AQA Geography - Plate Tectonics and Volcanoes

Facts about the crust
This is the outer and thinnest layer of the earth. It is between 5-10km thick below the oceans and up to 70km thick under the continents. There are 2 types; Oceanic & continental.This is the zone where tectonic plates are formed.
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What types of crust is there?
Oceanic is basaltic rocks known as sima (made of silica and magnesium) Continental is mainly consists of granitic rocks known as sial, which is much thicker than oceanic sima, but is less dense.
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Facts about the Mantle
The widest section of the earth- 2900km thick Due to the heat and pressure within this zone, the mainly silicate rocks are in a thick, liquid state. There are 2 parts- the upper and lower
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Facts about the Core
This layer is approximately 3490km in depth It is the hottest layer of the earth It is made up of a mixture of iron and nickel, with a little sulfur. There are 2 parts- the inner and outer
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Convection Currents
Heat rising and falling inside the mantle creates convection currents generated by radioactive decay in the core. The convection currents move the plates. Where convection currents diverge near the Earth's crust, plates move apart.
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Ridge push
As fresh magma wells up at mid-ocean ridges to form new young, oceanic lithosphere, a higher elevation is formed at spreading ridges. The new oceanic crust gradually cools and thickens with age and is pushed ‘downhill’ as new magma emerges
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Slab pull
In the current understanding of plate motion the movement is driven by the weight of cold, older, dense plate material sinking into the mantle at deep ocean trenches and pulling the rest of the plate slab with them as gravity causes them to slidedow
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A constructive plate boundary, sometimes called a divergent plate margin, occurs when plates move apart. Volcanoes are formed as magma wells up to fill the gap, and eventually new crust is formed.
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This occurs when oceanic and continental plates move together. The oceanic plate is forced under the lighter continental plate. Friction causes melting of the oceanic plate and may trigger earthquakes. Magma rises up through cracks and erupts
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A conservative plate boundary, sometimes called a transform plate margin, occurs where plates slide past each other in opposite directions, or in the same direction but at different speeds.
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what are the 3 main types of volcanoes?
Shield, Composite and Cinder
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Shield volcanoes are composed almost entirely of relatively thin lava flows built up over a central vent.
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How are most shields formed?
by low viscosity basaltic magma that flows easily down slope away form the summit vent. The low viscosity allows the lava to travel down slope on a gentle slope, but as it cools and its viscosity increases, its thickness builds up on the lower slopes
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What type of explosions forms shield volcanoes?
Shield volcanoes form by relatively non-explosive eruptions.
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How are most composite formed?
The steep slope near the summit is due partly to thick, short viscous lava flows that do not travel far down slope from the vent. The gentler slopes near the base are due to accumulations of material eroded from the volcano and to the accumulation
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What type of explosions forms composite volcanoes?
Due to the higher viscosity of magmas erupted from these volcanoes, they are usually more explosive than shield volcanoes.
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Why are composite dangerous?
Long periods of repose (times of inactivity) lasting for hundreds to thousands of years, make this type of volcano particularly dangerous, since many times they have shown no historic activity, and people are reluctant to heed warnings
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Cinder cones are small volume cones consisting predominantly of tephra that result from strombolian eruptions. They usually consist of basaltic to andesitic material.
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How are most cinders formed?
They are actually fall deposits that are built surrounding the eruptive vent.usually occur around summit vents and flank vents of composite.
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where are cinders found?
Cinder cones often occur in groups, where tens to hundreds of cones are found in one area.
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What determines the Explosivness of Eruptions?
If a magma has a lot of gas which becomes trapped in the magma, the pressure will build and build until eventually the magma erupts explosively out of the volcano.It's a bit like a bottle of fizzy soda.
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Phreatomagmatic eruptions
Phreatomagmatic eruptions are a type of explosive eruption that results from the magma erupting through water. The second phase of the Ejyafjallajökull eruption in 2010 was phreatomagmatic as a result of magma erupting under the ice.
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what can Explosive eruptions form?
Explosive eruptions can form pyroclastic flows which sweep down valleys destroying everything in their path. They also send ash high into the atmosphere forming plumes.
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Effusive Eruptions
If a magma has low viscosity (it is runny), gas can escape easily, therefore, when the magma erupts at the surface, it forms lava flows. These eruptions are gentle effusive eruptions.
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Where are effusive eruptions found?
Effusive eruptions commonly occur at hot spots, like Hawai'i and Iceland, and at areas of sea-floor spreading.
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How do you measure eruptions?
Volcanic explosivity is measured using the VEI scale (Volcano Explosivity Index) which is a measurement of the volume of erupted material. Hawaiian and Icelandic eruptions are commonly found near the bottom of the VEI scale
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How are hazard profile charateristed?
predictablity, frequancy, Duration, speed of onset, areal extent
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the probability of an event occurring over time and space
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How difficult is prediction?
Prediction can be very difficult. Volcanologists are able to monitor the volcano and they are able to look at the history of it and the type of activity it produces. These are important in understanding the magnitude and frequency of eruptions
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How are volcanoes measured?
Volcanoes are measured using the Volcanic Explosivity Index, or VEI.
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What charateristics does the scale have?
This scale uses several volcanic characteristics to judge the size of an eruption. It uses the eruption cloud height, the volume of products erupted and more subjective qualitative observations to attribute a number to the eruption from 0 to 8.
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What does a high VEI mean?
If the magnitude had a high VEI the n the likelihood of it being high again will pose a very serious threat to the environment
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it is possible to identify areas at greatest risk and land use planning can be applied to avoid building in high risk areas.
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Example of protection
In 1973, the inhabitants of Haeimaey (Iceland) were able to divert a lava flow by pouring seawater on the front so it would quickly solidify.
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What are the needs after the event?
Such aid is needed for monitoring, evacuation, emergency shelters and food, long-term resettlement of the population and the restoration of the area’s economic base and infrastructure.
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Immedaite responses
Immediate responses are those which take place just before or after the volcanic event such as warnings, evacuation and attempts to stop lava flows.
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Long term responses
In the long term, responses include the monitoring of a volcano, research into finding new methods of prediction and even building barriers in anticipation of the direction of lava flows and lahars.
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Short-Term responses to monserat
NGOs, such as the Red Cross, set up temporary schools and provided medical support and food.​ Warning systems were set up to alert inhabitants – sirens, speaker systems and via the media.​ Troops from the USA and the British Navy came to aid.
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How did the UK help monserrat short term?
£17 million in UK aid paid for temporary buildings and water purification systems.
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How did the UK help monserrat long term?
UK financial aid since 1995 has exceeded £420 million.
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How did the volcano effect vegetation?
Vegetation is slowly regrowing as the ash, lava and lahar deposits break down. Fertile soil means that land will again be used for cask crops such as cotton. But 2/3rds of the island remains an exclusion zone and warning sirens are tested daily at 12
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Long term effects of the eruption
Rebuilding tourism with the volcano itself as an attraction. Montserrat was selected by National Geographic Adventure Magazine as one of the ‘Top 25 New Trips of 2010’. A new airport, hotel and dive shop have been built.
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Other cards in this set

Card 2


What types of crust is there?


Oceanic is basaltic rocks known as sima (made of silica and magnesium) Continental is mainly consists of granitic rocks known as sial, which is much thicker than oceanic sima, but is less dense.

Card 3


Facts about the Mantle


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Card 4


Facts about the Core


Preview of the front of card 4

Card 5


Convection Currents


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