PLATE TECTONICS: FLASH NOTES

Tectonics Theory

Earth split into three sections:

  • Core, Mantle, Crust

Crust made of two types:

  • Continental and Oceanic - Continental is thicker (30-70km, 6-10km for Oceanic) and less dense.

Mantle made of two layers:

  • Asthenosphere - semi molten mantle (Below)
  • Lithosphere - rigid mantle and crust (Above)

Crusts form boundaries called plate boundaries or plate margins.

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Plate Movements

Asthenosphere heast up and sinks and rises, this causes CONVECTION CURRENTS

Sea Floor Spreading:

Rising convection currents diverge, the plates diverge and move away from each other.

Magma rises up and fills the gap created, the magma cools and forms a new crust.

Crust moves apart and forms a new crust.

The sea floor spreads...

Creates Mid Ocean Ridges. (ridges are higher terrain on either side of the margin)

(Q) Describe the structure of the earth (8 marks)

(Q) Describe and explain how convection currents cause tectonic plates to move (8 marks)

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Theory of Plate Tectonics

Theory of Plate Tectonics:

  • 1912 - Alfred Wegener proposed continental drift
  • Suggested all continents were once joined - Pangaea
  • First based on geological evidence and fossil records
  • 1950's - Continental drift evidenced by Palaeomagnetism
  • 1960's - Sea Floor Spreading provided further evidence for continental drift

 

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Evidence for Continental Drift

Evidence for Continental Drift:

  • Geology - South America and Africa have rocks of the same age and composition. Mountains in Scotland and East coast of North America very similar
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Types Of Plate Margin (1)

CONSTRUCTIVE:

Occurs when two plates are moving away from each other - Diverging

When the plates move away they produce pressure that melts the mantle, forming magma. This magma rises up as it less dense - can create a volcano

The plates dont diverge inn a uniform way, some move faster than others. This create pressure, when the pressure becomes too high, the plate cracks and forms a fault line - causing an earthquake

2 differnet landforms created:

  • Mid Ocean Ridge - diverging plates underwater causing a mid-ocean ridge. Example being the Mid-Atlantic Ridge. Also, underwater volcanoes occur along the mid ocean ridges and can build up above sea level - seen with Iceland.
  • Rift Valleys - Plates diverge, rising magama causes the continental crust to bulge and fracture, creating a fault line. The crust between these lines drops down, creating the valley. 
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Types Of Plate Margin (2)

DESTRUCTIVE MARGIN:

Occurs when plates are converging. WHat happends depends on the plates:

(1) Oceanic - Continental: Oceanic crust is subducted as it is denser, below the continetal crust. This forms a Deep Sea Trench. Fold Mountains  are formed when the plates meet. They are made up or sediments accumulated on the continetal crust, folding upwards. The oceanic crust is melted by friction and contact with upper mantle, turns into magama. The magma is less dense than the continental crust, and rises to the surface to form volcanoes. AS one plate moves over the other, it can get stuck, the pressure builds up and this can be realeased and causes an earthquake.

(2) Oceanic - Oceanic: The denser of the two is subducted, forming deep sea trench (where the crust melts, this is called the subduction zone) Earthquakes and volcanoes occur here. Volcanic eruptions create Island arcs - cluster of islands that sit in a curved line. 

(3) Continental - Continental: No volcanoes. Earthquakes occur. Fold mountains are formed (e.g. San Andreas Fault)

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Types Of Plate Margin (3)

CONSERVATIVE:

Occurs when 2 plates are moving PAST each other

The 2 plates get locked together in places. The pressure builds up and when it gets too much the pressure is released in a jolt. This release of energy causes an Earthquake. 

An example is the Pacific Plate moving past the North American Plate. Many earthquakes occur along this margin, along the fault lines - e.g. San Andreas Fault in California

(Q) Describe the features of constructive plate margins (8 marks)

(Q) Compare and contrast margins where two oceanic plates are converging with margins where two continental plates are converging (8 marks)8. Volcanic Activity (1)

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Volcanic Activity (1)

Volcanic Activity can be INTRUSIVE or EXTRUSIVE:

  • INTRUSIVE: Beneath the earth's surface. Formation of large chambers of magma and magma being forced into the crust.
  • EXTRUSIVE: Takes place on the Earth's surface. Volcanic eruptions of lava. Minor Types include hot springs, geysers and boiling mud pools. 
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Volcanic Activity (2)

INTRUSIVE ACTIVITY:

  • Large chambers of magma cool underground they form domes of igneous rock called batholiths.
  • When magma flows into gaps in the surrounding rock and cools. It forms a vertical dyke and horizontal sills.
  • Cracks form as magma cools - called cooling cracks.

(http://www.acegeography.com/uploads/1/8/6/4/18647856/1617860_orig.jpg)

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Volcanic Activity (3)

EXTRUSIVE ACTIVITY:

Lava Eruptions are a form of extrusive activity. There are 3 main types of lava:

  • BASALITIC
  • ANDESITIC
  • RHYOLITIC

Basaltic: Made at CONSTRUCTIVE plate margins. They have low viscosity, low silica content and very high temperatures. This causes frequent eruptions for long periods of time, but not violent.

Andesitic: Made at DESTRUCTIVE plate margins. Has medium silica content, temperature and viscosity.

Rhyolitic: Made at DESTRUCTIVE plate margins. Has high silica content and high viscosity, but low temperatures. Both (A+R) form violent eruptions, with large releases of pressure. Eruptions are intermittent and short lived.

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Types of Volcanoes (1)

(1) DOME:

(http://www.explorevolcanoes.com/volcanoimages/volcano%20dome.png)

  • Destructuve plate margins. 
  • Lava: Rhyolitic and Andesitic 
  • Steep sides, viscous lava that cools quickly
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Types of Volcanoes (2)

(2) SHIELD:

(http://www.explorevolcanoes.com/volcanoimages/shieldvolcanoUSGS.png)

  • Constructive plate margin and hot spots
  • Usually produce basaltic lava
  • Gentle slopes
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Types of Volcanoes (3)

(3) CALDERA:

(http://4.bp.blogspot.com/-ylRlbqWVksU/U3qv1lJOkXI/AAAAAAAAAFM/sXct19KFNa0/s1600/5103263.JPG)

  • Destructive margins, producing rhyolitic and basaltic lava.
  • Central part of volcano collapsed due to the eruption below.
  • Wide circular crater, often several km's across.
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Types of Volcanoes (4)

(4) FISSURE:

(http://go2add.com/images/Volcanoes/fissure-volcano.jpg)

  • Constructive margin, producing basaltic lava.
  • Long linear vent - few metres wide but several km long
  • Flat surface -  caused by low viscosity lava, flowing for long distance
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Hot Springs

HOT SPRINGS:

Springs are places where groundwater emerges at the surface

If the groundwater source is near an area of recent intrusive volcanic activity, then the water is heated up and the spring becomes a hot spring

Temperatures vary from 20 degrees to 90 degrees

Found all over the world

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Geysers

Geysers:

Geysers are a type of hot spring where hot water and stream are ejected at the surface in a fountain. They form in areas of intense volcanic activity.

  • Grounwater is heated to above boiling point by magma deep in the crust.
  • The hot water becomes pressurised and forces its way to the surface through cracks in the rocks. The hot water and steam spray out from a vent.

They erupt periodically - only when theres enough pressure to force it out of the ground.

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Boiling Mud Pools

Boiling Mud Pools:

Another type of hot spring.

They form in areas of very fine-grained soil - the hot spring water mixes with the soil to create a boiling mud pool.

Sometimes contain brightly-coloured mud because of the minerals deposisted by the hot water.

Found all over the world - Yellowstone National Park (USA)

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Hot Spots

Hot Spots:

Most volcanic activity occurs along the plate margin, however, volcanoes can form in Hot Spots.

  • A hot spot is caused by a magma plume - a verticial column of magma that rises up from the mantle
  • Volcanoes form above the magma plume
  • The magma plume remains stationary over time, but the crust moves above it
  • Volcanic activity in the part of the crust that was above the hot spot decreases as it moves away - soon becomes dormant and then extinct
  • New volcanoes form in the crust above the hot spot
  • As the crust continues to move, a chain of volcanoes are formed

Example: Hawaii (USA)

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Seismic Activity (1)

Earthquakes:

Earthquakes are caused by the tension that builds up on all 3 plate margins.

When the plates jerk past each other they send out seismic waves (vibrations) These vibrations are the earthquake.

The seismic waves spread out from the focus (point in the lithosphere where the earthquake starts) The waves are stronger and more damaging nearer the focus.

The epicentre is the point on the earth's surface where the earthquake is first felt (straight above the focus)

They can also be caused by:

  • Reactiviation of old fault lines, subsidence (from deep mining) and pressure on surface rocks from large water reservoirs.

They can damage by: tsunamis, avalanches, landslides, liquefaction, collapsing structures

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Seismic Waves

Seismic Waves:

There are 2 main types of seismic waves that travel through the Earths Interior:

  • P Waves (primary): Can travel through solids and liquids. The waves push and pull the earth in the same direction as the wave is travelling. They are the fastest type of seismic wave.
  • S Waves (secondary): Travel through solids but not liquids. Waves move the earth 90 degrees to the direction of travel. Cause a lot of damage due to their shearing effect.

Other waves can only travel on the Earth's surface (in the crust):

  • Surface Waves: travel more slowly than P and S waves. 2 types:
  • Love Waves - only through solids. Move the surface from side to side. Lot of damage.
  • Rayleigh Waves - Liquids and solids. Move the surface in a rolling motion.
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Measuring Seismic Activity

Seismometers:

Total energy released can be measured using a seismometer. They measure the magnitude (strength.) They also measure the direction and duration of the vibrations.

(1) Richter Scale:

Measured the magnitude. Doesnt have an upper limit and is logarithmic - magnitude 5 earthquake has an amplitude (wave size) 10x greater than a magnitude 4. Each value represents about 30x more energy than the last. 1-2 people dont tend to feel it, 7+ are major earthquakes.

(2) Mercalli Scale:

Measures the impacts of the earthquake. The impacts are measured using observations of the event (e.g. photos and reports.) The scale is between 1 to 12. 1 being earthquake only detected by instructions, and 12 being total destruction.

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Tsunami (Cause)

Cause of Tsunami:

Large waves caused by the displacement of large volumes of water.

Can be triggered by underwater earthquakes. These cause the seabed to move, displacing the water. The water radiates out from the Epicentre of the quake. The greater the movement of seabed - greater displacement of water - larger waves created.

Volcanic eruptions and landslides that slide into the sea also displace the water, causing tsunamis.

Tsunamis are more powerful if the epicentre is closer to the coast, this is because the wave loses energy travelling towards land.

Waves travel very fast in deep water, and so can occur with little warning - creating a high death toll.

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Tsunami (Characteristics)

Tsunami Waves Travelling to Shore:

Tsunami's affect the whole column of water from surface to seabed.

In open ocean with deep water, the wave:

  • Travels at around 500-950km/h
  • Wavelength of 200km
  • Small amplitude (wave height) of about 1m

Closer to land, in shallower water, the wave:

  • Wave compresses and energy becomes concentrated
  • Wave slows down to less than 80km/h
  • Wavelength decreases to less than 20km
  • Amplitude increases to many metres

Drawback occurs at the coast, when water is withdrawn down the shore. The wave hits with great force, but only travels a short distance as it loses energy fast!

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Managing the Impacts of Tectonic Hazards

hazard is something that is a potential threat to human life or property.

Tectonic Hazards (e.g. earthquakes, volcanoes and tsunami's) are hazards caused by the movement of tectonic plates.

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Managing Tectonic Hazards: Predicting

Earthquakes:

Currently impossible to predict earthquakes. However, there are clues such as cracks appearing in rocks and stange animal behaviour. Earthquake warning systems can detect P waves, but only after the earthquake has begun, e.g. Japan's Earthquake Early Warning System (2007) Its possible to predict where future earthquakes will occur using previous quake data, allowing preparation. 

Volcanoes:

Its possible to roughly predict when an eruption will occur, e.g. tiny earthquakes and changes in the shape of the volcano (bulges) Prediction allows time for evacuation, e.g. 60,000 were evacuated before Mt Pinatubo eruption in 1991. Even with prediction, it can still be hard to evacuate people.

Tsunami:

Tsunami warning systems rely of earthquake detection systems, I.e. is an earthquake occurs in the ocean, the tsunami warning centres inform people, rely on good communication.

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Managing Tectonic Hazards: Building Techniques

Earthquakes:

Buildings can be built to withstand earthquakes, often using strong materials such as reinforced concrete. Construction Laws in some earthquake prone countries (Japan, USA) have become strickter - so new buildings are more likely to withstand a quake.

Volcanoes:

Cant be designed to withstand lava flows, but can be strengthend to prevent collapse from ash fall.Possibel to divert the lava flow away from settlements, i.e. Mt Etna eruption of 1983, rubble barrier 10m high, 400m long diverted the flow effectively. However, only possible for slow moving lava.

Tsunamis:

Buildings designed with raised open foundations, and strong materials such a concrete to reduce damage from wave. Tsunami Walls have been built around settlements to protect them (Japan), however some tsunami's have had a larger amplitude than the wall itself.

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Managing Tectonic Hazards: Planning and Education

  • Future developments can be planned to avoid areas most at risk, i.e. active volcanoes
  • Emergency Services can train and prepare for disasters, allowing for more effective responses during an actual event, i.e. FEMA (USA organisation prepares services for disasters)
  • Governments can plan evacuation routes, e.g. Oregon, USA, leftlets are provided giving information on how to evacuate in case of a tsunami
  • Governments and organisations can educate people on what to do in the case of a tectonic disaster, e.g. earthquake and tsunami drills are regularly practised in Japan

All of these are very cost effective ways of reducing the impacts of tectonic hazards

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Factors Increasing Severity of Impacts

Development Level of Country:

  • Impacts higher in less developed countries, they dont have the money for preparation or response, e.g. response teams
  • Buildings are of lower quality, as they are cheaper to make, more easily damaged
  • Infrastructure (roads etc) of lower quality, more difficult for Emergency Services to gain access to affected areas
  • Health care often poorer - struggle to treat large amounts of people
  • More dependence on agriculture, lasting effects greater as loss of livelihood for many
  • BUT, economic impact often higher in more developed countries as the buildings and infrastucture damaged are worth a lot of money

Population:

  • More people = more people affected
  • Densely populated areas such as cities have a lot of buildings, posing more a risk, particularly during earthquakes
  • Difficult to evacuate large amounts of people, roads become blocked etc.

Timing: Whether its during the day or night, what year etc. If during the night, more people sleeping so less able to get out of buildings during earthquakes + harder to communicate warnings to people. 

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