Summary of all physical geography work covered in lectures as well as extra notes from the prescribed textbook. Summary includes all necessary images, graphs and tables. Please note these notes were made in 2018 and the course work covered may have changed slightly since then.
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1. PLATE TECTONICS
THE STRUCTURE OF THE EARTH
The earth consists of multiple layers of different composition and different strengths
Layers of different composition:
1. core (iron)
2. Mantle
3. Crust
Layers of different strength:
1. Mesosphere
2. Asthenosphere (hot and weak)
3. Lithosphere (the earth’s crust and the ridged upper part of the mantle)
THE INTERIOR OF THE EARTH
CORE:
- Inner core: solid and very dense, made up of iron and nickel
- Outer core: kept at a semi-molten state
MANTLE:
- Contains largest mass of any of the earths layer’s
- Takes up 24% of the Earth’s volume
ASTHENOSPHERE:
- Outer part of the mantle
- Ridged
- Mostly composed of magnesium-iron silicates.
OUTER LAYERS OF THE EARTH
OCEANIC CRUST:
- Sima
- Consists mainly of basalt
- 6 to 10 km thick
- Temperature of 1200 degrees c
CONTINENTAL CRUST:
- Sial
, - Up to 70km thick
- Along with the top layer of the mantle, the crust is known as the lithosphere.
EVIDENCE OF CONTINENTAL DRIFT
Wegener found evidence based on:
1. Common mesosaurus fossils were found both in South Africa and Brazil
2. Geological evidence; rocks of similar age, type, structure etc. found in South Africa and Brazil and in
North America and Europe
3. Deposits of coal found under Arctic ice caps
4. Glaciation scars in warm areas
Other, less significant evidences:
- Mid-Atlantic ridge
- 1000km wide
- 2.5 km high
- volcanic rocks
- relatively ‘young’ rocks
- Earth’s magnetic pole
- 171 reversals over the last 76 million years
- Sea floor spreading
PROBLEMS WITH CONTINENTAL DRIFT
Although there is evidence that supports continental drift – one can find fault in this evidence
1. Plant seeds could have travelled by other means (e.g. wind, ocean currents) other than directly over
land before the continents split.
2. Continental shorelines are constantly changing and eroding; how good a fit could one actually expect?
3. Fossil animals could have travelled in other ways (e.g. island hopping and when the sea level is low)
PLATE TECHTONICS
Plate tectonics: the study of the movement of plates and their resultant landforms.
MECHANISM FOR PLATE MOVEMENT:
, - The lithosphere (crust) floats on top of the asthenosphere (mantle)
- The lithosphere is made up seven large and more significant plate and multiple, less significant smaller
plates.
- Convective movement of magma in the earth’s mantle causes the plates to move.
PLATE MATERIAL: CONTINENTAL VS OCEANIC CRUST
CONTINENTAL CRUST OCEANIC CRUST
THICKNESS 35-40 km on average (can reach up to 70km 6-10 km on average
under mountains)
AGE OF Very old; mainly over 1500 million years Very young; mainly under 200 million years
ROCKS
WEIGHT OF Lighter; average density of 2,6 Heavier; average density of 3,0
ROCKS
NATURE OF Light in colour; most rocks contain silica and Dark in colour; many contain silica and
ROCKS aluminum; numerous types of rock (granite is magnesium; few rock types (mainly basalt)
most common)
*continental crust can extend beyond land mass
PLATE MARGINS
1. CONSTRUCTIVE MARGINS
- Spreading or divergent plates
- Two plates move away from each other
- New oceanic crust appears forming mid-ocean ridges with volcanoes
- E.g. Mid-Atlantic ridge (Americas moving away from the Eurasian and African plates)
2. DESTRUCTOVE MARGINS
- Subduction zones
- Oceanic crust moves towards continental crust; because the oceanic crust I heavier, it sinks and is
destroyed
- This forms deep-sea trenches and island arcs with volcanoes
- E.g. Nazca sinks under the South American Plate (forms the Andes mountains)
COLLISION ZONES
- Two continental crusts collide
- Being the same density and weight, both are forced up
- This process forms fold mountains
- E.g. African plate collided with the Eurasian plate forming the Alps
3. CONSERVATIVE OF PASSIVE MARGINS:
- Transform faults
- Two plates move sideways past each other
- Land is neither formed nor destroyed
- E.g. San Andreas Fault in California
, *The centres of tectonic plates are ridged
- This results in shields lands (cratons) of ancient worn-down rocks
- As well as depressions on the edges of the shield which then develop into large river basins
- E.g. Canadian (Laurentian) Shield, Brazilian Shield
There is however one main exception to this:
- Africa dividing to form a rift valley and possibly a new sea
- African Rift Valley and the Red Sea
VOLCANIC MATERIAL
BASALTIC LAVA:
- Hot
- Less viscous
- Low silica
- Constructive margins
PYROCLASTIC MATERIAL:
- Solid materials ejected from volcanoes
- Consists of ash, stones and rocks
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