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LIVING WITHTECTONIC HAZARDS

a. What is a natural hazard? b. What is the internal structure of the Earth?c. What is a tectonic plate?d. Why do tectonic plates move?e. What are the different types of plate boundaries?

• A natural hazard is a natural event that threatens human lives and causes damage to property.

a. What is a natural hazard?

Distribution of natural hazards


Types of natural hazards:• Tectonic hazards• Climate-related hazards

Impacts of natural hazards: • Large scale• Small scale


• Tectonic hazards are caused by movements in the Earth’s crust.

• Examples:–Earthquakes–Volcanic eruptions–Tsunamis


b. What is the internal structure of the earth?

Core• Composed mostly of iron and nickel • Divided into inner core + outer core

1. Inner core – about 1,200 km thick – solid layer

2. Outer core– about 2,100 km thick– liquid layer

• Temperature between 3,000⁰C and 5,000⁰C


Mantle• Mostly solid rock (flows under high temperature and pressure)• 2,900 km thick • Divided into upper mantle + lower mantle

Upper mantle – a layer of solid rock + asthenosphere – below uppermost mantle– rocks close to melting point, easily deformed

• Temperature between 800⁰C and 3,000⁰C


Crust• Outermost layer on which we live• Oceanic crust is found beneath oceans • Continental crust is found beneath the continents • Thickness ranges from a few km to more than 70 km


• Makes up the Earth’s rigid outer shell• When the rocks in the lithosphere melt,

hot molten rock called magma is formed.

Lithosphere = Crust + Uppermost mantle


• A tectonic plate is made up of the lithosphere (i.e. crust + uppermost mantle).

• The earth’s crust is broken into several pieces of tectonic plates.

• These plates move in relation to one another.• Tectonic plates can be made up of:

– oceanic crust– continental crust or – a combination of both

c. What is a tectonic plate?

Oceanic crust vs Continental crust

Oceanic Crust• Located beneath deep

ocean

• Very thin — between 5 and 8 km

• Denser (e.g. basalt)

Continental Crust• Located beneath land

masses and under shallow seas

• Very thick — between 30 and 60 km

• Less dense (e.g. granite)

c. What is a tectonic plate?

1. Convection currents•Convection currents are movements of heat within the mantle.•Material in the mantle is heated by the core.•This causes convection currents in the molten mantle material.•Mantle expands, rises and spreads out beneath the plates.•Plates are dragged along and move away from each other.•Subsequently, the hot molten mantle cools slightly and sinks, pulling the plates along •Hence plates move towards each other. •The sinking mantle material heats up again as it nears the core and the whole process repeats.

d. Why do tectonic plates move?

Tectonic plates float on molten mantle, driven by heat energy/convection currents

Plates moving away from each other

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Ocean floor


2. Slab-pull force• This occurs when an oceanic plate (denser) subducts

under a less dense plate and pulls the rest of the plate along.• The subducting plate drives the downward-moving

portion of convection currents.• While mantle material away from the subduction zone drives the rising portion of convection currents.


Changing positions of the earth’s continents• Plate movements have altered the distribution of the earth’s land masses over several hundred million years.


e. What are the different types of plate boundaries?

Types of movement

Types of plates Landforms Examples

Divergent - plates move away from each other

Oceanic-oceanic plate divergence

Oceanic ridges Mid-Atlantic Ridge

Continental-continental plate divergence

Rift valley, volcanoes The Great Rift Valley, Cascade Range

Convergent - plates move towards each other

Oceanic–oceanic plate convergence

Oceanic trenches, volcanoes, island arc

Mariana Trench, Mount Etna, Mariana Islands

Continental–oceanic plate convergence

Ocean trenches, mountain ranges

Sunda Trench, Barisan Mountains

Continental–continental plate convergence

Mountain ranges Himalayas

Transform - plates move past each other

Continental-continental plates sliding past each other

None St Andreas Fault, North Anatolian Fault

1. Oceanic-oceanic plate divergence


1. Oceanic-oceanic plate divergence


• Magma rises from the mantle to fill the gap between the plates as they diverge.

• New sea floor is formed when the magma cools and solidifies. This process is called sea-floor spreading.

• Magma rises at the zone of divergence/spreading zone to form a ridge of new ocean floor called mid-oceanic ridge.


• The newly formed (youngest) rocks are closest to the middle of the ridge/plate boundaries.

• At various points along the ridge, magma builds up above the ocean to form volcanic islands.

• E.g. the Mid-Atlantic Ridge is found in the middle of the Atlantic Ocean cutting across Iceland, a volcanic island.


2. Continental–continental plate divergence• Can result in the breakup of continents • E.g. Great Rift Valley (East Africa)

– a lowland with steep sides and flat valley floor– formed by Somalian boundary of the African Plate

moving away from the Nubia plate boundary of the African Plate

– 6,000 kilometres long – between 30 to 100 kilometres wide– Evidence of tectonic activity: active volcanoes and

earthquake fractures found


• Can result in the formation of linear sea• E.g. Red Sea and Gulf of Aden near the Great

Rift Valley – Elongated/linear shape– 1,900 km long– 300 km at its widest stretch– Average depth of 500 m– Evidence of tectonic activity — formation of new

volcanic island in Red Sea


1. Oceanic–oceanic plate convergence• When two oceanic plates converge, one subducts

under the other. • A subduction zone forms, creating a deep oceanic

trench.• The subduction of the oceanic plate causes the solid

mantle material to melt and magma is formed.• The magma rises through the mantle and ocean floor

to emerge as volcanoes.• Eventually a chain or arc of islands called island arc is

formed.


• Earthquakes may also occur.• E.g. the Pacific Plate converging with the slower-moving

Philippine plate

1. Oceanic–oceanic plate convergence


1. Oceanic–Oceanic plate convergence


Locate the Indonesia Archipelago and Japan. Explain the shape and distribution of these islands.


2. Continental-continental plate convergence

• Plates made largely of continental crust may collide with other plates made largely of continental crust.

• However, both plates have similar densities and hence, resist subduction.

• Instead, the plates break, slide along fractures in the crust and fold, forming fold mountains.

• E.g. the Himalayas - convergence of the Indian Plate and the Eurasian Plate.


E.g. the Himalayas


Continental crust

Continental crust

Himalayas

Tibetan Plateau

Uppermost mantle

Asthenosphere

EURASIAN PLATE

INDIAN PLATE

3.Oceanic-continental plate convergence• When an oceanic plate meets a continental plate, the

denser oceanic plate subducts under the less dense continental plate.

• A subduction zone forms, creating a deep oceanic trench along the plate boundary.

• The subduction of the continental plate causes the soild mantle material to melt and magma is formed.


• The magma rises through the mantle and crust to emerge as volcanoes on land.

• The edge of thick continental plate buckles to form fold mountains.

• Earthquakes may also occur.• E.g. the Australian Plate subducting under a section

of the Eurasian Plate near Sumatra formed the Sunda Trench.

3.Oceanic-continental plate convergence






Transform plate boundaries• Plates slide past each other. • As they do so, tremendous stress builds up. • This stress is eventually released, often as a violent

earthquake. • E.g. San Andreas Fault, United States of America &

North Anatolian Fault, Turkey


Transform plate boundaries


E.g. San Andreas Fault, United States of America

• In 1906, an earthquake occurred in San Francisco, southern California between the Pacific Plate and the North American Plate. – This caused several hundred kilometres of

North American Plate to move an average of 2.5 m,

– and at one point almost 7 m all in less than 1 minute.


E.g. San Andreas Fault, USA


E.g. North Anatolian Fault, Turkey

How would the plate movement affect the two cities shown in time to come?


a. Why are different landforms found at different plate boundaries and how are they

formed?

• The movement of plates at different plate boundaries can result in various landforms such as:- Fold mountains

- Rift valleys and block mountains

- Volcanoes

Fold mountains• Over millions of years, the folding of rocks creates a

landform called fold mountains. • The Himalayas, the Rocky Mountains and the Andes

are examples of fold mountains.

• Fold mountains are formed along convergent plate boundaries.

• The compressional force causes the layers of rocks to buckle and fold.

• This process is known as folding.

Fold mountains

• The upfold is called the anticline and

• The downfold is the syncline.

• When there is increasing compressional force on one limb of a fold, the rocks may buckle until a fracture forms.

• The limb may then move forward to ride over the other limb

Fold mountains

Fold mountainsThe Himalayas

Peak : Mount Everest (between Nepal and Tibet)Elevation : 8,848 metres

• Fold mountains are located along convergent plate boundaries

Fold mountains

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.


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

• E.g. East African Rift Valley


• A block mountain is a block of land with steep sides. It is formed when sections of the crust extend along fault lines and rock masses surrounding a central block sink due to tensional forces.



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.

• Distribution of rift valleys and block mountains


Volcanoes

• A volcano is a landform formed by magma ejected from the mantle onto the earth’s surface.

Mount Saint Helens volcano before and after an eruption in May 1980

• Magma is molten rock found below the earth’s surface.

• Parts of a Volcano

- magma chamber

- vents

• Magma that is ejected onto the surface is known as lava.

Volcanoes

Volcanoes

Formation, structure of, and features of a volcano

Shapes and sizes of volcanoes

• Volcanoes vary in shapes and sizes due to the characteristics of the lava.

• Viscosity refers to the stickiness of the lava

• The most common types of volcanoes are:

- Shield volcanoes; and

- Stratovolcanoes

Volcanoes

Shield volcanoes• Shield volcanoes have gently sloping sides and a

broad summit• E.g. Mount Washington, United States of America

Volcanoes

Stratovolcanoes

• Stratovolcanoes develop from successive eruptions of lava and ash.

• E.g. Mount Mayon, Philippines

Volcanoes

VolcanoesDistribution of earth’s major active volcanoes

• 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.

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

• 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

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


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.

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.

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.


Loss of lives• Earthquakes and their associated hazards often

threaten the lives of those living in earthquake zones.


Volcanic eruptions- Occurs on land occur on the sea floor.

Active, dormant or extinct• Active volcanoes refers to volcanoes which are currently

erupting or are expected to erupt in the future. • Dormant volcanoes are currently inactive but may erupt

in the near future.• Extinct volcanoes refers to volcanoes without current

seismic activity

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

Risks of living near volcanic areas

Some of the risks associated with living near volcanic

areas include:• Destruction by volcanic materials• Landslides• Pollution• Effects on weather

Volcanic eruptions

Destruction by volcanic materials• Volcanic materials can lead to widespread damage of

property.

Volcanic eruptions

Landslides• Landslides can occur due to the structural collapse of a

volcanic cone.• Obstruct the flow of rivers which causes floods, • block roads, and • bury villages and farmlands.

Pollution• Ash particles may block sunlight, suffocate crops, and

cause severe respiratory problems for people and

animals.• Release of gases may be harmful to people.

Volcanic eruptions

Volcanic eruptions

Effects on weather

• Sulphur dioxide released from volcanic eruptions has impacts on the environment

• It may react with water vapour and other chemicals in the atmosphere to form sulphur-based particles.

• These particles reflect the sun’s energy back into space and temporarily cool the earth for periods of time.

Volcanic eruptions

Benefits of living near volcanic areas• Fertile volcanic soil• Building materials, and precious stones and

materials• Tourism• Geothermal energy

Volcanic eruptions

Pompeii was partially destroyed and buried under 4 to 6 m (13 to 20 ft) of ash and pumice in the eruption of Mount Vesuvius in CE 79.

Volcanic eruptionsTourism

• Volcanic areas offer a variety of activities for tourists to engage in.

• The ruins of Pompeii, Italy, is one such example.

Geothermal energy

• Geothermal energy is derived from the heat in the earth’s crust.

• The hot water or steam can be harnessed to produce electricity.

Volcanic eruptions

Fertile volcanic soil

• Lava and ash from the volcanic eruptions break down to form fertile volcanic soils.

• Favourable to agriculture

Volcanic eruptions

Precious stones and minerals, building materials• Volcanic rocks can be rich in precious stones and

minerals. • These resources can only be from a volcanic area after

millions of years.• An example is diamond.

Education

Chapter 1 gateway 123 combined students