5.2- Theory Of Plate Tectonics

Question 1

The interior of the Earth can be divided into 3 layers:

Answer 1

Core, mantle, crust

Question 2

Describe the core

Answer 2

The core is made up of dense rocks containing iron and nickel alloys and is divided into a solid inner core and a molten outer core, with a temperature of 5000 degrees Celsius (this heat is produced mainly as the result of two processes: primordial heat left over from the Earth’s formation
and radiogenic heat produced from the radioactive decay of isotopes

Question 3

Describe the mantle

Answer 3

The mantle is made up of semi-molten rocks containing lighter elements such as silicon and oxygen

Question 4

Describe the crust

Answer 4

Crust is even lighter because of the elements that are present- the most abundant being silicon, oxygen, aluminium, potassium and sodium

Question 5

Explain how the crust varies in thickness

Answer 5

Below the oceans (oceanic crust) it is only 6-10km thick but below the continents (continental) this increases to 30-40km and under the highest mountain ranges this can be as high as 70km

Question 6

New research has suggested that the crust and upper mantle should be divided into:

Answer 6

Lithosphere and asthenosphere

Question 7

What does the lithosphere consist of?

Answer 7

The crust and the rigid upper section of the mantle; this is the section of the Earth that is divided into 7 very large plates and a number of smaller ones. Plates are divided into 2 categories: oceanic and continental depending on the type of material from which they’re made

Question 8

What is the asthenosphere?

Answer 8

Lies beneath lithosphere and is semi-molten on which the plates float and move

Question 9

What are some key differences between continental and oceanic crust?

Answer 9

• thickness of continental = 30-70km
• thickness of oceanic = 6-10km
• age of continental= over 1,500 million years
• age of oceanic= less than 200 million years
• density of continental= 2.6 (lighter)
• density of oceanic= 3.0 (heavier)
• composition of continental= mainly granite; silicon, aluminium (SIAL)
• composition of oceanic= mainly basaltic; silica and magnesium (SIMA)

Question 10

Plate tectonic theory revolutionised the study of Earth science. As soon as maps of the Atlantic Ocean were produced, people started to notice that the continents either side seemed to fit together remarkably well- the bulge of South America fitting into the indent below West Africa. Who noticed this fit?

Answer 10

Francis Bacon in early 17th century, but it did not attract any serious attention as no one beloved continents could move

Question 11

In 1912, Alfred Wegner published his theory that a single continent existed about 300 million years ago and he named this continent

Answer 11

Pangea

Question 12

Wegner maintained that Pangea later split into two continents of

Answer 12

Laurasia in the north and Gondwanaland in the south

Question 13

Wegner believed that today’s continents were formed from further splitting of Laurasia and Gondwanaland. He published this theory of continental drift and claimed it was supported by several pieces of evidence that these areas once joined- what kind of evidence?

Answer 13

Geological evidence and biological evidence

Question 14

Explain some geological evidence for theory of continental drift

Answer 14

• as soon as maps of the Atlantic Ocean were produced, people started to notice that the continents either side seemed to fit together remarkably well- the bulge of South America fitting into the indent below West Africa
• rock sequences in northern Scotland closely agree with those found in eastern Canada, indicating that they were laid down under the same conditions in one location

Question 15

Explain some biological evidence for theory of continental drift

Answer 15

• fossil remains of the reptile Mesosaurus are found in both South America and Southern Africa- it is unlikely that the same reptile could have developed in both areas or that it could have migrated across the Atlantic

Question 16

Wegner’s theories were unable to explain how continental movement could have taken place and his ideas gained little ground. From the ____s onwards however, evidence began to accumulate to show that Wegner could have been correct

Answer 16

1940

Question 17

Explain concept of sea-floor spreading

Answer 17

the Mid-Atlantic ridge was discovered and examination of the ocean crust either side of the mid-Atlantic ridge suggested sea-floor spreading was occurring

Question 18

What is sea-floor spreading?

Answer 18

process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge

Question 19

What evidence is there for occurrence of sea-floor spreading?

Answer 19

• iron particles in the lava erupted on the ocean floor are aligned with the Earth’s magnetic field
• as the lavas solidify, these particles provide a permanent record of the Earth’s polarity at the time of eruption (palaeomagnetism)
• however, the Earth’s polarity reverses at regular intervals
• the result is a series of magnetic ‘stripes’
• this striped pattern, which is mirrored exactly on either side of a mid-oceanic ridge suggests that the oceanic crust is slowly spreading away from this boundary
• not only this, but the oceanic crust gets older with distance from the mid-oceanic ridge

Question 20

Sea-floor spreading and palaeomagnetism would suggest the Earth is getting bigger were it not for the discovery of

Answer 20

Deep ocean trenches where the ocean floor was being subducted and destroyed (ocean trenches)

Question 20

Explain idea of hotspots and convection currents in tectonic movement

Answer 20

• hot spots around the core of the earth generate thermal convection currents within the asthenosphere which cause magma to rise towards the crust and then spread before cooling and sinking
• this circulation of magma is the vehicle upon which the crustal plates move
• the crust can be thought of as ‘floating’ on the denser material of the asthenosphere
• this is a continuous process with new crust being formed along the line of conservative boundaries between plates (plates moving away from each other- divergent) and older crust being destroyed at destructive boundaries (plates moving towards each other-convergent)
• where two crustal plates move past each other, there is no creation or destruction of crust

Question 21

For a long time, the widely accepted view of plate motion was that that it was brought about by convection currents. It is now believed that forces behind plate motion are not as simple as would be explained entirely by convection currents- forces that act on plates can also be generated

Answer 21

At their boundaries- these forces push from the ridge, drag the plates down at the trenches, or act along the sides of plates at conservative plate boundaries

Question 22

Explain concept of ridge push

Answer 22

• at constructive boundaries, the upwelling of hot material at ocean ridges generates a buoyancy effect that produces the ocean ridge which stands 2-3km above the sea floor
• here, oceanic plates experience a force that acts away from the ridge known as ridge push which is the result of gravity acting down the slope of the ridge
• some experts do not like the term ridge push and prefer to name the process of gravitational sliding

Question 23

Explain the concept of slab pull

Answer 23

• situation at destructive plate boundaries is more complex
• a major component is the downward gravitational force acting on the cold, dense descending plate as it sinks into the mantle
• this gravity-generated force pulls the whole oceanic plate down as a result of negative buoyancy of the plate- this force is known as slab pull

Question 24

Each plate however moves at its own rate which suggests that the relative importance of the driving forces must

Answer 24

Vary from plate to plate- therefore seems unlikely that any single agent is the sole driving mechanism of plate motion and plates must ultimately be controlled by a number of forces

Question 25

What are the different type of plate boundaries?

Answer 25

• constructive
• destructive
• conservative

Question 26

Explain constructive plate boundaries

Answer 26

• when two plates separate (diverge) due to convection currents operating in opposite directions in the mantle, they form a constructive margin- there are 2 types of divergence:
  1- in oceanic areas, sea-floor spreading occurs on either side of mid-ocean ridges e.g. Mid Atlantic ridge
  2- in continental areas, stretching and collapsing of the crust creates rift valleys

Question 27

What are some landforms associated with constructive plate boundaries?

Answer 27

Ocean ridges, volcanic islands and rift valleys

Question 28

It is at constructive plate boundaries that some of the youngest rocks on the Earth’s surface are to be found. This is because

Answer 28

New crust is being formed as the gap created by the spreading plates is filled by magma rising from the asthenosphere- as the magma cools, it solidifies to form dense, new basaltic rock

Question 29

What are ocean ridges?

Answer 29

• formed when plates move apart in oceanic areas
• the space between the plates is filled with basaltic lava upwelling from below to form a ridge
• volcanic activity also occurs along these ridges, forming submarine volcanoes which sometimes rise above sea level e.g. Surtsey to the south of Iceland

Question 30

What are rift valleys?

Answer 30

• form when plates move apart on continental areas
• in East Africa for example, the brittle crust fractures as sections of it move apart
• areas of crust drop down between parallel faults to form the valley
• an area between two parallel rifts valleys forms an upstanding block, known as a horst

Question 31

Why is the Great African Rift Valley particularly interesting?

Answer 31

It may eventually mark the formation of a new ocean as Eastern Africa moves away from the rest of the country

Question 32

Explain destructive plate boundaries

Answer 32

• when two plates collide (converge) they form a destructive plate margin
• three different types of convergence are possible:
  1- oceanic plate meeting continental plate
  2- oceanic plate meeting oceanic plate
  3- continental plate meeting continental plate e.g. Himalayas

Question 33

What are some landforms associated with destructive (convergence) plate?

Answer 33

• deep sea trenches
• island arcs
• young fold mountains

Question 34

What are deep sea trenches?

Answer 34

• where oceanic and continental plates meet, the denser oceanic plate is forced underneath the less dense continental one (subduction)
• the downwarping of the oceanic plate forms a very deep part of the ocean known as a trench
• e.g. the Nazca plate is subducted under the South American plate forming the Peru-Chile trench

Question 35

What are Island arcs?

Answer 35

• during subduction, the descending plate encounters hotter surroundings, and this coupled with the heat generated from friction begins to melt the plate
• as this material is less dense then the surrounding asthenosphere, it begins to rise towards the surface as plutons of magma
• eventually these reach the surface and form complex, composite, explosive volcanoes
• if the eruptions take place offshore, a line of volcanic islands form = island arcs
• e.g. Marianas Islands

Question 36

What are young fold mountains?

Answer 36

• collision of two continental plates (not much subduction when meet)
• their edges and sediment between them are forced up into fold mountains
• as there is little subduction there is no volcanic activity but shallow-focus earthquakes can be triggered
• young fold mountains such as the Himalayas are constantly compressing and growing higher from collision between Indian plate and Eurasian plate

Question 37

Explain conservative plate boundaries

Answer 37

• when 2 plates slide past each other they form a conservative plate margin
• along these margins, crust is not being destroyed by subduction and there is no melting of rock, and therefore no volcanic activity or formation of new crust
• despite the absence of volcanic activity, these margins are extremely active and associated with powerful earthquakes

Question 38

Explain how earthquakes form as a result of conservative plate margin

Answer 38

• friction between the two moving plates leads to stresses building up whenever any ‘sticking’ occurs
• these stresses may eventually be released suddenly as powerful shallow-focus earthquakes
• these earthquakes occur along California’s infamous San Andreas fault system