1.2 theory of plate tectonics

Question 1

structure of the Earth - crust

Answer 1

• continental is thicker and less dense (45-50km)
• oceanic is thinner and more dense (6-10km)
• seven major plates and some minor ones

Question 2

what is the Mohorovičić discontinuity (Moho)

Answer 2

the boundary between the crust and the mantle

Question 3

structure of the Earth - mantle

Answer 3

• widest layer
• lithosphere is upper mantle and crust
• asthenosphere is a semi-molten layer which moves under high pressure

Question 4

structure of the Earth - outer core

Answer 4

• semi-molten
• mostly liquid iron and nickel
• 4500-6000 degrees C

Question 5

structure of the Earth - inner core

Answer 5

• solid centre
• mostly iron
• 6000 degrees C

Question 6

what are convection currents

Answer 6

• magma at the bottom of the mantle is heated by the core, giving it energy and causing it to rise
• cooler rock sinks due to lower energy levels

Question 7

what is the theory of convection currents to explain plate movements

Answer 7

• heat from radioactive decay in the core moves up towards the crust
• magma under the crust moves sideways at the Moho
• this pulls the plates apart due to friction (divergent boundary)
• magma cools and sinks pulling the crust downwards (convergent boundary)

Question 8

what is the theory of sea floor spreading and slab pull to explain plate movements

Answer 8

• the ocean floor is getting progressivley older and newer crust must be produced by molten rock rising from the core
• as molten rock comes to the surface the crust is pushed outwards (ridge push)
• oceanic crust is destroyed by subduction
• in the subduction zone the oceanic crust is heaviest and pulled down by gravity under the continental crust
• pushing at ridges (divergent boundaries) and pulling at convergent boundaries results in ocean floor spreading

Question 9

how does paleomagnetism evidence the movement of tectonic plates

Answer 9

• the earths magnetic field switches north and south every few million years
• this means rock strata has alternating magnetization
• the magnetic minerals in lava line up with the magnetic field and set when cooled
• parallel strips of crust in opposite directions prove the floor has spread

Question 10

what are volcanoes like at divergent boundaries and hotspots

Answer 10

• shield volcanoes
• mild and effusive
• basaltic lava (low gas content, low viscosity, higher temperature)

Question 11

what are volcanoes like at convergent boundaries

Answer 11

• composite / strato
• high magnitude eruptions due to friction and pressure build up in Benioff zone
• explosive as magma is forcing its way to the surface
• andesitic or rhyolitic lava (high gas content, high viscosity, lower temperature)