Plate Tectonics

Question 1

Wegener’s Theory

Answer 1

• Atlantic Ocean didn’t exist 200Ma ago
• Formed when ancient super-continent called Pangaea (Greek for ‘all of earth’) broke apart
• Continents and ocean floors are fundamentally different (density)
• Present day continents have essentially the same outlines since the break up of Pangaea

Question 2

Wegener Evidence

Answer 2

1. fossils of the same plant and reptiles found in rocks of S. America, Africa, India and Australia (subsequently found Antarctica)
2. The distribution of tillites (lithified glacial deposits) consistent with a single large ice sheet covering the reassembled continents.
   S. America, Africa, India, Australia, Antarctica.
   Position- Similar glacial till deposits, palaeomagnetism.
   Age- formed Ca. 300million years ag, began to break apart ~100million years ago

Question 3

Wegener’s Mechanism

Answer 3

Harold Jefferey’s book ‘The Earth’ (1920s)-
Wegener’s mechanism of tidal forces could not be accounted for the separation of the continents producing movements in many different directions for millions of years on the surface of a rotating planet.

Question 4

Arthur Holmes

Answer 4

Almost sole supporter of Wegener: Arthur Holmes, University of Edinburgh

Arthur Homes 1930s: internal mechanism
-Convection currents in the upper mantle (now known as the asthenosphere) driven by heat produced by radioactive decay in the core of the Earth
Very few people believed Holmes.

Question 5

What about the Ocean Floor?

Answer 5

Arguments for and against continental drift had been based on the exposed 1/3 of the Earth’s surface, i.e. the continents
Many important discoveries of the ocean floors made during the world war 2 to the early 1960s
Ocean floors rugged and irregular (not as flat as had been thought) underwater mountain ranged and deep trenches identified.

Question 6

Mid-Atlantic Ridge

Answer 6

• Stands over 3km above the abyssal planes
• > 2400km wide
• down-dropped rift valley ~2.5km deep & 32-48km wide extends along the length of its crest

Trenches are very seismically active: 80% of the Earth’s shallow focus earthquakes occur near the trenches and over 90% of the Earth’s deep focus earthquakes.

Question 7

Sea Floor Spreading as evidence of plate tectonics

Answer 7

Proposed as a model by Harry Hess in 1960.

Upward moving convection currents in the upper mantle produce the ocean ridges of the earth (cf. Arthur Holmes, 1930)

Question 8

Convection Currents

Answer 8

Convection currents carry molten rock (basalt) to the surface, then ‘spread it’ outwards on either side- pushing the continents apart and forming young ocean basins.

But what happens to all of the new oceanic crust that is being created?? is the earth expanding?!?! :0

Hess suggested trench systems mark sites of descending limbs of convection cells- material swept back into the upper mantle where it is re-melted.

Question 9

Magnetism as evidence

Answer 9

1963- Magnetic stripes
First observed using magnetometers in the NE Pacific
Form a symmetrical pattern on each side of a mid-ocean ridge.
Alternate stripes of oceanic crust with normal and reversed polarity.
-Fe-bearing minerals act like tiny compass needles and crystallise in ocean floor basalt in the direction of the Earth’s magnetic field at the time of eruption
-Alternating stripes of normal and reversed polarity are evidence for periodic geomagnetic reversals
-Have now been found adjacent to all the mid-ocean ridges

Question 10

Plate Boundaries

Answer 10

Geologically active regions 
Three types of plate boundaries 
1-converging 
2-diverging or rift 
3-transform

Question 11

Converging

Answer 11

Oceanic to oceanic plate boundaries (e.g. Japan)

Oceanic to continental plate boundaries (e.g. Juan de Fuca)

Question 12

Cascadia Subduction Zone (Oregon Coast)

Answer 12

• Converging at ~3.8cm/year
• Movement not smooth or continuous
• Plate lock in place, and unreleased energy build up overtime
• Accumulated energy is violently released as megathrust earthquake

Question 13

Cascadia landforms

Answer 13

Estimated 7-12% chance of megathrust earthquake in the next 50 years assuming recurrence of 530 +/- 260years, and that the last one occurred about 312 years ago.

Question 14

Continent to continent (converging)

Answer 14

Himalayan orogeny (for example)
(orogeny= mountain-building)

Question 15

Drifting or rift

Answer 15

Continent- continent rifting

Rift- Iceland

Question 16

Transform/Strike-slip

Answer 16

Transform margin

Strike-slip - San Andreas Fault, California