3.2 Weathering

Physical weathering processes: freeze-thaw, heating/ cooling, salt crystal growth, pressure release, and vegetation root action.

Define weathering

Weathering is the physical or chemical breakdown of rocks in situ on earths surface.

• no transport is involved in weathering processes

• Moisture usually present

Not involve movement of rock material.

What are the 3 types of weathering?

1. Physical/ mechanical weathering

2. Chemical weathering

3. Biological weathering (plants and animals)

1. Physical/mechanical weathering

   • what does it involve?

   • Examples

• the physical disintegration and reduction of side of rocks without changing their chemical composition

Pressure release (exfoliation)

Rocks under great a mount of pressure.

Removal of weight (through erosion of above rocks) mean underlying rocks expand as pressure released → fractures to form on rock surface.

Freeze thaw

Water enter cracks of rocks - freezing temps - freezes - expands 9% →

melting temp → more eater → freezing temp etc.

Cycle puts pressure on rock.

Causing it shatter and break off.

Creates angular pieces of broken rock called scree.

Heating/cooling processes (insolation)

Onion skin/ spheroidal

repeated heating and cooling of rocks - rocks broken down and weathers away.

Rocks expand rapidly when hot

Contract rapidly when cold

P]break apart layers of rock.

Hot desert environments: days extremely hot and night extremely cold. Change in temps cause stress on outer layers of rock. Outer layer peel off in process called exfoliation.

• no moisture

• Diurnal temps

→ granular disintegration

→if minerals with different heat conductivities are involved.

Temp changes heating / cooling - granular disintergration

• formed by same process of heating and cooling

• Breaks down to little grains

• Different colour mineral absorb and emit different rates

Salt wedging

• Due to salt crystal growth inside cracks and pores in rocks

• When slain solutions get into cracks and evaporates - leaves behind salt crystals

• Salt crystals accumulate over time

• Build up pressure and expand gap in rocks

• Eventually causes rocks to break off and disintegrate

Vegetation root action

Vegetation grows, such as plants and trees, roots grow and thus need more space.

Growing roots exert pressure onto surrounding rock, eventually causing the rocks to break apart.

What processes are under chemical weathering

• hydrolysis

• Hydration

• Carbonation

• Oxidation

• Solution

When is chemical weathering most likely to occur

Warm and wet (moist)

High temps - increase rate of reaction

Water for hydrolysis and hydration etc needed for chemical reactions

Explain hydrolysis

Breakdown of rock by acidic water to produce clay and soluble salts I.e granite

Feldspar react with acid water to for, kaolin (china clay), silic acid and potassium hydroxyl.

China clay kaolin end product

Explain hydration

Water is absorbed - causing new substance to form. causing rocks to expand in size and volume - so then disintegrates.

Anhydrite (CaSO4) absorbs 2 water molecules water to form gypsum (CaSO4.2H20)

Explain carbonation

CO2 dissolved in water (carbonic acid) reacts with rocks and breaks them down.

affects limestone

CaCO3 + H2CO3 → Ca(HCO3)2 (calcium carbonate)

Limestone - Karst.

Explain oxidation

Oxygen and water react with iron-rich minerals and weaken the structure of the mineral

Take on rusty colour

Explain solution as type of chemical weathering

Minerals in rock dissolve directly into water

Tell me the different types of biological weathering

• organic activity from lichen and algae

• Rock disintegration by plant growth

• Burrowing and tunneling organisms

• Secretion of acids

Explain the biological weathering by lichen, algae and decaying plants

Bio-chemical weathering.

Growing of these plants exert small amount of abrasion .

Can increase temperature - increase chemical weathering (every 10C increase weathering doubles)

Explain how plant roots cause weathering

• penetrate into cracks and cause rock to split and break into smaller particles through mechanical weathering

Explain how organism activity is a type of biological weathering (chemical and physical too)

• animals burrow cause them to break and disintegrate

• Secret acids that chemically dissolve rocks

Explain chelation as type of biólogical weathering

Bio-chemical process - secretion of humid acid to decompose minerals and rocks by removal of cations.

State the factors affecting weathering

• climate

• Rock type and structure

• Relief and aspect

• Vegetation

• Animals

Explain how climate affects rate of weathering and rate

(Peligres diagram)

• hot temps and precipitation = chemical weathering

• Cold and fluctuating temps = freeze-thaw - mechanical weathering

• Hot diurnal temps = exfoliation, heating and cooling physical weathering

Explain how rock type and structure affect type and rate of weathering

(Granite and limestone)

different rocks composed of different chemicals so susceptible to different weathering

• granite: hard and non-porous so less susceptible to chemical weathering

• Limestone: porous and chemical weathering due to carbonation and reaction with acids (composed of calcite)

• Layered sedimentary rocks with cracks more susceptible to weathering

How does vegetation affect type and rate of weathering

Root action weathering higher rates with higher vegetation.

Where tree roots decay humid acid is produced = biochemical weathering

Lichen and moss growth

How does relief and aspect affect type and rate of weathering

Steep relief = overland flow so weathering reduced

Gentle relief = more susceptible weathering as standing water.

Peak of mountain = fluctuating temps and cold = freeze-thaw

Base = warm and accumulation of moisture so chemical weathering

Aspect - slope facing sun = more vegetation = more biological weathering

PELTIERS DIAGRAM - LOOK AT IT