Plate Tectonics And Associated Hazards- Plate movement Flashcards
Mantle
Surrounds the core. Most of the mantle (asthenosphere) is semi molten.
Lithosphere
Consists of the crust and the rigid upper section of the mantle
Asthenosphere
This is below the lithosphere and is semi molten.
Oceanic crust
Thinner
Younger
Denser
Basalt rocks known as sima
Continental crust
Thicker
Older
Less dense
Mainly granite rocks known as sial
Pangea
An ancient supercontinent (the theory which suggests that all contingents were once all joined together)
Alfred Wegener
In 1912, he published a theory suggesting that all the continents were once all joined together in a supercontinent called Pangea.
He said that all the land masses drifted apart until they occupied their current positions.
Evidence of continental drift
Jigsaw fit- some continents seem to fit together if placed beside each other (Africa and South America)
Geological evidence- rocks of the same age and type and displaying the same formations are found in South East Brazil and South Africa.
Biological evidence- similar fossil formations are found on either side of the Atlantic
Problem with Wegeners theory
His theories were unable to explain how continental movement could have taken place.
Core
Two parts: a semi molten outer core and a solid inner core.
Temperatures can reach 5000 degrees C
The heat generates convention currents within the mantle
Palaeomagnetism
The study of the record of he earths magnetic field in rocks.
Every 400,000 years or so, the earths magnetic field switches polarity. The rocks that form the ocean crust show this alternating polarity.
As the lavas solidify, the particles provide a permanent record of the earths polarity at the time.
The result is a series of magnetic ‘stripes’
Convection currents
The heat from the core causes the mantle to be heated at its base. The hot rock rises towards the crust. As it rises, it moves further away from its heat source at the core and it starts to cool down. When it reaches the crust, it is forced out sideways. It continues to cool and as it cools, it begins to sink back down towards the core.
Constructive plate margins landforms
Mid oceanic ridges
Rift valleys
Volcanoes
Destructive plate margins landforms
Oceanic trenches Told mountains Volcanoes Earthquakes Island arcs
Mid oceanic ridges
An underwater mountain range.
Convection currents move the plates apart which leaves cracks and fissures and lines of weaknesses that allow magma to escape.
Eventually magma fills the gap and eventually erupts on the surface and cools as new land
Rift valleys
A Rift Valley is a lowland region created by the process of sea floor spreading.
Eg the Great African Rift Valley
They are formed when the lithosphere stretches, causing it to fracture into sets of parallel faults.
The land between these faults then collapses into deep, write valleys which are separated by upright blocks of land called horsts
Ocean trenches
Ocean trenches are deep areas of sea which form at destructive plate margins.
2 plates move together due to convection currents and the denser ocean crust is subducted underneath the continental crust.
An ocean trench marks the point at which the plate is being subduction.
Eg the Chille-Peru trench which is over 8km deep
Active volcanoes
Have erupted in living memory
Intermittent/ dormant volcanoes
Have erupted within historical record
Inactive volcano
Haven’t erupted in the last 25000 years
Distribution of volcanoes
Most volcanic activity is associated with tectonic process and are mainly located along plate margins in belts.
Along the ‘ring of fire’ that surrounds the Pacific ocean
Over hot spots such as the one in the middle of the Pacific Ocean which has formed the Hawaiin islands
Benefits of volcanoes
Lava flow can create land
Igneous rocks contain valuable mineral deposits
Lava and ashes weather quickly into fertile soils
Volcanoes can be great tourist attractions
Volcanic hazards
Lava flows burn and bury crops
Flooding results from lava flows
Submarine, coastal or island eruptions can cause tsuniamis
Ash falls disrupt transport, pollutes the air and causes breathing difficulties
Violent eruptions of pyroclastic flow destroys life and property
Volcanic dust absorbs solar energy and so lowers atmospheric temperature
How volcanoes occur at destructive margins
Convection currents in the asthenosphere cause the plates to move together.
The denser oceanic plate is being moved towards the thicker lighter continental plate.
The subduction plate begins to melt due to friction and heat in the mantle. This occurs in the Benioff zone. Water from the ocean above also descends and mixes with magma to make it more acidic and viscous.
Lighter, gaseous magma under pressure rises up through weaknesses in the continental crust above.
The magma erupts at the surface as lags along with ash and gases.
Subsequent eruptions over time cause a volcano to form from layers of ash and lava.
Why volcanoes occur at constructive margins (oceanic-oceanic)
Convection currents in the aesthenosphere pull the two oceanic plates apart.
This allows magma to reach the surface and solidify to form new crust and volcanoes in some places.
Why volcanoes occur at hotspots
In places like Iceland, hotspots lead to the formation of volcanoes where a rising plume of superheated magma thins and heats the crust, allowing magma to reach the surface.
Over time, layers of lava build up to form volcanoes.
Why volcanoes occur at contrastive margins (continental-continental)
Convection currents beneath continental crust pulls the crust in opposite directions, creating faults and weaknesses.
Magma rises to the surface and solidifies as lava flows.
Material erupted from a volcano
Gaseous emissions- dominated by steam but also more dangerous gases such as carbon monoxide and chlorine.
Solids- included ash, dust and glassy cinders.
Liquids- Tephra which solidify in mid air. Tephra ranges in size such as large lava bombs and in its fine form of hair like trails called lapilli. Also surface lavas which can be acidic or basic
Basaltic lava
Formed from the upward movement of mantle material.
Magma is low in silica which makes it a more fluid magma that allows gas bubbles to expand on the way up- preventing sudden explosive activity
Andesitic lava
Formed at destructive margins where crust is being destroyed.
Formed from silica rich magma that is very visous
Lava often solidifies before reaching the surface, leading to a build up of pressure and a violent explosion.
Gases don’t escape from the viscous lava so pressure builds up.
Rhyolithic lava
Formed at destructive and collision margins
Same characteristics as andesitic lava (bit thicker)
Volcanic islands
Where there is a split in the crust, a lower pressure zone is created where lava can erupt to form submarine volcanoes.
If these eruptions continue, volcanoes may develop until they reach the surface, forming volcanic islands.
Island arcs
Island arcs are curved chains of volcanic islands.
Subduction processes result in the formation of volcanoes which over millions of years, pile up lava on the ocean floor until a submarine volcano rises avoid sea level to form an island volcano.
Such volcanoes are typically strung out in curved chains called island arcs.
The magma that forms island arcs are produced by the partial me,ting of the descending large and or the overlying oceanic lithosphere
