rocks and weathering

Question 1

abyssal plain

Answer 1

the deep, largely flat parts of the ocean floor well away from the plate margins; average water depth is about 4km.

Question 2

asthenosphere

Answer 2

partially molten plastic layer in the upper mantle below the lithosphere in which convection currents drive the movements of the overlying lithospheric plates.

Question 3

basalt

Answer 3

dark, microcrystalline igneous rock formed at mid-ocean ridges and by some volcanoes.

Question 4

Benioff zone

Answer 4

the contact zone between a colliding ocean plate and a continental plate in which earthquakes are generated by the friction between the moving plates.

Question 5

collision zone

Answer 5

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).

Question 6

conservative plate boundaries

Answer 6

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.

Question 7

constructive plate margins

Answer 7

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.

Question 8

continental drift

Answer 8

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.

Question 9

continental plate

Answer 9

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.

Question 10

core

Answer 10

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.

Question 11

cratons

Answer 11

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.

Question 12

crust

Answer 12

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.

Question 13

destructive plate margins

Answer 13

lines along which ocean and/ or continental plates are colliding.

Question 14

epicentre

Answer 14

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

Question 15

focus

Answer 15

the point at depth in the Earth’s crust where the dislocation of rocks occurs to cause an earthquake.

Question 16

fold mountains

Answer 16

mountains formed mostly of compressed and folded marine sediments thrust up when two continental plates collide, or one ocean and one continental plate.

Question 17

granite

Answer 17

a coarse-grained, speckled intrusive igneous rock formed in large masses (batholiths), composed of three minerals: quartz, mica and feldspar.

Question 18

island arc

Answer 18

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).

Question 19

isostasy

Answer 19

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.

Question 20

lithosphere

Answer 20

the crust and upper mantle above the asthenosphere

Question 21

mantle

Answer 21

the layer of high density rocks between the earth’s crust and its core.

Question 22

moho

Answer 22

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.

Question 23

ocean plate

Answer 23

the relatively thin (5 - 10km) crust underlying the oceans, made of a dense igneous rock called basalt.

Question 24

ocean trench

Answer 24

deep trough in the ocean floor where one ocean plate is being subducted beneath another, or beneath a continental plate.

Question 25

paleomagnetism

Answer 25

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.

Question 26

pangea

Answer 26

the supercontinent that broke up eventually to form the continents of today.

Question 27

plates

Answer 27

large sections of the earth’s lithosphere, separated by plate margins. 7 major plates and 12 smaller ones have been identified.

Question 28

plate margin

Answer 28

the edges of lithospheric plates marked by concentrations of earthquakes and volcanoes. They may be constructive, destructive or transformational in character.

Question 29

pyroclasts

Answer 29

airborne volcanic rock particles which form pyroclastic rocks when they fall to Earth and are deposited.

Question 30

Richter scale

Answer 30

the logarithmic scale used to measure earthquake intensity.

Question 31

ridge pull

Answer 31

the upthrust action of rising convection currents in the Earth’s mantle creating mid-oceanic ridges.

Question 32

sea floor spreading

Answer 32

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.

Question 33

seismic activity

Answer 33

earthquakes caused either by friction between moving plates or underground explosive activity.

Question 34

slab pull

Answer 34

: the down-dragging action of descending convection currents in the Earth’s mantle creating ocean trenches.

Question 35

subduction

Answer 35

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.

Question 36

tectonic activity

Answer 36

movement or deformation within the earth’s crust, leading to warping, fracturing or vertical movements such as mountain-building.

Question 37

vulcanicity

Answer 37

volcanic activity such as the eruption of ash and lava.

Question 38

basal undercutting

Answer 38

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.

Question 39

batholith

Answer 39

large dome-shaped igneous intrusion, usually made of Granite.

Question 40

bedding planes

Answer 40

dividing lines between the layers (beds) of sedimentary rock.

Question 41

biological weathering

Answer 41

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).

Question 42

carbonation

Answer 42

chemical weathering process in which dilute carbonic acid converts calcium carbonate in limestone into calcium bicarbonate, which is highly soluble in water.

Question 43

carboniferous imestone

Answer 43

hard, crystalline form of limestone of Carboniferous age which usually forms areas of high relief.

Question 44

chalk

Answer 44

white, porous form of limestone; in Britain it is often highly fractured by frost action making it more permeable.

Question 45

chelation

Answer 45

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.

Question 46

chemical weathering

Answer 46

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.

Question 47

clay

Answer 47

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).

Question 48

creep

Answer 48

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.

Question 49

cuesta

Answer 49

a hill or escarpment formed of steep scarp slope and gentle dip slope; generally made of tilted sedimentary rocks.

Question 50

debris flow

Answer 50

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).

Question 51

denudation

Answer 51

the combined effects of weathering, mass-wasting and erosion in lowering the landscape possibly eventually forming a peneplain (almost flat surface).

Question 52

dilation

Answer 52

pressure-release caused by erosion of overlying rocks causes joints (cracks) to develop parallel to the surface.

Question 53

dry valley

Answer 53

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.

Question 54

escarpment

Answer 54

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).

Question 55

exfoliation

Answer 55

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.

Question 56

falls

Answer 56

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.

Question 57

free face

Answer 57

a steep and mainly bare rock face exposed to active weathering and erosion.

Question 58

freeze thaw weathering

Answer 58

physical weathering process in which water enters joints in rocks, freezes and expands by c. 9%; repeated cycles cause rocks to disintegrate.

Question 59

frost shattering

Answer 59

the result of freeze-thaw weathering.

Question 60

growan

Answer 60

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.

Question 61

hydration

Answer 61

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.

Question 62

hydrolysis

Answer 62

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).

Question 63

impermeable rocks

Answer 63

: rocks which do not allow water to pass through them, either because they lack joints or bedding planes, or because they have no pores.

Question 64

insolation weathering

Answer 64

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.

Question 65

intrusive igneous rocks

Answer 65

rocks formed from the cooling of molten magma underground. Coarse grained.

Question 66

joints

Answer 66

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.

Question 67

karst scenery

Answer 67

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.

Question 68

landslide

Answer 68

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.

Question 69

lithology

Answer 69

the type or character of rocks.

Question 70

mass movement

Answer 70

general term for the downslope movement of weathered material under the influence of gravity. It includes heave, flow, slides and falls.

Question 71

mudflow

Answer 71

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.

Question 72

oxidation

Answer 72

chemical weathering process in which oxygen dissolves in rainwater and reacts with certain rock minerals, especially iron, to form oxides and hydroxides.

Question 73

periglacial

Answer 73

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.

Question 74

permeability

Answer 74

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).

Question 75

physical weathering

Answer 75

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.

Question 76

porosity

Answer 76

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.

Question 77

pseudo bedding planes

Answer 77

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.

Question 78

regolith

Answer 78

weathered rock material of all shapes and sizes overlying bedrock. Forms the parent material for soil.

Question 79

resurgent stream

Answer 79

a stream that emerges from underground, usually at the base of a limestone outcrop.

Question 80

rilling

Answer 80

type of mass-movement in which surface runoff creates numerous small channels (“rills”) and carries weathered material downhill, e.g. on coal tips.

Question 81

rotational slump/slip

Answer 81

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).

Question 82

salt weathering/crystallisation

Answer 82

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.

Question 83

scree

Answer 83

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.

Question 84

sheet wash

Answer 84

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.

Question 85

slope profile

Answer 85

the shape of a slope seen in cross-section.

Question 86

soil creep

Answer 86

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.

Question 87

solifluction

Answer 87

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.

Question 88

tallus

Answer 88

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;

Question 89

tor

Answer 89

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)

Question 90

weathering

Answer 90

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.