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6B GL1 & 2a Vocab List
An enormous list of all the words you should know
316
Geology
12th Grade
03/24/2013

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Term
KEY WORD
Definition
DEFINITION
Term
Abrasion
Definition
The wearing away of the Earth's surface by the action of wind, water or ice dragging sediment over it, or hurling it at a surface.
Term
Absolute Dating
Definition
Finding the age of a rock quantitatively. E.g. by radiometric dating.
Term
Adductor Muscle Scars
Definition
Marks place where the adductor muscles attached to inside of a bivalve or brachiopod shell. Adductor muscles contract to close the shell.
Term
Aeolian Sediments
Definition
Sediments deposited by wind.
Term
Alluvium
Definition
Fine sediments deposited on the edges of rivers, generally making up the floodplain.
Term
Ammonites
Definition
Group of Cephalopods that lived in the Jurassic and Cretaceous. Had complicated suture lines. Used as zone fossils.
Term
Ammonoids
Definition
Class of Cephalopods that includes Goniatites, Ceratites and Ammonites, all zone fossils.
Term
Amydaloidal
Definition
Igneous rock texture where the vesicles are later infilled by minerals deposited from groundwater.
Term
Andalusite
Definition
Mineral formed in metamorphism of rocks, usually medium grade contact metamorphism. Has four crystals joined together to form a diamond with a cross inside.
Term
Andesite
Definition
A finely crystalline intermediate igneous rock comprising feldspar, mica and amphibole.
Term
Andesitic Magmatism
Definition
Production of intermediate forms of magma. Occurs at subduction zones due to the introduction of water into the mantle which lowers its temperature. Leads to explosive volcanic eruptions.
Term
Angular Unconformities
Definition
A break in time recorded by a break in the stratigraphy, over which the angle of the dip of beds above and below changes.
Term
Anhedral
Definition
Crystal which Is poorly formed and has no clear shape.
Term
Anthracite
Definition
Compacted coal which is hard with a bright, shiny lustre and does not dirty your hands.
Term
Anticline
Definition
Folded rock, shaped like an "n", where the oldest rock are in the middle of the fold.
Term
Apparent dip
Definition
A dip that is measured to be less than the maximum inclination.
Term
Arkose
Definition
Sandstone that contains at least 25% feldspar, and mostly quartz.
Term
Asymmetrical fold
Definition
A fold with limbs of the different lengths, asymmetrical about the axial plane.
Term
Asthenosphere
Definition
Layer within the solid, upper mantle that is can flow very slowly (rheid). Corresponds to the seismic low velocity zone, beneath the 1300°C isotherm.
Term
Attrition
Definition
The wearing down of sedimentary grains due to collisions with other grains during transport.
Term
Axial Plane (fold)
Definition
Plane that joins all the hinges of all the beds in a fold. It bisects the fold.
Term
Axial Plane Trace
Definition
The line that marks the axial plane on the Earth's surface
Term
Axis (fold)
Definition
Line that marks the top of the axial plane along the fold.
Term
Baked margin
Definition
The thin layer of metamorphosed country rock around an igneous intrusion.
Term
Basalt
Definition
A finely crystalline mafic rock comprising plagioclase feldspar and pyroxene and sometimes olivine.
Term
Basaltic Magmatism
Definition
Production of mafic magma. Occurs at mantle plumes and Mid-Oceanic Ridges. Produces runny lava and shield volcanoes.
Term
Batholith
Definition
A major igneous intrusion consisting of many plutons joined together.
Term
Bed
Definition
A layer of sedimentary rock that can be cm- to m-scale.
Term
Bedding
Definition
The centimetre to metre-scale layers of sedimentary rock (also called strata)
Term
Benioff Zone
Definition
The top boundary of a subducted oceanic plate at a subduction zone, where earthquakes occur and indicate the position of the subducted slab.
Term
Bioclastic
Definition
A biologically formed sedimentar rock composed of fragmented fossils.
Term
Biological Weathering
Definition
The mechanical and chemical weathering of rocks by plant roots or burrowing.
Term
Bituminous coal
Definition
Coal that has been put under some compaction. The coal we burn.
Term
Bivalves
Definition
A class of molluscs with a shell made of 2 valves of equal size and shape. The line of symmetry runs between the 2 valves.
Term
Body Fossil
Definition
The hard parts of an organisms such as a skeleton or shell.
Term
Body Waves
Definition
A seismic wave that travels through the interior of the Earth. Includes P and S waves.
Term
Brachial Valve
Definition
The larger valve (shell) of a Brachiopod.
Term
Brachiopods
Definition
A phlum of marine invertebrates comprising two valves (shells), one of which is larger than the other. A line of symmetry runs down the middle of each valve.
Term
Breccia
Definition
A sedimentary rock dominated by angular clasts of greater than sand size, set within a finer grained matrix.
Term
C-14 Dating
Definition
Radiometric dating using the decay of radioactive Carbon-14 isotopes, which occurs at a constant rate. Requires organic material to be present in the rock and rock younger than 50,000 years as the half-life is short.
Term
Calcification
Definition
A type of fossilisation where the organic material is replaced by calcite.
Term
Calcite
Definition
A mineral comprising calcium carbonate. Makes up limestone, chalk and marble.
Term
Carbonation
Definition
Reaction between carbonic acid and minerals
Term
Carbonisation
Definition
The process by which pressure of burial removes the volatiles from organic material, leaving a film of carbon.
Term
Cast
Definition
The infill of a mould with sediment or mineral to form a replica of the original shape of the fossil or sedimentary structure.
Term
Cement
Definition
A material that binds sedimentary grains together.
Term
Cephalon
Definition
Head' part of a trilobite. Contains the compound eyes, glabella and can have a fringe around it.
Term
Cephalopods
Definition
Class of Molluscs including Ammonoids, Nautiloids, Squid, Octopus.
Term
Ceratites
Definition
A type of ammonoid that existed in the Triassic. Has a moderately elaborate suture line.
Term
Chalk
Definition
A sedimentary rock made from coccoliths and foraminifera (both microfossils that make a carbonate skeleton)
Term
Chemical Weathering
Definition
Weathering that produces ions that are removed in solution leaving an insoluble residue. Included carbonation and hydrolysis.
Term
Chilled margin
Definition
The edge of an igneous intrusion that has cooled rapidly so it has very fine crystals.
Term
Chondrites
Definition
A stony meteorite with the same overall composition of the Earth.
Term
Clast
Definition
A small piece of rock that has ben derived by weatherng and erosion. The building block for clastic sedimentary rocks.
Term
Clastic
Definition
Sediments made up of fragments of pre-existing rock
Term
Claystone
Definition
Sedimentary rock made up of layer of clay minerals.
Term
Cleavage
Definition
The line of weakness with a crystal structure.
Term
Coal
Definition
A sedimentary rock comprising layers of plant material laid down in anoxic conditions.
Term
Coarse
Definition
Crystals or grains that are bigger than 2mm.
Term
Coccoliths
Definition
Plankton that make a carbonate skeleton, that when they accumulate on the bottom of the sea, make up chalk.
Term
Colonial Corals
Definition
Many corals stuck together, forming a a large skeleton of polygonal individal coralets each with septa.
Term
Columnar Jointing
Definition
Joints formed as a result of contraction on cooling of igneous rocks, E.g. Giant's Causeway.
Term
Competent rock
Definition
Rock that is strong, does not deform under pressure, such as limestone or granite but will form fractures and joints if the stress is too great.
Term
Compound Eye
Definition
Eye of a trilobite, comprising many calcite plates that were used as lenses to focus the light.
Term
Compression
Definition
Force trying to push rocks together. The Earth's crust will be shortened.
Term
Conduction
Definition
Transfer of heat in solids.
Term
Conglomerate
Definition
A sedimentary rock comprising large, rounded clasts within a finer matrix.
Term
Conservative Plate Boundary
Definition
Plate boundary where no crust is made nor destroyed. Also known as a transform boundary. For instance, San Andreas Fault, California. Made of strike-slip faults, also known as wrench or tear faults.
Term
Constructive Plate Boundary
Definition
New material is created. Also called a divergent plate boundary. For instance, Mid Oceanic Ridge or Rift Valley.
Term
Contact Metamorphism
Definition
Solid state recrystallisation by heat without pressure which produces an unfoliated metamorphic rock.
Term
Continental Collision Zone
Definition
Plate margin where 2 continental plates meet and form fold mountains. E.g. Himalayas formed by India moving into Asia.
Term
Continental Crust
Definition
Less dense, thick crust that underlies continents. Can be 0-3950Ma old. Average composition similar to granite.
Term
Continental Drift
Definition
The large-scale horizontal movement of continents during geological time.
Term
Convection
Definition
Large-scale tranfer of heat through liquids or gases.
Term
Convergent Plate Boundary
Definition
Crust is 'destroyed'. Also called a destructive plate boundary. For instance, subduction zone or continental collision zone.
Term
Coprolites
Definition
Fossilised faeces.
Term
Coral
Definition
A marine organism.
Term
Country rock
Definition
Any rock into which an igneous rock is intruded.
Term
Crest
Definition
Highest point of a folded bed.
Term
Cross-Bedding (Cross-stratification)
Definition
Sedimentary structures formed by the migration of dunes by wind or water. Can be used as a way-up and palaeocurrent indicator.
Term
Crust
Definition
The uppermost rigid, brittle layer of the Earth above the Moho that less dense.
Term
Crystalline Texture
Definition
A rock comprising crystals rather than grains.
Term
Curie Temperature
Definition
The temperature at which the orientation of a magnetic minerals becomes fixed when they cool past it.
Term
Death Assemblage
Definition
Accumulation of fossils that have been moved between death and fossilisation. They are often aligned to the current, sorted in size and broken up.
Term
Deposition
Definition
Laying down of sediment that occurs when a transporting agent loses energy.
Term
Derived Fossils
Definition
Fossils that have been weathered out of an older sediment and then form clasts in a younger one.
Term
Desiccation Cracks
Definition
Cracks formed in clay-rich sediments as water evaporates from them. Can be used as way-up structures.
Term
Destructive Plate Boundary
Definition
Crust is 'destroyed'. Also called a convergent plate boundary. For instance, subduction zone or continental collision zone.
Term
Dextral Strike-Slip Fault
Definition
Fault with only horizontal movement, where the opposite side of the fault is seen to move to the right.
Term
Diagenesis
Definition
All physical, chemical and biological processes that occur in a sediment after deposition and before metamorphism that lithify the sediment to form a rock
Term
Diductor Muscle Scars
Definition
Marks place where the diductor muscles attached to inside of a brachiopod shell. Adductor muscles contract to open the shell.
Term
Diorite
Definition
A coarsely crystalline intermediate igneous rock comprising feldspar, mica and amphibole.
Term
Dip
Definition
Maximum angle between the horizontal and geological surface such as a bedding surface or fault.
Term
Dip-Slip Fault
Definition
A fault where the movement is in the same direction as the angle of dip, including normal and reverse faults.
Term
Dissolution
Definition
The process whereby minerals that make-up the fossils are dissolved away and removed in solution by groundwater.
Term
Divergent Plate Boundary
Definition
New material is created. Also called a constructive plate boundary. For instance, Mid Oceanic Ridge or Rift Valley.
Term
Dolerite
Definition
A medium crystalline mafic rock comprising plagioclase feldspar and pyroxene and sometimes olivine.
Term
Downthrow
Definition
Side of the fault that has moved down.
Term
Dyke
Definition
A discordant igneous intrusion, usually made of mafic rock
Term
Equigranular
Definition
All crystals in an igneous rock are the same size.
Term
Era
Definition
Major unit of geological time, e.g. Palaeozoic, Mesozoic and Cenozoic.
Term
Erosion
Definition
Removal of weathered material, usually by the physical action of transported fragments.
Term
Euhedral
Definition
Well-shaped crystals
Term
Evaporites
Definition
Minerals formed by the evaporation of saline water, for example, halite and gypsum.
Term
Exfoliation
Definition
Also known as onion-skin weathering, where sheets of rock split off due to expansion and contraction of minerals during daily heating and cooling.
Term
Extension
Definition
Force trying to pull rocks apart. The Earth's crust will be lengthened.
Term
Extrusive
Definition
Igneous rocks cooled from molten magma on the surface very quickly. Crystal size is very small. E.g Basalt, Rhyolite.
Term
Facies
Definition
The environment of deposition. E.g. Fluvial facies, would be sediments deposited in a river.
Term
Fault Displacement
Definition
The amount a fault has moved in the direction of the fault plane.
Term
Fault
Definition
A fracture in the rock along which there has been an observable amount of displacement.
Term
Fault breccia
Definition
A rock composed of fraggments produced by rocks fractured during faulting.
Term
Fault plane
Definition
Surface along which movement of a fault takes place.
Term
Fine
Definition
Sediment or crystals less than 0.5mm big, or cannot be made out.
Term
Flame structures
Definition
Combines with load structures. Formed by the deposition of heavy sand on top of water-logged mudstone which subsequently deforms and forms flame structure.
Term
Flow Banding
Definition
Igneous rock texture produced when silicic magma flows and the light and dark minerals line up with the flow as they cool.
Term
Fluvial Sediments
Definition
Sediments laid down by rivers
Term
Flute casts
Definition
Sedimentary structure found on the base of a bed, formed as a result of erosion caused by turbulent flow. Can be used as a way-up structure as well as a palaeocurrent indicator.
Term
Fold limb
Definition
Section of a fold between one hinge and the next, forming the side of a fold
Term
Foliation
Definition
A repeated planar feature in a rock which may be defined by composition or alignment of minerals
Term
Foot wall
Definition
Side of a fault that lies below the fault plane.
Term
Fossils
Definition
The remains of living organisms, either body fossils or trace fossils.
Term
Fracture
Definition
A crack in the rock along which no displacement can be observed.
Term
Freeze-thaw weathering
Definition
Weathering cause by the expansion of freezing water in fracture which forces the rocks apart.
Term
Gabbro
Definition
A coarsely crystalline, mafic igneous rock, comprising plagioclase feldspar, pyroxene and amphibole with minor olivine.
Term
Gape
Definition
A permanent gap between valves of infaunal bivalves which have a siphon so long they cannot withdraw it all into their shells.
Term
Genal Spine
Definition
Spines that come from the pointed part of a trilobite's head.
Term
Geological Column
Definition
The list of geological time periods in order.
Term
Geothermal Gradient
Definition
The rate of increase in temperature with depth._x000D_
Term
Glabella
Definition
The central part of the head of a trilobite that contains the stomach.
Term
Glacial Deposits
Definition
Sediments formed by a glacier.
Term
Glassy
Definition
Igneous rock texture that looks like glass.
Term
Gneiss
Definition
High temperature and pressure metamorphic rock. Coarsely crystalline with gneissose banding. Typically made of quartz and feldspar in the light bands, and biotite mica and other mafic minerals in the dark bands.
Term
Gneissose Banding
Definition
Metamorphic texture found in gneisses, showing band of light minerals and dark minerals.
Term
Goniatite
Definition
An ammonoid that evolved in the Carboniferous and Permian. Has a simple suture line.
Term
Graded Bedding
Definition
A sedimentary structure were the largest clasts are found at the base, and the smallest at the top. Commonly found in turbidites and used as a way-up structure.
Term
Granite
Definition
A coarsely crystalline, silicic igneous rock comprising quartz, feldspar and mica.
Term
Granoblastic
Definition
Texture of metamorphic rocks that contain interlocking crystals all the same size.
Term
Graptolite
Definition
A planktonic, organism from the Palaeozoic era. Used as a zone fossil, particularly in the Ordovician and Silurian.
Term
Gravity settling
Definition
Minerals that crystallise out from a magma are often more dense and sink to the bottom. For instance, there is often a layer of olivine at the base of large mafic intrusions.
Term
Greywacke
Definition
A sandstone with more than 50% feldspar and rock fragments.
Term
Growth Lines
Definition
Lines on a bivalve and brachiopod that mark its progressive growth.
Term
Gutenberg Discontinuity
Definition
The boundary between the solid, peridotite, lower mantle and the liquid, outer core made of iron-nickel alloy.
Term
Half – Life
Definition
The time taken for half the unstable parent atoms of decay and form stable daughter products.
Term
Hardness
Definition
Defined using the Mohs scale.
Term
Hanging wall
Definition
Side of a fault that lies above the fault plane.
Term
Head
Definition
A type of superficial deposit, basically a bucket term.
Term
Hinge (fold)
Definition
The line marking the most sharply curved part of the fold.
Term
Hinge line (Bivalve)
Definition
The line along which the two bivalve valves are joined by a ligament.
Term
Hornfels
Definition
Contact metamorphic mudstone, high-grade. Dark grey, brittle and hard rock. Compare with hornfels.
Term
Hot spot
Definition
A volcano within a plate, which is the surface expression of a mantle plume
Term
Hydrolysis
Definition
Reaction between minerals and water, causing the minerals to decompose.
Term
Igneous
Definition
Rocks formed from molten rock, either intrusively or extrusively.
Term
Inclined fold
Definition
A fold with an axial plane that is at an angle from vertical.
Term
Incompetent rock
Definition
Rock that is weak and deforms greatly under pressure, such as clay and mudstone. Folds easily.
Term
Index Fossils
Definition
Also known as a zone fossil. Fossils that can be used to tell the age of the rocks they are in. Need to be widespread, easily recognisable, had rapid evolution, abundant and are well preserved.
Term
Inner Core
Definition
The solid central part of the Earth, made of iron-nickel alloy that sit beneath the Lehmann discontinuity.
Term
Intermediate igneous rock
Definition
An igneous rock with medium grey colour, containing minimal quartz, plagioclase feldspar and some mafic minerals, e.g. andesite, microdiorite and diorite.
Term
Intrusive
Definition
Igneous rocks cooled slowly from molten magma under ground. Crystal size is large. E.g Gabbro, granite.
Term
Iron Meteorites
Definition
Meteorite derived from the core of larger bodies in space, made of the same material as our core, i.e. iron-nickel alloy.
Term
Island Arcs
Definition
The production of volcanoes above an oceanic-oceanic subduction zone. Comprising intermediate lavas.
Term
Isoclinal fold
Definition
Fold with near parallel limbs.
Term
Joint
Definition
A fracture in competent rocks along which no observable movement has occurred. Can be formed by folding (tension joints) formed parallel to the axial plane.
Term
K/Ar Dating
Definition
A type of radiometric dating based on the decay of radioactive K-40, to its stable daughter isotope Ar-40.
Term
Keel
Definition
A projecting rib around the edge of an ammonite.
Term
L Waves
Definition
A type of surface wave.
Term
Lava Flows
Definition
Molten rock erupted onto the Earth's surface
Term
Law of Cross Cutting Relationships
Definition
A geological surface that cuts across another must be younger than the one it cuts across.
Term
Law of Included Fragments
Definition
Fragments found within another rock, for instance large clasts within a sedimentary rock, must be older than the rock in which they are contained.
Term
Law of Superposition
Definition
Rocks beneath others are older than those above them, unless the area has been overturned.
Term
Lehmann's Discontinuity
Definition
Boundary between the liquid,outer core and the solid, inner core.
Term
Life Assemblage
Definition
Accumulation of fossils that have not been moved between death and fossilisation. They are preserved in the orientation they were in life.
Term
Lignite
Definition
Coal which has not been buried deeply and in which woody material can still be recognised. Also known as brown coal.
Term
Limb (Fold)
Definition
The sides of a fold.
Term
Limestone
Definition
A sedimentary rock made of calcite, commonly with fossils in it.
Term
Lithification
Definition
The process of changing unconsolidated sediment into rock.
Term
Lithosphere
Definition
Topmost layer of the Earth, including the crust and the topmost upper mantle. Base of the lithosphere is taken as the 1300°C isotherm which can vary between 5km at the Mid Oceanic Ridge to 300km below fold mountains at continental collision zones.
Term
Load Structures
Definition
Combines with flame structures. Formed by the deposition of heavy sand on top of water-logged mudstone which subsequently deforms and forms flame structure.
Term
Lustre
Definition
The way light reflects off a mineral, for instance, vitreous (glassy), pearly.
Term
Mafic
Definition
Igneous rocks with dark coloured minerals, such as amphiboles and pyroxenes and little quartz. Includes basalt, dolerite and gabbro.
Term
Magma
Definition
Molten rock within the Earth
Term
Magnetic Anomaly
Definition
A value for the Earth's magnetic filed that is different from the expected value.
Term
Magnetic Inclination
Definition
The angle of dip of the magnetic field lines. It is the angle made with the horizontal of a compass needle free to rotate in 3D.
Term
Mantle
Definition
Made of solid peridotite that can flow (especially the asthenosphere). Divided into upper and lower mantle.
Term
Mantle plume
Definition
A stationary area of high heat flow in the mantle, which rises from great depths and produces magma that feeds hot spot volcanoes.
Term
Marble
Definition
A metamorphic rock that can be either generated by contact or regionally metamorphism of limestone.
Term
Marine
Definition
In the sea
Term
Matrix
Definition
The background material in a rock, consisting of small particles in which larger fragments occur.
Term
Medium grained
Definition
0.5-1.5mm grains of sand or crystals in an igneous rock
Term
Metamorphic
Definition
Generated by heat and/or pressure.
Term
Metamorphic Aureoles
Definition
The area around an igneous intrusion that has been metamorphosed by heat. Width can be affected by things such as volume of intrusion, heat of intrusion and angle of the dip of the intrusion.
Term
Metamorphic Foliation
Definition
Solid state recrystallisation under pressure with or without heat causing the new minerals to align in a direction perpendicular to the dominant stress.
Term
Metamorphic Rock
Definition
Rock formed by recrystallisation in the solid state of any rock subjected to heat and pressure.
Term
Metaquartzite
Definition
Metamorphosed sandstone, either contact or regional metamorphism.
Term
Meteorite
Definition
Fragments of debris that falls to Earth. There are 2 sorts: stony and iron.
Term
Micro-diorite
Definition
A medium crystalline intermediate igneous rock comprising feldspar, mica and amphibole.
Term
Micro-granite
Definition
A medium crystalline, silicic igneous rock comprising quartz, feldspar and mica.
Term
Mid-Oceanic Ridge
Definition
Divergent plate margin generating new oceanic crust.
Term
Mineral
Definition
A naturally-occurring, inorganic, crystalline, chemical compound or element that have unique properties and defining characteristics.
Term
Mineral Veins
Definition
A fracture in the rock that has been filled by precipitation of minerals from groundwater. Often the target of mining.
Term
Moho
Definition
Boundary between the crust and the upper mantle
Term
Mould
Definition
The impression of the outside or inside of a fossil.
Term
Mudstone
Definition
Sedimentary rock made of mud-sized particles, includes shale (well layered), siltstone and claystone (no layering visible)
Term
Normal Faults
Definition
A fault with vertical movement, where the hanging wall moves down relative to the footwall. The area extends.
Term
Oceanic Crust
Definition
More dense, thin crust that underlies oceans. Can be 0-200Ma old. Average composition similar to basalt.
Term
Oceanic Trench
Definition
Deep ocean trench marking the position of a subduction zone.
Term
Oolith
Definition
Spherical, concentric calcium carbonate balls formed by chemical precipitation around a nucleus such as a sand grain in warm tropical shallow water, as they are moved by waves.
Term
Oolitic
Definition
Limestone made from ooliths
Term
Open fold
Definition
Fold with a large interlimb angle, i.e. wide spread limbs.
Term
Orogenic Belts
Definition
Mountains formed by continental collision.
Term
Outer Core
Definition
The liquid part of the Earth, made of iron-nickel alloy. Sits between the Gutenburg Discontinuity and the Lehman Discontinuity.
Term
Palaeomagnetism
Definition
Ancient magnetism preserved in rocks
Term
Pallial Line
Definition
The line on a bivalve shell that marks the limit of the attachment of soft tissue.
Term
Pallial Sinus
Definition
An indent in the pallial line that represents the area into which the siphon is withdrawn when the shell is closed.
Term
Partial Melting
Definition
Where a proportion of the minerals have a lower melting point, allowing them to melt while the rest remains solid.
Term
Peat
Definition
Plant debris that has been laid down but not compacted. Preliminary to coal.
Term
Pedicle
Definition
A fleshy stalk made mainly of muscle,which attached a brachiopod to the substrate.
Term
Pedicle Foramen
Definition
A hole in a brachiopod's valve out of which the pedicle emerges.
Term
Pedicle Valve
Definition
The largest valve of a brachiopod. Usually contains a hole for the pedicle to protrude through.
Term
Permeability
Definition
The ability of a fluid to pass through a rock.
Term
Peridotite
Definition
Ultamafic igneous rock composed of olivine and pyroxene with a coarse crystal size.
Term
Period
Definition
Unit of geological time, e.g. Cambrian, Silurian, Jurassic.
Term
Phenocryst
Definition
A large crystal that has grown very slowly at great depths, within a finer-crystalline matrix. Granite often has a large phenocrysts of orthoclase feldspar.
Term
Physical Weathering
Definition
Weathering by physical means, such as freeze-thaw, exfoliation etc.
Term
Pillow Lava
Definition
Produced when mafic lava is erupted underwater. Has a rounded top, and either a rounded base or shaped like a bulb, with a glassy rim.
Term
Planktonic
Definition
Organisms that float in sea water.
Term
Pluton
Definition
Large intrusive body of magma, that has risen through the crust because it is hot and less dense than the rocks around it, and then cooled slowly. Usually of granitic composition.
Term
Porosity
Definition
The volume of pore space in a rock.
Term
Porphyritic
Definition
Igneous rock texture where a rock has cooled in two stages, to form large phenocrysts that have cooled very slowly in a finer-crystalline matrix that has cooled quicker.
Term
Porphyroblast
Definition
A large crystal that has grown during recrystallisation in a metamorphic rock and is surrounded by a finer grained groundmass of other crystals. Compare with a phenocryst in igneous rocks.
Term
Pressure release weathering
Definition
Expansion and fracturing of rock due to removal of overlying rocks.
Term
P-Waves
Definition
Primary seismic waves, arrive first at a seismometer after an earthquake, compressional wave.
Term
Pygidium
Definition
The 'tail' part of a trilobite, made of fused segments.
Term
Pyritisation
Definition
The replacement of organic material with pyrite.
Term
Pyroclastic
Definition
All fragmental materials formed by explosive eruptions, including bombs, blocks, lapilli and ash.
Term
Radioactive Decay
Definition
The constant rate at which unstable radioactive isotopes decay to form their stable daughter isotopes.
Term
Radiometric Dating
Definition
Uses the ratio of radioative parent isotopes to their stable daughter isotopes, which decay at a constant rate, to calculate the age of a rock.
Term
Recrystalisation
Definition
Solid state process that changes minerals into new crystalline metamorphic minerals.
Term
Recumbent fold
Definition
A fold with a horizontal axial plane
Term
Regional Metamorphism
Definition
The solid state recrystallisation of a rock under heat and pressure, to create a foliated metamorphic rock.
Term
Relative Dating
Definition
Dating the age of rocks relative to one another, but without knowing the numerical ages of each rock unit. Uses fossils, law of superposition etc.
Term
Remnant Magnetism
Definition
Magnetism shown in the rocsk due to the alignment of their magnetic minerals according to the Earth's magnetic field at the time of their formation.
Term
Replacement
Definition
Atom by atom substitution of one mineral for another.
Term
Reverse Faults
Definition
Fault with vertical movement, where the hanging wall moves up and over the footwall. The area is compressed.
Term
Rheid
Definition
A solid material that flows
Term
Rhyolite
Definition
A finely crystalline, silicic igneous rock comprising quartz, feldspar and mica.
Term
Rift Valley
Definition
A divergent plate boundary that occurs within a continent. E.g. East African Rift Valley.
Term
Ripples
Definition
Smaller sedimentary structures (cm-scale) formed by the migration of ripples by medium-energy water or wind. Ripples can be symmetrical or asymmetrical depending on whether the current is two-way (wave) or one-way (current).
Term
Rock
Definition
An aggregation of minerals that show a textural relationship between them.
Term
S Waves
Definition
Secondary seismic waves, shear waves.
Term
Saltation
Definition
Transport of material by bouncing.
Term
Sandstone
Definition
A sedimentary rock comprising sand grains, variably compacted and cemented.
Term
Schist
Definition
Medium grade metamorphic rock with aligned mica crystals and commonly garnet porphyroblasts. Shiny.
Term
Schistosity
Definition
The texture in medium and coarsely crystalline metamorphic rocks formed by the preferred alignment of flat/tabular minerals. The alignement is perpendicular to the direction of maximum pressure applied during metamorphism. No traces of original bedding remain.
Term
Sedimentary Rock
Definition
Rock composed of clasts, grains or fossils that hav been lithified by the processes of compaction and diagenesis.
Term
Seismic Velocity
Definition
Speed at which a wave travels through the Earth.
Term
Seismic Wave
Definition
A wave that travels through the Earth. Includes P, S and L waves.
Term
Seismological Low Velocity Zone
Definition
The area within the upper mantle, that roughly corresponds to the asthenosphere, where the velocity of seismic waves is reduced as it is partially molted.
Term
Septa
Definition
Radiating calcite plates that support a coral skeleton.
Term
Shadow Zone
Definition
The area on the Earth's surface that cannot be reached by S-waves due to their inability to pass through the molten outer core, or P-waves due to their refraction by the liquid outer core.
Term
Shale
Definition
A layered sedimentary rock made of mud.
Term
Shell Symmetry
Definition
Bivalves have a line of symmetry between its two shells. A brachiopod has a line of symmetry down the middle of its shell.
Term
Silicification
Definition
The replacement of organic material with silica.
Term
Silicic Igneous Rock
Definition
Igneous rocks with a lot of quartz in them, light in colour and also contains feldspar and mica. E.g. granite, microgranite and rhyloite.
Term
Sill
Definition
A concordant igneous intrusion, usually made of mafic rock.
Term
Sinistral Strike-Slip Fault
Definition
Fault with only horizontal movement, where the opposite side of the fault is seen to move to the left.
Term
Siphon
Definition
A soft tissue part of a bivalve that takes in fresh water and filters it, and removes used water. Aids respiration and feeding.
Term
Slate
Definition
A low-grade, regionally metamorphic rock with slaty cleavage derived from mudstone.
Term
Slaty Cleavage
Definition
The texture in fine grained rocks formed by low grade regaional metamorphism. Platy minerals recrystallise perpendicular to the direction of stress applied during metamorphism so that the rock splits into thin sheets.
Term
Slickensides
Definition
Polishing and striations found on a fualt place indicating the direction of relative movement.
Term
Sole Structures
Definition
Sedimentary structures that form due to high-energy currents on a bed. Can be way-up structure.
Term
Solitary Corals
Definition
A singular coral, shaped like a variably bent ice-cream cone.
Term
Sorting
Definition
The degree to which particles are the same size.
Term
Spotted Rock
Definition
Contact metamorphic mudstone, medium-grade. Consists of medium grey rock with darker spots. Compare with hornfels.
Term
Stipe
Definition
The stack of thecae of a graptolite built up to form a colony.
Term
Stony Meteorites
Definition
Meteorites of a similar composition to the world as a whole, see Chondrites.
Term
Streak
Definition
The colour of the powder left by rubbing a mineral on a streak plate.
Term
Strike
Definition
Direction at right angles to the dip. There isa zero dip in this direction,. It is a bearing measured from north using a compass.
Term
Strike - Slip Fault
Definition
Fault where movement is only horizontal. Can be sinistral (left-handed) or dextral (right-handed).
Term
Subduction
Definition
The process by which oceanic crust is forced underneath other oceanic or continental crust
Term
Subduction Zone
Definition
A destructive plate margin, where an oceanic crust is forced down beneath either another oceanic or continental crust.
Term
Subhedral
Definition
A moderately formed crystal, somewhere between anhedral (poor) and euhedral (well) formed crystals.
Term
Superficial Deposits
Definition
The soil and subsoil layer that you get on top of the true bed rock if you dig down.
Term
Surface Waves
Definition
Earthquake waves that do not travel through the body of the Earth but only in the uppermost crust. Comprise both shear and oscillatory waves.
Term
Suspension
Definition
Transport of material in water or air without it touching the Earth's surface.
Term
Suture Lines
Definition
The line along which the internal walls of an ammonoid meet the outside wall.
Term
Symmetrical fold
Definition
A fold with limbs of the same length, symmetrical about the axial plane.
Term
Syncline
Definition
A "u"- shaped fold, where the youngest beds are in the middle of the fold.
Term
Teeth and sockets
Definition
Part of the hinge line in a bivalve or brachiopod that join the valves together.
Term
Texture
Definition
The inter-relationship and nature of grains or crystals within a rck.
Term
Theca
Definition
Individual cup in which one graptolite organism (zooid) lives.
Term
Thoracic Segments
Definition
Segment of a trilobite comprises a pair of legs, a pair of gills and a covering to protect them.
Term
Thorax
Definition
The 'body' part of a trilobite, divided into thoracic segments.
Term
Thrust fault
Definition
Low-angle reverse fault.
Term
Tight fold
Definition
Fold with a small interlimb angle, i.e. narrowly-spread limbs.
Term
Trace Fossil
Definition
The fossilised indication that an organism has lived there. For instance, burrows, footprints, resting traces.
Term
Transform Faults
Definition
Strike-slip fault with only horizontal movement.
Term
Trilobite
Definition
An organism from the Palaeozoic era. An marine arthropod.
Term
Trough (fold)
Definition
Lowest point of a folded bed.
Term
Throw
Definition
The amount of VERTICAL movement on a fault measured between the top of the same bed seen on both sides of the fault.
Term
Tuff
Definition
A sedimentary rock consisting of pyroclastic material.
Term
Turbidite
Definition
A deposit that reduces in grain size upwards of greywackes deposited by a turbidity current.
Term
Turbidity current
Definition
A high velocity current that flows down gentle gradients because the sediment dispersed within it makes it denser than sea water, like an underwater avalanche.
Term
Twinning
Definition
Occurs when two separate crystals share some of the same parts of another crystal. Like Siamese twins.
Term
Ultramafic igneous rocks
Definition
An igneous rock with no quartz in it, dominated by olivine and pyroxene. E.g. peridotite.
Term
Umbo
Definition
Area of a valve marking the point of first growth.
Term
Unconformity
Definition
A gap in time that is recorded by an absence of rock deposition, with younger rocks above and older rocks below. Usually there is a different dip and/or strike of the rocks above and below this surface and there may also be a conglomerate along the surface.
Term
Uniformitarianism
Definition
The present is the key to the past. The idea that geological processes that are acting today have acted in the same way through geological past.
Term
Uplift
Definition
Return of buried rocks to the Earth's surface by tectonic forces.
Term
Upright fold
Definition
A fold with a vertical axial plane.
Term
Upthrow
Definition
Side of the fault that has moved up.
Term
Vesicular
Definition
Igneous rock texture where gas bubbles are trapped in the lava as it cools rapidly.
Term
Xenoliths
Definition
A fragment of 'foreign' rock included in an igneous rock, which has come from a different source.
Term
‘Way up’ Criteria
Definition
Characteristic used to determine which way up a bed is, and therefore which beds are older and which younger. E.g. Roots, flute casts, Flame and Soul structures, Shape of vesicles in lava
Term
Weathering
Definition
The in-situ breakdown of rocks.
Term
Zone Fossils
Definition
Also known as an index fossil. Fossils that can be used to tell the age of the rocks they are in. Need to be widespread, easily recognisable, had rapid evolution, abundant and are well preserved.
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