Hazards- Plate Tectonics

Question 1

Lithosphere

Answer 1

The solid top layer of crust in which plates are formed

Consists of crust and upper mantle

Question 2

Asthenosphere

Answer 2

Soft, plastic like rock in the upper mantle just below the lithosphere

Question 3

Earths heat source

Answer 3

• Earth was hot when it formed
• formed when “planetesimals” condensed and stick together
• earth makes some of its own heat through RADIOACTIVE DECAY
• seismic waves and seismic data show what materials make up the core

Question 4

Continental crust

Answer 4

30-50km thick 
4 billion years old 
Density of 2.7*/cm3
Silicon, aluminium and oxygen 
Granite (metamorphic rock)

Question 5

Oceanic crust

Answer 5

7-10km thick 
200 million years old 
Density of 2.9*/cm3
Silicon, aluminium and oxygen 
Basalt (igneous rock)

Question 6

Jigsaw fit

Answer 6

Similarities in outline of the coastlines of Eastern South America and west Africa
Best fit when coastlines match at 100km below sea level
Areas where there are gaps/overlaps may be explained by:
-coastal erosion
-coastal deposition
-changes in sea level
-changes in land level

Question 7

Geological fit

Answer 7

• geology of eastern South America and west Africa mapped

- ancient rock outcrops over 2000 million years old were continuous from one continent to the other

Question 8

Tectonic fit

Answer 8

• fragments of old fold mountain belt (400-450 million years ago) found on widely separate continents today
• when fragments are reassembled, the mountain belt forms a continuous linear feature

Question 9

Fossil evidence

Answer 9

Many examples of fossils found on separate continents and nowhere else, suggesting continents were once joined

Question 10

What is Paleomagnetism?

Answer 10

When new material is made at mid-ocean ridges (this is a constant process), the new material takes the current magnetism of the earth into it at the time

Question 11

Paleomagnetism- process

Answer 11

RIDGE PUSH:
-as the sea floor is spreading there is a symmetrical pattern in the rock which is created as the rock gets increasingly far from the ridge
-due to the magnetism of the Earth changing (flipping), the striped patterns which are created in the rock can help tell the age of the rock
SLAB PULL:
-as new material is constantly being created at ridges for the Earth to remain how it is materials are destroyed at subduction zones
So.. SLAB PULL=RIDGE PUSH

Question 12

Subduction zones

Answer 12

• Where gravity pulls the oceanic plate down into the mantle (at destructive boundaries)
• This destroys crust material (helping maintain the Earths shape)

Question 13

Constructive boundary

Answer 13

• two plates are moving apart leaving a gap in the middle for magma to rise up through
• volcanoes form but don’t erupt with much force (magma can’t escape easily)
• rift valleys form

Question 14

Destructive boundary

Answer 14

Oceanic plate descends below the less dense continental plate

• as the plate descends it is melted due to friction (forms magma)
• magma then rises through cracks in the continental plate and forms volcanoes if it reaches the surface

Question 15

Collision boundary

Answer 15

• two plates of similar density (eg. Continental and continental plates)
• causes material to buckle up forming fold mountains eg. Himalayas

Question 16

Conservative boundary

Answer 16

• two plates don’t directly collide but slide past each other along a fault
• no volcanoes are found here but earthquakes do occur eg. San Andreas Fault

Question 17

How do rift valleys form? Eg. East African Rift Valley

Answer 17

• form at divergent plate boundaries
• basaltic lava erupted from hundreds of parallel fissures on the ocean floor
• convection currents cause the plates to move apart
• elevated heat flows from the mantle caused by thermal bulges
• as the bulges form they stretched and fractured the crust into a series of faults
• convection currents in the atmosphere causes the extension of the crust
• as the crust extends, graben (down dropped blocks) and horts (upthrown blocks) are formed
• rift continues to extend, part of the crust becomes thin enough for oceanic type basalts to be erupted, signalling the formation of new oceanic crust

Question 18

Destructive boundary: oceanic-continental

Answer 18

-denser oceanic plate subducts beneath the less dense continental plate
Fold mountains:
-pressure put on the continental plate causing it to buckle upwards- fold mountains form
-the descending oceanic plate is scraped off and folded to form fold mountains

Question 19

Benioff zone (oceanic-continental)

Answer 19

• the further the rock descends, the hotter the surroundings become
• this, together with the heat generated from friction, begins to melt the oceanic crust into the magma
• as it is less dense than the surrounding asthenosphere, the molten material begins to rise in great plumes up through fissures/faults in the buckled continental plate by burning their way through overlaying rock
• eventually these reach the surface to form explosive volcanoes (with viscous lava)

Question 20

Destructive boundary:oceanic-oceanic

Answer 20

• two oceanic plates moving towards each other
• the crust that is subducted may be marginally denser of the two plates, or it is the one moving quicker
• occurs in the same way as oceanic-continental but is less clear which is subducted
• earthquakes are frequent- subduction produces shallow-to-deep focus earthquakes (some very powerful sub sea ones may cause a tsunami)
• ocean trenching and island arcs are the main features

Question 21

Destructive boundary: continental-continental

Answer 21

• continental plates are of lower density than the asthenosphere beneath them
• so subduction can’t occur
• instead they collide with each other
• these boundaries are prone to very powerful earthquakes
• fold mountains also form