The Plate Tectonic Theory
- The earth's crust is divided into separate parts called tectonic plates.
- These plates float on the partially molten rocks of the upper mantle.
- The plates move due to convection cells in the mantle.
- Margins are the areas between the tectonic plates. Margins of converging plates are called constructive margins while margins of diverging plates are called constructive margins. Margins where the plates slide against each other are called conservative margins.
- Plates can be oceanic or continental. When an oceanic plate meets a continental plate, it's drawn beneath the margin of the continental plate and the rocks of the oceanic plate go into the mantle. When a continental plate meets another continental plate, folding may occur and fold mountains created.
The History of Continental Drift
- Continents drift due to the movement of the tectonic plates.
- Originally, there was one large continent called Pangea surrounded by a sea called Panthalassa. It is believed Pangea was centered around present day Africa.
- Pangea split into two other continents. The northern continent was Laurasia and the southern continent Gondwanaland. Between the two wad a sea called Tethys.
- Gondwanaland then split into the present continents.
- Folding is the process through which crustal rocks are bent.
- It produces anticlines (upfolds) and synclines (downfolds).
- Folding can produce very large mountains such as the Himalayas and the Rockies.
- Orogenesis is the process through which mountains are created. There are 3 periods of recent mountain building:
- Caledonian Period:310 million years ago. Most mountains created during this period have been destroyed through denudation. Remnants are the Scottish Highlands and the mountains of Scandinavia.
- Hercynian Period:240 million years ago. Examples include the Cape Ranges of South Africa, the Appalachians of the United States and Welsh mountains of the UK.
- Alpine Period:30 million years ago. Examples include the Alps and the Andes. Mountains created during this period are the highest because the have not yet been reduced by denudation.
- Folding takes place in three main stages:
- Initial bending of rocks as plates come together
- Simple Anticlines and Synclines are created.
- Formation of fold mountains-asymmetrical folds.
- Breaking of the folding results in faults/depressions.
- Peneplaination is the reduction of fold mountains to a level plain called a peneplain.
- Vulcanicity is the process through which gases and molten rock are either extruded on the earth's surface or intruded into the earth's crust.
- Magma is the molten rock originating from the upper plastic layer of the mantle. When it gets to the surface and loses its gases, it is known as lava.
- Pyrocrasts are the ashes, cinders and small particles of magma during an explosive volcanic eruption.
Intrusive Igneous Landforms
- Batholiths: Largest mass of magma crumbling in the crust. An example is the Stone Mountain of Georgia.
- Sills: Sheets of magma intruded onto bedding planes of sedimentary rocks.
- Dykes: Wall like features formed when magma cuts across several bedding planes.
- Pipes: The stems of volcanoes.
- Laccoliths: Dome shaped layers of magma formed when magma encounters rock more resistant at its sides than its center.
- Lopoliths: Bowl shaped layers of magma formed when magma encounters rock more resistant at its center than its sides.
Extrusive Igneous Landforms
- Composite Volcano: A cone shaped feature formed by alternating layers of lava and ash. Sometimes, a volcano erupts so violently, its crater is blown off enlarging the top depression. This is then known as a caldera. If filled with water, it is known as a Caldera Lake.
- Lava Plateau: This is an upland with a generally level summit made up off successive layers of lava and ash.
Types of Volcanoes
- Active: This is a volcano that has erupted within the last 500 years and still shows signs of activity.
- Dormant: This is a volcano that has not erupted within the last 500 years but still shows signs of activity such as hot springs. An example is Mt. Kilimanjaro.
- Extinct: This is a volcano that has not erupted within the last 500 years and shows no signs of activity. An example is Mt. Kenya.
- These are processes that take place on the earth's surface.
- They are subdivided in two categories:
- Processes of degradation
- Processes of aggradation
- Processes of aggradation are those that build up material. Processes of degradation are those that destroy what has been built up by endogenetic/aggradation processes.
- The destruction of the landscape is known as denudation. There are three processes involved:
- The exogenetic processes require the use of agents such as running water, ice and wind.
- Weathering is the breaking and decomposition of rocks at/near the earth's surface by physical & chemical processes.
- There are two main types of weathering:
- Chemical Weathering: This involves the decomposition of rocks due to chemical reactions between rock minerals, water and some atmospheric gases such as oxygen and carbon dioxide. The main types of chemical reactions are:
- Hydration: This is the absorption of water by rock minerals.
- Oxidation: This is when oxygen is taken up by a mineral compound.
- Carbonation: this is when hydrogen carbonate ions combine with a mineral to give a soluble compound which can then be carried away in solution.
- Solution: The solution of soluble minerals e.g. limestone in rocks.
- Hydrolisis: The reaction between the hydrogen ions in water and some metal ions.
- Mechanical Weathering:
- Temperature changes causing expansion and contraction of rocks.
- Freezing of water--frost action.
- Wetting and Drying
Factors Influencing the Character and Rate of Weathering
These factors include:
- The greater the amount of rainfall the higher the rate of chemical weathering.
- Temperature changes accelerate the rate of mechanical weathering.
- The nature of the parent rock.
- Vegetation cover
- The gradient of the slope
- Man's activities
Mass wasting is the movement of rock materials under the influence of gravity.
Factors Affecting the Speed of Mass Wasting
These factors include:
- The nature of the material
- The angle of the slope
- Vegetation cover
- Mans activities
Main Types of Mass Wasting
- Soil Creep: This is the downward movement of soil
- Mudflow: When continued heavy rainfall turns the soil into a semi-liquid state.
- Landslide: When large quantities of loosened surface rocks slide down a steep slope.
- Rockfall: When rocks fall off a steep cliff
The Action of Rivers
The Work of Ice