3.1 Elementary plate tectonics Flashcards
Define plates
pieces of crust that split up the surface of the Earth
Define plate boundary
where 2 plates meet
Define plate tectonics
The theory that helps to explain how the Earth’s crust has changed shape, and cause earthquakes, volcanoes, rift valleys and fold mountains
Define crust
pieces of plates; either continetal-land or oceanic plates
Define tectonic plate
a slab of lithosphere
Define Pangaea
the name for the crust when it was all joined in one piece 250 million years ago
Define lithosphere
A relatively inflexible and buoyant layer. It is the layer which floats on the material underneath as it moves, it carries the the tectonic plates
Define aesthenosphere
The layer below the lithosphere. Seismic waves decrease with distance through this region.
What’s the structure of the Earth?
Characteristics of continetal crust
- Thickness: 35-70 km on average
- Age of rocks: Very old, over 1500 million years
- Colour and density of rocks: Light in colour, average density of 2.6
- Minerals: silica, aluminium and oxygen
- Nature of rocks: Numerous types; many contain silica and oxygen, grantic is the most common
Characteristics of oceanic crust
- Thickness: 6-10km on average
- Age of rocs: Very young, mainly under 200 million years
- Colour and density of rocks: Dark in colour, heavier with an average density 3.0
- Minerals: Silica, iron and magnesium
- Nature of rocks: Few types and mainly balsaltic
What makes the plate move?
- Convection currents takes place in mantle
- Heat from the core hears the mantle. therefore, hot magma rises because it is less dense
- This current cools down as it comes closer to the surface of the Earth so cool magma sinks down
- This results in the horizontal movement along the bottom of the crust => plates move as well
- When current cools down more, the convection current descends and goes toward the core. This cycle repeats
Define ridge push
The hypothetical force caused by the horizontal spreading of the near surface asthenosphere at constructive margin, one of the 2 main driving forces of lithospheric plates
Define slab pull
The force caused by the sinking of the cold, dense lithosphere into the asthenosphere at a destructive margin, one of the 2 main driving forces of lithospheric plates
What is a constructive/divergent plate boundary (ocean to ocean)?
- As 2 plates move apart, faults are formed => rising magma which cools and creates new crust, they fill the gap and rises
- This new crust forms submarine volcanoes which create mid-ocean ridges
- If the submarine volcanoes breach the ocean surface => volcanic islands
- Features that are associated with:
- shallow focus earthquakes
- transform faults
- sea floor spreading
- Landforms that are associated with:
- mid-ocean ridges
- volcanic islands
- submarine volcanoes
What is a constructive/divergent plate boundary (continent to continent)?
- When 2 plates are forced apart, tension and stretching forms cracks (faults)
- Block of crust at the boundary drops into the gap (created by the stretching) then slides down (due to gravity). This block is called graben and as it goes down, a rift valley is formed
- The blocks on either side that do not slide down are the block faulted mountains or Horsts mountains
- Landform and features:
- rift valley
- horsts mountains
- volcanic activity
- shallow focus earthquakes
Define sea floor spreading
A geologic process in which ocean floors are spreading outward from vast underwater ridges.
How was sea floor spreading disovered?
- The earths magnetic field has different polarity in different periods of time
- Lava cooling on the sea floor acquires the polarity of the earth’s magnetic field at the time of cooling
Define mid-ocean ridge
- The uplifting of the ocean floor when convection currents rise in the mantle beneath the oceanic crust and create magma where 2 tectonic plates meet a divergent boundary.
- Consist of rock that is hotter and less dense than the older, colder plate
What is a destructive/convergent plate boundary (ocean to continent)?
- Oceanic plate colides with a continental plate then is subducted beneath it because it is denser
- An ocean trench (depression in the ocean floor) is formed
- The subducting oceanic crust is melted in the subduction zone due to friction caused by geothermal heat
- Newly formed magma rises (because it is less dense) and erupts through a weakness in the continental crust above
- Landforms and features:
- volcanoes
- deep ocean trench
- subduction zone
What is a destructive/convergent plate boundary (ocean to ocean)?
- Denser plate is subducted and an ocean trench is formed
- Subducted oceanic crust is melted by friction from geothermal heat
- Newly-formed magma rises as it is less dense than the surrounding lithosphere and erupt through a weakness in the crust
- A submarine volcanic island forms with repeated eruptions
- A string of these islands forms an island arc
Define subduction zone
- The name given to the area where one plate moves underneath another plate
- As the earth has not grown any bigger, the amount of subduction balances against the new crust created at divergent boundaries.
- The plate subducting tends to dip at an angle of between 30 and 70 degrees
- The older the crust, the steeper it dips.
Define ocean trench
Long narrow depressions on the seafloor with depths from 6000m to 11 000m. Formed at subduction zone
Define Benioff zones
- Dipping, roughly planar zones of increased earthquake activity produced by interaction of downgoing oceanic crustal plate wiith an overriding plate
- The zone extends from the surface of the ocean trench down to a depth of 680km.
- The deeper earthquakes occur further from the subduction zone
