Draw a diagram showing the structure of the earth
What is the difference between the lithosphere and the asthenosphere?
The lithosphere is cooler and more rigid, whereas the asthenosphere is hotter and flows more easily
Is the lithosphere above the asthenosphere or is the asthenosphere above the lithosphere?
The lithosphere is above the asthenosphere and therefore closer to the Earth's crust
What does the lithosphere consist of?
Either oceanic crust or continental crust (forming the seven large continental plates and many smaller ones in the world)
Is oceanic or continental crust thicker? How thick are both?
Continental crust is thicker at approx. 200km compared to approx 35km oceanic
How does the oceanic crust become thicker?
As time passes, more material spreads from mid ocean ridges and it cools and becomes thicker.
What causes convection currents?
Intense pressure created in the core of the Earth by gravity and the radioactive decay in the core
What is the Continental Drift theory of plate tectonics?
- Created by: Alfred Wegener
- Wegener, like other explorers saw that the continents could fit together like a jigsaw (e.g South America and Africa)
- He proposed that continents moved through ocean basins and that continents that are separated now may once have been joined together in the past
Why was the continental drift theory of plate tectonics dismissed?
- Idea was controversial
- No explanation for why the continents moved
- Relied on observational evidence
What evidence is there to support the theory of plate tectonics? (5)
- Fossils - Fossils of similar species found on separate continents (India and Australia)
- Glacier evidence - paleo-climate studies found that glaciers had covered large areas of the world that were now separated (S. America and Antarctica)
- Seismometers - showed movement such as earthquakes happened on plate boundaries and this shows movement
- Rock sequences - similar types of rock found in Scotland and Canada
What is paleomagnetism?
- Studies revealed a striped pattern of magnetic reversals in the crust of ocean basins
- The Earth has undergone several magnetic reversals in the past (north and south pole reverse for a period of time)
- As the crust of the earth showed these reversals, it is evidence that the crust is in motion.
- if they weren't in motion then the stripes would not exist
Describe the process of a convection current (6 steps)
- 1. Intense pressure and radioactive decay generates heat
- 2. Mantle is heated up making material less dense
- 3. Heat rises through dense material towards the Earth's surface
- 4. Cooling and spreads out, dragging plates in different directions
- 5. Material cools and increases in density sinking towards the core
- 6. Repeat
What is sea floor spreading?
- Idea put forward by: Harry Hess
- Continental crust separates to form constructive plate boundaries (where plates move away from each other).
- This starts by a plume of hot magma rising from the mantle pushing the crust up and causing the plate to separate and break.
- This means that rocks in the middle of the ocean are younger than the rocks at coastlines.
- e.g. the mid atlantic ridge
- e.g. Iceland was formed as a result of sea-floor spreading
Who first put forward the idea of convection currents?
English geologist Arthur Holmes in 1920
What is the lithosphere?
The outer layer of the Earth, either continental or oceanic crust. Forms at mid-ocean ridges and spreads outwards to become thicker over time.
What is the asthenosphere?
A layer of softer, almost plastic-like rock which moves slowly underneath the lithosphere
Which layers of the earth are liquid?
Which layers of the earth are solid?
What are the features of oceanic crust? (6)
- Dense and heavy
- Will sink
What are the features of continental crust? (6)
- Light/less dense
- Does not sink
- Always being created or destroyed
Where are nearly 99% of all volcanoes found?
- Along destructive or constructive plate boundaries
- The other volcanoes are found at hot-spots
What does a volcano's activity depend on?
- Whether there is a central or linear vent type
- What type of lava/ash is ejected
- Whether the volcanic area is active, dormant or even extinct
What is a constructive plate boundary? + example
- Two oceanic or continental plates move apart from one another
- e.g. Mid-Atlantic Ridge
- As the plates move apart, magma rises to fill the gap and form shield volcanoes
- This causes regular volcanic activity e.g. Hekla, Iceland
What type of earthquakes are found at constructive plate boundaries? Why?
Minor and shallow earthquakes (lower than 5) due to lack of friction between the plates
What is a destructive plate boundary?
- Where a continental plate meets an oceanic plate
- Oceanic plate is subducted and sinks under the continental plate because it is denser.
- The plates do not slide past each-other smoothly
- Intense friction locks the plates together until movement overcomes the friction and they slide past each other quickly = earthquake
Why do destructive plate boundaries have violent volcanoes?
- As the oceanic plate subducts, it drags down sediment and water which mixes with magma making it less dense.
- Under great pressure, this less dense but viscous magma is forced through the cracks in the continental plate.
- If it reaches the surface magma erupts violently
What is the Benioff Zone?
- The narrow zone indicating the subducting plate where intermediate and deep earthquakes occur
What is a collision boundary?
- Occur when 2 continental plates of similar densities move towards each-other
- causes material between them to buckle and rise u, forming fold mountains such as the Himalayas
- High magnitude earthquakes occur but not volcanoes
What is a conservative plate boundary?
- Occur when two plates grind past each other along a fault.
- Movement may be in the same direction, or different directions
- No lithosphere is created or destroyed so there are no volcanoes but high magnitude earthquakes do occur
- e.g. San Andreas Fault, California
What is a volcanic hot-spot?
Concentrated areas where plumes of magma rise through the Earth's mantle to form a volcano, such as in Hawaii