tectonics 2

What are earthquakes?

• the shaking of the Earth’s ground due to the

  • sudden release of energy in the Earth’s lithosphere

• earthquakes generally occur along plate boundaries

  • which contain faults 

How do tectonic processes result in an earthquake?

1) Rock masses on either side of a fault are pushed by

• tectonic forces

2) Friction causes them to get locked and

• stress builds up

3) When the stress built up exceeds the strength of the fault

• the rocks snap or

• suddenly move to a new position

4) This sudden movement causes seismic waves to be

• released

  ∴ results in ground shaking

Shaking is generally felt most strongly at the

• epicentre (first line of impact)

What are the features of an earthquake? (FYI check TB)

Focus

• the point in the Earth’s crust where seismic waves are released

Epicenter 

• the point on the Earth’s surface directly above the focus 

What is the relationship between an earthquake and the focus + epicentre?

Focus of an earthquake

• may be deep in the crust or

• shallow near Earth’s surface

  • generally, shallow-focus earthquakes do the most damage

    → cuz of their closeness to the surface

    → 0-70km (distance from focus to epicentre)

  • a deeper focus results in

    • weaker earthquake experience on Earth’s surface

    • seismic waves take longer to travel

      → would have lost the majority of their energy by then

      → 70-700km

      ∴ less damage

How are earthquakes measured?

via 

• seismometers 

  • sensitive instruments that detect ground vibrations 

    → determining the magnitude of an earthquake

• the greater the seismic energy released during an earthquake

  • the greater the magnitude 

scales used to rate the magnitude of earthquakes

• Richter Scale (ML)

• Moment Magnitude Scale (Mw)

How is the Richter Scale used to rate the magnitudes of earthquakes?

calculates earthquake magnitude using the

• height of the largest wave recorded on seismometers

∴ earthquake magnitude is measured based on (FYI)

• maximum seismic intensity reached

• rather than total seismic energy released through the entire earthquake

the scale is

• numbered from 1 - 10

  • 10 being the largest magnitude

• logarithmic

  • earthquake of magnitude 6 releases about 32x more energy than magnitude 5 earthquake

  • magnitude 1 - frequent earthquakes

  • magnitude >6 - powerful but less frequent earthquakes

Limitations

• rates earthquakes with a single drastic spike in wave energy as having a higher magnitude

  • than a long earthquake with many large, intense waves

∴ underestimates longer earthquakes which

• releases more overall energy

by rating them as having lower magnitudes though they are

• likely to do more damage

∴ The Richter Scale is no longer commonly used

• except for small, local earthquakes

TLDR - Richter scale measures local magnitude of earthquakes

How is the moment magnitude scale used to rate the magnitudes of earthquakes?

rates earthquake magnitude based on the 

• total energy released during the earthquake

• estimates the total energy released during an earthquake 

∴ generally more accurate in measuring earthquakes of magnitude ≥8 (than the Richter scale)

• scientists have adjusted the magnitudes of past earthquakes

  • which were initially measured by the Richter scale

the scale is 

• logarithmic

  • an earthquake of magnitude 6 releases about 32x more energy than a magnitude 5 earthquake

  • magnitude 1 - very frequent earthquakes

  • magnitude >6 - very powerful but less frequent earthquakes

How do tectonic processes result in a volcanic eruption?

volcano

• a landform created when lava erupts on the Earth’s surface

• may form a cone-shaped mountain as more lava

  • erupts and accumulates 

    → over time 

volcanic eruptions occur at

• divergent plate boundaries

• convergent plate boundaries 

How do volcanic eruptions occur at divergent plate boundaries?

1) Plates move apart

• the crust stretches and 

• fractures develop

2) The decrease in overlying pressure causes

• parts of the underlying mantle to melt

  → forming magma 

3) Magma contains dissolved gases and is 

• less dense than the

  • surrounding materials 

4) Hence, magma rises through weak areas in the crust to Earth’s surface to 

• erupt as lava

  • causing a volcanic eruption 

5) The lava cools, solidifies and accumulates over time

• forming a volcano 

How do volcanic eruptions occur at convergent plate boundaries?

1) Plates move towards each other

• the denser plate subducts under the other

2) As the subducting plate sinks into the mantle

• the high-pressure forces water out of the oceanic crust

• water lowers the melting point of the overlying mantle 

  → causing it to melt

  • forming magma 

3) Magma contains dissolved gases and is

• less dense than the 

  • surrounding materials

4) Hence, magma rises through weak areas in the crust to Earth’s surface to

• erupt as lava

  • causing a volcanic eruption

5) The lava cools, solidifies, and accumulates over time

• forming a volcano 

How is the explosivity of volcanic eruptions measured?

determined by how easily dissolved gases can

• escape from magma

  • can’t escape = explosive eruptions

  • escapes easily = effusive eruptions

and by

• magma viscosity

  • high viscosity = explosive eruptions

  • low viscosity = effusive eruptions 

What are the characteristics of high silica magma?

Magma is of

• high viscosity

• as magma rises towards Earth’s surface

  • dissolved gases in the magma

    → cannot escape easily

  • more pressure builds up until

    • gases escape explosively 

  ∴ volcanic eruptions are explosive and violent

• stratovolcanoes 

Exception

• however, there are volcanoes with viscous magma but

  • they don’t result in

    → explosive and violent eruptions

  • because the magma rises in a way that

    • enables the dissolved gases to escape

What are the characteristics of low silica magma?

Magma is of

• low viscosity

• as magma rises towards Earth’s surface

  • dissolved gases in the magma

    → can escape easily

  • less pressure builds up

  ∴ volcanic eruptions are gentle and effusive

• shield volcanoes

How do stratovolcanoes form?

• convergent plate boundaries

1) High viscosity magma rises through weak areas in the crust to Earth’s surface

• erupts explosively as

  • lava, ash and rocks

2) The ash and rocks settle on the sides of the volcano and are later

• covered by the lava

3) Over successive eruptions

• a tall volcano develops

  • consisting of alternating layers of ash and lava

4) The volcano has 

• steep sides

• narrow summit 

  → as the highly viscous lava travels a shorter distance 

  • before cooling and solidifying 

How do shield volcanoes form?

• divergent plate boundaries

1) Low viscosity magma rises through weak areas in the crust to the Earth’s surface

• erupts effusively

2) Over successive eruptions

• a volcano develops 

  • consisting of layers of lava

3) The volcano has

• gently sloping sides 

• broad summit

  → as the less viscous lava travels a longer distance

  • before cooling and solidifying 

How are volcanic eruptions measured?

The Volcanic Explosivity Index (VEI)

• measures the magnitude of volcanic eruptions based on

  • explosivity

    via the following criteria:

1) Volume of Ejected Material

• larger vol. of ejected material

  → higher VEI

2) Height of the Eruption Cloud

• higher height of the eruption cloud

  → higher VEI

3) Duration of Eruption

• longer eruption

  → higher VEI

Scale

• 0 - 8

  • usually shield volcano VEI rating ‘0 - 1’

• logarithmic 

  • an increase of VEI ‘1’ indicates eruption is 

    → 10x more powerful than the previous rating

How are earthquakes distributed?

occurs along

• all types of plate boundaries

  • more common at convergent PB at subduction zones

    → more stress is built up during subduction

• largest concentration of earthquakes is at the

  • Pacific Ring of Fire

e.g., 

Broad belt of earthquakes 

• along convergent PB 

  • in the Pacific Ring of Fire

Narrow belt of earthquakes

• along divergent PB 

  • at the Mid-Atlantic Ridge

• along the transform PB 

  • at the San Andreas Fault

• exception

  • there are some isolated earthquakes

    • occurs away from PB

    • such as within the Eurasian Plate

How are volcanoes distributed?

generally located near

• convergent PB

  • e.g., found along subduction zones in the

    • Pacific Ring of Fire

• divergent PB

  • e.g., found between NA and Eurasian plates

  → at these PBs, magma rises to the surface

  • forms volcanoes

• found away from PBs

  • an exception

    → hot spot volcanoes

Volcanoes not found at

• transform PB

• C-C convergent PB

  → cuz at these PBs, magma does not rise to the Earth’s surface

  • no volcanoes formed

How are tectonic hazards distributed? (1/2)

Earthquake Hazards

• ground shaking

• soil liquefaction

• landslides

• tsunamis

distribution

• ground shaking, soil liquefaction, landslides

  • often localised

  • generally located within the same geographical region as the earthquake

• as earthquakes are found along PBs

  • its hazards too are found there

• however tsunamis are not localised

  • may spread beyond the geographical regions

    • travelling far from where the earthquake originated from
      → without great loss of energy

How are tectonic hazards distributed? (2/2)

Volcanic Eruption Hazards

• tephra

• volcanic gases

• lava flows

• pyroclastic flows

• lahars

• volcanic landslides

  • all found around the volcanoes except ‘tephra’

  • volcanoes found along convergent + divergent PB

    → hazards found generally around there too

• however ‘tephra’ is not localised

  • may spread beyond the geographical regions

    • travelling far from where the volcanic eruption originated from

How does ground shaking caused by Earthquakes affect the natural and human systems?

Occurs when

• seismic energy is released from an earthquake

  • causing violent vibrations on land

Impacts on Natural Systems

(a) Destroys ecosystems

• oil + chemical factories

  • ruptured

    → land + water pollution

• trees

  • uprooted

  • fractured

    → widespread tree injury + death

    → damages wildlife habitats

Impacts on Human Systems

(a) Destroys Properties and Infrastructure 

• buildings, bridges, roads, and railways

  • weakened 

      → collapse

  ∴ difficult to rescue people + supply emergency aid

(b) Causes Injuries and Fatalities 

• people may get trapped under 

  • collapsed infrastructure 

    → injuries + loss of lives 

(c) Disrupts Services 

• water + gas pipes 

  • snaps

    → water shortages

    → gas supply disrupted 

• electricity + communication cables 

  • break 

    → affects important communication services 

    i.e., tsunami warnings + tv broadcasts 

e.g., 2010 Christchurch, New Zealand earthquakes

• many trees damaged 

  • >300 trees removed 

    → reduces availability of habitats for wildlife 

How does landslides caused by Earthquakes affect the natural and human systems?

Occurs when

• violent ground vibration causes

  •  cracks on steep slopes

    → loosening the rocks and soil

  • then further shaking triggers the loose articles to

    → move downslope

Impacts on Natural Systems

(a) Destroys ecosystems

• fast-moving debris 

  • buries and destroys huge areas of 

    • forest

    • wetlands

• debris can enter and pollute 

  • rivers 

    → kills aquatic life

• rivers can be

  • blocked 

    → floods that damage nearby 

    • ecosystems

    • properties

       

Impacts on Human Systems

(a) Destroys Properties and Infrastructure 

• debris can 

  • bury 

  • destroy

    → properties + infrastructure 

(b) Causes Injuries and Fatalities 

• debris can

  • bury

  • hit

    people 

    → injuries + fatalities 

• debris can 

  • block rivers

    → floods 

    • drown people

(c) Disrupts Services 

• debris can

  • snap

→ electricity + communication cables and gas + water pipes

∴ disrupting the supply of these services

• debris can

  • block

    → roads + railways

∴ difficult to rescue people + supply emergency aid 

e.g., 2018 Papua New Guinea Earthquake (Mw 7.5)

• triggered landslides that caused 

  • huge amounts of debris to 

    → enter rivers 

    ∴ flooding 

    → polluting waters and killing marine life

• forests were destroyed

How does soil liquefaction caused by Earthquakes affect the natural and human systems?

Occurs when

• violent ground shaking causes

  • saturated

  • loose 

    soil to lose its soil structure 

    → transforming into a thick liquid 

Impacts on Natural Systems

(a) Destroys Ecosystems

• trees on liquefied soil may

  • sink and tip over

→ damages wildlife habitats

∴ forest + biodiversity loss

• liquefied soil may enter rivers 

  • smothers aquatic plants

∴ causing them to die 

• sewage pipes may be unbroken

  • untreated waste materials 

    • pollute rivers

∴ killing aquatic life

Impacts on Human Systems

(a) Destroys Properties and Infrastructure

• buildings and other infrastructure can

  • sink in and tip over

    → collapses

∴ difficult to rescue people and supply emergency aid

(b) Cause Injuries and Fatalities 

• people can get trapped in

  • collapsed infrastructure

    ∴ injuries + fatalities  

(c) Disrupts Services 

• electricity + communication cables and water + gas pipes

  • sinks in and snaps

    → disrupts supply of these services 

• roads and railways above liquefied soil

  • sink in and get damaged 

∴ difficult to rescue people and supply emergency aid

e.g., 2010 Christchurch, New Zealand earthquakes

• triggered severe soil liquefaction

• > 60,000 residential infrastructure (approx. 1/3 of the city)

  • were damaged

• liquefied soil entered

  • rivers

    +

• untreated sewage from 

  • broken pipes 

∴ polluted rivers

→ killed many aquatic species

How do tsunamis form?

1) An undersea earthquake causes the seabed to be

• displaced 

2) A large volume of water is lifted

• forms waves of

  • great wavelength 

  • low height (<1m)

3) Waves travel towards land at high speeds 

• 800km/h 

4) When approaching the coast

• greater friction with the shallower seabed

  • slow the waves 

5) Waves cluster together and increase in height 

• reach up to >15m height 

• travel at a speed of 30-50km/h

  → devastate shorelines hit by the waves 

6) Before a tsunami occurs

• The sea may recede from the shore

  • as the water fills the void caused by the 

    → displacement of the seabed

How does tsunamis caused by Earthquakes affect the natural and human systems?

Impacts on Natural Systems

(a) Destroys Ecosystems

• seawater

  • floods

→ huge areas of coastal wetlands + forests

∴ damages habitats

• large amounts of debris carried in by waves

  • pollute these areas

∴ damages ecosystems + kills wildlife 

Impacts on Human Systems

(a) Destroys Properties and Infrastructure

• fast-moving waters + large amounts of debris carried by the waves

  • sweeps away

    • buildings + infrastructure

∴ destroys them

(b) Cause Injuries and Fatalities 

• sweeping waters

  • drown people

• large amounts of debris carried by the waves

  • hit

  • kill

    → people

(c) Disrupts Services 

• fast-moving waters + large amounts of debris carried by the waves

  • snaps cables

    • communication + electricity

    → disrupts the supply of these services 

• fast-moving water

  • sweeps away

    • roads + railways

∴ difficult to rescue people and supply emergency aid

e.g., 2011 Tohoku Earthquake, Japan

• tsunami 

  • 40m tall

• waves carried debris inland

  • flooding

  • polluting

→ large areas of the land

• Sendai, a coastal city

  • suffered extensive damage

  • half of its population died 

• Iwate

  • 70k pine trees were knocked down

→ loss of forest + biodiversity

How does tephra caused by volcanic eruptions affect the natural and human systems?

Impacts on Natural Systems

(a) Destroys Ecosystems

• ash

  • carried by thousands of km by

    • prevailing winds 

→ pollutes forests, rivers + habitats 

∴ destroys ecosystems

• ash

  • suffocate + kill wildlife

  • cause blindness to birds

    → their eyelids get gummed together

Impacts on Human Systems

(a) Destroys Properties and Infrastructure

• volcanic bombs (a few cm - size of vehicles)

  • hit properties

→ damages them

• ashfall

  • accumulates on the roofs of buildings 

  • when saturated with water, ash’s weight doubles

    → roofs collapse

• ash

  • corrosive

→ weakens building structure 

∴ more susceptible to collapsing 

• thick blankets of ash

  • suffocates crops 

∴ damage farmland + destroys livelihoods

(b) Cause Injuries and Fatalities 

• volcanic bombs

  • hit people

→ injuries + fatalities

• ashfall causes

  • respiratory problems

  • eye irritation

  • suffocation

(c) Disrupts Services 

• ash particles can 

  • damage plane engines 

    → closure of airspaces 

∴ disruption of air transportation services

e.g., 1991 Mount Pinatubo Eruption, Philippines 

• emission of huge amounts of ash

• buried >180km2 of forests in

  • ash of 25cm

• destroyed 800km2 of rice fields

  • livelihoods of many farmers affected

• 7 airports had to close down + many flights cancelled

  • disruption of transport

• some planes → damaged

How does volcanic gases caused by volcanic eruptions affect the natural and human systems?

Characteristics

• releases large amounts of toxic gases 

  • sulfur dioxide 

  • carbon dioxide

Impacts on Natural Systems

(a) Destroys Ecosystems

• when sulfur dioxide reacts with water in the air 

  • acid rain 

→ damages vegetation + soil + kill wildlife 

• cold carbon dioxide is concentrated in 

  • low-lying volcanic areas 

    → as it is heavier than air 

∴ abundance of carbon dioxide

→ kills wildlife + soil + destroys vegetation

Impacts on Human Systems

(a) Destroys Properties and Infrastructure

• sulfur dioxide causes

  • acid rain

→ corrodes infrastructure

(b) Causes Injuries and Fatalities and Threatens Public Health

• sulfur dioxide irritates

  • skin

  • eyes

  • nose

  • throat

• air with >3% carbon dioxide causes

  • headaches

  • breathing difficulties

  • even death when it exceeds 15%

e.g., 1979 Dieng Volcano Eruption, Indonesia

• released deadly amounts of CO2

  • killed about 150 people

How do lava flows caused by volcanic eruptions affect the natural and human systems?

Characteristics

• extremely hot 

• travel some distances within the volcano it originated from

• deaths caused directly by lava flows are

  • uncommon 

    → people can easily move outta the way 

Impacts on Natural Systems

(a) Destroys Ecosystems

• hot, low-silica lava 

  • travel some distances 

∴ destroys forests + habitats + ecosystems in them

Impacts on Human Systems

(a) Destroys Properties and Infrastructure

• lava burns through

  • homes

  • infrastructure etc.

(b) Cause Injuries and Fatalities 

• indirect death via

  • destruction of homes

    → lava burning through

• deaths are uncommon

  → easy to move out of the lava’s way

(c) Disrupts Services 

• lava destroys

  • communication + electricity cables

  • water + gas pipes

→ disrupts the supply of these services

e.g., 2018 Kilauea Eruption in Hawaii

• far-reaching lava flows destroyed

  • >600 homes

  • forests + ecosystems 

• telephone + power lines

  • damaged

    → widepread communication outages 

How do pyroclastic flows caused by volcanic eruptions affect the natural and human systems?

Characteristics

• hot cloud of 

  • gas

  • ash

  • rocks

→ travelling down the slopes of a volcano at high speeds (200km/h)

• temperature of up to 700 degrees Celsius 

  • Perhaps the most devastating volcanic hazard

Impacts on Natural Systems

(a) Destroys Ecosystems

• huge areas of forests

  • destroyed

∴ biodiversity loss

• thick layers of ash

  • cover the path of the flows 

∴ pollutes huge areas of forests

Impacts on Human Systems

(a) Destroys Properties and Infrastructure

• hot flows burn

  • everything in their path

∴ destroying any infrastructure in its way

(b) Cause Injuries and Fatalities and Threatens Public Health

• extreme temperatures

  • burn

  • kill

→ people in a fraction of a second

(c) Disrupts Services 

• hot flows

  • destroys

→ electricity + communication cables and water + gas pipes

∴ difficult to rescue people and supply emergency aid

e.g., 2010 Merapi Eruption, Indonesia

• 350 people killed

  • mostly due to burns + blast injuries 

• huge areas of forests were burnt

How do lahars caused by volcanic eruptions affect the natural and human systems?

Characteristics 

• mudflows

  • comprising of water + volcanic ash

• formed when ash + rocks ejected during an eruption

  • mix with water

• occurs when 

  • intense rainfall washes down the ash

  • pyroclastic flows melt snow + ice

    → mixes with water on the volcanic cone as they travel down

Impacts on Natural Systems

(a) Destroys Ecosystems

• lahars can cover

  • forests

  • ecosystems

→ with thick mud that hardens like concrete

∴ destroying them

• lahars can 

  • pollute rivers

→ kills aquatic life

Impacts on Human Systems

(a) Destroys Properties and Infrastructure

• lahars

  • buries

  • destroys

→ houses

• lahars can cover

  • farms

→ with thick mud that hardens like concrete 

∴ destroys them

(b) Cause Injuries and Fatalities and Threatens Public Health

• lahars can bury

  • houses and

  • its residents

∴ killing them

(c) Disrupts Services 

• lahars can damage

  • communication + electrical cables

  • water + gas pipes

→ disrupts the supply of these services

• lahars can block

  • railways + roads

∴ difficult to rescue people and supply emergency aid

e.g., 2010 Mount Merapi Eruption, Indonesia 

• Heavy rain occurred on November 4th

  • lahars 

    → flowed into towns 

    → destroyed bridges

How do volcanic landslides caused by volcanic eruptions affect the natural and human systems?

Impacts on Natural Systems

(a) Destroys Ecosystems

• fast-moving debris

  • buries

  • destroys

→ huge areas of forests + wetlands

• debris from landslides 

  • pollutes rivers

  • blocks rivers 

∴ kills aquatic life + causes floods 

→ damages properties + ecosystems

Impacts on Human Systems

(a) Destroys Properties and Infrastructure

• debris 

  • buries 

  • destroys

→ infrastructure + properties

(b) Cause Injuries and Fatalities 

• debris

  • bury

  • hit 

→ people 

∴ injuries + fatalities

• debris blocks rivers

  • floods 

→ drowns people

(c) Disrupts Services 

• electricity + communication cables and water + gas pipes

  • snapped by debris

    → disrupts the supply of these services 

• roads and railways

  • blocked by debris

∴ difficult to rescue people and supply emergency aid

e.g., 1980 Mt St Helens Eruption, USA

• triggered volcanic landslides 

  • traveled as far as 23km away from its origin

• destroyed huge areas of land

  • killing many wildlife

What are the benefits of living near volcanoes and volcanic eruptions?

volcanic eruption provides

• fertile soil for farming after volcanic materials are broken down and weathered

• and makes available valuable minerals and building materials

living near volcanoes allows harnessing

• of geothermal energy and

• tourism activities

Farming with Fertile Soil

over thousands of years

• volcanic rocks

  → break down + undergo weathering

∴ forms fertile soils

• suitable for agriculture

  • produces a higher crop yield

∴ locals benefit → earn more revenue

                          and grow a larger variety of crops

e.g., Indonesia

• best rice-growing regions located near volcanoes 

  • Bali → several volcanoes like Mount Agung

Extracting Precious Minerals and Fossil Fuels

some magma may cool and solidify beneath volcanoes

• forms precious minerals

  → copper, silver, gold

• locals living near volcanoes can be

  • employed to mine these minerals

→ providing locals with a source of income

• governments may sell the minerals

  • generate revenue

→ can fund social services for locals (education+healthcare)

• volcanic materials like ash + sand

  • may be mined 

→ used as construction materials

e.g., Mount Merapi, Indonesia

• thousands of locals living there are

  • employed to mine volcanic sand

→ as it is suitable for constructing buildings

Harnessing Geothermal Energy

geothermal energy is derived from heat in 

• Earth’s crust 

it can be harnessed in tectonically active areas 

• beneath Earth’s surface

  • hot rocks heat groundwater

    → steam formed is harnessed 

    • generates electricity 

∴ locals benefit from cheaper electricity 

• produced locally 

→ locals can also be employed to work in

• geothermal power plants

e.g., Iceland

• majority of its electricity is generated from

  • geothermal power

→ due to the many volcanoes in the country

Tourism Activities 

tectonic environments

• attracts tourists 

• tourists can engage in many activities

  • hiking

  • sightseeing

Tourism provides locals with employment opportunities 

• tour guides

• food vendors

• souvenir seller

• homestays

∴ boosts local economic development

e.g., Hawaiian Islands

• popular tourist destination for its

  • scenic volcanic landscapes 

→ tourism generates an annual income of US$88mil

• provides locals with thousands of jobs