Plates moving away from each other Ocean floor Plates

Magma Convection Current

Plates moving away from each other

Plates

Ocean floor

• Plates move because of convection currents in the mantle

• Heat from Core of Earth causes magma in mantle to expand and rise

• Magma rises, spreads out below plates

• Plates are pulled along with the currents

Magma Convection Current

Magma Convection Current

• The spreading magma cools and sinks

• As magma sinks, plates are dragged towards each other

• Thus, repeated heating and cooling causes plate movements, and thus

• Convergence, divergence and slipping of plates

Rift valleys and block mountains

• Rift valleys and block mountains are formed at divergent plate boundaries.

• A fault is a fracture in the rocks along which the rocks are displaced.

• The tensional forces result in parts of the crust being fractured.

• This process is called faulting.

Rift valleys and block mountains

• A rift valley is a valley with steep sides formed along fault lines.

• E.g. East African Rift Valley

Rift valleys

• It is formed at a divergent plate boundary when two plates move away from each other.

• The central block of land subsides (sinks) between two parallel faults due to tensional forces.

• A rift valley, which is a valley with steep sides is formed.

Rift valleys

• A block mountain is a block of land with steep sides.

Block mountains

• It is formed at a divergent plate boundary when two plates move away from each other.

• When the crusts on either side of the normal faults move apart, they sink and leave the central block standing higher than the rest.

• This forms a block mountain, which is a block of land with steep sides.

Block mountains

Rift valleys and block mountains

The East African Rift Valley

Is formed from the Nubian section of the African Plate and the Somalian section of the African Plate pulling away from one another.

3. Fig. 3 shows a landform.

Rift valleys and block mountains

Identify the landform formed as a result of the plate movement.

Name landform A and explain its formation. [5]

A

3. Fig. 3 shows a landform. Identify the landform formed as a result of the plate movement. Name landform A and explain its formation. [7]

Rift valleys and block mountains

• Rift Valley

• East African Rift Valley

• It is formed at a divergent plate boundary when African Plate move away from Somali sub-plate.

• The central block of land subsides (sinks) between two parallel faults due to tensional forces.

• East African Rift Valley, which is a valley with steep sides is formed.

4. With the help of well-annotated diagrams, explain the formation of a block mountain. [5m]

Rift valleys and block mountains

5. With the aid of well-annotated diagrams, explain the formation of a rift valley. [5]

Rift valleys and block mountains

• An earthquake is a vibration in the earth’s crust caused by the sudden release of stored energy in the rocks found along fault lines.

What phenomena are found at plate boundaries and how are they formed?

• Extent of earthquakes

- The Richter Scale is used to measure the magnitude of an earthquake

Earthquakes

• Apart from its magnitude, the extent of damage caused by an earthquake may vary based on other factors:

- Population density

- time of occurrence

- level of preparedness

- distance from the epicentre

- type of soil

Earthquakes

The distribution of the earth’s major earthquakes

Earthquakes

• Measuring earthquakes

- A seismograph records the seismic waves released by an earthquake.

Earthquakes

Hazards associated with living in earthquakes zones

- Threat of tsunamis

- Disruption of services

- Fires

- Landslides

- Destruction of properties

- Destruction of infrastructure

- Loss of lives

Earthquakes

Threat of tsunamis

• Tsunami refers to an usually large sea wave.

• Tsunamis may be formed by:

- The movement of the sea floor during a large earthquake at subduction zones;

- An underwater volcanic eruption;

- An underwater landslide; and

- A landslide above sea level which causes materials to plunge into the water.

Hazards associated with earthquakes

Hazards associated with earthquakes

• In 2004, a 9.2 magnitude triggered a tsunami with waves spreading throughout the Indian Ocean.

Aceh before and after the tsunami hit the area

Hazards associated with earthquakes

The spread of the tsunami waves caused by the tsunami

Hazards associated with earthquakes

Disruption of services

• An earthquake can disrupt services such as the supply of electricity, gas and water.

• The earthquake in Kobe, Japan, in 1995 disrupted electricity, gas and water supplies to about a million of Kobe city’s 1.4 million residents.

Fire

• Earthquakes may rupture gas pipes and this can provide fuel to start fires.

• For example, the earthquake in Kobe, Japan, in 1995 caused extensive fires.

Hazards associated with earthquakes

Landslides

• Landslides are rapid downslope movements of soil, rock and vegetation.

• Mudflows may also occur when there is heavy rainfall.

Destruction of properties

• Earthquakes can cause destruction to many homes.

• People may be without homes after the disaster.

Hazards associated with earthquakes

Destruction of infrastructure

• Earthquakes may cause cracks to form in infrastructure such as roads and bridges.

• Transportation can be disrupted as it is unsafe to use the damaged roads.

Hazards associated with earthquakes

Hazards associated with earthquakes