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 refer to 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
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, geologists 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 experiments,
and numerical modelling.
In practical terms, geology is important for
mineral and hydrocarbon exploration and exploitation,
evaluating water resources, understanding
of natural hazards, the remediation of environmental
problems, and providing insights into past
climate change.
Geology, a major academic discipline, also
plays a role in geotechnical engineering.
== Geologic materials ==
The majority of geological data comes from
research on solid Earth materials.
These typically fall into one of two categories:
rock and unconsolidated material.
=== Rock ===
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,
sedimentary, and metamorphic.
The rock cycle
illustrates the relationships among them (see
diagram).
When a rock crystallizes from melt (magma
or lava), it is an igneous rock.
This rock can be weathered and eroded, then
redeposited and lithified into a sedimentary
rock.
It can then be turned into a metamorphic rock
by heat and pressure that change its mineral
content, resulting in a characteristic fabric.
All three types may melt again, and when this
happens, new magma is formed, from which an
igneous rock may once more crystallize.
==== 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: Measurement of the amount of light
reflected from the surface.
Luster is broken into metallic and nonmetallic.
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 scratch.
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,
like halite (which tastes like table salt).
Smell: Minerals can have a distinctive odor.
For example, sulfur smells like rotten eggs.
=== Unconsolidated material ===
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.
== Whole-Earth structure ==
=== 
Plate tectonics ===
In the 1960s, it was discovered that the Earth's
lithosphere, which includes the crust and
rigid uppermost portion of the upper mantle,
is separated into tectonic plates that move
across the plastically deforming, solid, upper
mantle, which is called the asthenosphere.
This theory is supported by several types
of observations, including seafloor spreading
and the global distribution of mountain terrain
and seismicity.
There is an intimate coupling between the
movement of the plates on the surface and
the convection of the mantle (that is, the
heat transfer caused by 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.
For example:
Mid-ocean ridges, high regions on the seafloor
where hydrothermal vents and volcanoes exist,
are seen as divergent boundaries, where two
plates move apart.
Arcs of volcanoes and earthquakes are theorized
as convergent boundaries, where one plate
subducts, or moves, under another.Transform
boundaries, such as the San Andreas Fault
system, 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.
=== Earth structure ===
Advances in seismology, computer modeling,
and mineralogy and crystallography at high
temperatures and pressures give insights into
the internal composition and structure of
the Earth.
Seismologists can use the arrival times of
seismic waves 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 and
lithosphere 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
modelling 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.
== Geologic time ==
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 Ga
(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).
=== Time scale ===
The following four timelines show the geologic
time scale.
The first shows the entire time from the formation
of the Earth to the present, but this gives
little space for the most recent eon.
Therefore, 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, and the most recent
period is expanded in the fourth timeline.
Millions of Years
=== 
Important milestones ===
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, 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
542 ± 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 – 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 ka (kiloannum: thousand years ago): First
modern Homo sapiens appear in East Africa
== 
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
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,
laccoliths, batholiths, sills and dikes.
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 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 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 vertical time line,
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 geologic events using radioactive isotopes
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 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 lattice.
These are used in geochronologic and thermochronologic
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 emplacement.
Thermochemical techniques can be used to determine
temperature profiles within the crust, the
uplift of mountain ranges, and paleotopography.
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 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 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 deformed
and/or metamorphosed.
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 and
folding.
In the shallow crust, where brittle deformation
can occur, thrust faults form, which causes
deeper rock to move on top of 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 "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
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 lavas, 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 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.
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 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 undiscernable 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 field, 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 biogeochemical pathways,
and use geophysical methods to investigate
the subsurface.
Sub-specialities of geology may distinguish
endogenous and exogenous geology.
=== Field methods ===
Geological field work varies depending on
the task at hand.
Typical fieldwork could consist of:
Geological mappingStructural 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 (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
River-channel change through migration and
avulsion
Hillslope processes
Subsurface mapping through geophysical methodsThese
methods include:
Shallow seismic surveys
Ground-penetrating radar
Aeromagnetic surveys
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 and logging
Biogeochemistry and geomicrobiologyCollecting
samples to:
determine biochemical pathways
identify new 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,
twinning, 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 evolution.
=== Structural geology ===
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 mountains 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.
=== 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, 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.
== 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 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".
Specialised 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 ==
=== 
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, 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
the 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
samples, and ice cores.
Ice cores and sediment cores are used to 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 of geology ==
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 period, Pliny the Elder wrote
in detail of the many minerals and metals
then in practical use – even correctly noting
the origin of amber.
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 erosion
of the mountains and by 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 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 (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.James Hutton 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 (Vol. 1, Vol. 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 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 ==
== See also
