Gradational Processes

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

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

BASE LEVEL

• Death Valley, 282 ft. b.s.l.

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

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

• Most of these processes occur so slowly that they often go unnoticed

• However processes such as landslides & floods can change the landscape very quickly

AGGRADATION

• involves the building up of the earth surface by the deposition of rock materialsDEPOSITION – occurs when the transporting agent

loses the energy to carry its load and the material is dropped

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

Types of Weathering

MECHANICAL (disintegration)• 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

CHEMICAL (decomposition)• 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

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

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

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

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

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

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

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

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

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)

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

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