Section 1: Weathering, transport, grain size

Question 1

% reduction of biomass and abundance following dredging

Answer 1

90

Question 2

Particle size before and after dredging

Answer 2

Before: Gravel rich, poorly sorted sediment - species rich assemblage
After: mobile (more easily eroded) sandy sediment - sparse species-poor assemblage

Question 3

Define weathering

Answer 3

The disintegration and decomposition of rock and sediments by mechanical or chemical processes acting at or near the Earth surface in situ

Question 4

Define erosion

Answer 4

The process whereby particles primarily of rock into smaller fragments, each with the same properties as the original. Occurs mainly by temperature and pressure changes

Question 5

Define mechanical/physical weathering

Answer 5

Physical disintegration of rock into smaller fragments, each with the same properties as the original. Occurs mainly by temperature and pressure change.

Question 6

Define chemical weather

Answer 6

Alteration or change in composition of a mineral due to the action of chemical agents

Question 7

Types of mechanical/physical weathering

Answer 7

Frost wedging
Salt wedging
Thermal expansion & contraction
Exfoliation: Spalling of surficial layers due to the release of lithostatic pressure and expansion as rock (esp. plutonic rock) is exhumed. Also occurs as a release of chemical weathering eg along join systems
Root wedging
Frosting: A lustreless ground-glass or mat surface on rounded grains (esp quartz) results from innumerable impacts of other grains during wind action

Question 8

Weatherability series
Most and least stable rocks

Answer 8

Reverse of Bowen’ reaction series for crystallisation from melts
Halite, Olivine least stable
Quartz, Hematite most stable

Question 9

Factors influencing chemical weathering

Answer 9

Mineral composition (weathering susceptibility)
Climate (temperature, contrasts, rainfall)
Living organisms (bioturbation, acid production, mineral decomposition)
Time

Question 10

Chemical weathering products

Answer 10

Clays
Metal ores
Rounding of boulders (chemical exfoliation)

Question 11

Composition of topsoil and subsoil

Answer 11

Top soil: O Horizon: organic debris, humus
A Horizon: humus mixed with minerals derived from bedrock
Subsoil: B Horizon: accumulation of dissolved material and fine clays, hardpan
C Horizon: weathered and broken bedrock

Question 12

Define regolith

Answer 12

Layer of unconsolidated particulate material covering bedrock (also used for bedrock cover of other rocky planets)

Question 13

What is pedalfers

Answer 13

Soil type: developed in humid, temperate regions e.g. central and northern Europe

Question 14

What is pedocals

Answer 14

develop in dry warm regions like the western U S. A horizon is thin. B horizon contains minerals such as calcite that would never precipitate in moister environments. these form nodules of calcite (caliche)

Question 15

What is laterite

Answer 15

oil formed in the tropics where rainfall is so intense all soluble minerals completely leached even silica Rainfall is so heavy, and decomposition so fast that there is no significant O horizon. A horizon may be thick, but lacks soluble materials - typically rich in insoluble iron and aluminium oxides including bauxite (aluminium ore). B horizon is absent

Question 16

Properties of an alluvial fan

Answer 16

Production of sedimentary particles
Initial sorting, angular fragments (not been moved far, less erosion), wide size range (poorly sorted)

Question 17

How are grain sizes measured

Answer 17

Roundness: smoothness of grain surface - changes through abrasion
Sphericity: how closely it approximates a sphere - related to original properties of the grain

Question 18

Lower size boundary of gravel

Answer 18

2mm

Question 19

Lower size boundary of sand

Answer 19

62μm

Question 20

Lower size boundary of silt

Answer 20

4μm (sometimes 2μm)

Question 21

Size boundary of clay

Answer 21

<4μm (sometimes 2μm)

Question 22

When sorting gran size graphically, what do you expect well sorted and poorly sorted to look like

Answer 22

Well sorted: low sigma, low standard deviation eg beach or end of river
Poorly sorted: big sigma, high standard deviation

Question 23

What is skewness

Answer 23

The degree of asymmertry of distribution
Positively skewed: coarse tail
Negatively skewed: fine tail, beach sand, fine particles washed away (windowed)

Question 24

Define platykurtic

Answer 24

Better sorted in tails than central part

Question 25

Define leptokurtic

Answer 25

Better sorted in the central part than tails

Question 26

Direct measurement:
Technique
Size range

Answer 26

Callipers etc
Pebble +

Question 27

Indirect measurement:
Technique
Size range
Method

Answer 27

Sieving
Sand and granule (silt>10μ)
Pre-treatment- wet-sieving
Split into equal sizes (coning or quartering)
Weigh sediment in each pan

Question 28

What concentration should be used in all settling techniques and why

Answer 28

Concentration less than 1%
High concentrations result in local bulk density of fluid being raises and a fluid grain suspension will settle as a mini density current

Question 29

Settling:
Technique
Size range
Method

Answer 29

Sedigraph
silt/(clay)
X-ray retardment
Attenuation of fine fractions
Produced cumulative size distribution from coarse to fine
A measure of settling diameter (fluids ‘eye view’ of settling particle)

Question 30

Particle volume (resistivity):
Technique
Size range
Method

Answer 30

Electrical sensing zone, Coulter Counter
Silt ((clay)(sand))
One electrode inside tube, one electrode out-side, electrolytes, suspended sample, drawn through aperture, changes resistance between the electrode
Produces size and number data
Doesn’t tell you size (volume)

Question 31

Diffraction of light:
Technique
Size range
Method

Answer 31

Laser size analyser
Silt (clay)(sand)
Particles passing through a laser beam will scatter light at an angle that is directly related to their size
Larger particles scatter at low angles with high intensity
Smaller particles scatter at wider angles but low intensity
Can measure size populations, looks and side view (longest side)
Can be used in situ