2.1 Plate Tectonics

syllabus_link_buttonDescribe the general distribution of fold mountains, volcanoes and earthquakes and explain how this distribution is related to movements at plate boundaries.

Fold mountains, volcanoes and earthquakes occur along cracks in the earth´s crust.

Pacific Ring of Fire: Volcanoes surround the Pacific Ocean running up the West of the Americas and then down through Japan, the Philippines, Indonesia and New Zealand.  Earthquakes also occur in these locations.

Mid-Atlantic Ridge: a line of volcanoes and earthquake locations run up the center of the Atlantic Ocean as the African and South American plates move apart.

The Himalayas: Earthquakes occur throughout the Himalaya region resulting from the Indo-Australasian plate colliding with the Eurasian plate.

 

syllabus_link_buttonShow a basic understanding of plate tectonics, describing the global pattern of plates, their structure, and an awareness of plate movements and their effects – constructive (plates moving away from each other), destructive (subduction plates moving towards each other) and conservative (plates sliding past each other).

Structure of the Earth

structure

  • The crust is the outer layer, broken into sections called tectonic plates. It is the thinnest layer.
  • The mantle is the thickest layer and consists of molten rock
  • The outer core is molten & about 3000 degrees C.
  • This inner core is solid due to the immense pressure and is about 5000 degrees C.
  • Both the inner & outer core consist of iron & nickel

 

Plate Boundaries

Constructive Boundary

  • Convection currents in the mantle drag the plates apart.constructive
  • Magma rises to fill the gap and solidifies to form new crust.
  • As the process repeats a ridge is formed and this slowly gets wider as the plates continue to separate.
  • This creates fissure volcanoes which are long cracks, they are less explosive.
  • Example: the Eurasian and North American plates are moving apart creating the Mid-Atlantic Ridge.

 

Destructive Boundary

destructive

  • Oceanic & continental plates collide. The oceanic plate is denser and so sinks under the continental plate.
  • As the oceanic plate sinks it takes some sand, water and other material from the sea bed with it. This melts and is gaseous which causes it to force its way up to the surface as a volcano.
  • The continental plate crumples at the edge creating fold mountains in addition to the volcanoes.
  • Example: The Nazca plate (oceanic crust) is colliding with the South American plate (continental crust) and subducting.  The South American plate is crumpling up forming the Andes mountain range.

Collision Boundary

collision

  • Convection currents in the mantle pull two plates of continental crust together.
  • Since both crusts are made of the same material and have equal density neither sub-ducts. They crumple up.
  • This process creates fold mountains. There are no volcanoes at these boundaries but earthquakes occur.
  • Example: the Indo-Australasian plate is colliding with the Eurasian plate.  Since they are both continental crust neither is forced under. They crumple as fold mountains creating the Himalayas.

 

Conservative Boundaries

These occur when two plates are sliding past each other.  Crust is not created or destroyed but the movement causes earthquakes.  The San Andreas fault in California as the most famous example.

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Demonstrate an understanding of the main features of volcanoes (and their eruptions) and earthquakes.

Volcanoes

volcano_diagram

Volcanic Structures

  • Volcanoes vary in shape and structure depending on the reason for their existence.
  • Destructive boundary volcanoes are often cone shaped and explosive.
  • Constructive boundary volcanoes are often more gently sloped and have less violent eruptions.
  • Lava, ash, cinders and smoke may be ejected from the vent.

Pyroclastic flows: these are very fast moving and extremely hot clouds of smoke, steam and ash that race down the slopes of the volcano.  They occur when hardened lava domes collapse in on themselves and shatter.

Secondary vents/ fumaroles exist on some volcanoes where the vent has split and made a secondary exit.

Lava flows: these can move quickly if the lava is very hot and runny, thicker lava moves more slowly.  Lava destroys everything in its path.

Lahars or mudflows: volcanoes that are snow capped or have crater lakes often create mudslides as ash mixes with the water.  These can devastate the surrounding area.  Heavy rain after ash eruptions can also create mudflows, this can be a big problem in tropical locations.

Case Study: Mt. St. Helens (May 18th 1980)

Mt. St. Helens is located in the Cascade Mountain range in Washington State. It lies on a destructive boundary (the Juan de Fuca plate subducting below the North American plate). The main eruption was preceded by a series of earthquakes which allowed scientists to implement exclusion zones around the volcano.

Effects

Pyroclastic clouds and the force of the eruption flattened the surrounding forest for over 8 miles in each direction, over 10 million trees were destroyed.

61 people were killed in the explosion.

Spirit lake and the surrounding rivers were choked with ash and fallen trees.

An estimated 540 million tonnes of ash fell on seven States damaging cars, causing breathing problems, closing transport routes (roads, railways and flight paths), crops were also damaged from the ash.

 

Earthquakes

earthquake

  • Focus: the exact place in the crust that the earthquake occurs,  it is often deep underground.
  • Epicenter: the point on the earth´s surface directly above the focus.
  • Seismic waves: the shockwaves that spread out from the focus of the earthquake.
  • Seismometer: an instrument that measures the magnitude of seismic wave.
  • Seismograph: the graph created to show the strength (magnitude) of vibrations in the earth.

Case Study: Haiti (12th Jan 2010)

Haiti lies on a conservative boundary in the Caribbean Sea. The Caribbean plate and the North American plate move at different speeds. The earthquake measured magnitude 7 on the Richter scale and the focus was only 5km below the surface.

Effects

Haiti suffered badly from the quake with large scale building collapse and damage to road, water and power infrastructure. Poor quality building design and regulation meant that most buildings were not able to withstand severe quakes.

Over 300 000 people were killed and more than a million left homeless. The problem became much worse as hospitals were unable to cope with the injured, fresh water and food supplies quickly ran short and disease (cholera) started spreading.

The damage to the road networks made search and rescue very difficult.

A major international aid effort from many countries supplied skilled emergency workers, bottled water and food and medicines.

Prediction, Preparation and Responses to Disasters

Countries at risk from tectonic hazards aim to minimise the impact of them.

Predicting an eruption allows people to be evacuated and businesses/property to be protected which saves lives and money. Prediction is difficult and often requires scientists and equipment to be used over long periods of time which is expensive.

Preparing for disasters can significantly reduce the impact that they have. Emergency evacuation procedures, supplies of water, food and medicine, emergency shelters all help reduce panic, save lives and reduce the spread of disease after a disaster.

Responding to a disaster effectively is key to minimising its impact. People who are trapped or injured have a limited amount of time before they die. Buildings and roads may be damaged, phone lines down and electricity disrupted. Well rehearsed and well-equipped emergency service teams can make a huge difference.

Test Yourself

Past Paper Style Questions

Grab a pen and start practising for the exam

1) Describe the global distribution of volcanoes [3 marks]

2) Explain the formation of fold mountains [4 marks]

3) Describe the main features of a volcano [3 marks]

4) Explain using a labelled diagram why volcanoes and earthquakes occur at destructive boundaries [5 marks].

4) With reference to an example you have studied, describe and explain the effects of volcanic eruptions [7 marks].