1. Gradational Processes

2. Landforms result from the uplift caused by tectonic forces AND the processes of gradation that are constantly at work
Uplift is powered by energy from below
Gradation is powered by energy from above

3. BASE LEVEL
The point where a river does not deepen a valley any more but diverts its energy to widening it
Gradational forces work to reduce the land surface to base level
Sea level is the ultimate base level – the lowest level to which a land surface can be eroded

4. Death Valley, 282 ft. b.s.l. BASE LEVEL
Sea level is the ultimate base level however there are some areas that are below sea level
Death Valley, CA
Dead Sea, Israel & Jordan
All the continents would be worn down in 10 – 20 million years without constant uplift.
Dead Sea, 415 metres below sea level

5. Gradation DEGRADATION
Weathering – the wearing away and breaking down of the surface of the lithosphere, influenced by temperature & precipitation
Erosion – the removal of rock debris and organic matter (weathering + transport)
Transportation – the movement of weathered material from one place to another

6. Most of these processes occur so slowly that they often go unnoticed
However processes such as landslides & floods can change the landscape very quickly

7. AGGRADATION
involves the building up of the earth surface by the deposition of rock materials
DEPOSITION – occurs when the transporting agent loses the energy to carry its load and the material is dropped

8. WEATHERING Two major sources for the breakdown of rock materials are:
SOLAR ENERGY – powers chemical activity, the hydrologic cycle, and wind
GRAVITATIONAL ENERGY – (kinetic) brings down moisture as rain and causes stream flow and other downhill movements

9. Types of Weathering MECHANICAL (disintegration)
CHEMICAL (decomposition)
Rock is broken down into smaller particles without any change in composition
Common in arctic & desert environments
Running water, flowing ice, wind
Temperature changes
Plant growth
The rock is decomposed as the chemical properties of the rock-forming minerals are changed
Takes place more rapidly in hot, moist environments
Some rocks are more susceptible to chemical reactions
Water is the most important agent

10. Mechanical - FROST SHATTERING
AKA FROST WEDGING
Occurs when ice expands within cracks in rock and forces them to break apart
Common in temperate & mountainous regions where daily freeze-thaw cycles exist
Result in talus slopes when broken rock fragments fall to the base of a cliff
This is also what causes potholes in our roads

11. Mechanical – Thermal Expansion
The daily cycle of temperature change is thought to weaken rocks in hot, dry regions such as deserts
Extreme changes in temperature of 300C
Heating causes rock to expand
Cooling causes rock to contract

12. Mechanical – Unloading
SHEETING
When igneous rocks are exposed over time by erosion, the pressure of overlying rocks is removed
Slabs of rock break loose and peel off like an onion
EXFOLIATION
Outer layers expand more than the rock below and make a domed shape

14. Mechanical – Organic Activity
Plant roots in search of minerals and water grow into fractures and the roots wedge the rock apart as they grow
Burrowing animals further break down rock by moving fresh material to the surface
Decayed organisms produce acids which contribute to chemical weathering

15. Chemical Weathering - Solution
Certain minerals dissolve in water
Example: halite (common salt), calcite
Carbonic acid is produced when CO2 is dissolved in rain
Or sulfuric acid from acid rain
The acid reacts with the minerals and they are dissolved

16. Chemical Weathering - Hydrolysis
Happens when carbonic acid acts on rocks that contain silicates (the most common mineral group)
Hydrogen ions attack and replace other ions in the crystals
The mineral decomposes without structure

17. Chemical Weathering - Oxidation
Occurs when metallic minerals such as iron react with oxygen to form a reddish-brown oxide (rust)
Many rocks contain iron
Also occurs with sulfide minerals like pyrite
Pale green olivenite coating dark blue azurite within a cavity in limestone

18. Resistance to weathering
Most resistant – quartzite & granite
Medium – basalt, slate, conglomerate
Low – shale, rock salt, marble, limestone
Some rocks are more resistant to weathering than others.
The same kind of rock may weather quite differently in varying climate conditions
Breaking a rock into smaller pieces increases the surface area available for chemical attack

19. Influences on Rates of Weathering
Rock structure – types of minerals, solubility, fractures in rock
Climate factors – temperature & moisture influence weathering rate as well as vegetation cover (more carbonic acid)
Topography – influences the amount of rock exposed to the forces of weathering (i.e. mountainous vs smooth)

20. Differential Weathering
Layers of softer rock wear away more quickly than harder ones
Causes collapse of cliff faces like the Niagara Escarpment
Creates unusual landforms like the hoodoos in Alberta and flowerpot islands in Georgian Bay

21. Conclusion
Mechanical & chemical weathering combine in all environments to wear away the surface of the earth and create spectacular and unique landforms