plate tectonics Flashcards
What is the earths structure
Core
Mantle
Crust
Lithosphere
What elements are in the crust
Silicon, oxygen, aluminium, pottasium
How thick is the crust below the ocean
6-10km
How thick is the crust below land
30-40km
How thick is the lithosphere
60-70km
What are the two types of crust
Continental and oceanic
Which type of crust is denser
Oceanic
What is the composition of continental crust
Mainly granite, silicon,aluminium and oxygen
What is the composition of oceanic crust
Basalt, silicon,magnesium and oxygen
How do the plates move
Convection currents which push and pull the plates
What was Alfred wegeners theory of tectonic
That tectonic plates have evolved from pangea, 300 million years ago
How is sea floor spreads evidenced
Examination of the oceanic crust either side of the mid Atlantic ridge (alternating polarity of magnetic rock)
What create thermal convection currents
Hot spots in the mantle
Biological evidence for continental drift
-Fossil remains of mesosaurus in Brazil and South Africa
-fossil brachiopods found in Indian limestone and Australia
-fossil remains of plant which existed when coal was being formed found in India and Antarctica (coal only forms in tropical conditions)
Evidence for continental drift- climatology
Evidence of glaciation found in South America and India
Evidence for continental drift- geology
Rocks of similar age,formation and structure found in Brazil and South Africa
Coal sandstone and limestone could not have been formed in Britain with its current climate
Palaeomagnetism
In underwater eruptions basaltic lava cools
Rock aligns with earths magnetic field
This is reversed every 400,000 years
Bands of rock facing different poles are formed
Constructive margins occurs where
Two plates move away from eachother
What do constructive boundaries form
Rift valleys
Mid oceanic ridges
Volcanic activity at constructive boundaries
Gentle slides
Basaltic lava
Gentle eruptions
Frequent
Effusive
Transform faults
Occur at right angles to boundary as new crust is formed
Friction and earthquakes )shallow focused)
Rift valleys
-form when plates move apart on continental areas
-areas of crust drop down parrallel faults
-forms an upstanding block known as a horst
-in east african rift valley it is said to be an emergent plate boundary and forming a new ocean
Example of rift valley
East African rift valley
Extends for 5000km from Mozambique to the Red Sea and Jordan
Width varies from 10-50km
Conservative plate margins occur when
Two plates move past eachother
What hazards occur at conservative margins
Earthquakes only
Example of conservative margin
San Andreas fault in california
Destructive margins occur when
Two plates move towards each other and the heavier plate sinks beneath the lighter plate
Example of destructive margin
Andes, west coast of South America
Nazca plate subducts under South American plate
Molten magma rises forming explosive volcanoes
What are island arcs and how are they formed
- subducting plate becomes hot
-pressure and friction melts the plate
-rises to the surface as a pluton of magma
-composite volcano islands are formed
Where are fold mountains formed
-Destructive continental and continental boundaries
-there is no subduction
-edges and sediments are forced upwards
-there is no volcanic activity
-indo-australian plate travelling nirthwards into eurasian
-formed the himalayas
How do fold mountains form
Lower density continental courts collide
Plate edges forced upwards
No subduction so no volcanic activity e.g. the Himalayas
Hotspots
Areas on the crust of volcanic activity that are not related to hazards
What do hot magma plumes do
Burn through weaker parts of the crust and states in the same plates as the plate moves
Hawaiian chain of volcanic islands facts and figures
60° bend in the shape of the islands
3,700 miles long
Travelling NW
Are getting eroded
Up to 5 million years old
What does Tuzo Wilson say creates magma plumes
Static radioactive elements
What does Foulger 2003 suggest forms hotspots
Weakness in tectonic plates
Faults in the crust allow plume of magma through
What is a mantle plume
A pocket of magma which has melted through the asthenosphere from the core due to static radioactivity
What is the most common rock type formed by the eruption of Hawaiian volcanoes
Basaltic rock (low silica and low gas content)
ocean ridges
longest continuois uplifted features
when plates move apart in oceanic areas
space is filled with basaltic lava, cools and forms magnetic rock
volcanic acitivity - submarine volcanoes
deep sea trenches
where oceanic and continental plates move
denser plate subducts
nazca under south american
formed peru-chile trench
island arcs
during subduction the descending plate enters hotter surroundings generated by frcition
begins to melt the plate
material is less dense and rise towards surface as plutons of magma
eventually reach surface and form compositive explosive volcanoes
Sea floor spreading theory
After discovery of mid Atlantic ridge
Altenratating polarity of the rocks - iron/magnetic particles in erupted lava
Poles reverse every 400,000 years
Results in magnetic stripes that are aligned to the poles
Oceanic crust gets older with the distance from the ridge
Ridge push / gravitational sliding
Constructive boundaries
Upwelling of hot material at ocean ridges - creates buoyancy effect
Plates experience force that acts away from the ridge
Gravity acting down the slope of the ridge
Slab pull
Destructive boundaries
Downward gravitational force on the cold dense decending plate
Pills the whole oceanic plate down as a result of negative buoyancy
Frictional resistance - gives rise to shallow and deep earthquakes
