Geology – Important Short Notes from Past Papers

Contents

Acidic Rock:

An igneous rock that has a relatively high silica content. Examples are granite and rhyolite. Also see entries for basic, intermediate and ultrabasic rocks.

Acre-Foot:

The volume of water needed to flood one acre of land to a depth of one foot. Equivalent to 43,560 cubic feet, 1,233 cubic meters or 325,851 gallons. One of the most common units of measure used for reservoir capacity. Also used in mineral resource calculations (an acre-foot of coal is a block of coal one acre in area and one foot thick – it weighs approximately 1,800 tons).

Alkali:

Used in reference to materials that are rich in sodium and/or potassium.

Alluvial Fan:

A fan-shaped wedge of sediment that typically accumulates on land where a stream emerges from a steep canyon onto a flat area. In map view it has the shape of an open fan. Typically forms in arid or semiarid climates.

Alluvium:

An unconsolidated accumulation of stream-deposited sediments, including sands, silts, clays or gravels.

Angle of Repose:

The maximum angle that a soil, sediment or other loose material can be placed or accumulate and be stable. The angle of repose varies for different types of materials and different moisture conditions.

Angular Unconformity:

An erosional surface that separates rock units of differing dips. The rocks below the surface were deposited, deformed and eroded. The younger rocks above then accumulated upon the erosional surface.

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Anthracite:

The highest rank of coal. By definition, a coal with a fixed carbon content of over 91% on a dry ash-free basis. Anthracite coals have a bright luster, break with a conchoidal fracture, a semi-metallic luster and are difficult to ignite. Frequently referred to by the layman as “hard coal”.

Aquiclude:

A subsurface rock, soil or sediment unit that does not yield useful quanties of water.

Aquifer:

A subsurface rock or sediment unit that is porous and permeable. To be an aquifer it must have these traits to a high enough degree that it stores and transmits useful quantities of water.

Aquifer (artesian):

An aquifer that is bounded above and below by impermeable rock or sediment layers. The water in the aquifer is also under enough pressure that, when the aquifer is tapped by a well, the water rises up the well bore to a level that is above the top of the aquifer. The water may or may not flow onto the land surface.

Aquifer (confined):

An aquifer that is bounded above and below by impermeable rock or sediment layers. There may or may not be enough pressure in the aquifer to make it an “artesian aquifer”.

Aquifer (unconfined):

An aquifer that is not overlain by an impermeable rock unit. The water in this aquifer is under atmospheric pressure and is recharged by precipitation that falls on the land surface directly above the aquifer.

Arkose:

A sandstone that contains at least 25% feldspar. Easily recognized because the feldspar grains are typically pink and angular in shape.

Arroyo:

A flat-bottom gully with steep sides that is a channel for an intermittent stream.

Asthenosphere:

A portion of the upper mantle that is directly below the lithosphere. A zone of low strength in the upper mantle defines the top of the asthenosphere. This weak zone allows the plates of the lithosphere to slide across the top of the asthenosphere.

Astrobleme:

An ancient circular scar on Earth’s surface produced by the impact of a meteorite or comet. Use our Google maps page to get close up images of these meteor impact structures.

Atoll:

A ring-shaped group of coral islands that are surrounded by deep ocean water and that enclose a shallow lagoon.

Backwash:

The seaward rush of water down a beach that occurs with a receding wave.

Banded Iron Ore:

A rock that consists of alternating layers of chert and iron oxide mineral (usually hematite) with the iron oxide in high enough concentration to be of economic value.

Bankfull Stage:

A height of water in a stream that completely fills the natural channel. If the water rises any higher a flood will occur.

Bank Storage:

Water that seeps into the ground along the banks of a stream during a time of high flow. This loss of water into the ground slightly reduces the height that the stream will attain and then slowly seeps into the stream as the high water level subsides – hence the term “bank storage”.

Bar:

An underwater ridge, usually of sand and/or gravel, that forms from the deposition and reworking of sediments by currents and/or waves. Bars occur in rivers, river mouths and in offshore waters.

Barchan:

A sand dune that is crescent-shaped in map view. Barchan dunes form in areas of limited sand supply. They move across the desert floor with their gently sloping convex sides facing upwind and their steeply sloping concave sides facing downwind.

Barrier Island:

A long, narrow island that parallels a shoreline.

Basalt:

A dark-colored fine-grained extrusive igneous rock composed largely of plagioclase feldspar and pyroxene. Similar in composition to gabbro. Basalt is thought to be one of the main components of oceanic crust.

Base Flow:

Water that seeps into a stream through a permeable rock or sediment unit that outcrops in the bottom or banks of the stream.

Base Level:

The lower limit of erosion by a stream. Sea level is the ultimate base level. However, lakes can serve as a temporary base level in upstream areas.

Basement:

The igneous and metamorphic rocks that exist below the oldest sedimentary cover. In some areas such as shields the basement rocks may be exposed at the surface.

Basic Rock:

An igneous rock that has a relatively low silica content. Examples are gabbro and basalt. Also see entries for acid, intermediate and ultrabasic rocks.

Basin:

In tectonics, a circular, syncline-like depression of strata. In sedimentology, the site of accumulation of a large thickness of sediments.

Batholith:

A very large intrusive igneous rock mass that has been exposed by erosion and with an exposed surface area of over 100 square kilometers. A batholith has no known floor.

Bathymetry:

The measurement of ocean depths and the preparation of topographic maps of the ocean floor.

Bauxite:

The principal ore of aluminum. A mixture of aluminum oxides and hydroxides that forms from intense chemical weathering of a soil in tropical environments.

Bedding:

The characteristic structure of sedimentary rocks in which layers of different composition, grain size or arrangement are stacked one on top of another in a sequence with oldest at the bottom and youngest at the top.

Bedding Plane:

A distinct surface of contact between two sedimentary rock layers.

Bed-Load:

The larger heavier particles that are being transported by a stream. Instead of being dissolved or suspended, these are being rolled or bounced along, spending at least part of their time in contact with the stream bottom. See also: load, suspended load, dissolved load.

Bedrock:

Solid rock present beneath any soil, sediment or other surface cover. In some locations it may be exposed at Earth’s surface.

Beta-Particle:

An electron emitted with high energy and velocity from the nucleus of an atom during radioactive decay.

B-horizon:

A layer in the soil, below the A-horizon, where materials leached from above accumulate. Typically enriched in clay and oxides.

Biochemical Rocks:

A sedimentary rock that forms from the chemical activities of organisms. Organic (reef and fossiliferous) limestones and bacterial iron ores are examples.

Bioturbated:

An adjective used in reference to a sediment or sedimentary rock. Bioturbated sediments have been disturbed by animals (such as burronwing worms or shell fish) or plant roots. These have penetrated the sediment and disturbed any or all original sedimentary laminations and structures. Bioturbated rocks were disturbed in this way while still in the soft sediment phase of their formation.

Bituminous Coal:

A rank of coal that falls between anthracite and semi-bituminous. The most abundant rank of coal. Frequently referred to by the layman as “soft coal”.

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Block Fault Mountain:

A linear mountain that is bounded on both sides by normal faults.

Blowout:

A shallow, round or trough-shaped depression in sand or dry soil that is formed by wind erosion. The material removed by the wind may also be referred to as “blowout”.

Butte:

A conspicuous hill with steep sides and a flat top. The top is usually a cap-rock of resistant material. This structure is frequently an erosional remnant in an area of flat-lying sedimentary rocks.

Caldera:

A large, bowl-shaped crater associated with a volcanic vent. A caldera can form from a volcanic blast or the collapse of a volcanic cone into an emptied magma chamber.

Carbonate Rock:

A rock made up primarily of carbonate minerals (minerals containing the CO3 anionic structure). Limestone (made up primarily of calicite – CaCO3) and dolostone (made up primarily of dolomite – CaMg (CO3)2 are the most common examples.

Carbonic Acid:

A weak acid (H2CO3) that forms from the reaction of water and carbon dioxide. Most rain water is a very weak carbonic acid solution formed by the reaction of rain with small amounts of carbon dioxide in the atmosphere.

Cataclastic Rock:

A breccia of powdered rock formed by crushing and shearing during tectonic movements.

Cement:

A solid precipitate of calcium carbonate, silica, iron oxide, clay minerals or other materials that forms within the pore spaces of a sediment and binds it into a sedimentary rock.

Cementation:

The processes through which chemical precipitates form within the pore spaces of a sediment and help bind it into a sedimentary rock.

Chemical Sedimentary Rock:

A rock that forms from the precipitation of mineral material from solution. Examples are chert and rock salt.

Chemical Weathering:

The breaking down of surface rock material by solution or chemical alteration. Common alteration processes are oxidation and hydrolysis.

Chert:

A microcrystalline or cryptocrystalline sedimentary rock material composed of SiO2. Occurs as nodules and concretionary masses and less frequently as a layered deposit.

C-horizon:

The lowest horizon of a soil profile. It is below the B-horizon and is made up of weathered bedrock.

Cinder Cone:

A cone-shaped hill that consists of pyroclastic materials ejected from a volcanic vent.

Cirque:

A bowl-shaped depression with very steep sides that forms at the head of a mountain glacier. Forms from cold-climate weathering processes including frost wedging and plucking.

Clastic:

A sedimentary rock (such as shale, siltstone, sandstone or conglomerate) or sediment (such as mud, silt, sand, or pebbles). An accumulation of transported weathering debris.

Clay:

A clastic mineral particle of any composition that has a grain size smaller than 1/256 mm. The term is also used in reference to a broad category of hydrous silicate minerals in which the silica tetrahedrons are arranged into sheets.

Coal:

A brown or black sedimentary rock that forms from accumulated plant debris. A combustible rock that contains at least 50% (by weight) carbon compounds.

Coastal Plain:

An area of low relief along a continental margin that is underlain by thick, gently dipping sediments.

Compaction:

A compression process that reorients and reshapes the grains of a sediment in response to the weight of overlying deposits.

Composite Cone:

A cone-shaped volcanic mountain composed of alternating layers of cinders and lava flows. Also known as a stratovolcano.

Cone of Depression:

A cone-shaped lowering of the water table around a producing well.

Contact Metamorphism:

Alteration of a rock, mainly by heat, which occurs adjacent to a dike, sill, magma chamber or other magma body.

Contour Line:

A line on a map that traces locations where the value of a variable is constant. For example, contour lines of elevation trace points of equal elevation across the map. All points on the “ten foot” contour line are ten feet above sea level.

Contour Map:

A map that shows the change in value of a variable over a geographic area through the use of contour lines. For example, a contour map of elevation has lines that trace points of equal elevation across the map. See also: contour line and topographic map.

Cubic Feet Per Second:

A unit of measure frequently used to quantify the rate of flow of a stream. It is equal to a volume of water one foot high and one foot wide moving a linear distance of one foot in one second.

Datum:

A reference location or elevation which is used as a starting point for subsequent measurements. Sea level is a datum for elevation measurements. Datums can also be arbitrary such as the starting point for stream stage measurements or based upon a physical feature such as the base of a rock unit.

Daughter Element:

The element produced through the radioactive decay of a parent element.

Debris Avalanche:

The sudden downslope movement of rock and soil on a steep slope.

Deflation:

The removal of clay- and silt-size particles from a soil by wind erosion. The term can also be used in reference to the removal by wind of any unconsolidated material.

Delta:

A deposit of sediment that forms where a stream enters a standing body of water such as a lake or ocean. The name is derived from the Greek letter “delta” because these deposits typically have a triangular shape in map view.

Dendritic Drainage:

A stream drainage pattern that resembles the veins of a leaf in map view. Occurs mainly where the rocks below have a uniform resistance to erosion.

Density Current:

A gravity-driven flow of dense water down an underwater slope. The increased density of the water is a result of a temperature difference, increased salinity or suspended sediment load.

Deposition:

The settling from suspension of transported sediments. Also, the precipitation of chemical sediments from mineral rich waters.

Desert Pavement:

A ground cover of granule-size and larger particles that is typically found in arid areas. This ground cover of coarse particles is a residual deposit – formed when the wind selectively removes the sand-, silt- and clay-sized materials.

Detrital:

A word used in reference to sediments or sedimentary rocks that are composed of particles that were transported and deposited by wind, water or ice.

Diagenesis:

All of the changes which happen to a sediment after deposition, excluding weathering and metamorphism. Diagenesis includes compaction, cementation, leaching and replacement.

Diatom:

A one celled plant that lives in the shallow waters of lakes, streams or oceans. Many of these secrete a shell or internal parts composed of silica. Diatoms can occur in very large numbers and can make significant contributions to sea-floor or lake sediment.

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Diatomite:

A light colored, fine-grained siliceous sedimentary rock that forms from a sediment rich in diatom remains.

Diatom Ooze:

A seafloor sediment that consists of at least 30% diatom remains.

Differentiated Planet:

A planet that has layers composed of elements and minerals of different densities. As an example, Earth is a differentiated planet because it has a metal-rich core, surrounded by a rocky mantle, and covered by a crust of low-density minerals.

Dip:

The angle that a rock unit, fault or other rock structure makes with a horizontal plane. Expressed as the angular difference between the horizontal plane and the structure. The angle is measured in a plane perpendicular to the strike of the rock structure.

Discharge:

The volume of water in a flowing stream that passes a given location in a unit of time. Frequently expressed in cubic feet per second or cubic meters per second. Calculated by the formula Q = A x V where Q is the discharge, A is the cross sectional area of the channel and V is the average velocity of the stream.

Discontinuity:

A surface separating rock layers of differing properties or compositions. (see seismic discontinuity)

Dissolved Load:

The dissolved material being carried by a stream. See also: load, suspended load, dissolved load.

Divide:

A ridge that separates two adjacent drainage basins.

Dome:

An uplift that is round or elliptical in map view with beds dipping away in all directions from a central point.

Drainage Basin:

The geographic area that contributes runoff to a stream. It can be outlined on a topographic map by tracing the points of highest elevation (usually ridge crests) between two adjacent stream valleys.

Drainage Divide:

The boundary between two adjacent drainage basins. Drainage divides are ridge crests (or less obvious locations where slope of the landscape changes direction). Runoff produced on one side of the ridge flows into stream “A” and runoff on the other side of the ridge flows into stream “B”.

Drawdown:

A lowering of the water table around a producing well. The drawdown at any given location will be the vertical change between the original water table and the level of the water table reduced by pumping.

Drift:

A general term for all sedimentary materials deposited directly from the ice or melt water of a glacier.

Drumlin:

A low, smoothly rounded, elongate hill. Drumlins are deposits of compacted till that are sculpted beneath the ice of a flowing glacier. The long axis of a drumlin parallels the flow direction of the ice.

Dune:

A mound or ridge of wind-blown sand. Typically found in deserts and inland from a beach. Many dunes are moved by the wind.

Earthflow:

A detached mass of soil that moves downslope over a curved failure surface under the influence of gravity. An earthflow is more complex than a slump; it has a higher moisture content and the moving mass of soil has some internal movement or “flow”. Rates of movement are typically a few inches per year but faster rates can occur.

Earthquake:

A trembling of the earth caused by a sudden release of energy stored in subsurface rock units.

Ebb Tide:

A tidal current that generally moves seaward and occurs during the part of the tide cycle when sea level is falling. (see also: flood tide)

Effluent Stream:

A stream that gains water from ground water flow. These streams are typical of humid climates where water tables are high. The discharge of an effluent stream can be sustained by ground water flow for long periods of time between runoff-producing rainfall or snowmelt. Effluent streams generally increase in discharge downstream and contain water throughout the year. The opposite is an influent stream.

Elastic Limit:

The maximum stress that can be applied to a body without resulting in permanent deformation – the rock reverts to its original shape after the stress is removed. In the case of a fault or a fold the elastic limit is exceeded and the deformation becomes a permanent structure of the rock.

Elastic Rebound Theory:

A theory that explains the earthquake process. In this theory, slowly accumulating elastic strain builds within a rock mass over an extended length of time. This strain is suddenly released through fault movement, producing an earthquake.

Electron:

A subatomic particle with a negative charge and of negligible mass that orbits the nucleus of an atom.

Elevation:

The vertical distance between mean sea level and a point or object on, above or below Earth’s surface.

Eolian:

A term used in reference to the wind. Eolian materials or structures are deposited by or created by the wind.

Eon:

The major divisions of the geologic time scale. Eons are divided into intervals know as “eras”. Two eons of the geologic time scale are the Phanerozoic (570 million years ago to present) and the Cryptozoic (4,600 million years ago until 570 million years ago).

Epicenter:

The point on the Earth’s surface directly above the focus of an earthquake.

Epoch:

A subdivision of geologic time that is longer than an age but shorter than a period. The Tertiary Period is divided into five epochs. From most recent to oldest they are: Pliocene, Miocene, Oligocene, Eocene and Paleocene.

Era:

A subdivision of geologic time that is longer than a period but shorter than an eon. Precambrian, Paleozoic, Mesozoic, and Cenozoic are the eras of the time scale from oldest to youngest.

Erosion:

A general term applied to the wearing away and movement of earth materials by gravity, wind, water and ice.

Esker:

A long winding ridge of sorted sands and gravel. Thought to be formed from sediment deposited by a stream flowing within or beneath a glacier.

Eustatic Sea Level Change:

A rise or fall in sea level that affects the entire earth. Thought to be caused by an increase/decrease in the amount of available water or a change in the capacity of ocean basins.

Evaporation:

The process of liquid water becoming water vapor. Includes vaporization from water surfaces, land surfaces and snow/ice surfaces.

Evaporite:

A chemical sediment or sedimentary rock that has formed by precipitation from evaporating waters. Gypsum, salt, nitrates, and borates are examples of evaporite minerals.

Evapotranspiration:

All methods of water moving from a liquid to water vapor in nature. Includes both evaporation and transpiration.

Exfoliation:

A physical weathering process in which concentric layers of rock are removed from an outcrop.

Expansive Clay (Expansive Soil):

A clay soil that expands when water is added and contracts when it dries out. This volume change when in contact with buildings, roadways, or underground utilities can cause severe damage.

Extrusive:

Igneous rocks that crystallize at Earth’s surface.
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For correction and improvements please use the comments section below.

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SEE ALSO:  Important Minerals

One thought on “Geology – Important Short Notes from Past Papers

  • July 11, 2018 at 9:16 pm
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    Ok, Guys, the list is open to editing. Share latest Geology Terminologies / Short Term from Recent Papers in comments below so that the list could be updated.

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