Restless Earth - Case Studies

Knowledge of case studies are key to gaining high grades in GCSE Geog. you need to know these real world examples of the course content and you need to be able to refer accurately and in detail to them in examinations.

The studies here are mainly derived from the core text, but have been added to where information or structure has been deemed shallow or weak respectively.

Where diagrams have been used, these may have been altered from their originals to make them conform to the language or details of the syllabus.

Links to useful web sites have been included for your use/reference.

Case Studies:

Types of plate margins:

Constructive Plate Margins

• Constructive margins typically are found in Oceanic crust (e.g. Mid-Atlantic Ridge)
• Two Oceanic plates separate (move apart) due to the divergent flow of convection currents in the mantle.
• As they separate, magma from the Mantle erupts through either Shield Volcanoes or Fissure Vents to form new oceanic crust (Hence "Constructive" margin).
• Upward pressure and volcanoes elevates the ocean floor into a long ridge of mountains and volcanic cones.
• Due to the separating movement of the crust and its curved form, these margins are often associated with Transverse Faults that run at right angles to the main margin.
• As a consequence of the movement and creation of new oceanic crust at the margin, rocks further from the margin are progressively older and may thus be covered in increasingly thick layers of sediments.
• An example of a Constructive margin is the Mid-Atlantic Ridge where the North American Plate is moving west away from the Eurasian Plate that is moving East.
• The lava extruded along such margins is basic and of low viscosity. It produces Shield volcanoes such as the Hekla system in Iceland.
• Due to the movement and faulting at these margins, earthquakes are likely.

A Constructive Margin

Destructive Plate Margins

• Destructive margins involve the slow collision between denser Oceanic crust and less dense Continental crust.
• As such, they are typical of the zone between continents and oceans e.g. Pacific Ocean and South America where the Nazca plate meets the South American plate.
• Here, because of the higher density of the Oceanic crust, the Nazca plate is forced under the lighter South American.
• Below the margin, in the Subduction Zone, the Nazca plate (and chunks of the South American plate that have been broken off) are destroyed (hence "destructive") by  melting due to pressure and heat  from the Mantle.
• Some molten crust is squeezed up through cracks in the crust and eventually is extruded forming Composite Cone Volcanoes.
• The collision and deformation along Destructive margins creates two further features: Fold Mountains and Ocean Trenches. 
• Trenches result from the downward deformation between the two plates as the Oceanic is subducted and fold mountains result from the squashing, crumpling and buckling of the crusts as they are slowly pushed together. Thus, where you find Fold Mountains, you generally can expect to find Ocean Trenches. Composite Cone Volcanoes, may be found among or adjacent to the fold mountains. Examples in South America's Andes include Cotopaxi, Cayambe and Chimborazu in Equador. Mt St Helens is also a CCV resulting from the collision between the Juan de Fuca oceanic plate and the North American plate (Only a small part of the Juan de Fuca plate now remains as most has been subducted and destroyed).
• The Chile-Peru Trench and the Andes Mountains are associated with the boundary between the Nazca and South American plates.
• In addition to volcanoes, earthquakes are common at these margins because of the movement and fracturing of the crust and due to the movement of magma.

A Destructive Margin

Collision Plate Margins

• When two plates of the same type and thus density collide, there is no subduction. Such boundaries are termed Collision margins.
• The best known example of this is between the Indian Sub-continent and Eurasia and is marked by the Himalayan fold mountains.
• The mountains are formed by the deformation and buckling of the two crustal plates as they slowly "collide".
• Thickening of the crust at these margins means that volcanoes are not present, although earthquakes are likely.

A Collision Margin

Conservative Plate Margins

• As the earth is a sphere experiencing both together and apart movements between plates, there must also be places where movement between plates is side-by-side.
• Such margins are termed Conservative, the best known example being the San Andreas Fault system in western USA (Los Angeles and San Fransisco Bay area).
• Here the Pacific Oceanic plate and the Continental North American plate are both moving toward the north-west, but at different speeds. Consequently, there is relative movement between them.
• The movement causes earthquakes and surface deformation including small hills and the miss-alignment of linear features such as rivers, roads and fences...

A Conservative Margin

Fold Mountains

Example: Andes Mountains, Western South America.
The tectonics of these mountains are covered above (Destructive Plate Margins). This case study outlines way in which humans use and adapt to living in fold mountains.

Human activity in the Andes can be divided into four main types:

1. Farming
2. Mining
3. Hydro-electric power
4. Tourism

Farming

Farming necessitates adaptation to both steep relief and high altitudes. The traditional inhabitants of such places as the Bolivian Andes grow a variety of crops suited to the different conditions...

On the steep slopes - Potatoes, the staple food crop of the area, are grown on terraces. The terraces are steps constructed on the side of mountains that create patches of flat land that make it easier to work the land and harvest. They also serve to retain both soil and moisture that would otherwise be lost downhill due to gravity.


In the lower valleys -
a wide variety of crops are grown including cash crops such as Soybeans, rice and cotton. Subsistence farmers (who produce food for themselves and their families) grow beans, squashes and corn. They also keep livestock such as pigs, goats and sheep. Poultry are to be seen pecking about most homesteads and villages.

The animal most closely associated with the Andes is, of course, the Llama. Llamas are used for...

• Larger males carry loads (pack animals) e.g. building materials, ore from mines, goods for trade etc.
• Females provide milk and meat.
• Llama wool is valuable for making clothes and rugs.

Mining

Fold mountains, due to the effect of pressure, heat and the relocation of soluble mineral salts, are places where valuable metals are often concentrated. these include:

• Tin
• Nickle
• Gold
• Silver

Such valuable resources are important sources of foreign income for Andean LEDCs. Over half of Peru's exports are from mining and the world's biggest gold mine is located near Cajamarca in the northern half of the country...

The Yanacocha gold mine, Peru:

• Biggest gold mine in the world
• Developed as a joint venture between the Peruvian mining company and a US-based one (which holds a 51% share).
• Yanacocha is an open pit mine where the gold-bearing rock is loosened by blasting with explosives and the gold extracted using cyanide. 
• The blasting is destructive and disturbing to the local area
• Cyanide is toxic and has caused problems of pollution (including water supplies).
• The town of Cajamarca has grown rapidly from 30,000 to 240,000 (2005).
• The mine brought new employment opportunities (most jobs have gone to migrants from other parts of Peru).
• Rapid urban growth means that services are inadequate e.g. schools, health care, housing.
• Prostitution has increased (largely male workforce)
• Crime rates have increased.
• Local people have lost access to land from which they used to gather herbs and medicines.
• Fish have been killed or poisoned by chemicals.
• Spillages of toxins such as Mercury have gone unreported and local people have suffered ill health as a result.

A glance through the above list will reveal that mining does not just benefit an area... it can bring a range of environmental and social problems. 

Mercury Poisoning - News article

Aerial View of the Yanacocha Gold Mine, Peru.

Hydro-Electric Power

The Andes are perfect for the generation of HEP for the following reasons:


1. Lack of occupied land - no protests about flooding land.
2. Poor farming potential - not used for growing food etc
3. Steep slopes - provides a rapid fall (flow) of water to produce electricity.
4. Plenty of water - Rain plus seasonal snow-melt (seasonal flow can be more of a problem as the amount of power generated varies too)
5. Deep valleys - good water storage potential
6. Firm foundation - for the construction of dams to hold back the water in the reservoirs.

The Yuncan HEP project dams the Paucartambo and Huachon rivers in north-east Peru

 Tourism

Ocean Trenches
Example: Chile-Peru Trench, Off the west coast of South America. The dark blue stripe indicates the trench.






Composite Cone Volcanoes (also known as Strato-volcanoes)
Example: Mount St Helens, Washington State, USA

Shield Volcanoes
Example: Nyiragongo, Democratic Republic of Congo, Africa.

Super-volcanoes

Example: Yellowstone, Western USA