From Wikipedia, the free encyclopedia
(from the Greek
, "earth" and λόγος, logos
, "speech") is the science
and study of the solid and liquid matter that constitutes the Earth
. The field of geology encompasses the study of the composition, structure
, physical properties
, dynamics, and history
of Earth materials
, and the processes by which they are formed, moved, and changed. The field is a major academic discipline
, and is also important for mineral
extraction, knowledge about and mitigation of natural hazards
, some engineering
fields, and understanding past climates
History and etymology
A mosquito and a fly in this Baltic
amber necklace are between 40 and 60 million years old
Some modern scholars, such as Fielding H. Garrison
, are of the opinion that modern geology began in the medieval Islamic world
. Abu al-Rayhan al-Biruni
(973–1048 AD) was one of the earliest Muslim geologists
, whose works included the earliest writings on the geology of India
, hypothesizing that the Indian subcontinent
was once a sea. Ibn Sina
(Avicenna, 981–1037), in particular, made significant contributions to geology and the natural sciences (which he called Attabieyat
) along with other natural philosophers
such as Ikhwan AI-Safa
and many others. He wrote an encyclopaedic work entitled “Kitab al-Shifa
” (the Book of Cure, Healing or Remedy from ignorance), in which Part 2, Section 5, contains his essay on Mineralogy and Meteorology, in six chapters: Formation of mountains, The advantages of mountains in the formation of clouds; Sources of water; Origin of earthquakes; Formation of minerals; The diversity of earth’s terrain
. These principles were later known in the Renaissance
as the law of superposition
of strata, the concept of catastrophism
, and the doctrine of uniformitarianism
. These concepts were also embodied in the Theory of the Earth by James Hutton
in the Eighteenth century C.E. Academics such as Toulmin
(1965), commented on Avicenna's contribution: "Around A.D. 1000, Avicenna was already suggesting a hypothesis about the origin of mountain ranges
, which in the Christian world, would still have been considered quite radical eight hundred years later".
Avicenna's scientific methodology
of field observation
was also original in the Earth sciences, and remains an essential part of modern geological investigations.
In China, the polymath Shen Kua
(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
(1494–1555), a physician, wrote the first systematic treatise about mining
works, De re metallica libri XII
, with an appendix Buch von den Lebewesen unter Tage
(Book of the Creatures Beneath the Earth). He covered subjects like wind energy
, hydrodynamic power
, melting cookers, transport of ores, extraction of soda
, and administrative issues. The book was published in 1556.
(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.
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 in order 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
, who believed that all rocks had settled out of a large ocean whose level gradually dropped over time.
Sir Charles Lyell
first published his famous book, Principles of Geology
, in 1830. Lyell continued to publish new revisions until he died in 1875. The 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 100,000 to billions of years.
The most significant advance in 20th century geology has been the development of the theory of plate tectonics
in the 1960s. Plate tectonic theory arose out of two separate geological observations: seafloor spreading
and continental drift
. The theory revolutionized the Earth sciences
Geological time put in a diagram called a geological clock
, showing the relative lengths of the eons
of the Earth's history.
The geologic time scale encompasses the history of the Earth.
It is bracketed at the young end by the dates of the earliest solar system
material at 4.567 Ga
(gigaannum: billion years ago) and the age of the Earth
at 4.54 Ga
, at the beginning of the informally-recognized Hadean eon
. At the young end of the scale, it is bracketed by the present day in the Holocene epoch
Brief time scale
Millions of Years
The second and third timelines are each subsections of their preceding timeline as indicated by asterisks. The Holocene
(the latest epoch
) is too small to be shown clearly on this timeline.
Relative and Absolute Dating
Geological events can be given a precise date at a point in time, or they can be related to other events that came before and after them. Geologists use a variety of methods to give both relative and absolute dates to geological events. They then use these dates to find the rates at which processes occur.
Methods for relative dating
were developed when geology first emerged as a formal 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 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. There are a number of different types of intrusions, including stocks, laccoliths
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 which contains them.
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 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 at the same time period throughout the world, their presence or (sometimes) absence may be used to provide a relative age of the formations in which they are found. Based on principles laid out by William Smith
almost a hundred years before the publication of Charles Darwin
's theory of evolution
, the principles of succession were developed independently of evolutionary thought. The principle becomes quite complex, however, given the uncertainties of fossilization, the localization of fossil types due to lateral changes in habitat (facies
change in sedimentary strata), and that not all fossils may be found globally at the same time.
Geologists can also give precise absolute dates to geologic events. These dates are useful on their own, and can also be used in conjunction with relative dating methods or to calibrate relative dating methods.
A large advance in geology in the advent of the 20th century was the ability to give precise absolute dates to geologic events through radioactive isotopes and other methods. The advent of isotopic dating
changed the understanding of geologic time. Before, geologists could only use fossils to date sections of rock relative to one another. With isotopic dates, absolute dating
became possible, and these absolute dates could be applied fossil sequences in which there was datable material, converting the old relative ages into new absolute ages.
Fractionation of the lanthanide series
elements is used to compute ages since rocks were removed from the mantle.
The majority of geological data come from research on solid Earth materials. These typically fall into one of two categories: rock and unconsolidated material.
This schematic diagram of the rock cycle shows the relationship between magma and sedimentary, metamorphic, and igneous rock
There are three major types of rock: igneous, sedimentary, and metamorphic. The rock cycle
is an important concept in geology which illustrates the relationships between these three types of rock, and magma. When a rock crystallizes
from melt (magma
), it is an igneous rock
. This rock can be weathered
, and then redeposited
into a sedimentary rock, or be turned into a metamorphic rock
due to heat and pressure that change the mineral
content of the rock and give it a characteristic fabric
. The sedimentary rock can then be subsequently turned into a metamorphic rock due to heat and pressure, and the metamorphic rock can be weathered, eroded, deposited, and lithified, becoming a sedimentary rock. Sedimentary rock may also be re-eroded and redeposited, and metamorphic rock may also undergo additional metamorphism. All three types of rocks may be re-melted; when this happens, a new magma is formed, from which an igneous rock may once again crystallize.
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.
On this diagram, subducting slabs are in blue, and continental margins and a few plate boundaries are in red. The blue blob in the cutaway section is the seismically-imaged Farallon Plate
, which is subducting beneath North America. The remnants of this plate on the Surface of the Earth are the Juan de Fuca Plate
and Explorer plate in the Northwestern USA / Southwestern Canada, and the Cocos Plate
on the west coast of Mexico.
The development of plate tectonics provided a physical basis for many observations of the solid Earth. Long linear regions of geologic features could be explained as plate boundaries. Mid-ocean ridges
, high regions on the seafloor where hydrothermal vents
and volcanoes exist, were explained as divergent boundaries
, where two plates move apart. Arcs of volcanoes and earthquakes were explained as convergent boundaries
, where one plate subducts
under another. Transform boundaries
, such as the San Andreas fault
system, resulted in widespread powerful earthquakes. Plate tectonics also 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 layered structure. (1) inner core; (2) outer core; (3) lower mantle; (4) upper mantle; (5) lithosphere; (6) crust
Earth layered structure. Typical wave paths from earthquakes like these gave early seismologists insights into the layered 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
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 at depth to reproduce these conditions in experimental settings and measure changes in crystal structure. These studies explain the chemical changes associated with the major seismic discontinuities in the mantle, and show the crystallographic structures expected in the inner core of the Earth.
Geological evolution of an area
An originally horizontal sequence of sedimentary rocks (in shades of tan) are affected by igneous
activity. Deep below the surface are a magma chamber
and large associated igneous bodies. The magma chamber feeds the volcano
, and sends off shoots of magma
that will later crystallize into dikes and sills. Magma also advances upwards to form intrusive igneous bodies
. The diagram illustrates both a cinder cone
volcano, which releases ash, and a composite volcano
, which releases both lava and ash.
An illustration of the three types of faults. Strike-slip faults occur when rock units slide past one another, normal faults occur when rocks are undergoing horizontal extension, and thrust faults occur when rocks are undergoing horizontal shortening.
The geology of an area evolves through time as rock units are deposited and inserted and deformational processes change their shapes and locations.
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 cause 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 the rock layers are dragged along, forming drag folds, as slip occurs are 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
, 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; and 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 being placed below older units. Stretching of units can result in their thinning; in fact, there is a location within the Maria Fold and Thrust Belt
in which the entire sedimentary sequence of the Grand Canyon can be seen 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.
Geologic 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 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 understand the processes that occur on and in 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
, and glaciers
; investigate past and current life and biogeochemical
pathways, and use geophysical methods
to investigate the subsurface.
A typical USGS
field mapping camp in the 1950's
Geological field work
varies depending on the task at hand. Typical fieldwork could consist of:
- Geological mapping
- Structural mapping: the locations of the major rock units and the faults and folds that led to their placement there.
- Stratigraphic mapping: the locations of sedimentary facies (lithofacies and biofacies) or the mapping of isopachs of equal thickness of sedimentary rock
- Surficial mapping: the locations of soils and surficial deposits
- Surveying of topographic features
- Subsurface mapping through geophysical methods.
- These methods include:
- They are used for:
- High-resolution stratigraphy
- Biogeochemistry and geomicrobiology
- Collecting samples to:
- 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
- Collection of samples for geochronology and thermochronology
- Glaciology: measurement of characteristics of glaciers and their motion
Petrologists use fluid inclusion
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
A diagram of an orogenic wedge. The wedge grows through faulting in the interior and along the main basal fault, called the 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 geologic samples to observe the fabric
within the rocks which gives information about strain within the crystal structure of the rocks. They also plot and combine measurements of geological structures in order to better understand the orientations of faults and folds in order 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 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 several 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 the mountain range. These studies can also give useful information about pathways for metamorphism through pressure, temperature, space, and time.
Main article: Stratigraphy
Exploration geologists examining a freshly recovered drill core. Chile
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 in order 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.
Surface of Mars as photographed by the Viking 2
lander December 9, 1977.
With the advent of space exploration
in the twentieth century, geologists have begun to look at other planetary bodies in the same way as the Earth. This led to the establishment of the field of planetary geology
, sometimes known as Astrogeology, in which geologic principles are applied to 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 all aspects of the planets, a significant focus is in the search for past or present life on other worlds. This has led to many missions whose purpose (or one of their purposes) is to examine planetary bodies for evidence of life. One of these is the Phoenix lander
, which analyzed Martian
polar soil for water and chemical and mineralogical constituents related to biological processes.
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 consists of the extractions of mineral resources from the Earth. Some resources of economic interests include gemstones
, and many minerals such as asbestos
, and silica
, as well as elements such as sulfur
, and helium
Petroleum geologists study locations of the subsurface of the Earth which 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 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
Main article: Hydrogeology
Geology and geologic principles can be applied to various environmental problems, such as stream restoration
, the restoration of brownfields
, and the understanding of the interactions 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.
Main article: Natural hazard
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:
By mountain range
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