rocks and weathering Flashcards
abyssal plain
the deep, largely flat parts of the ocean floor well away from the plate margins; average water depth is about 4km.
asthenosphere
partially molten plastic layer in the upper mantle below the lithosphere in which convection currents drive the movements of the overlying lithospheric plates.
basalt
dark, microcrystalline igneous rock formed at mid-ocean ridges and by some volcanoes.
Benioff zone
the contact zone between a colliding ocean plate and a continental plate in which earthquakes are generated by the friction between the moving plates.
collision zone
an area where two plates are moving in opposite directions towards each other to form either a subduction zone (ocean/continental convergence e.g. off west coast S. America) or a range of fold mountains (continent/continent convergence e.g. Himalayas).
conservative plate boundaries
where lithospheric plates are moving parallel to each other; friction resists movement so that pressure builds up until stress is suddenly released, resulting in earthquakes e.g. San Andreas Fault.
constructive plate margins
a line along which two plates are moving away from each other and new material is added to each plate; also known as a “spreading centre” or “spreading margin” e.g. Mid Atlantic Ridge.
continental drift
the hypothesis (advanced by Wegener) that an original large landmass (Pangaea) broke apart, and the resulting continents have moved very slowly (at a few cm per year) over the asthenosphere, to their present positions.
continental plate
a large, thick (30-70km) piece of the earth’s crust and upper mantle, the former mostly made of less dense rocks, such as Granite.
core
the innermost part of the earth, with a radius of c. 3,400km; the inner core is solid; the outer core is liquid; very hot : probably about 2000 oC.
cratons
large, old, stable parts of the earth’s crust, unaffected by mountain building and earthquakes; the stable interiors of continents, well away from plate margins.
crust
the outermost shell of the earth. Continental crust is mostly composed of granitic rocks and is usually 30 to 70 km thick; ocean crust is mostly made of basalt and is usually 5 to 10 km thick.
destructive plate margins
lines along which ocean and/ or continental plates are colliding.
epicentre
the point on the Earth’s surface immediately above the focus of an earthquake.
focus
the point at depth in the Earth’s crust where the dislocation of rocks occurs to cause an earthquake.
fold mountains
mountains formed mostly of compressed and folded marine sediments thrust up when two continental plates collide, or one ocean and one continental plate.
granite
a coarse-grained, speckled intrusive igneous rock formed in large masses (batholiths), composed of three minerals: quartz, mica and feldspar.
island arc
where two ocean plates collide, the magma in the upper mantle above a subducted plate melts and becomes buoyant and so rises and erupts to form a gently curving line of volcanic islands (e.g. Japan, West Indes).
isostasy
the floating of the lower density plates on the underlying higher density asthenosphere, rather like a block of wood floating in a tank of water.
lithosphere
the crust and upper mantle above the asthenosphere
mantle
the layer of high density rocks between the earth’s crust and its core.
moho
the junction between the crust and the upper mantle. It modifies the velocities of earthquake shock waves owing to differences in density of the crust and mantle.
ocean plate
the relatively thin (5 - 10km) crust underlying the oceans, made of a dense igneous rock called basalt.
ocean trench
deep trough in the ocean floor where one ocean plate is being subducted beneath another, or beneath a continental plate.
paleomagnetism
the results of studies of changes in the earth’s magnetic field over geological time. Magnetic polarity reversals form a symmetrical pattern either side of constructive margins which, when combined with the age of the rocks, is strong evidence for sea-floor spreading and continental drift.
pangea
the supercontinent that broke up eventually to form the continents of today.
plates
large sections of the earth’s lithosphere, separated by plate margins. 7 major plates and 12 smaller ones have been identified.
plate margin
the edges of lithospheric plates marked by concentrations of earthquakes and volcanoes. They may be constructive, destructive or transformational in character.
pyroclasts
airborne volcanic rock particles which form pyroclastic rocks when they fall to Earth and are deposited.
Richter scale
the logarithmic scale used to measure earthquake intensity.
ridge pull
the upthrust action of rising convection currents in the Earth’s mantle creating mid-oceanic ridges.
sea floor spreading
the process by which molten magma wells up from the mantle at constructive (divergent) plate margins to form new basaltic rocks on the ocean floor along a mid-ocean ridge so that the oceanic plates are pushed apart.
seismic activity
earthquakes caused either by friction between moving plates or underground explosive activity.
slab pull
: the down-dragging action of descending convection currents in the Earth’s mantle creating ocean trenches.
subduction
the process occurring at destructive plate boundaries where a dense oceanic plate is forced below a less dense oceanic or continental crust, and is destroyed by melting in the mantle.
tectonic activity
movement or deformation within the earth’s crust, leading to warping, fracturing or vertical movements such as mountain-building.
vulcanicity
volcanic activity such as the eruption of ash and lava.
basal undercutting
undercutting at the foot of a slope by an erosive force such as a river or the sea; also known as sapping. Makes the slope above steep and unstable.
batholith
large dome-shaped igneous intrusion, usually made of Granite.
bedding planes
dividing lines between the layers (beds) of sedimentary rock.
biological weathering
physical or chemical weathering caused by biological organisms; either bio-physical (e.g. action of tree roots) or bio-chemical (e.g. action of urine).
carbonation
chemical weathering process in which dilute carbonic acid converts calcium carbonate in limestone into calcium bicarbonate, which is highly soluble in water.
carboniferous imestone
hard, crystalline form of limestone of Carboniferous age which usually forms areas of high relief.
chalk
white, porous form of limestone; in Britain it is often highly fractured by frost action making it more permeable.
chelation
bio-chemical weathering process in which rain water passes through leaf-litter, acquiring an acidic organic extract (chelating agent) capable of combining with metallic substances, such as iron and aluminium, found in many rocks.
chemical weathering
the decomposition of rocks, usually caused by acids dissolved in rainwater reacting with rock minerals; includes carbonation, hydrolysis, hydration, oxidation (and reduction), chelation and solution.
clay
very fine-grained rock that is impermeable. Clay particles are <0.002mm in size, they originate from the weathering of rock minerals such as feldspar, and are generally composed of combinations of oxygen, hydrogen, aluminium and silica and one or more basic cations (such as potassium).
creep
the slow downslope movement of soil or other loose surface material such as scree, as a result of freezing and thawing or wetting and drying cycles.
cuesta
a hill or escarpment formed of steep scarp slope and gentle dip slope; generally made of tilted sedimentary rocks.
debris flow
type of mass-movement involving fast flowing materials of a wide range of sizes, lubricated by water or, occasionally, by a cushion of compressed air; such flows may cover considerable distances (up to several km).
denudation
the combined effects of weathering, mass-wasting and erosion in lowering the landscape possibly eventually forming a peneplain (almost flat surface).
dilation
pressure-release caused by erosion of overlying rocks causes joints (cracks) to develop parallel to the surface.
dry valley
valley without a river, common on permeable/ porous rocks such as chalk and limestone. Formed as a result of lowered water table or when during a very cold climate, the ground was frozen and impermeable, allowing considerable runoff of meltwater in the spring; now dry since water travels underground.
escarpment
an outcrop of tilted rock strata with a steep (scarp) face in one direction and a gentle (dip) slope in the other, controlled by the dip of the bedding plain (see cuesta).
exfoliation
weathering process in which the outer layers of a rock outcrop are broken, split and peeled-off, often caused by the hydration process, by alternate wetting + drying and/ or the expansion of salt crystals within the rock pores.
falls
type of mass-movement in which weathered rock material suddenly becomes detached from a rock face and falls freely to the ground, to accumulate as scree.
free face
a steep and mainly bare rock face exposed to active weathering and erosion.
freeze thaw weathering
physical weathering process in which water enters joints in rocks, freezes and expands by c. 9%; repeated cycles cause rocks to disintegrate.
frost shattering
the result of freeze-thaw weathering.
growan
an accumulated mass of sandy fragments of weathered granite derived from chemical weathering (break-down of the feldspars). In areas other than Dartmoor in Devon, it may be called gruss.
hydration
weathering process on the boundary between physical and chemical weathering, in which whole water molecules are absorbed by rock minerals, leading to expansion and disintegration. May cause exfoliation and granular disintegration.
hydrolysis
chemical weathering process in which water reacts chemically with certain types of rock minerals (e.g. feldspar) to produce insoluble precipitates (usually clays) and insoluble residue (usually sand).
impermeable rocks
: rocks which do not allow water to pass through them, either because they lack joints or bedding planes, or because they have no pores.
insolation weathering
physical weathering process supposedly caused by the alternate heating of rocks by the sun (= insolation) causing expansion during the day, and severe cooling at night, causing contraction. Laboratory experiments indicate that this process alone is ineffective and that exfoliation/ granular disintegration requires the presence of water, causing hydration or salt crystallisation within the rock pores.
intrusive igneous rocks
rocks formed from the cooling of molten magma underground. Coarse grained.
joints
cracks or partings within a mass of rock caused by stresses such as contraction on cooling, shrinkage on drying out, or the release of overlying pressure.
karst scenery
classic pattern of landforms developed on (Carboniferous) limestones; includes limestone pavement (clints + grykes), swallow holes, dolines, caves, stalagmites, stalactites etc and cockpit karst in tropical regions.
landslide
type of mass-movement in which a large mass of rock/ regolith slides quickly over a rock surface, maintaining contact with it these often occur along a bedding plane.
lithology
the type or character of rocks.
mass movement
general term for the downslope movement of weathered material under the influence of gravity. It includes heave, flow, slides and falls.
mudflow
type of mass-movement in which a mass of weathered rock material, including a high proportion of clay liquefies, perhaps after heavy rain, and flows downhill; sometimes they form distinctive so-called “mud-glaciers”, e.g. above Stair Hole, Dorset.
oxidation
chemical weathering process in which oxygen dissolves in rainwater and reacts with certain rock minerals, especially iron, to form oxides and hydroxides.
periglacial
an area found on the edges of ice sheets and glaciers. Permafrost (permanently frozen ground) is often present. Today it occurs in high latitudes (the tundra of Asia and North America) and at high altitudes. In the last Ice Age southern Britain was a periglacial area.
permeability
the rate at which water is able to pass through a rock, ranging from totally impermeable (e.g. saturated clays) to highly permeable (e.g. weathered limestones).
physical weathering
the disintegration of rock into angular pieces as a result of freeze-thaw, insolation weathering, salt crystallisation, pressure release, wetting & drying processes, and the action of plant roots. Exfoliation, granular and blocky disintegration and slaking helps form scree or talus deposits.
porosity
the volume of pore spaces within a rock or soil; expressed as the ratio of the volume of pore spaces to the total volume of the material; it varies from 25 – 35% for alluvial gravels, to 0.001% for slate.
pseudo bedding planes
structures in igneous rocks which resemble the bedding planes found in sedimentary rocks; usually caused by expansion of the rock on pressure release, resulting from erosion of the overlying rocks.
regolith
weathered rock material of all shapes and sizes overlying bedrock. Forms the parent material for soil.
resurgent stream
a stream that emerges from underground, usually at the base of a limestone outcrop.
rilling
type of mass-movement in which surface runoff creates numerous small channels (“rills”) and carries weathered material downhill, e.g. on coal tips.
rotational slump/slip
type of mass-movement in which masses of surface material moves along a curved (semi-circular) plane; common on cliff-tops in which permeable (e.g. sandy) material overlies impermeable (e.g. clay).
salt weathering/crystallisation
physical weathering process in which water containing salts in solution enters the joints or pores of a rock; when the water evaporates, salt crystals can grow and break the rock apart; occurs commonly on coasts and in hot deserts.
scree
a sloping accumulation of angular rock fragments below a cliff or rock face (free face) produced largely by weathering and subsequent rock falls; also called talus.
sheet wash
type of erosion in which surface debris, such as gravel, sand , silt and clay is moved downslope by overland flow. It is initiated by rain-drop impact, then the loosened debris is moved downslope by water flowing over the surface.
slope profile
the shape of a slope seen in cross-section.
soil creep
type of mass-movement in which cycles of freezing + thawing, heating + cooling, wetting + drying cause surface materials to rise and fall in a downslope zig-zag motion.
solifluction
type of mass-movement in which surface material which has been frozen during the winter thaws, becomes plastic, and flows very slowly over the frozen subsurface in periglacial regions.
tallus
the same as scree. a sloping accumulation of angular rock fragments below a cliff or rock face (free face) produced largely by weathering and subsequent rock falls;
tor
rocky outcrop, often several metres high, usually made of granite or sandstone. Large blocks of rock, in situ, are separated by distinct vertical joints and horizontal bedding planes (pseudo-bedding plane if granite)
weathering
the disintegration and/or decomposition of rocks in situ creating a layer of waste rock (regolith). Weathering can be chemical or mechanical and is often aided by biological factors such as plant roots.
