Weathering: Freeze-thaw, Heating/Cooling, Salt

• Weathering affects rocks and rock formations; it is also known as mechanical weathering.

Freeze-thaw

• Is a weathering process occurring due to freezing temperatures.
• Water enters rock cracks, freezes, and expands by around 10%.
• The expansion puts pressure on the rock, causing it to break apart.

Heating/Cooling Processes

• Causes rocks to break down due to repeated cycles.
• Rocks expand rapidly when hot and contract rapidly when cold.
• The changes in temperature cause stress on the outer layers of rock eventually causing exfoliation.
• Exfoliation is typically caused in hot desert environment when days are extremely hot and nights are extremely cold.

Salt Weathering

• Occurs due to salt crystal growth inside rock cracks and pores.
• Saline solutions enter cracks, evaporate, and leave behind salt crystals.
• Over time, salt crystals accumulate and expand the gaps in rocks, causing them to break or disintegrate.
• Honeycomb weathering is caused by salt crystal growth.

Pressure Release (Dilatation)

• Is caused when rocks under great pressure no longer bear a heavy load, leading to expansion and fracturing.
• When weight is removed (e.g., through erosion), the underlying rocks expand, forming fractures on the surface.
• Occurs when rocks formed under tremendous pressure beneath the Earth's surface are exposed due to erosion.

Vegetation Root Action

• Plant and tree roots grow and exert pressure on surrounding rocks.
• The pressure from growing roots eventaully causes rocks to break apart.

Chemical Weathering

• Chemical reactions cause the breaking down of rocks.

Hydrolysis

• Hydrolysis weathering is the breaking down of rock when it reacts with water.
• Rocks soluble in water, such as chalk, can react and break down.

Hydration

• Happens when water is absorbed into a substance, causing a new substance to form.
• Anhydrite (CaSO4) reacts with water to form gypsum (CaSO4·2H₂O).
• Hydration causes rocks to expand in size and volume, affecting other forms of weathering.

Carbonation

• Carbonation is when CO₂ dissolved in water (carbonic acid) reacts with rocks and breaks them down.
• Carbonic acid reacts with various rocks, including limestone.
• CO₂ dissolves into rainwater from atmospheric emissions or groundwater, creating caves/sinkholes.

Factors Affecting Weathering

• Rate, type, and severity of weathering can be affected by different factors.

Climate

• Temperature and precipitation play an important role in weathering.
• Alpine climates experience more severe freeze-thaw weathering due to rapid temperature fluctuations, more than polar environments.
• Salt crystal growth favors dry climates, allowing water to evaporate and leave salt crystals.

Rock Type and Structure

• Rock type and structure dictate weathering rate and type (physical or chemical).
• Water-soluble rocks are more vulnerable to chemical erosion.
• Permeable rocks with pores are vulnerable to many types of weathering.
• Hard, impermeable rocks may have lower weathering rates due to more energy being required to break the rock.
• Layered sedimentary rocks and rocks with natural weakness areas are more vulnerable to weathering.

Vegetation

• Vegetation affects weathering in multiple ways.
• Root action weathering occurs rapidly in areas with high vegetation, especially trees with large roots.
• Root action is very prevalent in rainforests.

Relief

• Relief can affect the weathering rate and type.
• Gentle relief areas are subject to standing water and water infiltration.
• Steep relief areas experience overland flow, reducing the prevalence of weathering caused by precipitation.

The Peltier Diagram

• Illustrates the relationship between temperature and rainfall and the different weathering types that prevail.
• The diagram shows how weathering changes in severity and type (physical or chemical) when the temperature and precipitation changes.

Slope Processes

• Different processes can occur on slopes that affect slopes in different ways.

Slope Processes Include

• Erosion and weathering that change the landscape.
• The movement of water and sediment down a slope.
• The mass movement of material.

Water and Sediment Movement

• Due to the effect of gravity on slopes, water and sediment moves downhill resulting in rainsplash:
• The impact of rain droplets can displace soil particles.
• Water and displaced soil granules move downhill due to gravity.