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Glossary of Geology Terms and Definitions

Glossary of Geology Terms and Definitions

Geology is the study of Earth science. This ScienceStruck article will guide you with the definitions of the basic geological terms, thereby, helping you to know more about this subject.
Mayuri Kulkarni
Last Updated: Dec 10, 2017
Blink of an Eye!
If the entire geological time period since the formation of our planet is compressed in a span of 24 hours, the evolution of humans or Homo sapiens will only be one second out of the entire 24 hour time frame! Such a representation is known as an geological clock.
The study of the Earth, the solid and liquid materials it is made up of, as well as the structure of these materials are together termed as geology. The field also consists of the history of formation of volcanoes, rivers, mountains, rocks, glaciers, etc., and the different processes that take place on our planet's surface, along with the effect of these processes on human life. So, geology also consists of the study of living things/organisms that have inhabited the Earth. The main aim of geological research is to study the history of our planet, along with the gradual changes in the Earth materials and living organisms, due to various processes that took place over vast time periods. The study of these processes helps foresee events that can happen in future due to these aspects.
Earthquakes, landslides, volcanoes, floods, etc., are all studied under this field. This helps in avoiding future casualties due to natural disasters by taking proper preventive measures. The exploration of oil, minerals, and metals helps find out the presence of these materials on our planet. The Earth's history helps us to understand the related climate change and how the present climate will influence the future. Geology, if taken up as a career, promises a very bright future. Given below is an alphabetical glossary of all geological terms that are commonly used while studying it.
A
Aa
It is a type of basaltic lava that has a high speed of flowing and cools down to form rough and scraggy-surfaced igneous rocks. It is a fragmented type of lava that becomes highly viscous on melting. The lava is named "aa" due to the natural response of sound one makes on touching it. The term "aa" has a Hawaiian origin.

A Horizon
It is the uppermost layer of soil that contains leached and organic minerals. It contains less amount of clay, and is a layer where any of the biological processes take place. It is also called biomantle, as it contains many of the soil organisms like earthworms, fungi, bacteria, etc.

Ablation
It means the removal of the surface material below the snow line due to various erosive processes. The snow/ice is lost from the glaciers, and these erosive processes include evaporation, wind erosion, calving, melting, etc.

Absolute Time
The time period used to define the age of a geological term is known as absolute time. This age can be of any term that includes geological events or related parameters. The age can be in seconds, minutes, years, etc. Radiometric dating techniques are used to determine the age of any geological term.

Abrasion
It is a mechanical weathering process that takes place when loose particles of rocks are released, and these particles collide with each other or move along the surfaces of bedrocks and boulders. Thus, abrasion is the frictional process between moving particles and stationary rocks. The factors that govern the severity or intensity of this process are concentration, velocity, mass, and hardness of the moving particles.

Abyssal Plain
The deepest regions of oceans that are characterized by large, flat, or gentle sloping areas are called abyssal plains. These regions are very difficult to explore due to their extreme depth. These plains are usually found at a depth of 6,500 ft below the mean sea level.

Accretion
It is defined as the growth of a body due to accumulation and adhesion. In geological terms, accretion is a process wherein geological material is added to the landmass or a tectonic plate. This can be in the form of sea mounts, volcanic arcs, or sediments. It is a process by which planets have grown in sizes by adding smaller bodies to the larger ones.

Accretionary Wedge
It is also called accretionary prism, and it is formed by the process of accretion. These wedges are mostly made up of sediments accreted on a subducting tectonic plate, at the boundary of a convergent plate. They can also be made up of marine sediments and volcanic island arcs.

Acidic Rock
Acidic rock is a term that refers to an igneous rock, which is made up of light-colored minerals. This rock consists of more than 66% of combined or free silica. Thus, acidic rock can also be defined as an igneous rock that has maximum silica content. Some of the examples are rhyolite and granite.

Acid Rain
This type of rain is produced by a combination of water present in the atmosphere and oxides that are released on burning of hydrocarbons. It contains acidic compounds like nitric acid and sulfuric acid. The emission of chemical compounds reacts with atmospheric water to form sulfuric and nitric acid that mix with the rainwater. Acid rain leads to damage of crops, infrastructure, and forests.

Active Volcano
It is defined as the type of volcano that is still producing lava, and it can erupt in the near future. Mauna Loa is the world's largest active volcano.

Aftershock
They are tremors in the ground that take place just after the main earthquake. These shocks may even take place two years after the main earthquake has occurred. The magnitude is usually less than the main earthquake, and near the epicenter of the main earthquake.

Algal Mat
This is a layer of filamentous algae formed in a sequential manner of wetting, algal formation, and drying, at the bottom of freshwater or marine water. It is also a layer formed by a group of algae that are found in fossils, that are related to carbonate sedimentation.

Alpine Glacier
Alpine glaciers are formed on the slopes of a mountain. When the size of the alpine glacier grows and fills a valley, it is called a valley glacier.

Alluvial Fan
It is a fan-shaped wedge that is formed by the accumulation of sediments left by a fast-flowing stream, when it slows down and spreads in a fan-shaped fashion at the canyon exit.

Alluvium
This type of sedimentary deposition occurs due to a running water source (like streams and rivers) and consists of clay, gravel, sand, and silt particles. The sediments deposited are present in an unconsolidated form.

Amphibolite
This metamorphic rock is formed by the process of recrystallization. It mainly consists of the amphibole mineral, some amount of plagioclase feldspar, and little or no amount of quartz.

Amygdaloidal
It is defined as the texture of volcanic rocks containing a large number of amygdules (almond-shaped vesicles filled by secondary minerals).

Amygdule
Amygdule is a gas cavity formed in a volcanic rock that is filled with secondary minerals like quartz, chlorite, zeolites, etc.

Andesite
This igneous rock mainly consists of plagioclase laths along with other minerals like biotite, pyroxene, hornblende, etc. It is the most common type of rock found in volcanic regions of the Earth.

Angle of Repose
The maximum angle at which a heap of an unconsolidated material like soil, sediments, or any loose material can stay stable is called angle of repose. It can also be defined as the maximum angle, when such loose material is added to the heap and the material comes to rest.

Angular Unconformity
An unconformity is formed due to the deposition of horizontally parallel strata of sedimentary rocks on eroded or tilted layers. This deposition leads to an angular discordance with horizontal layers that are overlaid.

Anthracite
It is a type of coal that has the maximum percentage of carbon (92 - 98%). It burns with blue flames and less smoke. It is commonly referred to as "hard coal".

Anticline
Anticline is a fold in a rock and is convex-shaped. It is the oldest section of the rock. The term is confused with the general term "antiform", that is used to describe any convex-up fold.

Aphanitic
It is a type of texture seen in igneous rocks that are made up of very fine crystals and/or mineral particles that cannot be identified with naked eyes. It is mainly seen in extrusive igneous rocks that undergo quick cooling to inhibit the growth of crystals.

Aplite
It is an intrusive igneous rock that contains maximum percentage of feldspar and quartz. It is a light-colored and fine-grained rock.

Aquiclude
This type of groundwater reservoir absorbs water slowly, but does not transmit it with great speed enough to act as a supply for a spring or well.

Aquifer
The kind of underground rock formation that stores groundwater, which can be extracted from wells and pumps, is known as an aquifer. It is basically of two types, confined and unconfined, depending on the pressure under which the groundwater is present in it.

Aquitard
This underground reservoir has low permeability, and hence, the movement of water through these rock formations is slow.

Archean
It is a geologic eon that represents or describes the time period between 3.9 billion years to 2.5 billion years ago.

Aridity Index
The index that represents the ratio of annual evaporation to the solar radiation received by an area is called aridity index. It gives the degree of deficiency of water in a specific region.

Arkose
It is a sedimentary rock that has at least 25% feldspar minerals. This rock can be recognized due to its typical pink color that is imparted because of the presence of angular-shaped feldspar grains. Arkose is formed by the weathering of rocks that contain feldspar.

Arroyo
It is a dry bed or gulch that fills with water seasonally due to heavy rains. An arroyo can be man-made as well as natural.

Artesian Well
This type of well is characterized by pumping of water from confined aquifers to the surface due to the high pressure acting on the water level.

Aseismic
The regions where earthquakes do not occur are called aseismic regions.

Ash
Ash is made up of fine particles of volcanic gas and rock that are thrown up in the atmosphere after a volcanic eruption. It is also referred to as volcanic ash.

Assemblage
It is a group of minerals that describe the characteristics of a rock/facies.

Asthenosphere
It is the upper mantle of the Earth that is present just below lithosphere. It is located at 100 to 200 km below the Earth's crust, and extends approximately up to 400 km. This layer is made up of soft and mobile rocks, and it is called the 'weak zone'.

Astrobleme
An old/ancient scar left on the Earth's surface which is circular in shape and is produced due to an impact of comet or meteorite is called an astrobleme.

Atoll
It is a circular ring-shaped structure of coral islands that encloses a lagoon and is surrounded by ocean water.

Augen
Augen means "eyes" in German. These are mineral grains that are large and eye-shaped in appearance. They are found in metamorphic rocks. Some of the common minerals that form augen structures are quartz, feldspar, and garnet.

Aureole
This zone surrounds an igneous intrusion, and is characterized by an occurrence of contact metamorphism.

Authigenesis
In this process, new minerals are formed due to the deposition of sedimentary rocks.

Avalanche
A rapid flow of ice down a slope that takes place naturally or due to human activity is called an avalanche.

Aggradation
This process consists of an increase in the elevation of landforms due to sediment deposition. Usually, aggradation takes place in areas where the amount of sediment deposited is more than the amount of sediment that the area can transport.
B
Back-arc Basin
These depressions are present between subduction zones and island arcs. These can be found at the boundaries of a few convergent plates that are currently located in the Western Pacific Ocean. They are a characteristic feature of island arc tectonics.

Back-arc Spreading
In this process, the speed of the subducting plate is more than the overriding plate. The overriding plate of the subduction zone is stretched to a point of rifting to make the magma rise into the gap formed by the rift.

Backswamp
This region of floodplains is characterized by fine silt and clay deposition that occurs after a flood. Such areas are generally found behind natural levees of a stream.

Banded Iron Formation
Also referred to as banded iron ore, it is a typical type of rock that is made up of repeated thin layers of iron oxides (hematite/magnetite) and chert (silica oxide/dioxide). Most of the banded iron formations are estimated to have formed 2 billion years ago.

Bar
Bar is a deposition of sediments that generally consist of sand or gravel and is usually formed in river mouths, river, border of a stream, or offshore waters. A bar is also a unit of pressure.

Barchan Dune
It is a crescent-shaped dune formed due to wind action, and moves across the desert. Its convex side faces upwind, and the steeply sloped concave side faces downwind. It is usually formed in desert regions with less sand and stable wind directions.

Barrier Island
A narrow and long island, which is made up of sand due to the action of waves that run parallel to the shore is known as a barrier island. The height of this geomorphic structure can be about 6 meters, and its length can be up to 100 km.

Basal Sliding
In this process, the glacier undergoes thawing at its base, resulting in its movement. This process usually takes place in warmer or temperate areas, and due to the slipping movement, it is known as basal sliding.

Basalt
It is an extrusive dark-colored igneous rock that is characteristically formed at the mid-oceanic regions. During fissure eruptions, the lava consolidates in the form of flows and columns. This rock is fine-grained in appearance and mainly consists of plagioclase feldspars, augite, and glass fragments.

Base Flow
Base flow is a part of streamflow that arises from precipitation due to soil infiltration and gradually flows through the stream. It is the flow of water that is contributed by drainage of springs, seepage, or large lakes.

Base Level
Base level is the elevation where any flowing water source cannot erode any more. It can be called the lowest level up to which a land area can be reduced or eroded by the action of running water.

Basement
Rocks like igneous and metamorphic rocks present below sedimentary cover or platform are called basement.

Basin
Any large depression-filled sediment can be called basin. With reference to tectonics, it can be a depression of layers that are circular and syncline (downward curving fold). While with reference to sedimentology, it is an area with a large thick deposition of sediments.

Batholith
Batholith is a very huge mass of intrusive igneous rock that is exposed due to erosion, and the eroded surface is spread over an area of about 100 sq km.

Bathymetry
It is the study of measuring the depths of ocean floors and preparing topographic maps.

Bauxite
It is an important ore of aluminum. Bauxite is a rock that is made up of hydrous aluminum oxides and is a product of weathering in tropical areas.

Bed
Bed is the smallest part of stratigraphic rock series and is a layer of volcanic material or sediment that can be recognized from other layers.

Bedding
The arrangement of layers with different compositions and grain sizes, one above the other in specific order, with the youngest layer at the top and the oldest layer at the bottom, constitutes the bedding of a particular sedimentary rock.

Bedding Plane
It divides different layers of a sedimentary rock, which is composed of various minerals.

Bedrock
The oldest and bottommost rock layer of a sedimentary succession is known as a bedrock. It is rarely exposed, and it forms the base of an entire sedimentary formation.

Berm
Berm is a layer or a narrow pile formed due to deposition of sediments on a backshore by the action of storm waves.

B Horizon
B horizon is also called subsoil and is a soil layer that is composed of minerals or clay. It is composed of minerals, like aluminum or iron, or any organic material that is deposited by the process of leaching. B horizon is also referred as zone of accumulation.

Bioturbation
The displacement or mixing of sediments by animals and plants is called bioturbation.

Black Smoker
A sea vent formed due to superheated water that flows from the Earth's crust to the ocean floor is called black smoker. This water is mineral-rich, and when it comes in contact with cold ocean water, the minerals deposit on the vent and form a black chimney-like structure.

Blowout
It is a sandy, shallow, circular, depression formed due to the action of wind.

Body Wave
A seismic wave that travels or transmits energy through the body of the Earth and not along the Earth's surface is called body wave. This wave transmits energy in all the directions from the earthquake's focus through the interior of the Earth.

Bottomset Bed
It is a layer formed due to the deposition of silt and clay near the boundary separating the delta and shore areas.

Bowen's Reaction Series
It was developed by the geologist Norman L. Bowen, and it explains the process of evolution of minerals on our planet, especially in case of mafic magma. In this progression, the latter minerals are complex in nature, and they form due to reactions between the former or simpler ones of the series.

Braided Stream
It is made up of a network of number of water channels, which are separated by a narrow band of sand and gravel. These channels divide and combine at various parts of the main stream, forming a braid-like pattern.

Branchwork Cave
It consists of passages, which are formed in a pattern that resemble the flow of tributaries in the downward direction or in an areal pattern.

Breakwater
Breakwater is a wall that is built parallel to a coast to cease the incoming waves and protect the shore or harbor.

Breccia
Breccia is a type of clastic rock that is made up of angular fragments of minerals or rocks in a matrix form. There are different types of breccia depending on the type of its parent rock, like tectonic breccia, sedimentary breccia, impact breccia, igneous breccia, etc.

Butte
In French, butte means "small hill". Butte is a hill that has a flat top, small vertical steep sides, and is located at an isolated place. The hill is formed due to erosive action in an area that contains flat sedimentary rocks.

Beheaded Stream
When the lower part of a stream loses its upper portion due to stream piracy or diversion, it is known as a beheaded stream.

Bankfull Stage
Bankfull stage is the height of the water or the water level at which the natural channel gets filled completely. Rise of the water above this level indicates occurrence of flood.

Biogenic Sediment
Biogenic sediment is a skeletal deposition that results due to physiological processes or activities of living organisms.
C
Calcareous
Any soil, sediment, or sedimentary rock that either contains a high percentage of calcium carbonate, or is entirely made up of calcium carbonate, can be attached with the adjective "calcareous." It mainly means that the geological structure contains a high amount of calcium.

Calcarenite
It is a rock that is formed by the process of water percolation through a mixture of fragments of calcareous shell and quartz sand. During this process, the lime that dissolves in water acts as a cementing material to bond together the total mass. Calcarenite is also known as dune limestone or dune rock.

Caliche
A sedimentary rock that is formed due to the hardening of the calcium carbonate deposition is called caliche. During the hardening process, the calcium carbonate in the rock cements together other sediments like clay, silt, sand, and gravel.

Calving
Also referred to as ice calving, calving is a process wherein a mass of ice breaks away from bodies like glaciers, icebergs, ice shelves, crevasse. This mass ends up falling into a water source, and usually the ablation of ice creates more icebergs.

Caprock
It is highly resistant, and it overlies a rock that is comparatively less resistant. Ultramafic rocks and sandstones are few examples of caprocks.

Carbonate Conservation Depth
The water level in the ocean below which calcium carbonate does not exist is called carbonate conservation depth. Below this level, CaCO3 in the ocean is in a completely dissolved form.

Carbonate Rocks
A group of sedimentary rocks that are mainly composed of carbonate minerals are called carbonate rocks. Major examples are dolomite, limestone, and aragonite.

Cataclastic Metamorphism
It is a very uncommon type of metamorphism that takes place due to mechanical deformation. It is also called dynamic metamorphism, and is related to chief fault planes. The metamorphism takes place due to heat generated by the friction of fault movements.

Cementation
In this process, dissolved minerals get deposited in the openings of sediments, wherein a new rock is formed. Carbonates, iron oxides, silica, and quartz are some of the cementing materials.

Cenozoic
Out of the three eras of human history, Cenozoic era is the latest one or the present geological era. It began around 65.5 million years ago, and it continues till date.

Chalcedony
This silicate mineral is cryptocrystalline in nature, and it is made up of very fine inter-growths of moganite and quartz. Both these minerals have different crystal structures. Chalcedony has a waxy luster, and can be translucent or semitransparent.

Chalk
It is a type of limestone that is made up of calcite. It is a sedimentary rock that is white, porous, and soft in nature. Magnesium silicate and calcium sulfate can also be called chalk.

Chemical Remanent Magnetism
Abbreviated as CRM, this type of magnetization is acquired by rocks, during the process of diagenesis of sedimentary deposition. Thus, the magnetic minerals aligned themselves to the global magnetic field.

Chemical Sediment
It is formed due to the chemical precipitation of water. This sediment is made up of dissolved minerals, which can be precipitated due to vaporized water or due to extraction of water by living organisms and deposition of minerals upon the death of an organism.

Chemical Sedimentary Rock
It is formed by precipitation of minerals from water or any other solution, and is composed of chemical sediments.

Chemical Weathering
It is a process or a set of chemical reactions that alter the chemical composition of rocks. Oxidation and hydrolysis are some of the most common chemical alteration processes that take place. Exposure of rocks to water and atmosphere leads to chemical weathering.

Chert
This sedimentary rock is cryptocrystalline and is made up of fine-grained silica. Chert is found in different colors like brown, gray, white, rusty red, etc.

C Horizon
It is also called regolith, and is the layer that lies below the B horizon. This layer is less organic in nature, and is characterized by very less weathering.

Cinder Cone
Also called scoria, this conical-shaped mountain is made up of lava fragments known as cinders that are formed from volcanic eruptions. The cinders are accumulated around the volcanic vent.

Clastic
They are sedimentary rocks that are made up of particles derived from pre-existing rocks. These particles are called clasts.

Clay
It is made up of fine grained materials that are produced during the weathering process. Clay shows the characteristic of plasticity, depending on the percentage of water added to it. This naturally occurring material can be hardened when heated or dried.

Claypan
It is a layer that contains a higher percentage of clay. This layer is compact, impervious, and stiff. The speed of water slows down when passing through a claypan.

Cleavage
Certain rocks or minerals to break along specific planes where weak bonds are present. Such planes are called cleavages.

Coal
It is a combustible sedimentary, and is black or brown in color. It is formed by the decomposition of dead plants, and contains more than 50% of carbon compounds.

Coast
It is a dry part of land that is located at the margin of a water body like an ocean.

Compaction
It is the process of reduction in porosity of a sediment due to the effect of loading of the deposition above it. This compression process changes the shape and orientation of a sediment due to the weight of deposition that overlies it.

Composite Volcano
Also called stratovolcano, this structure is characterized by the presence of cinders, lava, and other eruptive material layers, which are arranged in an alternating pattern.

Comprehensive Soil Classification System (CSCS)
This classification is based on the physical and chemical properties of different types of soil.

Compression
It is a system of forces that reduce the length or volume of rock. Compressive forces act on tectonic plates to create folded mountains.

Concordant
Rocks that lie parallel to the strata are referred to as concordant.

Cone of Depression
It is an area created below a well or an underground water source due to pumping action. This area is formed due to faster speed of pumping, as compared to the speed required by water to flow through the aquifer.

Conglomerate
It is a sedimentary clastic rock, which is made up of lithic beds of round gravel. The fragments are cemented together with the help of sand.

Constancy of Interfacial Angles
Also called "first law of crystallography", it actually states that "the angles between the crystal faces of a given species are constant, whatever the lateral extension of these faces or the origin of the crystal, and are characteristic of that species."

Contact Metamorphism
Also known as thermal metamorphism, this process results due to an extreme or sudden change in temperature of the surrounding environment of the specific rocks. This results in the entire recrystallization of the mineral grain of the parent rocks, and ultimately leads to the formation of formed metamorphic rocks like granulites and marbles.

Continental Arc
It is a strip or a belt of volcanic mountains lying above a subduction zone on a continental mainland.

Continental Crust
It is a layer or a part of the Earth's crust that is made up of metamorphic, sedimentary, and igneous rocks, forming the continental portions. This layer is on an average 35 km thick, and can be maximum 70 km thick in few large mountain ranges.

Continental Deserts
These deserts are located in the interior part of a continent, and remain dry for most part of the year.

Continental Ice Glacier
The rivers of ice that are formed at high latitudes are called continental ice glaciers. These are located in regions where the temperature is too cold even in summer so that snow that was stored in the previous winter does not melt. Such glaciers are found in Arctic and Antarctic regions.

Continental Rise
It is the area that is present between the ocean floor and the continental slope.

Crust
It is defined as the outermost or uppermost layer of our planet and is a part of the lithosphere. There are two types of the Earth's crust: continental crust and oceanic crust. The thickness of the Earth's crust lies between 7 to 70 km. It is made up of rocks formed by igneous processes, and these are low-density rocks.
D
Dacite
It is a kind of volcanic rock that is rich in iron content. It is an extrusive igneous rock, and is made up of crystalline minerals and plagioclase feldspars. The composition of dacite lies between rhyolite and andesite.

Dendritic Drainage
It is a type of stream/river drainage pattern, which appears like dendrites that are joined to each other at acute angles. This pattern is usually formed in areas where the surface rocks are resistant to erosion. It is the most common type of drainage system that is mostly observed in igneous rock terrains.

Desertification
It is a process that is basically initiated due to human activities, and implies conversion of a non-desert land into desert area. This conversion usually takes place due to loss of vegetation or its gradual destruction. Overgrazing, increase in the rate of erosion, reduction in water table due to overdrafting, overpopulation, etc., are the causes of this process.

Detrital Sedimentary Rock
It is made up from particles that are derived due to weathering, and are transported by air or water. These rocks are composed of organic debris particles that rederived from preexisting rocks.

Diagenesis
This process consists of chemical, biological, or physiological changes of sediments, and includes their stages of deposition and rock formation. Thus, it can be called a set of processes that brings a change in sediments once they are deposited.

Devonian
It is a geologic period of time that existed about 416 to 359.2 million years ago. This period is said to have occurred during the Paleozoic era.

Differential Weathering
There are a few rocks that weather at different rates as compared to the ones that exist in their surroundings, in spite of being exposed to the same climate or environment. This happens due to the different compositions and resistance of rocks to weathering.

Dolostone
It is a sedimentary rock that is mainly made up of dolomite mineral, which consists of calcium, carbon, oxygen, and magnesium.

Dome
An upheaval of the Earth's surface, which consists of symmetrical anticlines is a called a dome. These structures consist of the oldest part of a rock, and they are formed throughout the process of their deformation.

Dust Storm
These storms take place when dust particles travel in large amounts and spread over a very large area of an arid region.
E
Earth System
It can be simply defined as the interconnection of biological and physical components of our planet.

Earthquake
The sudden release of energy due to the movement of rigid rocks under the Earth's crust, which causes the propagation of seismic waves towards the ground surface is called an earthquake.

Eon
An eon can be called the major or primary division of geologic time scale. Hadean, Archean, Proterozoic, and Phanerozoic eons are the ones in chronological order, with Hadean being the oldest and Phanerozoic being the present eon.

Epicenter
The point located on the Earth's surface that is present exactly above the focus point of an earthquake is called epicenter.

Era
An era is a unit or division of geologic time, and is made up of two or more periods. Paleozoic, Mesozoic, and Cenozoic are names of geological eras in chronological order, with Paleozoic being the oldest era.

Erosion
In this process, the surface particles of a rock get worn off or loosened, and are transported to some other place by air (wind), water, gravity, or ice.

Eutrophication
In this process, water bodies like lakes, streams, or any slowing moving river acquire excess nutrients that results into increased plant growth, and thereby, reduces the amount of oxygen in the water, leading to the death of other organisms living in the same body.

Exfoliation
In this process, weathering of a large rock mass takes place, such that the scales, plates or shells of this rock mass get peeled off.

Exfoliation Dome
It is a dome-shaped structure formed on a rock as a result of exfoliation. Such domes are usually formed in igneous rocks.

Extrusive Rock
They are igneous rocks that are formed by quick solidification of flowing lava from the Earth's surface. They can also be defined as fine-grained igneous rocks formed due to crystallization of magma on the Earth's surface.
F
Fault
It is defined as a crack or a fracture that divides a rock into two parts. The movement of the rock takes place along this crack.

Fault Gouge
It is a type of tectonic rock that is soft, has very small grain size, and is found near the faults.

Ferromagnesian
Minerals that contain magnesium and iron are called ferromagnesian. For example, olivine and pyroxenes are included under this category.

Flint
This silicate rock is a type of chert that is hard in nature, and is also known as flintstone. It is cryptocrystalline and sedimentary, and can be found in rocks like limestone and chalk in the form of nodules.

Flowstone
This term is used for all the cave deposits that are made up of carbonate compounds or calcite materials. These rocks are formed along the floor or walls of the caves along which water flows.

Foliation
It is defined as the structure or arrangement of a class of minerals in a parallel manner, such that they form sheet-like layers. These structures are usually formed in metamorphic rocks as a result of exposure to direct pressure.

Foreshock
The first tremor felt just before the main body of an earthquake occurs is known as foreshock. It is very difficult to detect such tremors on seismographs, and generally a full-scale earthquake follows a foreshock with the same epicenter or somewhere near the epicenter. An earthquake may take place after occurrence of a number of foreshocks.

Fossil
It is an evidence or a trace of dead animals and plants that are buried under sedimentary strata. Footprints imprinted on rocks, bones of ancient humans and animals, shells, etc., are a few examples.

Fractional Crystallization
In this process, crystals are produced from magma and are separated from the previous magmatic fluids. This changes the composition of the magma and generates igneous rocks with different properties and composition. Due to removal of crystals, the percentage of silica in the magma increases gradually.

Frost Wedging
It is a type of physical or mechanical weathering, wherein rocks that were joined together are separated due to expansion of water. This process takes place in rock fractures as a result of freezing. Formation of ice in the cracks increase the wedging effect.
G
Gabbro
The type of intrusive igneous rocks that are coarse-grained, dark-colored, and mafic in composition are called Gabbroic rocks. This group is similar to basalt in composition, and is mainly made up of plagioclase, pyroxene, amphibole, and olivine minerals.

Geode
A rock formation that is spherical in shape, hollow, and contains some amount of minerals in it. These are usually observed in sedimentary and volcanic rocks. They mainly are cavities that are lined with crystals from the inside.

Geologic Time Scale
It is a chronological order of the events that help to explain the formation of rocks and evolution of organisms on Earth, over different units of time. Period, era, and eon are the three basic units or divisions of this scale.

Geyser
A natural hot spring that shoots out hot water and steam in a periodic manner is called geyser. When the molten magma heats the water present beneath the Earth's surface, a lot of steam and high pressure is generated, and on releasing the pressure, the water along with steam instantly rises upwards and shoots out of the geyser vent.

Glaciation
It is the process of formation, development, and establishment of glaciers on the Earth's surface.

Glacier
It is defined as a moving mass of ice that is formed due to the accumulation of snow. It moves as a result of gravity or due to its own weight.

Gneiss
It is a metamorphic rock that is coarse and foliated with alternate bands of granular minerals and flaky or dark minerals. Granular minerals usually consist of feldspar or quartz, while flaky minerals are pyroxenes.

Gneissosity
The pattern of foliation that takes place in gneisses is called gneissosity.

Greenstone
It is a meta-igneous rock type that is made up of green-colored minerals like epidote, amphiboles, or chlorite. It is mainly formed in association with basement gneisses.

Guyot
A seamount whose top is flat due to weathering is called Guyot.
H
Hardness
This is a property of minerals, which indicates its degree of resistance. The strength of holding the bonds of the mineral's atoms is specified by the hardness of that mineral. To measure this property, the mineral is rubbed with another one of known hardness. The relative scale is known as Mohs Hardness Scale.

Hot Spot
It is an area present in the upper mantle of Earth (lithosphere), and it contains magma that rises in a plume to form volcanoes. Such structures may persist for 10 million years or more.

High Level Nuclear Waste
The primary waste that originates from the manufacturing and development of nuclear weapons is called high level nuclear waste. It is the by-product left in the nuclear reactor after the nuclear fuel has been consumed.

Hinge Fault
Angular or rotational movement of a fault on one side of the hinge is called hinge fault, and it is always perpendicular to the fault plane. In case of the hinge fault, the displacement increases with the distance from the hinge.

Hydrologic Cycle
It is a model that describes the movement of water in the hydrosphere, lithosphere, atmosphere, and biosphere. The various processes that are described by the hydrologic cycle are evaporation, percolation, transpiration, infiltration, and run-off.

Hydrothermal Deposits
The ones that are formed by the action of hot water and gases related to magmatic source are known as hydrothermal deposits.

Hypocenter
It is a point beneath the Earth's surface, which is said to be the origin of the seismic waves. The term is used to describe the origin of vibrations in case of earthquakes or nuclear explosions.

Hydraulic Gradient
It is the slope between two or more hydraulic head measurements, and is divided by the lateral distance between the respective hydraulic heads. This characteristic property is dimensionless, and is also called 'Darcy Slope'.

Hydrograph
It is a plot or graph that shows the variation of discharge of water over time. Discharge is the volume of water flowing at a particular location per unit time.

Hanging Valley
A tributary in an U-shaped glacial valley that enters the valley at a higher elevation, instead of the same level as that of a main stream, is known as a hanging valley.
I
Ice Age
It is the name given to the period when the temperature of the Earth was too low, and caused expansion of glaciers and ice sheets in various continents and especially in polar regions.

Ice Cap
It is a small dome-shaped sheet of ice that covers a large area of the peak of a mountain.

Igneous Rock
Those rocks that are formed from molten magma are known as igneous rocks. They are formed on crystallization of the magma, once it cools down.

Index Fossil
They are also called guide fossils, and are remnants of living organisms that were present in a particular geologic period. With the help of these fossils, the age of rock layers in which they are found can be identified.

Index Mineral
It helps to find out the degree of metamorphism underwent by the parent rock. This includes the temperature, pressure, and composition of the rock.

Inner Core
It is the innermost portion of the Earth's core, and has a radius of 1,220 kms. It is mainly made up of alloys of iron and nickel, and is situated around 6,370 km below the Earth's crust.

Intermediate Rock
An igneous rock whose chemical composition lies between acidic and basic rocks is known as an intermediate rock. It is characterized by the presence of silica content between 50-60 percent.

Intrusive Rock
An igneous rock that is formed from cooling and solidifying of magma inside the Earth's crust is called intrusive rock. They are formed by intrusion of magma into preexisting rocks.

Isograd
A line on the Earth's surface or a geological map, which joins the points where the degree of metamorphism is same, is known as an isograd. Thus, it can also be defined as a line that divides two different metamorphic zones.

Isostasy
It is the state of balance of gravitational forces (floating of tectonic plates ) that depress and elevate the Earth's crust. It can be defined as the equilibrium between the forces that tend to depress, and buoyancy that raises the lithosphere segment to float on asthenosphere.
J
Jade
It is a green colored gemstone that mainly consists of the metamorphic rocks called jadeite and nephrite, along with silicate minerals. This stone is mainly used in ornaments.

Jasper
It is a type of silicate mineral and is opaque and cryptocrystalline in nature.

Jetty
It is a protective structure built perpendicular to the coastline, in order to protect the shore from erosion and deposition of sediments, due to the action of waves.

Joint
A crack or fracture on the surface of a rock, along which displacement does not take place is called a joint. This fracture divides a rock into two planes without any relative displacement.

Juvenile Water
The type of water body formed due to the magmatic and volcanic processes that take place on our planet is known as juvenile or magmatic water.
Jurassic
It is the geologic time period that existed between 150 million and 200 million years ago. It is a part of Mesozoic era, and is sometimes also referred to as "Age of Reptiles" during which dinosaurs existed.
K
Kame
It is an irregular geological feature formed by the deposition of gravel, sand, or till in the sunken parts of a melting glacier, which eventually ends up on a land surface, due to the melting of ice.

Karst
This kind of landscape or topography is formed or developed in a region, where limestone and dolomite rocks are mainly present. Thus, due to the effect or action of ground water and surface water, sinkholes, caves, underground drainage, disappearing streams, etc., are some of the typical characteristics that develop in such regions.

Kettle
It is a depression formed on the surface of ground due to the melting of buried glacier ice.

Komatiite
An igneous rock that is derived from the mantle, which contains a high percentage of magnesium oxide is known as komatiite.
L
Laccolith
Laccolith is an igneous intrusion forced between two layered rocks. The bottom of the intrusion is flat enough, but the top of the intrusion is curved. It is dome-shaped as the pressure of the released magma is very high.

Lateral Moraine
It is usually found along the sides of glaciers that occupy a valley. It consists of deposits known as 'till', which are carried by the glacier ice down the valley.

Laterite
It is a type of residual deposit, and it mainly consists of oxides of aluminum and iron because of the weathering of underlying parent rock, mainly due to the action of groundwater and surface water. This rock is found in tropical soils, where the water table fluctuates as per seasonal changes.

Lava
The molten magma that is discharged through fissures or volcanic vents on the Earth's surface is known as lava.

Lava Dome
It is a circular structure that is formed by the slow extrusion of lava from the volcanic vents or fissures developed when a volcano erupts.

Lava Lake
The accumulation of molten lava in a crater that is formed due to volcanic eruption is known as a lava lake.

Lignite
It is a type of soft, brown-colored, and low-grade coal. Lignite is commonly called brown coal.

Limestone
It is a sedimentary rock that is mostly made up of calcium carbonate, along with impurities of carbon and magnesium.

Lithification
The process of consolidation of sedimentary rocks due to the aggregation of lithic and mineral fragments is known as lithification. Along with diagenesis, the modes of transportation of wind and water play a major role in this process.

Lithosphere
It is the rigid outer shell of our planet that consists of the crust and the upper portion of upper mantle.
M
Mafic
It is a term used to define an igneous rock that contains higher percentages of magnesium and iron. Hence the name "mafic" (magnesium + iron). These rocks contain mafic minerals like olivine, pyroxene, augite, etc., and are dark in color. Main examples include gabbro and basalt.

Magma
The molten rock material formed beneath the Earth's surface is known as magma.

Mantle

The layer that lies between the core and the crust of our planet is known as mantle. It is approximately 2780 km thick, and occupies almost 84% of the Earth's volume.

Marble
it is a rock mainly formed by the metamorphism of limestone, and it consists of a high percentage of calcite. It is widely used for construction related purposes.

Mesozoic Era
It is a part of Phanerozoic eon that existed for a period of 180 million years. It consists of three periods that include Triassic, Jurassic, and Cretaceous. This era lies between the Paleozoic and Cenozoic.

Metamorphic Rock
Rocks that are formed due to the process of metamorphism are called metamorphic rocks. These rocks have undergone structural changes due to pressure exerted, extensive heat, or action of any chemical.

Metamorphism
It is a process that is mainly characterized by changes in pressure and temperature of the surrounding environment of igneous and sedimentary rocks, leading to the formation of metamorphic rocks. This alteration takes place without melting of the parent rock. The factors that are altered include chemical composition, structure/texture, and mineral content of the parent rock.

Meteoric Water
It results due to precipitation in the form of rain, snow, hail, etc.

Monocline
It is a fold that is made up of different layers with every layer oriented in the same direction.

Mudstone
It is also called mudrock, and is a sedimentary rock that is made up of clay-sized particles.
N
Nannofossils
They are studied with the help of a small resolution, mostly under an electron microscope.

Natural Bridge/Arch
It is a naturally formed arch-shaped rock formation that is a result of weathering or erosion.

Nodule
It is a mass of different compositions, and is more resistant to weathering as compared to the surrounding rocks. These masses are usually round in shape.

Nonconformity
It occurs between metamorphic or igneous and sedimentary rock, and marks a change in the type of depositional environment.

Normal Fault
It is a type of fault that extends in a vertical direction, such that the hanging wall moves downwards with respect to the footwall. Basically, this fault is a sign of tectonic extension, and hence, it is also known as extension fault.

Normal Polarity
A rock is said to be of normal or positive polarity, when its magnetic field is same as that of the Earth's magnetic field.

North Magnetic Pole
The northern end of the magnetic field of our planet is known as the north magnetic pole.

Nuclear Power
It is generated by fusion or by controlled fission reactions. The heat produced by this power is mainly used to produce steam to drive the turbines.

Nuée Ardente
It is a hot, dense cloud of volcanic ash and gas generated in a Plinian volcanic eruption.
O
Obsidian
Obsidian is a type of igneous rock formed by the process of sudden or quick cooling of lava. This rock has a glass-like texture, and hence, it is also called volcanic glass.

Oceanic Crust
This layer covers the ocean floor or oceanic basins. It is quite thin as compared to the continental crust.

Oolite
These are small and circular features present in sedimentary rocks, and are mainly made up of calcium carbonate forming ooids.

Ore
An aggregate of a commercially important minerals, along with other impurities is known as an ore of the mineral. It can be a single rocks, or also a combination of various rock types.

Original Horizontality
It is one of the laws or principles of geology, which states that sedimentary rocks were deposited in horizontal layers, and if any inclination is observed in these layers, then it is due to gravity or other forces, which acted upon them after the depositional activity.

Outer Core
It is the outer part of the Earth's core, and is in liquid form. This portion has a thickness of about 2,260 km.

Outwash
The deposition of sediments due to the melting of glacial ice is known as an outwash. These deposits mainly consist of sand and gravel.

Oxbow Lake
It is a crescent-shaped or U-shaped lake that is formed when a meander separates out from the main body of a river. This type of lake is formed due to erosion and deposition in the river channel. The term "oxbow" is used to refer to a part of the river that is U-shaped.

Ozone Hole
The damage caused due to ozone destructing substances like CFCs and HFCs is generally represented as the ozone hole. Simply said, it is the thinning of the ozone layer in the Earth's atmosphere due to high level of pollution.
P
P Wave
The primary wave or P wave is the first to be generated during any seismic event, and also the first to be recorded by a seismogram. It travels through a series of compressions and dilations, which are along the same direction as that of the wave.

Pahoehoe
It is a type of basaltic lava flow formed due to quick cooling and solidification of lava.

Paleomagnetism
It is the remnant magnetism that has been preserved in rocks, especially in the form of mineral grains aligned in the direction of the magnetic field, which was present during formation of the rock.

Paleosol
A layer of soil that has formed many years ago than the present is known as paleosol. It is studied to interpret the climate and depositional environment of that particular time when it was formed.

Paleozoic Era
It is an era marked by the presence of fish, amphibians, insects, and marine invertebrates.

Parent Material
It is a source from which a soil is derived, and generally consists of bedrock and sediments.

Pangaea
It is an ancient continent from which the present continents are formed, due to breaking up and spreading of the sea floor.

Parabolic Dune
These dunes have a convex leeward slope and concave windward slope. They are generally formed on lake shores and sandy oceans.

Partial Melting
Also known as incomplete melting of rocks, this process is characterized by the melting of rock minerals whose melting point is same as that of the magma, which is in contact with that rock.

Peat
It is the lowest grade coal, and is formed because of partial decomposition of plant and animal matter, under varying degrees of overlying pressure.

Pebble
A small round-shaped rock fragment that is larger than a granule is called pebble.

Pedalfer
It is a common type of soil found in humid regions, and is characterized by the abundance of iron oxides and clay minerals. This type of soil is mainly deposited in the B horizon due to leaching process.

Pedocal
It is a soil type found in arid regions, and is characterized by the accumulation of calcium carbonate in the A horizon.

Pegmatite
It is a plutonic igneous rock, i.e., it is formed at great depths beneath the Earth's surface. Pegmatites are coarse-grained in nature as the mineral grains grow slowly due to slow cooling of the magma under the crustal portions.

Plinian Eruption
It is a type of volcanic eruption, which is specially characterized by nuées ardentes and lava domes.

Perched Water Table
When the level of groundwater is higher than the normal level, it is known as perched water table or an perched aquifer.

Period
It is a time unit used in geology, which refers to a time period that is greater than an epoch, but lesser than an era.

Permafrost Table
It defines the depth of a region, where the maximum temperature attained is 0°C.

Petroleum
This term is used to refer to the field involved in study of oil and natural gas.

Phanerozoic
It is the current eon (longest division of geological time) that began 570 million years ago, and is continuing till date.

Phyllite
It is a low-grade metamorphic rock, which is mainly formed due to regional metamorphism of sandstone. This rock mainly consists of minerals like chlorite and epidote, along with carbonaceous impurities.

Pirate Stream
The type of stream that captures the origin or headwaters of another stream is known as pirate stream. This mainly happens due to the tectonic activities that are taking place in the particular regions.

Placer Deposits
The type of mineral deposits that results due to the flow of transporting medium like streams and rivers is known as placers.

Plate Boundaries
The edges or boundaries of tectonic plates wherein they interact with the neighboring ones are known as plate boundaries.

Plate Tectonics
It is a theory that states the lithosphere is made up of portions that are known as tectonic plates. These portions comprise the crust (both upper and lower) and the upper part of the upper mantle.

Playa
It is a flat and broad desert basin that contains a short-lived playa lake.

Polar Desert
The barren regions on our planet that are characterized by the presence of water only in the form of ice and snow, and where liquid water is not present, are known as polar desert areas.

Polar Glacier
These glaciers do not melt, and their temperature is always below the freezing period.

Polymetamorphism
It is a series of events of metamorphic episodes, which have made a distinctive influence on the same rock.

Polymorphism
When two different crystalline structures have identical chemical composition, the phenomenon is known as polymorphism.

Potentiometric Surface
It is the surface to which the groundwater level rises in an artesian system, when the limiting aquitard is pierced.

Proterozoic
It is sandwiched between the Archean and Phanerozoic eons. It started 2.5 billion years ago and ended 0.57 billion years ago.

Pyroclastic
It is a term that refers to the material, which is ejected into the atmosphere during a volcanic eruption. Such material mainly consist of glass fragments, ash, volatile substances, etc.
Q
Quarry
The area from where stones and rocks are extracted is known as a quarry. These areas are also used to extract sand, gravel, and various aggregates.

Quarrying
It is the process by which building materials are extracted from quarries.

Quartz
One of the most important minerals to be found on our planet, quartz is can be formed in metamorphic, igneous, as well as in sedimentary rocks. Its chemical composition is characterized by the arrangement of silicon and oxygen in a tetrahedral pattern.

Quartz Arenite
It is a type of sandstone with a maximum percentage of quartz.

Quartzite
It is a metamorphic rock that is made up of quartz grains. It is derived from a pure form of sandstone, and is a durable and non-foliated type of metamorphic rock.

Quaternary Ice Age
It started somewhere around 2 million years ago, and is still continuing till the present (according to some theories).
R
Rain Shadow Region
These areas are characterized by very low precipitation, due to the presence of an obstacle such as a mountain range. The mountains block the passage of clouds, resulting in the precipitation on one side, whereas the other side receives scanty or absolutely no rainfall. The latter side or area is known as a rain shadow region.

Rayleigh Wave
It is a type of surface seismic wave that is produced during an earthquake.

Rectangular Drainage
This type of drainage pattern is characterized by flow of tributaries, which are at right angles or near perpendicular to each other at their junctions.

Regional Metamorphism
Also called Barrovian metamorphism, it takes place mainly through temperature changes in the surrounding environment of the parent rocks.

Reservoir Rock
It is a permeable and porous rock, from which natural gas or oil can be obtained.

Rhyolite
An extrusive igneous rock that is formed due to fast consolidation of magma, spreading over large distances is called rhyolite.

Richter Scale
The logarithmic scale that measures the intensity or strength of an earthquake is called Richter scale. The magnitude of the earthquake is measured by finding the logarithm of the amplitude of waves that are generated by the seismograph. A division on this scale represents a ten-fold elevation in the amplitude that is recorded by the seismogram.

Rock
It is defined as an aggregate of different minerals in varying percentages. Igneous, metamorphic, and sedimentary are the basic three different classes of rocks.

Rock Avalanche
Also called rock-slide, this avalanche is characterized by the sudden fall of a large volume of rocks, resulting from an earthquake or temperature changes. During such a slide, rock fragments roll over a steep slope due to the action of gravity.

Rock Cycle
It is a closed continuous chain of all the geologic processes right from the formation of a rock till its destruction. It consists of the change of rocks into its particular classes, i.e., igneous, sedimentary, and metamorphic. The processes that are involved in this cycle include metamorphism, alteration, weathering, lithification, crystallization, etc.

Rock System
It is similar to rock cycle, but it is not a continuous one, i.e., it does not return to the starting point.

Rock Glacier
It is also called talus glacier, and is an ice mass that contains rocks and other material with a layer of ice beneath the surface.
S
S Wave
Also called secondary wave, it is a type of seismic wave, which makes the rock particles move in a perpendicular direction of movement, as compared to the direction of wave propagation.

Sandstone
It is a sedimentary rock of clastic nature, which is made up of fine sand particles. These sand fragments are held together with silicate material that acts as cement.

Schist
It is a metamorphic rock formed through the process of regional metamorphism. This rock is coarse, and it can be easily split into sheets because of its foliated structure.

Sea-floor Spreading
It is a process wherein two oceanic plates get separated. It forms a new sea floor as magma that rises through the cracks due to separation and spreads on the ocean floor.

Sedimentary Facies
They are the arrangement of certain minerals in deposits, along with sedimentary structure, which help in categorizing different types of sedimentary rocks according to their depositional environment.

Sedimentary Rock
The class of rocks that are formed due to deposition of sediments or due to chemical precipitation is known as sedimentary rocks. Almost 80 percent of the Earth's surface is covered by these rocks. Chalk, sandstone, limestone, dolostone, etc., are some examples of sedimentary rocks.

Seismic Gap
The area or segment of an earthquake susceptible zone that has not undergone tremors, in spite of them occurring in the other regions, is known as a seismic gap.

Seismic Sea Wave
Popularly known as a Tsunami, these waves are produced due to seismic activities of the ocean floor. They can be devastating due to the displacement of large volumes of water at very high speed.

Seismic Tomography
This technology uses computers to prepare a three-dimensional image of the Earth's interior. To construct this image, seismic wave velocities recorded at different stations are used.

Seismic Wave
These waves are generated due to an eruption, explosion, or an earthquake, and that travel through the Earth.

Seismograph
It is an instrument that detects and records the magnitude of an earthquake. It records the amplitude of the seismic waves generated during such a natural disaster.

Seismology
It is the branch of geology that deals with the study of earthquakes with the help of seismic waves, which are generated artificially or naturally.

Seismoscope
This instrument helps us to know the occurrence and time of an earthquake, by measuring various velocities of the seismic waves.

Shale
It is a type of sedimentary rock that consists of fine clay and carbonaceous particles, which cannot be distinguished with the naked eye. Shale is categorized under clastic sedimentary rocks.

Shield Volcano
It is a large cone-shaped volcano that is built completely from lava.

Shock Metamorphism
It is also called impact metamorphism, and it takes place when extremely high pressure acts on a rock for a few minutes or few seconds. It is usually takes place when a meteorite strikes the Earth.

Silica
It is a compound that is crystalline in nature, and is made up of quartz, flint, sand, etc.

Sill
It is a flat or horizontal mass of igneous rock present between two layers of sedimentary rock.

Sinkhole
Several depressions or pits in the ground surface are formed due to erosion of rocks like limestone and dolomite. This arises from the action of water on them. The pits or depressions are known as sinkholes.

Slate
It is a metamorphic rock of very low-grade, and is made up of very fine particles. It is developed from shale, and can break into thin sheets due to its foliated nature.

Soil
The topmost unconsolidated layer of the Earth's crust, which is forms due to the erosion of preexisting rocks is known as soil. It is very crucial for the existence of living beings, and is further divided into several sub-layers.

Soil Texture
It can be defined as a physical characteristics of a soil that is related to the grain size of the minerals present in the soil.

Solifluction
It is also called soil fluction, and is measured when a mass of water-saturated soil moves down a slope area.

Specific Gravity
It can be defined as the ratio of density of a substance to the density of water at specific temperature.

Star Dune
It is a type of dune that has more than three sides and forms a star-like shape. This type of dune is formed when wind blows in more than three directions, or when the direction of wind changes frequently.

Stratification
It is the deposition of sediments in a layered manner, such that the materials can be easily differentiated.

Stratigraphy
This branch of geology can be defined as the study of rock layers and their formation. This study also includes the age of the rock and other related parameters.

Striations
Striations are small grooves that are formed on the surface of sediments, rocks, pebbles, etc. These are formed due to glacial action.

Subduction Zone
They are the areas beneath the Earth's crust, wherein two tectonic plates collide with each other and one of them goes underneath the second one. This is known as subduction, and such zones are formed below mountain ranges, island arcs, and oceanic trenches.

Subtropical Deserts
These deserts are present in the subtropical regions of our planet. For example, the Thar desert in India.

Surging Glacier
It moves rapidly and stagnates at alternate intervals of time, and hence, it is called a surging glacier.

Suture
Suture zones are the areas where two continental or converging plates meet through collision. Usually, these zones are very high mountain ranges. Example: Himalayas and Alps.

Syncline
It is a fold that is convex in downward direction, and the central part of this fold is considered as the youngest portion of the rock.
T
Taconite
It is a type of silicate rock, and is also mined as an iron mineral.

Tar Sand
The type of sand containing traces of tar and asphalt is known as tar sand.

Terminal Moraine
It is the maximum extent of a glacier, and is characterized by several deposits of 'till' material that have been carried by the moving ice.

Texture
It is defined as the physical appearance or characteristics of rocks like, shape, size, arrangements of the materials present in it, etc.

Thermoremanent Magnetism
The type of magnetism present in rocks that has resulted from the orientation of the Earth's magnetic field during the formation of rocks is known as thermal magnetism.

Tidal Power
The type of energy generated by utilizing the force of tidal waves is known as tidal power.

Topography
The set of physical features, both natural and man-made, present on the surface of the Earth is known as topography. Mountains, valleys, rivers, etc., characterize the topography of a particular area.

Transform Fault
It is a type of strike-slip fault found along mid-ocean ridges. These faults are caused due to the movement present between crustal plates.

Transverse Dune
This type of dune is deposited perpendicular to the direction of the wind. It is more or less straight and elongated.

Triple Junction
In plate tectonics, triple junction is the region where three plate boundaries or associated rifts are joined together at approximately 120 degrees angle.

Trench
It is defined as a depression in the ocean floor formed due to the subduction of oceanic plates.

Turbidite
It is a type of sedimentary deposition caused due to turbid water carrying particles of varying sizes. The coarse grains get deposited at the bottom, while the finer ones are at the top.
U
Unconformity
It is defined as the boundary formed between two lithologies due to a period of erosion or non-deposition.

Uniformitarianism
It is the principle that encompasses the laws of nature that are constant.

Unstratified Drift
It is a glacial drift composed of rock fragments ranging from clay to boulders.

Upwarped Mountain
These mountains comprise a broad area of the Earth's crust, which has gently arched itself upwards. This deformation or alteration is made up of igneous, metamorphic, and sedimentary rocks.

Uranium-Thorium-Lead Dating
It is a type of dating technique, which is dependent on the half-life of the radioactive isotopes of uranium. In other words, the U isotopes decay into Pb isotopes.

USDA Soil Classification System
This system is based on the processes and conditions that are responsible for the formation of different soils.
V
Velocity Profile
It is a plot of seismic velocity versus the depth of Earth.

Ventifact
It is a cobble, pebble, or a boulder that is faceted by wind-driven sand.

Vesicle
It is a cavity in consolidated lava that has a gas bubbles entrapped during solidification.

Vesicular
This term refers to an igneous rock containing many vesicles, which were formed due to the expansion of gases (which were earlier dissolved in lava).

Volcanic Arc
It is an arc-shaped chain of islands or mountains formed by plate tectonics. There are two types of volcanic arcs, viz., oceanic volcanic arc and continental volcanic arc.

Volcanic Ash
It comprises sand, rock granules, and powder that are ejected in the atmosphere after a volcanic eruption.

Volcanic Cinder
Volcanic cinders are cone-shaped, and are made from rock fragments, ash, and fragments of volatile matter.

Volcanic Cone
It is formed due to the piling up of ejected volcanic material.

Volcanic Dome
It is a dome formed over the vent, from which the volcano erupts. In case the lava is very viscous, it cannot flow out of the vent, thus, forming a dome over it.

Volcanic Rock
The rocks that are formed from the consolidation of lava on the Earth's surface are called volcanic rocks.

Volcanism
It is the process in which the molten lava escapes through the Earth's core and becomes cool to form hard rocks.

Volcanogenic Massive Sulfide Deposit
These metal sulfide deposits are formed after a volcanic eruption takes place.
W
Wadati-Benioff Zone
It is a deep and active seismic area near the subduction zone. Also known as Benioff zone, it can be up to 700 km deep. The foci of most deep-seated earthquakes cluster along this zone.

Water Table
It is defined as the area or surface present between the zone of aeration and the zone of saturation. The ground is saturated with water below the surface of the water table.

Watershed
It can be defined as the area through which water flows to join a stream or a river. Watershed, also known as drainage basin, separates two adjacent water systems.

Wave Crest
The highest part of a wave is known as the wave crest. It is opposite to the wave trough, which is the lowest part of a wave.

Weathering
It is defined as the process of breaking down or erosion of rocks caused by exposure to atmospheric agents or weather. Weathering takes place on the surface of the Earth.

Well
It is a deep vertical hole or a shaft that is dug to obtain water or oil.

Wilson Cycle
It is defined as the cycle of formation and destruction of ocean basins and associated mountain ranges.

Wind Abrasion
It is the weathering or erosion of rocks caused by the sand particles that are carried by the wind.

Wind Farm
It is a group or cluster of wind turbines that are located in the same place for the generation of electricity. The power plant that makes use of wind turbines for the production of electricity is also known as wind farm.

Wind Power
It is a renewable source of energy derived from the wind with the help of windmills.
X
Xenolith
Small fragments of host rock or any other foreign mineral present in igneous rocks is known as xenoliths.

X-ray Diffraction
This technique utilizes the phenomenon of diffraction produced in a crystal, which helps us to know the crystal's internal structure.

X-ray Fluorosis
This technique uses the penetration power of X-rays, wherein they are made to penetrate the powder samples of the rocks under study, to get an idea about their elemental composition.
Y
Yardang
It is a sharp and irregular ridge, which is carved by wind, and is oriented parallel to the wind direction.

Yazoo-type River
It is a tributary stream that is unable to enter the main stream due to natural levees along the main stream. Such a kind of river flows in the backswamp area, parallel to the main stream.

Yield Point
It is the stress limit that produces a deformation in a non-brittle material.

Yucca Mountain
This mountain is situated in Nevada for storing high level nuclear waste.
Z
Zone of Ablation
This zone is characterized by the loss of ice due to melting, calving, or sublimation into water.

Zone of Accumulation
It is the area wherein the ice accumulates in a glacier.

Zone of Aeration
The area below the Earth's surface, where air and water are both present in the rock pores and soil is known as the zone of aeration.

Zone of Leaching
It is present in the upper horizon of soil, where the soluble decomposition products are removed due to the movement of gravitational moisture.

Zone of Saturation
This zone lies just below the zone of aeration, and is characterized by filling up of all the pore spaces with water.
To be an expert in Earth Sciences and Geology, you need to have knowledge about the terms and definitions that are a part of such fields. You also need to get your fundamentals cleared regarding the same. The above-described detailed glossary will be very useful for your further studies in these fields.