3.1 Elementary plate tectonics

Question 1

Define plates

Answer 1

pieces of crust that split up the surface of the Earth

Question 2

Define plate boundary

Answer 2

where 2 plates meet

Question 3

Define plate tectonics

Answer 3

The theory that helps to explain how the Earth’s crust has changed shape, and cause earthquakes, volcanoes, rift valleys and fold mountains

Question 4

Define crust

Answer 4

pieces of plates; either continetal-land or oceanic plates

Question 5

Define tectonic plate

Answer 5

a slab of lithosphere

Question 6

Define Pangaea

Answer 6

the name for the crust when it was all joined in one piece 250 million years ago

Question 7

Define lithosphere

Answer 7

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

Question 8

Define aesthenosphere

Answer 8

The layer below the lithosphere. Seismic waves decrease with distance through this region.

Question 9

What’s the structure of the Earth?

Answer 9

Question 10

Characteristics of continetal crust

Answer 10

• 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

Question 11

Characteristics of oceanic crust

Answer 11

• 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

Question 12

What makes the plate move?

Answer 12

• 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

Question 13

Define ridge push

Answer 13

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

Question 14

Define slab pull

Answer 14

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

Question 15

What is a constructive/divergent plate boundary (ocean to ocean)?

Answer 15

• 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

Question 16

What is a constructive/divergent plate boundary (continent to continent)?

Answer 16

• 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

Question 17

Define sea floor spreading

Answer 17

A geologic process in which ocean floors are spreading outward from vast underwater ridges.

Question 18

How was sea floor spreading disovered?

Answer 18

• 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

Question 19

Define mid-ocean ridge

Answer 19

• 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

Question 20

What is a destructive/convergent plate boundary (ocean to continent)?

Answer 20

• 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

Question 21

What is a destructive/convergent plate boundary (ocean to ocean)?

Answer 21

• 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

Question 22

Define subduction zone

Answer 22

• 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.

Question 23

Define ocean trench

Answer 23

Long narrow depressions on the seafloor with depths from 6000m to 11 000m. Formed at subduction zone

Question 24

Define Benioff zones

Answer 24

• 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

Question 25

Define island arcs

Answer 25

• Chain of volcanic islands on the continetal side of an ocean trench
• Fromed when an oceanic crust subducts below another oceanic crust. Pieces are scraped off from a subduction complex (accretionary prisms) which can break through the surface of ocean to form an arc
• The descending plate starts to heat up, the rocks are melted to form magma and rise toward the surface
• The volcanoes pile up until they poke through the surface of the ocean and form an arc
• A dip called a fore-arc basin is formed between island arc and subduction zone

Question 26

What is a conservative/transform plate boundary?

Answer 26

• When 2 plates slide past each other in opposite direction or in the same direction at different speeds
• Associated with shallow earthquakes and transform fault

Question 27

Define transform fault

Answer 27

Locations where 2 plates slide past one another

Question 28

What is a collision plate boundary?

Answer 28

• When 2 continetal plates meet, both may be folded and buckled
• Associated with mountain formation

Question 29

Define fold mountains

Answer 29

• Mountains that form mainly by the effects of folding on layers within the upper part of the Earth’s crust

E.g. Himalayas -

• The Himalayas were formed when two continental plates collided as Indian plate moved towards the rest of Asia during the last 70 million years
• The leading edge of the Indian plate have been thrust beneath the edge of the Asian continent
• In the collision zone the Asia overrides India and is therefore uplifted and folded to form mountains
• As no magma escapes to the surface in the Himalaya region there is little volcanic activity

Question 30

Evidence for plate tectonics theory

Answer 30

• Fossil remains in India match Australia’s
• Glacial deposits in Brazil is the same as in West Africa
• Geological sequence of rocks in Scotland matches those in Newfoundland
• Jigsaw - how continents’ borders can be arranged to fit to the others