Tectonics Flashcards
Structure of earth
Crust: Continental crust and Oceanic crust
Mantle
Outer core
Inner core
Crust
A thin layer of solid rocks around the outside of the earth.
2 types of crust:
* Continental crust is a crust made up of land and is mainly composed of Granite.
* Oceanic crust is a crust made up of oceans it is composed of Basalt.
* The earth’s crust is broken up into large slabs of rock called tectonic plates
Mantle
A layer of semi-molten (melted rock)
* The thickest layer.
* Heat currents called convection currents to rise and fall in the mantle.
Outer core
A layer of molten (melted) rock.
* Average temperatures of 3000 degrees C.
* Composed of iron / nickel.
Inner core
The centre of the earth
* Temperature of about 2700 degrees C
* Described as a solid ball of iron and nickel
* It is in a solid state due to pressure from the other layers.
Major tectonic plates
- The Pacific
- The Eurasian.
- The African
- The American
- The Indo Australian
- The Nazca Plate
- The Antarctic plate.
What are tectonic plates?
Tectonic plates are sections of very thin crust.
* They float like rafts on the semi-molten material that makes up the earth’s mantle.
* These plates move on top of the mantle by a series of heat (convection) currents.
* The movement is very small at less than 1 cm per year, but it can result in volcanic
eruptions and earthquakes.
* Earthquakes and volcanoes are found near plate boundaries. They can happen on the
seabed as well as on land.
Different plate movements and why they happen
There are 4 ways in which the plates can move due to the convection currents:
Destructive/convergent plate margin
Constructive/divergent plate margin
Conservative/transform margin
Collision (convergent/destructive)
Destructive/convergent plate margin:
- This happens when oceanic and continental plates move together.
- The oceanic plate is denser (heavier) than the continental one and is forced down (subducted).
- The continental plate is forced up (Fold Mountains).
- When the oceanic plate reaches the mantle, it melts forming magma.
- The heated magma is less dense and so rises through cracks to the surface, it erupts forming a volcano.
- Plates can become locked together, pressure builds up as the plates are trying to move and earthquakes occur when the tension is released.
Constructive/divergent plate margin:
- Convection currents at a constructive margin are pulling plates apart.
- The sea floor spreads, and magma wells up to fill the gap.
- The magma erupts forming a volcano.
- New crust is created by the rising magma, this creates mid–ocean ridges – chains of submarine mountains and volcanoes
E.G the Mid-Atlantic Ridge. - New crust is continually being made a constructive boundary and over a long period of
time it can rise forming volcanic islands e.g., Iceland. - Minor earthquakes occur as the plates pull apart.
Conservative/transform margin:
- Convection currents cause two plates slide past one another.
- Plates can become locked together, pressure builds up as the plates are trying to move and earthquakes occur when the tension is released. E.G. Turkey-Syria Earthquake 2023.
- No crust is created or destroyed, and no volcanic
eruptions take place.
Collision (convergent/destructive):
- Continental plates converge (move towards each other).
- Since two plates of land are of a similar density neither is subducted.
- Instead, plates are buckled and pushed up to form Fold Mountains (e.g., Himalayas).
- There is no subduction and so no volcanoes.
- However, violent earthquakes can occur. As plates push together pressure builds and earthquakes occur when the tension is released. E.G. Nepal Earthquake 2015.
Volcanic Hotspots
- Hotspot volcanoes can be found both along and away from plate boundaries.
- Hotspots are unusually hot magma plumes that rise in the mantle and erupt through the crust.
- This creates seamounts which are volcanic mountains beneath the sea.
- Seamounts can rise above the ocean surface to create islands.
- The Hawaiian Island chain formed as the Pacific crustal plate moved over a stationary source of magma or hotspot.
- There are many volcanoes on the Hawaiian islands, however, only a few are active.
- When volcanoes move off the hotspot they lose their source of magma and become extinct.
Volcanoes at the Hawaiian hotspot include:
- Loihi – a submarine volcano that is continuing to grow in size.
- The Big Island of Hawaii is at the hotspot and has several active volcanoes e.g. Mauna Loa and Kilauea.
Types of volcano
Shield volcanoes
Composite/strato volcanoes
Volcanoes are classified as:
- Active – if they have erupted recently
- Dormant – resting – have not erupted recently
- Extinct – will not erupt again
Shield volcanoes
- Shield volcanoes have very runny lava (non-viscous); because of this they do not have an ‘explosive’ eruption.
- Lava spreads quickly across the landscape.
- With each eruption a new layer of rock is built on the previous one.
- Gradually a wide dome of rock is built up.
- It is called a shield volcano because it looks like a curved shield lying on the ground (or an upside down dinner plate).
- The slopes of a shield volcano are very gentle.
- The Hawaiian islands are a chain of shield volcanoes
Composite/strato volcanoes
- Composite volcanoes are the most common type of volcano.
- When you think of a volcano you are probably picturing the classic cone shape of the composite volcano.
- They are formed by hardened layers of lava and ash from successive eruptions.
- The lava is viscous (therefore thicker than with shield volcanos) and it cools and hardens before spreading very far, therefore the volcanoes are steep-sided.
- The eruptions tend to be very violent, capable of producing deadly pyroclastic flows.
- Mount Vesuvius in Italy is an example of a Composite Volcano.
Features of a volcano
Ash, Dust and Gas
Eruption
Crater
Vent
Magma Chamber
Cone
Lava
Prediction for volcanoes erupting
Volcanic eruptions can destroy people and the environment. We need to predict and plan for the
eruption. This helps reduce the loss of life and damage. Some of the methods scientist use to
predict are:
* Lasers to detect the physical swelling of the volcano
* Chemical sensors to measure the increases in sulphur levels
* Seismometers to detect the large number of earthquakes that occur due to the magma rising up
* Satellite images to record the warming of the ground surface as the magma edges towards the ‘breaking through point’.
Some of the methods scientists and local authorities use to plan for eruption are:
- Have an evacuation plan, e.g,. supplies of food and water, medical facilities, face mask to prevent inhaling ask and temporary homes (tents).
- Be ready to establish exclusion zones if needed. For example, in Montserrat in 1997 an exclusion zone covering 2/3 of the island was established to protect lives.
- Government agencies such as the police organise the evacuations. These need to be practiced and publicized (in the eruption of Eyjafjallajökull in 2010 800 people who were at risk of flooding were evacuated).
- Have plans to manage lava flows if they threaten valuable areas e.g., in Sicily Mount Etna regularly produces lava flows, the authorities may use powerful jets of water to attempt to divert the flow away from valuable areas.
Eyjafjallajökull Volcanic Eruption (2010)
Location: Southern Iceland
Date of eruption: April 2010
Location of the volcano Eyjafjallajökull Volcanic Eruption (2010)
The mountain lies within the country’s East Volcanic Zone.
Its name originates from an Icelandic phrase meaning “the island’s mountain glacier,” and the
volcano itself lies beneath Eyjafjallajökull (Eyjafjalla Glacier).
Its highest point rises to 5,466 feet (1,666 metres) above sea level.
Causes of the eruption Eyjafjallajökull Volcanic Eruption (2010)
Iceland has formed at a divergent plate boundary due to the North American and Eurasian plates moving apart. In addition, Iceland also lies over a hotspot or mantle plume – an upsurge of abnormally hot rock in the Earth ́s mantle.
As the plates moved apart, excessive eruptions of lava constructed volcanoes and filled rift valleys.
This has created the Mid Atlantic Ridge, a ridge of mountains and volcanoes that are mostly below sea level.
However, there are also volcanic islands along the ridge, with Iceland being the largest because of
the additional volcanism caused by the hotspot under the country.
The plates are moving apart at a rate of 1cm to 5 cm per year.
This has created a chain of volcanoes along the SE Rift zone of Iceland, which runs from NE to SW across Iceland, even passing underneath some of the countries ice caps.
