Chapter 1 gateway 123 combined students

LIVING WITH
TECTONIC 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.

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

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)

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.

Tectonic plates float on molten mantle, driven by heat
energy/convection currents
Plates moving away from each other
Platesmoving
towardseachother
Plates
moving
towards
eachother
Plates
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.

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
boundaries?

• 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.
boundaries?

• 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.
boundaries?

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
boundaries?

• 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
boundaries?

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.
boundaries?

• Earthquakes may also occur.
• E.g. the Pacific Plate converging with the slower-moving
Philippine plate
1. Oceanic–oceanic plate convergence
boundaries?

1. Oceanic–Oceanic plate convergence
boundaries?

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

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.
boundaries?

E.g. the Himalayas
boundaries?
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.
boundaries?

• 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.
boundaries?

boundaries?

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
boundaries?

Transform plate boundaries
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.
boundaries?

E.g. San Andreas Fault, USA
boundaries?

E.g. North Anatolian Fault, Turkey
How would the plate movement affect the two
cities shown in time to come?
boundaries?

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 mountains
The 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

Volcanoes
Distribution 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

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 eruptions
Tourism
• 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.

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