Hazards 1a &1b- evidence for continental drift and plate tectonics

Question 1

Definition of hazards

Answer 1

Hazards occur where physical bad human systems meet.
The concentrated release of energy poses a huge threat to life and the built environment .

Question 2

Mechanical Layers of Earth

Answer 2

1) Lithosphere (crust combined with top of mantle) 100km
2) Asthenosphere (middle section of mantle. Ductile, moves plates.) Up to 700km
3) Mesosphere (bottom layer of mantle) 350-2900km

Question 3

Compositional Layers of Earth

Answer 3

Crust
Mantle
Inner core - 5100km to 6370km, solid Iron and nickel.
Outer core- 2900km to 5100km, molten nickel and iron.

Question 4

Oceanic vs Continental

Answer 4

Oceanic: Dense, heavy, 3.5g/cm^3 (subducts). Mafic intrusive igneous rocks (basalt). Age: <200Ma Thickness: 4miles

Continental: Lighter, 2.7g/cm^3. Granitic intrusive igneous rocks. Age:1.8bn years. Thickness: 40-70KM

Question 5

Wegner’s theory of continental drift

Answer 5

In Carboniferous period, 250mn yrs ago, single large continent named Pangea existed.
Slowly drifted apart into 2 large land masses.
Theory ignored as he couldn’t explain reason for movement.

Question 6

Evidence for continental drift:
1) JIGSAW FIT

Answer 6

Similarity in outline of coastlines of east South America and west Africa. Best fit if coastlines match 100m deep. Gaps and overlaps explained by coastal erosion, deposition and eustatic change.

Question 7

2) Paleoclimatic evidence

Answer 7

Similarities found between coal in Brazilian mines and that found in Africa.
- Glaciers spread from central point
-Glacial striations are recorded in areas that are too warm to have glaciers. Occur when large boulders are dragged beneath glaciers, creating grooves parallel to direction of travel of glacier. When striations aligned, all spread from singular point.

Question 8

3) Fold mountain evidence

Answer 8

Fragments of an old fold mountain from 450mn yrs ago found on widely separated continents. E.g Appalachian Mnts disappear off Canada, match mnts in Britain, Ireland, and Scandinavia. When land masses reassembled, mnt belt forms continuous linear feature.

Question 9

BIOLOGICAL EVIDENCE
Fossils

Answer 9

Mesosaurus, a freshwater reptile, existed around 270mn years ago in South America and South Africa. It lived in freshwater lakes and ponds. Remains found in these two locations but impossible for it to swim between continents.

Question 10

Hess Theory of Sea Floor Spreading

Answer 10

-Sonar recordings showed oceans were shallower in the middle due to mid ocean ridges (raised ocean floor by 1.5km.
-Found deepest part of ocean were very close to continental margins in pacific, and ocean trenches up to 11km deep.
-Molten material oozes up from Earth’s mantle, creating new sea floor and spreading from the ridge.

Question 11

Evidence of Sea floor Spreading

Answer 11

1) When basalts of sea floor were dated, were same age at similar distances from ridge on each side.

Question 12

2) Paleomagnetism

Answer 12

Magnetic minerals in the crust created during periods of normal polarity are aligned with those in the crust aligned in the opposite direction that are created in times of reverse polarity. Creates alternating magnetic stripes along sea floor. Can follow the bands of crust and determine the furthest away from the ridge is oldest.

Question 13

Convection Currents

Answer 13

CC within mantle driven by radioactive decay adding heat to earth interior.
Convection creates flow in semi molten asthenosphere. Some plates move faster than others due to other factors : SLAB PULL & RIDGE PUSH.

Question 14

SLAB PULL

Answer 14

• Strongest force driving plate motion
  -occurs at subduction zones at CONVERGENT plate boundaries
  -Subducting oceanic plate becomes denser as lighter minerals melt off.
  -The denser, sinking plate means gravity and cc work more effectively .
  -Slab pull does 85% more work than ridge push.

Question 15

RIDGE PUSH

Answer 15

1-convection currents lead to spreading ridges. Upwelling magma at DIVERGENT boundary on ocean floor. Magma cools= new, elevated crust.
2- spreading ridges stand 2500m higher than ocean basins, so large mass on slopes. Gravity will pull the weight down—> slides down asthenosphere . <— —> direction

Question 16

Types of plate margin

Answer 16

Divergent
Convergent
Transform (Conservative)

Question 17

ICELAND- Divergent margin

Answer 17

-Iceland appears as volcano extruded at m-o ridge.
-1975 to 1984, numerous episodes of rifting along Krafla fissure zone.
-Rifting accompanied by volcanic activity
- plate movement easy to see around Thingvellir, SW Iceland.
-Thingvellir wide straight rocky valley and fissure zone and is on land exposure of mid Atlantic ridge.
-Right of the fissure, N American plate pulling westward away from Eurasian plate.

Question 18

DIVERGENT:
Continental- Continental rifting system

Answer 18

-Rifting on continent
- Crust must be thinned to rift, occurs if plume of magma rises beneath a continent
-Crust stretched to form sunken valley, a graben, with higher blocks remaining on each side.
-Rift widens, magma may erupt through faults to surface. Valley then sinks below sea level.
-Unexpected EQ’s can reactivate faults.
-Rifts may not fully detach, and instead become valleys for rivers .
EXAMPLE: EAST AFRICAN RIFT VALLEY, a line of continental volcanoes.

Question 19

CONVERGENT:
Oceanic- Continental

Answer 19

-Denser oceanic plate forced under continental (subduction)
-Subduction forms narrow trench (6000-11,000km deep). Steepest on continental side.
-Some oceanic crust melts and lighter minerals bubble off, forming plutons that become new crust. Rest slides due to slab pull.
-As oceanic plate descends into asthenosphere, sediment from ocean floor are plastered onto edge of C plate
-C plate is bent and uplifted, forming fold mountain chains, e.g Mt.Helens USA
-As O plate melts, lighter magma is injected into fold mountains to form volcanoes.
-Increasing pressure and friction causes fracturing in Benioff Zone, when descending plate at angle of 45 degrees. This process releases lots of energy —> EQs

Question 20

CONVERGENT:
Oceanic- Oceanic

Answer 20

-When 2 oceanic plates meet, the denser subducts, creating a trench. As the descending plate melts, magma rises to form chain of islands, island arcs.
-The NW zone of the ring of fire contains island arcs such as Aleutian Islands.
-

Question 21

CONVERGENT:
Continental- Continental

Answer 21

-When 2 continental plates converge, no subduction, as plates have similar densities.
-Leads to fold mountains building.
-Collision do Indo-Australian plate and Eurasian plate created Himalayas.

Question 22

Hotspots:

Answer 22

An active volcano usually exists at one end of an island chain, with progressively older extinct volcanoes occurring along the rest of the chain. Hotspots are thought to be the surface expression of giant plumes of heat, termed mantle plumes, that ascend from deep within the mantle, some 2,900 km (1,800 miles) below the surface. These plumes are thought to be stationary relative to the lithospheric plates that move over them. A volcano builds upon the surface of a plate directly above the plume. As the plate moves on, however, the volcano is separated from its underlying magma source and becomes extinct. Extinct volcanoes are eroded as they cool and subside to form fringing reefs and atolls, and eventually they sink below the surface of the sea to form a seamount. At the same time, a new active volcano forms directly above the mantle plume.

Question 23

Conservative/Transform boundaries

Answer 23

-Plates slide past each other in relatively horizontal motion.
- left move= sinistral right move=dextral.
-Lithosphere is neither created or subducted, and whilst conservative plate and result in shallow focus earthquakes.
EXAMPLE: San Andreas Fault