AS Module GL1
DEFINITIONS
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AS GEOLOGY
Important Definitions
Minerals
Mineral - A solid element or compound found naturally in the Earth.
Form / habit – The 3D shape of a mineral.
Lustre – The way in which a mineral reflects light.
Cleavage – The ability of a mineral to split along planes of weakness.
Fracture – When a mineral breaks, not along planes of weakness.
Transparency – The ability of a mineral to let light pass through it.
Density – The mass per unit volume of a mineral (or anything else). Density = mass / volume
Streak – The colour of the powder of a mineral, obtained by scraping the mineral along an unglazed
white tile.
Twinning – When two (or more) crystals have grown together.
Sulphide – A group of minerals which contain sulphur (S). These minerals react with acid to produce
hydrogen sulphide gas (smells of rotten eggs).
Carbonate – A group of minerals which contain carbon and oxygen (CO3). These minerals react with
acid to produce carbon dioxide gas.
Oxide – A group of minerals which contain oxygen (O).
Silicate – A group of minerals which contain silicon and oxygen (SiO…)
Rock forming minerals – Minerals which commonly form rocks.
Igneous
Igneous rock - A rock which has formed by magma or lava cooling and crystallising.
Partial melting - the process which forms magma. Part of the rocks at depth melt - the minerals with
the lowest melting points.
Magma – A liquid mixture of molten rock (mainly molten silicates) and dissolved gases, below the
surface of the Earth
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Lava- A liquid mixture of molten rock (mainly molten silicates) and dissolved gases, above the
surface of the Earth.
Volatiles – Gases which escape from magma or lava as it comes to the surface.
Pyroclastic – Solid material sent out into the air in a volcanic eruption.
Texture – What grains the rock is made of and how those grains relate to each other.
Crystalline – The word used to describe the texture of a rock made of interlocking crystals.
Intrusive rock – An igneous rock made when magma forces its way into other rocks, cools and
crystallises.
Extrusive rock – An igneous rock made when lava, erupted from a volcano, cools and crystallises.
Pluton – A large, usually deep intrusion.
Batholith – A large intrusion with a surface area >100 km2.
Dyke – A tabular shaped (flat, parallel sided) intrusion which cuts across beds.
Lava flow – A mass of molten rock flowing across the surface.
Sill – A tabular shaped intrusion which follows bedding.
Vent – The hole that lava and gas escape from in a volcano.
Crater – A large, bowl-shaped depression at the summit of the volcano.
Shield volcano – A broad, flat shaped volcano made from basic lava (e.g. Mauna Loa, Hawaii).
Composite cone volcano – A steep sided volcano made of intermediate lava and pyroclastics (e.g.
Etna).
Blocks – large pieces of solid rock thrown into the air in a volcanic eruption.
Bombs – large pieces of liquid lava thrown into the air in an eruption.
Ash – Fine material (dust) thrown into the air in an eruption.
Agglomerate – A rock made of a mixture of fragments of volcanic rock deposited from the eruption.
(Welded) Tuff – A pyroclastic rock formed from ash which has been stuck together by solidifying
lava droplets.
Coarse grained – Crystals are > 5 mm
Medium grained – Crystals are 1 - 5 mm
Fine grained – Crystals are < 1mm
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Euhedral – Well shaped crystals
Subhedral – Moderately well formed crystals.
Anhedral – Poorly shaped crystals
Equigranular – All grains the same size
Porphyritic – A rock texture which is made of two distinct sizes of crystals (big = phenocrysts, small =
groundmass).
Vesicular – A rock which has holes left by volcanic gas.
Glassy – An igneous rock with no crystals – it has cooled too quickly to form crystals.
Acidic – A rock with a relatively high proportion of silica (>66%).
Intermediate – A rock with a medium proportion of silica (52 – 58%).
Basic - A rock with a low proportion of silica (<52 %).
Granite – Acidic, coarse grained igneous rock.
Gabbro – Basic, coarse grained igneous rock.
Dolerite – Basic, medium grained igneous rock.
Basalt – Basic, fine grained igneous rock.
Andesite – Intermediate, fine grained igneous rock.
Pillow structures – “Blob” shaped structures formed when lava is erupted under water.
Columnar jointing – Cracks which form in lava flows and sills, perpendicular to the base.
Country rock - The rock into which magma has intruded.
Chilled margin – The edge of an igneous body which has smaller crystals due to the edge cooling
faster than the rest of it.
Baked margin – The edge of the country rock touching the magma, which has become changed
(metamorphosed) due to the magma’s heat.
Surface processes
Weathering – Break up of rocks at the surface of the Earth dues to physical, chemical and biological
processes. Rock fragments and soluble substances are produced. This does not involve movement of
the products (in situ).
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DEFINITIONS
Erosion – Break up of rocks due to transport by rivers, sea, glaciers etc.
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Abrasion – The process where the bedrock is ground down by clasts transported in a river.
Attrition – The grains themselves become smaller and rounded due to impacts between grains.
Saltation – The transport of grains by bouncing along the river bed.
Suspension – Fine material like clay carried along in the water.
Bedload – Clasts transported along the river bed by rolling or sliding.
Entrainment velocity – The minimum velocity of a river necessary to start a particular grain size
moving.
Chemical weathering – The weathering processes caused by chemical reactions between minerals and
liquids and gases in the environment.
Physical weathering – The weathering processes caused by changes in temperature and pressure.
Carbonation - A form of chemical weathering when carbonate minerals (like calcite) react with acidic
water to form soluble hydrogencarbonates.
Hydrolysis – A chemical weathering reaction in which minerals like feldspar break down to form clay.
Insolation – Heating and cooling of rocks caused by heat from the sun. This causes the minerals to
expand and contract.
Freeze-thaw – Physical weathering process in which the rock is broken due to water freezing and
expanding in cracks.
Biological weathering – weathering processes caused by animals and plants.
Mass movement – Movement of loose rock and soil down slope under the influence of gravity.
Sedimentary
Sedimentary rocks – Rocks formed from the fragments derived from the break down of other rocks.
Clasts – Rock fragments.
Sorting – A way of describing the mixture of grain sizes in a sedimentary rock.
Well sorted – A rock composed of mostly one grain size (texturally mature).
Poorly sorted – A rock composed of a mixture of grain sizes (texturally immature).
Diagenesis – All the processes that convert soft sediment into hard sedimentary rock.
Compaction – Squashing a sediment due to deposition of more sediment above.
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Dewatering – Water being squeezed out of pores in sediment.
Cementation – Water leaves behind minerals which sticks grains together.
Matrix – Fine grained material deposited with coarse grained clasts.
Fluvial – Processes associated with rivers and streams.
Aeolian – Processes associated with wind.
Glacial – Processes associated with glaciers.
Biochemical – Rocks formed from biological or chemical processes.
Evaporites – Rocks formed by evaporation of salty water.
Limestone – A rock made of calcium carbonate.
Ooid – A spherical, concentrically layered ball of calcium carbonate (formed in a tropical lagoon).
Oolitic limestone – A limestone composed of ooids.
Bioclastic limestone – A limestone formed from shell fragments.
Coal – A biochemical sedimentary rock, used as a fossil fuel. Formed from accumulated vegetation
which has been compressed and heated.
Breccia – A coarse grained sedimentary rock made from angular clasts.
Conglomerate – A coarse grained sedimentary rock made from rounded clasts.
Sandstone – A medium grained sedimentary rock.
Arkose – A type of sandstone containing a significant amount of feldspar.
Greywacke – A sandstone with a high proportion of clay (“muddy sandstone”).
Shale – A fine grained sedimentary rock made from tiny clay fragments. It splits into thin layers
(fissile).
Coarse grained – Fragments > 2mm
Medium grained – Fragments 0.5 - 2mm
Fine grained – Fragments < 0.5mm
Halite – Rock salt. This is precipitated from evaporated sea water.
Bed – A packet of sediment (or sedimentary rock) deposited horizontally.
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Bedding plane – Surfaces which separate beds. They represent breaks in deposition.
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Graded bedding – A sedimentary structure in which the coarsest grains are at the base and gradually
become finer higher up.
Desiccation cracks – Cracks in the surface of a muddy sedimentary layer formed due to the mud drying
out due to exposure to the sun’s heat. Usually they form polygonal shapes.
Sole structures – Sedimentary structures on the base of a bed. (Includes flute casts, prod marks, groove
casts).
Flute casts – Sole structures formed by eddy currents scouring (eroding) the sediment beneath the flow
and then being filled in by more sediment.
Cross bedding – Curved lines in a bed which represent the movement of the front of a dune.
Cross laminations – Curved lines in a bed which represent the movement of the front of a ripple.
Turbidity current – A mixture of sediment and water which flows down the continental slope onto the
abyssal plain (also called a density flow).
Phi scale - an alternative scale for measuring grainsize. Uses whole numbers (including negative
numbers) rather than fractions e.g 2 mm = phi -1, 1mm = phi 0, 0.5 mm = phi 1
Metamorphic
Metamorphism – A process of change in which the texture of a rock changes due to the effects of high
temperature and high pressure
Metamorphic rocks – Rocks which have been formed by metamorphism
Contact metamorphism – Metamorphism that is caused by heat from an intrusion of magma.
Regional metamorphism – Metamorphism that is cause by high temperature and high pressure due to
deep burial.
Foliation – Minerals in bands.
Aureole – The region of metamorphic rocks around an intrusion.
Hornfels – A contact metamorphic rock formed from a mudrock (like shale). It has crystalline, random
texture.
Spotted rock – A rock found at the edge of an aureole, formed by the partial metamorphism of
mudrocks. It has a texture of white crystals in a matrix of clay minerals.
Slate – A metamorphic rock with a crystalline texture with very small crystals in thin bands. It splits
into thin sheets (fissile). It forms from mudrocks by regional metamorphism.
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Schist – A rock made by regional metamorphism of mudrocks. It is a higher grade rock than slate
(higher pressure and temperature). Texture is foliated and crystalline, crystals and bands are larger
than in slate.
Gneiss – A high grade, regional metamorphic rock, formed by the metamorphism of mudrocks.
Texture is coarse grained, crystalline with wide bands.
Marble – A metamorphic rock formed by either regional or contact metamorphism of limestone. It
has a crystalline (often sugar-like) texture. No bands.
Slaty cleavage – The ability of a metamorphic rock to split into thin sheets like slate. This is usually
due to plate-like minerals lining up parallel to make thin bands.
Schistosity – Thicker bands of minerals seen in schists.
Gneissose banding – The type of thick, usually wavy, mineral bands seen in gneiss.
Coarse, medium, fine grained – as for igneous.
Chiastolite – A form of andalusite which has dark material in it, usually in a cross shape.
Polymorphs - Minerals with the same chemical composition but different crystal structures ( e.g.
aluminium silicates).
Andalusite – A polymorph of aluminium silicate that shows the rock formed under low pressure and
high temperature.
Garnet – A metamorphic mineral, usually a red colour, rounded shape.
Geological structures
Folds – Rock layers which have been bent.
Antiforms – An “up-fold”.
Anticlines – An “up-fold” where the oldest layers are in the core of the fold (the beds have not been
inverted).
Synforms – A “down-fold”.
Synclines – A “down-fold” where the youngest layers are in the core of the fold (the beds have not
been inverted).
Tectonic – Any processes involving movements of the Earth’s crust.
Fault – A break in the rock.
Reverse faults – A fault in which the hanging wall moves up relative to the footwall.
Normal faults – A fault in which the hanging wall moves down relative to the footwall.
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Joint – A break in the rocks in which there is no significant movement.
Throw – Vertical displacement.
Unconformity – A surface which represents a period of uplift and erosion. There will be a time gap
between the layers above and below the unconformity. The layers may also be dipping at different
angles.
Earthquakes
Focus - Point of origin of an earthquake.
Epicentre - Point on the Earth's surface directly above an earthquake.
P-wave - Primary wave. The fastest type of wave. Longitudinal wave motion (particles vibrate
parallel to wave direction).
S-wave - Secondary wave. This is a slower wave (always arrives at seismometer second). Transverse
wave motion (particles vibrate perpendicular to wave direction).
L-wave - Surface waves (sometimes called long or Love waves). Slowest earthquake waves, travel
near to surface of the crust, cause the most damage.
Seismograph / seismometer - Instrument for recording earthquake waves.
Shadow zone - Regions on the Earth surface where earthquake waves are not recorded, due to
refraction inside the Earth. P-wave shadow zone extends from 103 - 142, S-wave shadow zone starts
at 103.
Refraction - the way in which earthquake waves change direction as they move deeper into the Earth,
due to changes in the rock properties like rigidity.
Structure of the Earth
Crust – The outer layer of the Earth made of rock. (There are two types of crust – continental and
oceanic).
Mantle – The thick layer of rock beneath the
Outer core – The layer beneath the mantle. Made of iron alloy (iron with nickel and sulphur). Liquid
layer due to high temperature. Causes Earth's magnetic field due to movement and electrical currents.
Inner core – Central part of the Earth. Same composition as outer core, but a solid due to high
pressure.
Lithosphere – The crust and the rigid part of the upper mantle.
Asthenosphere – The ductile part of the mantle below the lithosphere.
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Low velocity zone – The asthenosphere - it's called this because earthquake waves slow down due to
its ductile behaviour.
Discontinuity – Boundaries between layers, identified by sudden changes in earthquake wave speed.
Moho – The discontinuity that divides the crust and mantle.
Pate Tectonics
Constructive (divergent) – Where two lithospheric plates are moving apart and new rocks are made
between them.
Rift – The valley formed as two plates are pulled apart. The edges of the valley are formed by normal
faults.
Ridge – The underwater mountain chain at a divergent margin.
Destructive (convergent) – Where two lithospheric plates are moving towards each other, and one plate
is subducted and recycled into the mantle.
Active continental margin – The edge of a continent at a plate boundary where volcanoes and
earthquakes occur due to subduction (e.g the Andes, South America.)
Island arc – A chain of volcanic islands formed by magma produced at a convergent margin. They
follow a curved (arc) shape.
Trench – Deep depression on the sea floor where the two plates meet at a convergent margin.
Conservative – Where two plates are moving past each other. Also called transform faults.
Convection – The way in which heat is transferred in the mantle by circulating ductile rock.
Palaeomagnetism – The record of the Earth’s magnetic field, preserved by magnetic minerals in rocks.
Curie temperature – The temperature below which a solid can become magnetised.
Remnant magnetism – A magnetic signature preserve in the rock as magma cools.
Hot spot – A place where hot material from deep in the Earth (probably base of the mantle) rises and
causes partial melting under the lithosphere – creates volcanoes (e.g. Hawaii).
Polar wander curves – Using records of ancient magnetism to plot the points where the poles seemed
to be in the past. Joining these points produces a curve. They are caused by plates moving, not the
poles moving.
Anomalies – Variations in physical quantities such as gravity, heat flow and magnetism. Positive
anomaly – greater than the mean, negative anomaly – less than the mean.
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Continental drift – The way in which continents have moved their positions over time due to plate
tectonics.
Sea floor spreading – Creation of new oceanic crust at the constructive plate boundaries makes the
ocean get wider.
Geological time
Uniformitarianism – “The present is the key to the past”. Processes which occur today also happened
in the past.
Radiometric dating – Determining the age of a rock by measuring the proportions of radioactive
isotopes present in the rock.
Relative dating – Determining the age of a rock (or structure) in relation to others (ie. What is the
sequence in which the rocks formed?)
Correlation – Matching rocks of the same age, usually by identifying the same mix of fossils in the
rock.
Half-life – The time taken for half of the radioactive isotope to decay.
Stratigraphic column / geological timescale – The division of the history of the Earth into eras and
periods.
Fossils
Fossil – The remains of living organisms preserved in sedimentary rock.
Body fossil – The preserved remains of the body of the organism (e.g. bones, shells).
Trace fossil – Activity of the organism preserved in rock (e.g. footprints, resting traces).
Zone fossils – Fossils that are particularly good for dating rocks.
Coprolites – Fossil dung.
Petrification – “Turning into stone”. Organic remains preserved by being replaced by inorganic
minerals.
Silicification – Mineral replacement by quartz (silica).
Pyritisation – Mineral replacement by iron pyrites.
Calcification – Mineral replacement by calcite.
Assemblage – The collection of fossils in a sedimentary rock.
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Derived fossils – Fossils in the wrong rock layer. They have been eroded from older beds of rock and
incorporated into newer rock, so they cannot be used to find the age of the rock.
Brachiopods – Invertebrate animals which live in the sea. Usually have two valves of different sizes.
Pedicle – Fleshy stalk used by brachiopods to attach themselves to the sea bed.
Pedicle foramen – The opening in the pedicle valve through which the pedicle emerges.
Pedicle valve – The valve which has the opening for the pedicle (usually the bigger valve).
Brachial valve – The other valve!
Hinge line – The region where the two valves are attached to each other.
Adductor muscle scars – Marks on the internal surface of the valves where muscles used to close the
shell were attached.
Diductor muscle scars – Marks on the internal surface of the valves where muscles used to open the
shell were attached.
Bivalves – A group of invertebrates with a shell made of two valves, usually the same size and shape
(symmetrical).
Muscle scars – Marks on the internal surfaces of the valves where the adductor muscles were attached.
Hinge line – the region where the two valves were attached.
Teeth & sockets – “Knobs” and pits on the hinge line which allowed the shell to articulate correctly.
Gape – A permanent opening at one end of the shell. Shows that the bivalve had large siphons to
allow it to live deeply buried in the sediment.
Pallial line – A line on the internal surface of the valves which show the limit of the animal’s body
attached to the shell.
Pallial sinus – A “dent” in the pallial line which shows that the bivalve had siphons to allow it to live
buried in sediment.
Umbones – The pointed tips of the valves in either bivalves or brachiopods (one is called an umbo).
Ammonoids – A group of molluscs with a coiled shell.
Chambers – Spaces within the Ammonoid shell.
Septa – walls which divide up the chambers.
Suture lines – Patterns on the shell formed where the internal septa meet the shell.
Goniatitic suture – A simple suture line pattern.
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Ceratitic suture – A more complex suture line, later in time.
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Ammonitic suture – Very complex suture lines which are the most evolved formed.
Keel – The flattened edge of an ammonoid shell.
Whorl – One complete revolution of the shell.
Siphuncle – A fleshy tube that connects the body of the ammonoid to the internal chambers.
Septal neck – Small extensions to the septa which support the siphucle.
Corals – Soft bodied animals that produce a skeleton of calcium carbonate to house the animal (polyp).
The animals have tiny plants in their bodies which have a symbiotic relationship.
Colonial – Corals which grow together to form large groups. Many of these may form reefs.
Solitary – Corals which exist as single animals.
Septa – Vertical walls which divide up the coral skeleton.
Trilobites – An extinct group of marine arthropods. They had a hard outer skeleton made of chitin.
Cephalon – The head of the trilobite.
Thorax – The body of the trilobite, made of many articulated segments (plurae), each of which has a
jointed leg.
Pygidium – Tail of the trilobite.
Compound eyes – Eyes made of many lenses.
Glabella – Rounded central region of the cephalon.
Genal spines – The rear edge of the cephalon, extended into backward pointing spines.