of the Earth's history

Timescale of the Moon

Main article: Lunar geologic timescale
Early ImbrianLate ImbrianPre-NectarianNectarianEratosthenianCopernican period
Millions of years before present


Timescale of Mars

Main article: Geological history of Mars
Pre-NoachianNoachianHesperianAmazonian (Mars)
Martian time periods (millions of years ago)

Epochs:

Dating methods

Relative dating

Main article: Relative dating
normal fault
(cutting through A, B, C & E).

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.[17] 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."[18]

The

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, sills and dikes
.

The

normal fault or a thrust fault.[19]

The

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
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 Permian through Jurassic stratigraphy of the Colorado Plateau area of southeastern Utah is an example of both original horizontality and the law of superposition. These strata make up much of the famous prominent rock formations in widely spaced protected areas such as Capitol Reef National Park and Canyonlands National Park. From top to bottom: Rounded tan domes of the Navajo Sandstone, layered red Kayenta Formation, cliff-forming, vertically jointed, red Wingate Sandstone, slope-forming, purplish Chinle Formation, layered, lighter-red Moenkopi Formation, and white, layered Cutler Formation sandstone. Picture from Glen Canyon National Recreation Area, Utah.

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).[19]

The

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.[19]

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.[20]

Absolute dating

Main articles: Absolute dating, radiometric dating, and geochronology
The mineral zircon is often used in radiometric 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.[21]

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

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,

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.

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

composite volcano
, which releases both lava and ash.
An illustration of the three types of faults.
A. Strike-slip faults occur when rock units slide past one another.
B. Normal faults occur when rocks are undergoing horizontal extension.
C. Reverse (or thrust) faults occur when rocks are undergoing horizontal shortening.
The San Andreas Fault in California

The geology of an area changes through time as rock units are deposited and inserted, and deformational processes alter their shapes and locations.

Rock units are first emplaced either by deposition onto the surface or intrusion into the

lava flows blanket the surface. Igneous intrusions such as batholiths, laccoliths, dikes, 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

convergent boundaries, divergent boundaries
, and transform boundaries, respectively, between tectonic plates.

When rock units are placed under horizontal

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.

A diagram of folds, indicating an anticline and a syncline

Even higher pressures and temperatures during horizontal shortening can cause both folding and

crystalline rocks
.

Extension causes the rock units as a whole to become longer and thinner. This is primarily accomplished through

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 faults develop in shallow regions, and become shear zones
at deeper depths where the rocks deform ductilely.

Geological cross section of Kittatinny Mountain. This cross-section shows metamorphic rocks, overlain by younger sediments deposited after the metamorphic event. These rock units were later folded and faulted during the uplift of the mountain.

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 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

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.

Investigative methods

A standard Brunton Pocket Transit, commonly used by geologists for mapping and surveying

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

biogeochemical pathways, and use geophysical methods to investigate the subsurface. Sub-specialities of geology may distinguish endogenous and exogenous geology.[24]

Field methods

USGS
field mapping camp in the 1950s
digital geological mapping
).
petrified log in Petrified Forest National Park, Arizona
, US

Geological

field work
varies depending on the task at hand. Typical fieldwork could consist of:

A petrographic microscope
A thin section in cross polarized light
In optical mineralogy, thin sections are used to study rocks. The method is based on the distinct refractive indexes of different minerals.

Petrology

Main article: Petrology

In addition to identifying rocks in the field (

geochemical
evolution of rock units.

Petrologists can also use fluid inclusion data[35] and perform high temperature and pressure physical experiments[36] to understand the temperatures and pressures at which different mineral phases appear, and how they change through igneous[37] and metamorphic processes. This research can be extrapolated to the field to understand metamorphic processes and the conditions of crystallization of igneous rocks.[38] This work can also help to explain processes that occur within the Earth, such as subduction and magma chamber evolution.[39]

Folded rock strata

Structural geology

Main article: Structural geology
décollement. It builds its shape into a critical taper
, in which the angles within the wedge remain the same as failures inside the material balance failures along the décollement. It is analogous to a bulldozer pushing a pile of dirt, where the bulldozer is the overriding plate.

Structural geologists use microscopic analysis of oriented thin sections of geological 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 geological structures.

Among the most well-known experiments in structural geology are those involving orogenic wedges, which are zones in which mountains are built along convergent tectonic plate boundaries.[40] 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.[41] 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.[42] 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.[43]

Stratigraphy

Different colors caused by the different minerals in tilted layers of sedimentary rock in Zhangye National Geopark, China
Main article: Stratigraphy

In the laboratory, stratigraphers analyze samples of stratigraphic sections that can be returned from the field, such as those from

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.[46] Stratigraphers can then use these data to reconstruct ancient processes occurring on the surface of the Earth,[47]
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.[44] 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.[48] Magnetic stratigraphers look for signs of magnetic reversals in igneous rock units within the drill cores.[44] Other scientists perform stable-isotope studies on the rocks to gain information about past climate.[44]

Planetary geology

Surface of Mars as photographed by the Viking 2 lander December 9, 1977
Main article: Planetary geology

With the advent of

terrestrial planets, icy moons, asteroids, comets, and meteorites. However, some planetary geophysicists study the giant planets and exoplanets.[49]

Although the Greek-language-origin prefix

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.

Applied geology

Mokelumne. Harper's Weekly
: How We Got Gold in California. 1860

Economic geology

Main article: 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

Main article: Mining

silica, as well as elements such as sulfur, chlorine, and helium
.

Petroleum geology

Mud log in process, a common way to study the lithology when drilling oil wells
Main article: 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,[50] 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

Main articles: Engineering geology, Soil mechanics, and Geotechnical engineering

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, particularly in North America.

A child drinks water from a well built as part of a hydrogeological humanitarian project in Kenya.

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.[51]

Hydrology

Main article: Hydrogeology

Geology and geological principles can be applied to various environmental problems such as

natural habitat and the geological environment. Groundwater hydrology, or hydrogeology, is used to locate groundwater,[52] which can often provide a ready supply of uncontaminated water and is especially important in arid regions,[53] and to monitor the spread of contaminants in groundwater wells.[52][54]

Paleoclimatology

Main article: Paleoclimatology

Geologists also obtain data through stratigraphy,

global climate change outside of instrumental data.[57]

Natural hazards

Rockfall in the Grand Canyon
Main article:
Natural hazard § Geological 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.[58] Examples of important natural hazards that are pertinent to geology (as opposed those that are mainly or only pertinent to meteorology) are:

History

Main articles: History of geology and Timeline of geology
geological map of England, Wales, and southern Scotland. Completed in 1815, it was the second national-scale geologic map, and by far the most accurate of its time.[59][failed verification
]

The study of the physical material of the Earth dates back at least to

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 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.[60][61]

deposition of silt.[65]

Georgius Agricola (1494–1555) published his groundbreaking work De Natura Fossilium in 1546 and is seen as the founder of geology as a scientific discipline.[66]

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

Mikkel Pedersøn Escholt (1600–1669), who was a priest and scholar. Escholt first used the definition in his book titled, Geologia Norvegica (1657).[73][74]

rock strata (layers) by examining the fossils contained in them.[59]

In 1763, Mikhail Lomonosov published his treatise On the Strata of Earth.[75] 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.[76]

James Hutton (1726–1797) is often viewed as the first modern geologist.[77] 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.[78]

Followers of Hutton were known as

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.[79] 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.[80] 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.[81] This antedates William Smith
's geological map of England by six years, although it was constructed using a different classification of rocks.

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.[83] 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

Earth sciences. Today the Earth is known to be approximately 4.5 billion years old.[16]

See also

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