OCR-Plate tectonics (Higher)


What causes volcanoes and earthquakes?

Little is known about the inside of the Earth. Most of what we know comes from studying the waves earthquakes make through the Earth.

There are three main parts: Core, Mantle and Crust.

Core: Central part of the earth. Temperature and pressure is enormous. Inner Core solid. Outer Core semi-liquid.

Mantle: made up of rocks. occupying over 80% of earth's volume. mainly solid, but upper mantel is a layer of semi-liquid molten rock (MAGMA). The magma is moved slowly underneath the crust by convection currents.

Crust: outer layer of rock. Floats on mantle. Solid rock. Broken into several pieces+ smaller pieces; AKA plates. fit together like a jigsaw. Some move around 2cm a year. The theory that describes this is Plate Tectonics. The place where 2 plates meets is a Plate Boundary (where most earthquakes/ volcanoes occur).

There is 2 types of crust: Continental (older/lighter/cannot sink/permanent) and Oceanic (younger/heavier/can sink/constantly being destroyed and replaced).

1 of 15

A cross section through the Earth


2 of 15

Plate Boundaries

There are 3 types of plate boundaries: Convergent, Divergent, Transform.

Convergent (destructive): One plate collides with another. When Oceanic crust collides with Continental crust, the oceanic crust is forced downwards into the mantel and destroyed; a subduction zone (found where the pacific plate meets the Eurasian plate). Where to plates made ofContinental crust collide they are pushed upwards forming fold mountains (Himalayas/Andes).

Divergent (constructive): plates are forced apart by movement of magma in the mantle- new crust is formed in between (Mid-Atlantic volcanic ridge).

Transform (conservative): where two plates move horizontally past each other (San Andreas Fault).

3 of 15

What is an earthquake?

When two plates collide or slide past each other you get an earthquake. This means they occur at convergingand transform plates.

1. two plates move towards each other

2. one gets pushed under the other. If the plate gets stuck it causes strain in the surrounding rock. Sideways moving rocks can also get stuck.

3. when tension is released it produces strong shock waves; aka seismic waves.

4. Theshock wavesspread out form the focus (like a ripple effect). The waves get weaker as they move away from the focus.

Epicentre- point on the Earth's surfacedirectly above the focus.

Focus- The point where the earthquake starts.

A diagram on the following page shows the focus, epicentre, and seismic waves.

4 of 15


the orange lines are the seismic waves

the black arrows show the direction of sudden movement.

5 of 15

How are Earthquakes measured?

Size or Magnitude of an earthquake is measured using a seisometre. Earthquake vibrations are measured by a sensetive arm which moves up and down on a seismograph (like a heart monitor).

Readings from the seismographare measured using the Richter Scale; a logarithic scale (earthquake measuring 5 is 10 tmes more powerful than one measuring 4 and 100 times more powerful than one measuring 3). This dertermins the strength of an earthquake.

Most serious earthquakes range between 5 and 10

6 of 15

why live at risk of earthquakes?

> Job oportunities

> Standard of living much better

> Education is better

> More medical facilities

basically because the benefits outweigh the dangers in many cases.

7 of 15

Preparing for an Earthquake

Earthquake proof buildings are being introduced in MEDC cities; i.e. San Francisco. The buildings sway resulting in damages to the buildings being reduced, leading to a decrease in the number of people being injured or killed.

Improve preparation- in many places prone to earthquakes people try to prepare themselves. In Japan, 1st September is disaster day, the anniversary of the Tokyo earthquake of 1923; 156,000 were killed. The day is a public holiday which earthquake drills are practised. They learn how to protect themselves during an earthquake, how to fight fires and how to give first aid. The media is also used in countries like the USA, to give guidance about what should be done during an earthquake and the internet also provides guidance.

Emergency supplies- people should keep updating their emergency supplies to ready themselves for any shortages during a disaster.

8 of 15

Case Study: Earthquake MEDC

Location: Kobe, Japan Date: 17th January 1995

Cause: lies above a destructive plate margin. The Philippines plate was forced down under the Eurasian plate.

Epicentre: Kobe = VERY DESTRUCTIVE Magnitude: 7.2 on the Richter Scale

Damage: Primary- 200,000 buildings collapsed, bridges collapsed, 120 of 150 quays in the port of Kobe destroyed, trains derailed. Secondary- Electricity, gas, and water supplies destroyed, fires raged for several days- destroying 7,500 homes, 230,000 people made homeless, shortage of blankets, clean water and food, 716 aftershocks recored, industry forced to close (including Mitsubishi).

After: water, gas, telephone services fully operational by July. Fire areas cleared. Commercial buildings repaired. rail services back to normal. Port of Kobe 80% functional within a year. Replacement buildings had stronger earthquake resistance. Highrise buildings fitted with flexible steel frames. houses built with fire resistant materials. Increase in seismic instruments to record earth movements fitted.

9 of 15

Case Study: Earthquake LEDC

Location: Afghanistan Date: 4th February 1998

Epicentre: Takhar Province Magnitude: 6.1 on the Richter Scale

Causes: Lies on collision plate boundary where the Iranian and Eurasian plates meet. Iranian plate constantly moving- tension released by frequent earthquakes.

Damages: Takhar is so remote it took 3 days for news of the quaketo reach Kabul. 27 Villages destroyed. 9th Feb, food supplies still not come. Landslides blocked mountains. 1000's starved.4000 deaths. 10,000 injuries. 15,000 homeless.

After: 16th Feb, helicopters dropped supplies into isolated villages.Latter in 1998, anotherearthquake struck> 300 deaths. more buildings destroyed.

Why unable to cope? Natural severe drought- water supplies low before earthquake. They have no technology to predict an earthquake. Not enough money to train people in first aid. Not enough money to provide emergency services. Poorly built housing. Roads generally rough tracks. Mountainous terrain & severe climate. Lack of electricity and telephones- distrupts comunication.

10 of 15

What is a Volcano?

Volcanoes, usually cone shaped, are formed bysuccessive eruptions of lava, volcanic bombs,ash flows and tephra (airborne ash and dust). They are built around a vent that connects with reservoirs of magma below the Earth's surface.

Volcanoes can be: Extinct (will never erupt; i.e.Devil's tower, Wyoming), Dormant (hasn't erupted in the last 2000 years; i.e. Santorini, Greece)or Active (erupted recently and likely to erupt again; Mt.St.Helens).

3types of volcano: Composite Volcano, Shield Volcano and Dome Volcano.

Composite: made from ash and lava. ejects- Ash, gas, volcanic bombs and lava. Material once ejected cools and hardens to form the volcanoes mountain from the layers of ash and lava. An example, Mt. Etna, Sicily.

Shield: Lava isalkaline and runny. Flows quicky and easily. Spreads over wide areas- giving it wid, flat features. An example, Mauna Loa, Hawaiian Islands.

Dome: Lava is acidic and thick. flows slowly and hardens quicky to form steep sided features. An example is Mt.St.Helens, USA.

11 of 15


some eruptions are explosive others are not, it depends on how runny or sticky the magma is.

If magma is thin and runny, gas can escape > when eruption occurs the lava flows out of the volcano. <<< NON-EXPLOSIVE

If the magma is thick and sticky, gas cannot escape > Pressure builds until gas is released violently and explodes. <<< EXPLOSIVE

12 of 15

Why live near a volcano?

People live near volcanoes because...

there is rich farm land, it is extremely fertile...

tourist attraction > jobs > money > oppurtunities...

geothermal energy...

Good health > bathing in volcanic ash...

no choice because they were born there...

minerals > lava in volcaoes can crystallise to form minerals like gold, silver, diamonds.

13 of 15

Monitoring and predicting eruptions

Reason: so that people living near active volcanoes are aware when they are likely to erupt.

Problem: although all volcanoes are similar, each one behaves differently and has its own refined hazards.

Scientists Monitor: They map past volcanic deposits and use satellite pictures to look at volcanic features, ash clouds and gas emissions. they monitorseismic activity, thermal changes, gas emissions, flow rate, sediment transport and water levels of streams and lakes near the volcano.

By studying volcanic deposits, scientists can produce hazard maps. This indicates hazards in certain areas when avolcano erupts.

Close monitoring of a volcano over a long period of time will indicate changes in a volcano occur before an eruption. This will help with future predictions of when it will next erupt.

14 of 15

Case Study: Volcano

Location: Mount. Saint. Helens, USA Date: 18th May 1980

Cause of eruption: on a convergent plate boundary. The Jaun de Fuca Ridge collided with the North American Plate; being oceanic crust, the Jaun de Fuca Ridge was forced downwards into the mantle. The increased temperature turned the crust into magma which increased the pressure in the mantle andcaused themagma torise to the Earth's surface; resulting in the eruption on 1980.

Trigger: An earthquake on the 20th March 1980 measuring 4.2 on the Richter Scale.

Effects: 61 people killed, more than 200 houses and cabins destroyed, 185 miles of highway destroyed, 15 miles of railway destroyed, 27 bridges destroyed, estimated 7,000 big game animals (deer, elk and bear)perished, Birds and small mammals effected, some crops destroyed, poor visibility > roads closed and flights cancelled, $1.1 billion damages, poor comunications for a while.

15 of 15


No comments have yet been made

Similar Geography resources:

See all Geography resources »See all Natural hazards resources »