Weathering and Soils

Question 1

What is Weathering?

Answer 1

The break up (physical disintegration) and decomposition (chemical alteration) of solid rock by surface processes

Solid rock → loose unconsolidated sediment and/or dissolved ions (weathering does not involve transport)

Question 2

where does weathering occur?

Answer 2

at interface: solid earth meets hydrosphere, atmosphere, biosphere

Question 3

Why is weathering important?

Answer 3

Provides sediment
-Contributes to soil formation
-Wears down mountains built by plate tectonic processes
-Shapes landforms
Important role in carbon cycle (reactions consume CO2, produce carbonate)
Some weathering processes (e.g. of mining waste piles) lead to toxic run-off

Question 4

what are some products of weathering?

Answer 4

Loose debris (regolith)
Secondary minerals
Ions in solution
Weathering also leads to rounded surfaces and is essential for most landform & landscape development

Question 5

what are the two types of weathering processes?

Answer 5

1. Physical (disintegration → smaller pieces)
–no change in chemistry
–increases surface area
2. Chemical (decomposition/alteration)
–breaking of bonds
–reactions alter or ‘stabilise’ minerals

both processes often work simultaneously
water is usually required

Question 6

Physical/mechanical weathering increases the surface area exposed for …

Answer 6

chemical weathering

Question 7

what are the two main processes of physical weathering?

Answer 7

(1) Ice wedging

2) Pressure unloading (exfoliation

Question 8

what is ice wedging?

Answer 8

Water seeps into fractures and other openings
Freezes → expands volume by 9%
–exerts huge amount of stress…
–wedging, heaving, shattering
Significant in climates with freeze-thaw cycles – with each freeze, the rock is wedged further apart

Question 9

what is pressure unloading?

Answer 9

Rocks formed at depth expand & ‘relax’ when exposed at surface
‘Unloading’ → exfoliation
Sheeting, exfoliation
Rock layers break off like an onion

Question 10

what is chemical weathering?

Answer 10

Breakdown of minerals through chemical reactions with the atmosphere or hydrosphere

Question 11

what are the three main processes of chemical weathering?

Answer 11

(1) Dissolution
(2) Hydrolysis
(3) Oxidation

Question 12

what are the products of chemical weathering?

Answer 12

soluble ions (e.g.Ca2+)
new secondary minerals e.g. clays, oxides

Question 13

what is dissolution?

Answer 13

Minerals completely dissolve into soluble ions in the presence of water
e.g. calcite dissolves in weak acid

Question 14

what is hydrolysis?

Answer 14

minerals react with water
H+ ‘attacks’ and replaces other cations in mineral
→ alteration to form new clay minerals

Question 15

what is oxidation?

Answer 15

Iron (Fe) bearing minerals react with oxygen

e.g. rusting of solid iron → iron oxide

Question 16

What minerals does oxidation affect?

Answer 16

Fe-bearing silicate minerals e.g. olivine, pyroxene, biotite, garnet
Sulfide minerals e.g. Pyrite FeS2 (when exposed to oxygen and water → sulfuric acid (H2SO4) + iron & hydrogen ions → acid mine drainage)

Question 17

what is biological weathering?

Answer 17

(physical/chemical)
Tree roots: wedge into rock fractures → growth exerts sufficient pressure to split rock
Animal burrowing, boring
Lichens: secrete acidic compound

Question 18

How does weathering shape landforms?

Answer 18

All landscapes experience weathering
Landforms result from differential weathering
–at the mineral scale up to rock outcrop scales
–more resistant ≈ ridges;
–weaker ≈ valleys

Question 19

what creates karst topography?

Answer 19

dissolution of carbonate

e.g. Limestone pavement, W. Ireland or Karst towers, S. China

Question 20

What controls weathering rates?

Answer 20

How susceptible is the rock?

Do the conditions promote weathering?

Question 21

what are the primary controls of weathering rates?

Answer 21

rock type & mineral composition

climate (temperature, precipitation)

Question 22

what are the secondary controls of weathering rates?

Answer 22

rock texture (grain size, porosity)
topography (relief, aspect, drainage)
vegetation & soil chemistry

Question 23

chemical weathering is strongest with…

Answer 23

high precipitation

high temp.

Question 24

physical weathering is strongest with…

Answer 24

moderate precipitation

low temp.

Question 25

What is Soil?

Answer 25

Thin veneer covering most land surfaces
Soil develops on regolith (rock and mineral fragments produced by weathering)
Interface in the Earth system: bridge between life and rocks
Supports the growth of plants and people

Question 26

how is Soil used as a resource

Answer 26

Medium for plant growth
Ecosystem habitat
Nutrient recycling
Water storage & filtration
Engineering medium

Question 27

What is soil made up of?

Answer 27

45% mineral matter
25% air
25% water
5% humus, decayed remains of plants & animals

Question 28

what are the controls on soil formation?

Answer 28

Parent Material
Climate - temperature and precipitation
Time
Topography - thin soils on steep slopes
Plants and animals - supply organic matter to the soil, fungi and bacteria active in decay

Question 29

what are the Soil horizons (O,A,E,B,C)

Answer 29

O: organic layer (humus) - decomposition of organic matter
A: organic and mineral matter - High biologic activity
E: zone of eluviation & leaching - Little organic matter
B: mineral ‘subsoil’…zone of accumulation
- deposition of clays, oxides, carbonates from A
- commonly red or brown in colour
- little organic matter – often rich in clays
C: Weathered parent material

Question 30

soil profiles are dependent on…..

Answer 30

climate

Question 31

what is an example of a soil profile in a cool and moist climate?

Answer 31

BC, Quebec & Maritimes: Coniferous forests
Strong leaching
O, A, E, B, C

Question 32

what is an example of a soil profile in an arid climate?

Answer 32

AB, SK: prairie ecosystems

O, A, B, C