Plate Tectonics Flashcards
How thick is the continental crust?
30-70km thick
How thick is the oceanic crust?
6-10km thick
How dense is continental rust?
2.6
How dense is oceanic crust?
3
Which type of crust is denser?
Oceanic
What is the composition of continental crust?
Granite
Silicon
Aluminium
Oxygen
What is the composition of oceanic crust?
Basalt
Silicon
Magnesium
Oxygen
What alloys are found in the core?
Iron and nickel
What is the physical state of the inner core?
Solid
What is the physical state of the outer core?
Liquid
Why does the core produce heat?
Primordial heat
Radiogenic heat
What is meant by primordial heat?
This is heat left over from the Earth’s formation.
What is radiogenic heat?
Heat from the radioactive decay of isotopes.
What type of rock makes up the mantle?
Molten and semi-molten rocks
What elements are present in the mantle?
Lighter elements like silicon and oxygen.
What does the lithosphere consist of?
The crust and the rigid upper section of the mantle.
How thick is the lithosphere?
80-90km thick
What is the lithosphere divided into?
Large and small plates.
Where is the asthenosphere located?
Beneath the lithosphere.
In what state is the asthenosphere?
Semi-molten
What do the plates do in the asthenosphere?
They float and move.
What did Francis Bacon notice about the continents?
They all fit into each other.
Name two continents that fit together.
The South American bulge fits into the indent below West Africa.
What was Wegner’s contribution to continental drift?
Pangea existed about 300million years ago.
This split into two.
Further splitting resulted in the continents that we have today.
What were the names of the continents that originally split from Pangea?
Laurasia
Gondwanaland
Where was Gondwanaland located?
In the south.
Where was Laurasia located?
In the north.
Give 4 pieces of geological evidence for continental drift.
South America and West Africa appear to fit together,
Late carbon-ferous glaciation deposits found in Antartica, South America and India.
Striations on rocks in Brazil and West Africa point to a similar situation.
Similar rock sequencing in Scotland and eastern Canada.
List 3 pieces of biological evidence for continental drift.
Branchiopod fossils found in India and Australia.
Mesosaurus reptile fossils found in South America and South Africa.
Fossilised remains of a plant that grew when coal was formed have only been located in India and Australia.
Explain how the discovery of the mid-Atlantic ridge develops the theory of continental drift.
The mid-Atlantic ridge is evidence for sea floor spreading and shows the alternating polarity of the rocks in the crust which suggests that the seafloor is spreading.
Why does iron particles in lava suggest sea floor spreading?
A permanent record of the Earth’s polarity at the time of a volcanic eruption. This creates alternating strips showing the Earth’s polarity and therefore magnetic field at a specific time. The magnetic strips get further and further away from the boundary because of sea floor spreading.
If sea floor spreading is happening, what else must be happening?
Some of the crust must be being destroyed.
Where does the destruction of crust occur?
At trenches.
What happens at oceanic trenches?
Large areas of ocean floor are pushed down by subduction.
How do plates move?
By convection currents.
How do convection currents work?
Warm magma is less dense and rises to the surface whilst cool magma contracts and becomes more dense so it sinks, creating a movement known as a convection current.
What happens to the plates at a constructive plate boundary?
Plates move apart
What happens as the plates move apart at a constructive plate boundary?
Volcanoes form as magma wells up to fill the gap and eventually, new crust is formed.
Give an example of a constructive plate boundary
The mid-Atlantic Ridge
What happens to the plates at a destructive plate boundary?
The denser oceanic plate is sub ducted under the continental plate.
What happens as the oceanic plate is sub ducted under the continental plate?
Friction causes the oceanic plate to melt and may cause earthquakes.
Magma rises through cracks and erupts.
Give an example of a tectonic plate being subducted.
The Nazca plate is being subducted under the South American plate.
Where do collision plate boundaries occur?
Where two continental plates collide.
Why do fold mountains form at collision plate boundaries?
As both plates are continental, neither is subducted meaning that both plates are forced upwards.
Give an example of fold mountains.
Himalayas
What happens at conservative plate boundaries?
Plates slide past each other in opposite directions but at different speeds.
Why do earthquakes occur along conservative plate boundaries?
When plates slide past each other, the friction builds up. This friction is then eventually released as a shockwave which produces an earthquake.
Give an example of a conservative plate boundary.
San Andreas fault, California
What creates a buoyancy effect at constructive boundaries?
The upwelling of hot material at ocean ridges which causes the material to become less dense.
Why does ridge push take place?
Gravity acts down the slope of the ridge as a result of the magma cooling and becoming denser.
On what type of boundary does ridge push occur?
Constructive
What is ridge push also known as?
Gravitational sliding
What indicates that there is some frictional resistance to ridge push?
Shallow earthquakes occur when new crust is repeatedly torn apart.
What is slab pull?
A large gravitational force that action the dense oceanic plate, creating a negative buoyancy and pulling the whole plate down.
Where does slab pull take place?
At destructive plate boundaries
Why is there frictional resistance at destructive plate boundaries?
Due to the pushing of the sub ducting plate against the overriding plate.
What can frictional resistance at destructive plate boundaries lead to?
Both shallow and deep earthquakes in the subduction zones
What suggests that the relative importance of driving forces varies from plate to plate?
Plates move at different speeds.
True or false: plates are moved by one single agent
False, plates are moved by a variety of forces
When are oceanic ridges formed?
When two oceanic plates move apart at a constructive plate boundary.
What type of lava fills the gap between the two oceanic plates that are drifting apart at the constructive plate boundary?
Balsatic lava
How high can the mountain ranges (oceanic ridges) get?
Heights can reach up to 4km
How wide can oceanic mountain ranges get?
4000 km
What can form as a result of two oceanic plates moving apart (not mountain ranges)?
Submarine volcanoes
Give an example of a submarine volcano that peaks above sea level.
Surtsey, Iceland
When do rift valleys form?
When two continental plates move apart.
At what type of boundary can rift valleys be found?
At constructive plate boundaries.
How are rift valleys formed?
As the continental plates move apart, they fracture and small sections of crust drop down at parallel fault lines.
What is a horst?
The raised area between each Rift Valley
What can form as a result of the creation of rift valleys?
New oceans
Give an example of a Rift Valley.
Eastern Africa is splitting from the rest of Africa, creating a new ocean
What are deep sea trenches?
Parts of the ocean where subduction has occurred or is occurring.
Give an example of a deep sea trench
The Peru-Chile deep sea trench
How has the Peru-Chile deep sea trench been formed?
The Nazca plate is being subducted by the South American plate.
Can a deep sea trench be formed at a destructive plate boundary with two oceanic plates?
Yes- the larger plate is subducted under the smaller plate.
Give an example of a deep sea trench formed by two oceanic plates.
The Marianas trench has been formed by the western part of the Pacific plate being subducted under the Philippine plate.
How do island arcs form?
The plate that is being subducted melts. This melted material is less dense than the asthenosphere and therefore rises to the surface as plutons of magma. When this magma reaches the surface, volcanoes form and when they erupt offshore, a line of volcanic islands form.
What type of volcanoes form at island arcs?
Composite
Give an example of an island arc.
The Marianas Islands and Guam which run parallel to the Marianas trench.
Why is there no volcanic activity when fold mountains form?
There is no subduction
True or false: the Himalayas are still forming today
True
How did the Himalayas form?
The Indo-Australian plate moved towards the Eurasian plate.
Where did the sediment for the Himalayas come from?
The Sea of Tethys (the sea that originally separated the plates)
What is another potential source of sediment for the Himalayas?
The continental shelf
What generates hot temperatures witching the Earth’s core?
Radioactive decay
What happens if the radiative decay is concentrated?
Hot spots will form
What causes magma plumes?
The hot spots heat the lower mantle creating localised thermal currents where magma plumes rise vertically.
How can magma plumes create volcanic activity on the crust?
The plumes may rise within the centre of plates and burn through the lithosphere.
Why can hotspots form chains of volcanoes?
The hotspot remains stationary however, the plate carries on moving.
How were the Hawaiian Islands made?
From previous volcanic eruptions.
On what ridge are the Hawaiian Islands located.
They are the tops of volcanoes on the Hawaiian Ridge.
What happens when a volcano moves past a hotspot?
It becomes extinct
Whereabouts are the oldest Hawaiian Islands?
In the north
Where are the youngest Hawaiian Islands?
In the southeast
What causes solid rock to rise to the crust?
Hot spots
What causes the solid rock that has risen to the crust to melt?
Low pressure
When was the last eruption in Hawaii?
Mauna Loa, 1984
What type of volcano is Mauna Loa?
Shield
How many volcanoes make up Hawaii?
5
