Plate Tectonics

Structure of the Earth

There are 3 main layers of the Earth. 

1. The Core
2. The Mantle
3. The Crust

1. The Core is divided into the inner core which is a solid body that can heat up to 6000 degrees celcius and a semi-molten outer core. Rocks in the core contain the minerals Iron and Nickel.

2. The Mantle is the largest section of the Earth and surrounds the core. It is made up of mainly silicate rocks which are rich in iron and magnesium. The mantle closest to the core is quite rigid and the Athenosphere (Upper Mantle) is semi-molten however the very top of the mantle which, with the crust forms the lithosphere, is rigid. 

3. The Crust is the outermost layer of the Earth and there are 2 types. Oceanic crust is thin (only 6-10km thick) but dense whereas Continental crust is thicker (30-70km) and less dense. 

The lithosphere is divided into many different slabs (Tectonic Plates). The main ones are:

- Eurasian
- North American
- South American
- Pacific
- African
- Indo-Australian

 However there are many smaller ones inbetween. 

The plates move around due to convection currents. 

Convection Currents: 

• The radioactive decay of elements in the mantle generates heat which then causes rocks to become less dense and therefore slowly rise to the surface. 
• Moving further away from the core leads them to cool down and, as they do, they become more dense and sink. 
• The circular motions of heated rock rising and cooling rock sinking are the convection currents. 
• These currents drag the tectonic plate above them and cause them to move.
• Depending on the direction of the convection current, the plate margin can either be constructive (plates diverging), destructive (plates converging), or conservative (plates sliding past each other).

When plates diverge, new crust is created as magma rises up to fill the gap left. When this occurs at sea it is called sea floor spreading.
When plates converge, crust is destroyed as it gets heated up or it's pushed up into mountains. 

The theory of plate tectonics grew out of continental drift which occurs after a very long period of sea floor spreading as points on the tectonic plates are gradually moved away from their original position. 

Alfred Wegener proposed this theory of continental drift as he believed all the continents were originally joined together in a super continent (Pangea) and, through millions of years of sea floor spreading, moved to their current positions. The main argument for this theory is that the continents of the world today roughly fit together as a jigsaw. 

The other evidence to support the theory is:

1. Geology
2. Fossils
3. Living Species
4. Climatology
5. Paleomagnetism
 

Geology

Rocks found in South America and Africa are of the same age and the same type and, when the continents are aligned, their geographical distribution matches.

Fossils

Like with geological evidence, there have been discoveries of fossils belonging to land dwelling reptiles (Cynognathus) in both South America and Africa and these animals are very unlikely to have swum across the ocean or flown. 

Living Species

There are species living thousands of miles away such as a particular family of earthworms which have been found on several different continents (Asia, North America, Oceania). 

Climatology

Areas such as Antarctica and the UK have deposits of coal that was formed in the tropics and which are of a similar age. These areas are no longer in the tropics and so must have drifted apart.

Paleomagetism

The Earth's polarity switches around once every 200,000 years and the magnetic alignment of rocks outwards from the Mid-Atlantic ridge shows evidence of sea floor spreading. The alignment alternates on both sides of the ridge and the alignment of the rock depends on the polarity of the earth at the time it solidified. 

Constructive margins are where plates diverge.

Example:
Under the Sea: Mid-Atlantic Ridge (North American and Eurasian plates)
Under the land: East African Rift Valley (African Sub-plates - Nubian and Somalian) 

• As the two plates diverge pressure on the mantle reduces, causing the mantle to melt and magma to form. 
• The magma rises up as it is less dense than the plates and erupts as a Volcano. 
• The plates don't move apart uniformly and therefore stresses can be placed on some areas which eventually crack, create a fault line and cause an Earthquake.

Mid-Ocean Ridges: Volcanoes erupt and the lava which settles forms a ridge. If the eruptions continue, the ridges can emerge from the sea and form islands (E.g. Iceland)

Rift Valley: Heating and up-doming of the crust causes fracturing and rifts form. Central sections of the crust drop down and form the rift valley. Active volcanoes can be found on the higher plateaus. 

Destructive plate margins occur where tectonic plates converge.

The processes and hazards associated with convergence margins depends on the types of plate involved. Oceanic-Continental, Oceanic-Oceanic, or Continental-Continental. 

Oceanic-Continental (E.g. Andes: South American and Nazca plates)

• Oceanic crust is denser than continental and so, when they meet the oceanic is subducted underneath the continental. 
• Where the oceanic begins to descend a deep ocean trench is formed and rocks from the ocean are pressed against the continental plate to form fold mountains. 
• The friction between the two plates leads to earthquakes at varying depths.
• The heat generated by friction partially melts the subducted plate to create magma. As magma is less dense than the rock and so it forces its way to the surface where it erupts as a volcano.

Oceanic-Oceanic (E.g. Mariana Islands: Pacific and Philippine plates)

• Similarly to Oceanic-Continental convergence margins, when two oceanic plates collide, one subducts beneath the other. 
• The plate that subducts is marginally denser or the faster moving of the two. 
• A deep ocean trench is created at the point of subduction with earthquakes of variable depths also occuring. 
• Volcanoes occur at this margin too and tend to form island arcs (Mariana Islands).

Continental-Continental (E.g. Himalayas: Indo-Australian and Eurasian plates)

• When two continental plates meet neither is subducted as they have similar density however they are still converging and thus collision occurs. 
• This collision leads to the plates 'crumpling' and tall fold mountains being formed (Himalayas). 
• Despite there being no volcanoes at this margin, shallow earthquakes do occur. 

Conservative margins are where two plates neither converge or diverge but instead slide past each other. 

Example: San Andreas Fault, California (North American and Pacific plates)

The frequent build up of friction as plates stick as they move past each other leads to frequent seismic activity.

No volcanoes occur at passive margins such as this. 

Type of Margin                 Example            Landforms    Earthquakes(Y/N)   Volcanoes(Y/N)

Constructive:   

Under Water:        Eurasian/North American      Mid-Ocean Ridge         Yes                    Yes

Under Land:          African (Somalian/Nubian)     Rift Valleys                 Yes                    Yes

Destructive:

Oceanic-Continental:  Nazca/South American    Deep sea trench           Yes                    Yes

                                                                   Young fold mountains

Oceanic-Oceanic:     Pacific/Philippine       Deep sea trench/Island Arc    Yes                  Yes

Continental-Continental: Indo-Austr./Eurasian        Fold mountains          Yes                    No

Conservative:            North American/Pacific            Low Ridges             Yes                   No

Hot Spots are areas where volcanic activity occurs far away from a known plate-margin. 

The best example of a hot spot is the Hawaiian islands. 

A hot spot is where a (fixed) plume of magma breaches the surface of the crust after originating within the mantle.

Islands are formed by a series of eruptions allowing magma to emerge from the sea.

Island chains are formed as the crust moves whereas the hot spot stays relatively still and the next series eruption occurs in a different place to the previous. 

In Hawaii the plate is moving north-west and so new islands are formed to the south-east..

There are various types of volcano which can be distinguished from each other most obviously by shape but also by the type of lava they produce, the nature of their explosion and the plate margin along which they occur. 

Classification by Shape:

• Dome 
• Caldera
• Fissure
• Shield
• Composite

Classification by lava:

• Basaltic (Contructive plate margins)
• Andesitic (Destructive plate margins)
• Rhyolitic (Destructive plate margins)

Dome volcanoes are steep sided volcanoes which form as a result of very viscous lava that doesn't travel far from the point of eruption. 

Rock Type: Rhyolitic

Location: Continental crust

Eruptions: Explosive and unpredictable

Caldera volcanoes usually form when a catastrophic eruption has caused much of the volcanic summit to collapse and leave an enormous crater behind. New eruptions may form smaller cones.

Rock type: Andesitic

Location: Destructive margins

Eruptions: Very explosive and unpredictable

Fissure volcanoes occur where an elongated crack allows lava to flow out over a large area. The low viscosity of the lava allows the volcano to remain fairly flat as the lava flows a long distance.

Rock type: Basaltic

Location: Rifts/early constructive margins

Eruptions: Gentle and persistent

Shield volcanoes are incredibly wide volcanoes which have gently sloping sides thanks to the low viscosity of the lava they produce. 

Rock type: Basaltic

Location: Hot spots and areas where two oceanic plates converge

Eruptions: Gentle and predictable 

Composite volcanoes are the most common type of volcano. They are cone shaped as the lava is fairly viscous and are formed with alternating layers of ash and lava. 

Rock type: Andesitic

Location: Destructive margins

Euptions: Explosive and unpredictable

                                              Basaltic                     Andesitic                     Rhyolitic

Silica content                            Low                         Medium                         High

Viscosity                              Low (Runny)                   Medium                High (Thick and sticky)

Temperature of eruption           High                        Medium                           Low 

                                     (Over 950degreesC)        (750-950degreesC)    (Less than 750degreesC)

In volcanic regions much of the magma never reaches the surface. Instead, the magma cools beneath the ground and forms intrusive features such as Batholiths, Dykes, and Sills. 

Batholiths:

These are large chambers of magma which slowly cool to form huge domes of igneous rock. They can be several hundred km wide and the slow cooling allows large crystals to form.

Dykes and Sills:

The magma from a magma chamber may flow into gaps in the surrounding rock before cooling. If the cooling occurs vertically it is called a Dyke whereas if it occurs horizontally it is a Sill.

During the cooling of the magma cracks may form and these are called cooling cracks.  

Extrusive activity is volcanic activity which takes place above ground and the major form is volcanic eruptions however minor forms can include Hot Springs, Geysers, and Boiling Mud. 

Hot Springs: Where groundwater passes an area of recent intrusive volcanic activity it becomes heated. The groundwater becomes a hot spring at the point it emerges at the surface and can be anywhere between 20 and 90degreesC. 

Geysers: Areas where hot water and steam are ejected from the ground in a fountain. When groundwater is heated to above boiling point it turns into steam which increases the pressure. The pressure forces water and steam up through cracks in the rock before it sprays out from a vent.

Boiling Mud: Boiling mud is created when a heated water mixes with mud and surface deposits to form a muddy pool. 

Solfatara: Created when suphurous gases escape onto the surface.

Fumaroles: Heated water turns to steam when it reaches the surface due to the drop in pressure.

Lava flows aren't the only hazard created by a voclanic eruption. Ash clouds, Tephra, Acid rain, Pyroclastic flows, Landslides, Lahars, and Jokulhlaups can also occur.

Ash clouds can travel many km high and cause disruption to air traffic.

Tephra is solid material of varying sizes that is ejected into the atmosphere during an eruption.

Acid rain is caused by the volcanic gases and tephra can lead to erosion of alkaline rocks. 

Pyroclastic flows are very hot (over 1000degreesC) and very fast (200km/h) flows which contain a mixture of tephra and volcanic gases and often destroy everything in their path. 

Landslides can occur if the volcanic eruption weakens the side of the volcano. 

Lahars are mixtures of rock, water, and mud that are caused when an eruption disturbs a lake or icefield and the water is mixed into the erupted particles. Lahars can be very deadly as they follow valleys, where human settlements are concentrated.

Jokulhlaups occur when the eruption happens beneath an icefield which leads to huge volumes of ice melting causing massive floods. 

Earthquakes are caused by the stresses that build up as tectonic plates interact with each other and can occur at all 3 types of margin.

When plates jerk past each other they send out seismic waves which is the earthquake.

Focus: The place in lithosphere where the earthquake occurs and where seismic waves spread out from.

Epicentre: The first place where the earthquake is felt, directly above the focus and on the surface.

There are 3 types of seismic wave:

• P (Primary) - Fastest type of seismic wave. Push and pull the earth in the same direction as the wave travels. Can travel through both solids and liquids.
• S (Secondary) - Slower than P waves. Move rock particles up and down. Can travel through solids only.
• Surface (Love & Rayleigh) - Can travel through the crust only and no deeper. Love waves travel through solids only and move side-to-side. Rayleigh waves can travel through liquids and solids and move like sea-waves. 

There are 2 main ways that earthquakes are measured: The Richter Scale and the Mercalli Scale.

Richter Scale

• Measures the magnitude of an earthquake. Records on a scale increasing by 0.1 from 1. 
• Logarithmic. A magnitude (X) quake is 10 times as powerful as a magnitude (X-1). 
• Energy released is 30x more in a magnitude (X) than magnitude (X-1).

Mercalli Scale

• Measures the impact an earthquake has using observations of the event. Scale from 1 - 12
• Example: Magnitude 1 is only detected by instruments, Mag. 6 is felt by everyone and many objects moved, and a Mag 12 is total destruction.
• Subjective and not scientific however. 

Tsunamis are caused by huge volumes of water being displaced. This displacement can be due to underwater earthquakes as well as volcanic eruptions and landslides which slide into the sea. 

A tsunami loses energy as it moves however they can travel vast distances such as all the way across the pacific and therefore they present a huge danger to people. 

The waves of a tsunami can start travelling at over 500 km/h with a large wavelength (200km) but have a small amplitude (1m).

As they travel closer inland the sea becomes shallower and the waves get compressed. The water is now travelling at around 80km/h, the wavelength has decreased (less than 20km), and the amplitude increased (several metres). 

Earthquakes

Prediction:

Currently impossible to predict when one will occur but there can be small clues e.g. small tremors, cracks, and weird animal behaviour. 

Less damaging P waves can be detected after the earthquake has occured and give people a very short notice to prepare for strong tremors. 

Can predict where future earthquakes will occur (fault lines) so they can prepare for them.

Protection:

Buildings can be designed to withstand earthquakes (reinforced concrete/materials that absorb the shock)

Construction laws can ensure that buildings in earthquake prone areas can withstand the tremor.

Volcanoes

Prediction:

Possible to predict volcanoes through study of the volcano. Looking for changes in shape and tiny earthquakes. 

Remote sensing can be used to detect emissions of dangerous gases which may signify an eruption.

Prediction like this mean people can evacuate. E.g. Mt. Pinatubo - 60,000 people evacuated in time.

Protection:

Buildings can't withstand lava flows however you can strengthen them to withstand the ash. 

Lava flows can be diverted away from settlements using barriers (made of rubble) if it is moving slow enough. The lava can also be forced to change course by using explosives. 

Tsunamis 

Prediction:

Tsunami warnings are issued if a seismic event occurs in an area likely to cause a tsunami. 

There are tsunami warning centres around the world however they rely on good communication and if people don't get the message in time then they can't evacuate. 

Protection:

Buildings on raised, open foundations and made of strong materials less likely to be damaged. 

Tsunami walls have been built to protect areas extremely prone to tsunamis. 

Tsunamis 

Prediction:

Tsunami warnings are issued if a seismic event occurs in an area likely to cause a tsunami. 

There are tsunami warning centres around the world however they rely on good communication and if people don't get the message in time then they can't evacuate. 

Protection:

Buildings on raised, open foundations and made of strong materials less likely to be damaged. 

Tsunami walls have been built to protect areas extremely prone to tsunamis. 

Planning and Education

• Future developments can be planned to minimise the risk.
• Emergency services can train and prepare for disasters so that when one does occur they know exactly what to do.
• Governments can issue evacuation plans and educate on what to do. In Japan there are regular (annual) earthquake and tsunami drills

Development Level of the Country

Poorer countries

Don't have the money for disaster prevention or response.

Buildings and infrastructure of poorer quality so they are more susceptible to damage.

Healthcare isn't as good - struggle to treat as many casualties well.

More people depend on agriculture which is usually badly affected by the hazards.

Rich Countries

Economic impact greater as buildings and infrastructure are worth more.

Population

More people in the area = more affected by hazard.

Densely populated areas have taller buildings usually. Collapse is a big problem.

Large numbers of people make it harder to evacuate everyone safely. Congestion as everyone tries to escape the area. 

Timing

Time of day or year can alter the severity of the impact.

More people would be asleep and unable to escape in time if the disaster occured at night.

Winter weather means that many might freeze to death before they are rescued.