Plate Tectonics

Question 1

uniformatarianism

Answer 1

• when things operate the same way today as they did in the past (ie. wind)
• can use this to interpret past environments

Question 2

what makes earth unique?

Answer 2

plate tectonics and life

Question 3

earth’s layers

Answer 3

• core
• mantle
• crust
• lithosphere

Question 4

core (and its 2 layers)

Answer 4

• Iron and nickel (and some sulfur)
• Most dense
• 2 layers:
• Inner core: solid
• Outer core: liquid
• Northern lights due to earth’s liquid iron outer core

Question 5

mantle (and its 3 layers)

Answer 5

• is rock, but can flow b/c of heat & high pressure
• Iron and magnesium silicates
• Rock type: peridotite
• Less dense
• 3 layers of mantle:
• Uppermost mantle: rigid
• Middle mantle (asthenosphere): plastic -> solid that is able to flow (like ice/glaciers)
• Lower mantle (mesosphere): rigid

Question 6

crust (and its 2 types)

Answer 6

• Abundant silicates
• Richer in potassium and sodium
• Least dense
• 2 types of crust: continental and oceanic

Question 7

continental crust

Answer 7

• FELSIC

- Granite-like composition, rich in feldspar and silicate minerals

Question 8

oceanic crust

Answer 8

• MAFIC
• Basaltic-like composition, rich in ferro-magnesium minerals
• Generally thinner than continental crust, but denser

Question 9

earth’s layers: chemical vs. physical properites

Answer 9

• chemical: crust (granite and basaltic rocks), mantle (silicates), core (iron, nickel, sulfur)
• physical: lithosphere, asthenosphere, mesosphere, outer core, inner core

Question 10

lithosphere

Answer 10

• Crust and upper mantle
• Rigid, less dense than underlying layers, ranges in thickness (50-200km)
• Moves over the plastic asthenosphere

Question 11

plates

Answer 11

• fractured blocks of lithosphere floating on asthenosphere
• Physical properties: cool, strong, brittle, fractured
• Lithosphere is broken into 12 large and several small plates, which move on top of the weak asthenosphere
• Oceanic crust & continental crust can sit on the same lithospheric plate (ex. South American plate)
• Majority of geological activity happens at plate boundaries (ex. Earthquakes, volcanoes, mountain ranges)

Question 12

plate tectonics

Answer 12

• theory that explains the movement of the plates and the geological features it produces
• proposed by Alfred Wegener -> continents like a jigsaw
• Proposed supercontinent “Pangaea” -> existed 225 million years ago, rifts happened 175 years ago
• This theory was rejected by the scientific community
  He had the overall idea correct, but he did actually get the mechanism wrong -> he thought the continents were moving, and that this was due to centrifugal force and gravity of sun & moon, but these are far too weak to cause this

Question 13

support for Wegener’s theory

Answer 13

• Fossil, climate, continental edges, rock types, and styles of deformation and glaciation match across different continents
• ex. Mountains like the Appalachians & Mountains in the UK are very similar (in age, deformation, and rock types) -> “The Caledonian Mtn Chain”
• ex. Same fossils found in Africa & South America
• Ex. glaciation: striations, scratches & leftover sediment on rocks from glacial ice -> found in India, S. Am, Africa, Australia -> evidence that those continents were all once connected to Antarctica -> “Permo-Carboniferous glaciation

Question 14

the mechanism of plate tectonics

Answer 14

• started to be discovered during WWII after invention of echolocation -> helped us map topography of ocean floor
• Captain Harry Hess used his boat’s SONAR to plot ocean floor
• Hess proposed that sea floor was spreading, generated at the Spreading Centers represented by the oceanic ridges -> continents don’t plough through ocean crust – they ride along on plates

Question 15

why was using SONAR to map the ocean floor important (other than for mechanism of plate tectonics?)

Answer 15

• Discovered ridges (volcanic mountain chains), sunken volcanoes, deep trenches, etc.
• This was important as people have previously thought that the ocean floor was flat and deepest in the middle and shallowest along the coast -> This is untrue! (ex. Marianas Trench – deepest body of water, right along coast)

Question 16

sea floor spreading

Answer 16

• Magma oozes up at ridges to form new ocean crust -> pushes older ocean floor to either side -> ocean crust descends into the earth at the ocean trenches
• Continents carried as new ocean crust spreads (rather than plowing through the crust as Wegner had proposed)
• From this, the modern theory of plate tectonics developed

Question 17

paleomagnetism

Answer 17

• used to measure age of ocean floor and see if sea floor spreading is true
• Ridges are younger, trenches are older
• can date it using magnetism/magnetosphere
• Rocks can record earth’s reversed or normal polarity (paleomagnetism) based on the magnetic polarity found in the cooled lava
• Normal and reversed magnetism creates a “zebra skin” pattern on a magnetometer
• Can date patterns on ocean floor by comparing them with those on land

Question 18

magnetism/magnetosphere

Answer 18

• Magnetosphere extends into space to protect us from sun’s radiation
• This is generated by the Dynamo effect in earth’s core (core is like a giant bar magnet)
• Magnetic flux lines deep in ground at poles; parallel to ground surface around equator
• Earth’s magnetic field is not constant -> flips every so often (North and South pole switch positions) -> last one was 780,000 yrs ago (indication/clue of impending switch: weakening of earth’s magnetic field)
• these flips are recorded by rocks

Question 19

proof that sea floor spreading occurs

Answer 19

Youngest rocks are at the spreading ridges and get older symmetrically on either side of the ridge. Oldest rocks found at convergent or passive plate margins.

Question 20

which are older - oceanic or continental rocks?

Answer 20

• continental!
• Oldest continental rock is Canadian (4 billion years)
• Oldest oceanic rock is in Mediterranean (280 million years)

Question 21

plate margins (and their 2 types)

Answer 21

• Most geological action happens at margins of plates -> not all plate boundaries are active though
• 2 types of margins:
• Active margins: Regions where lithospheric plates interact with each other
• Passive margins: Transitions between oceanic and continental crust where there is no interaction between plates (ex. At the border of South America and the Atlantic ocean). Typically marked by thick accumulation of sediment due to lack of activity -> This creates oil -> good economically

Question 22

types of active plate margins

Answer 22

• convergent
• divergent
• transform

Question 23

divergent margins

Answer 23

• Plates are spreading apart
• Can split continents apart
• Heating below continental crust causes crust to thin until oceanic crust starts to form and a body of water forms in between -> can eventually create a whole new ocean
• Associated with shallow earthquakes
• Examples: East Africa rift, Red Sea, Atlantic ocean

Question 24

convergent margins (and its 3 types)

Answer 24

• Two plates converge -> subduction
• Accompanied by mountains, trenches, volcanoes, and earthquakes
• 3 types:
• Oceanic and continental meet: oceanic goes underneath (since it’s denser), creating a deep trench and the continental to get pushed up into mountain chain (ex. Coastal BC)
• Oceanic and oceanic meet: the older plate goes underneath (since it’s colder and therefore denser), creating a trench, earthquakes, and a chain of volcanic islands (ex. Eastern Caribbean)
• Continental and continental meet: neither plate is dense enough for subduction, so they go up, forming mountain chains and earthquakes

Question 25

transform margins

Answer 25

• Plates are sliding past one another -> no material destroyed or created
• Associated with earthquakes
• Usually link different boundaries together or cut across mid-ocean ridges
• Ex. San Andreas fault (note: LA is on Pacific Plate -> continent and ocean on same plate)
• Note: transform boundaries mean plates are moving in opposite directions on either side of boundaries -> if they’re moving in the same direction, it’s called a fracture zone

Question 26

measuring plate movement

Answer 26

• Today, GPS is used (shows 3D absolute movement)

- Helps us understand that plates can rotate – they don’t just move in lines

Question 27

4 potential mechanisms of plate movement

Answer 27

1. Mantle convection
   - Hot material rises from the core-mantle boundary and then cools down and sinks, stirring the mantle
   - Could be causing plate motion, or could be caused by plate motion
2. Ridge push
   - Plate moves away from ridge and cools, thickens, and subsides, forming a slope
3. Slab pull
   - Cold dense plate subsides into mantle and drags plate behind it (like a wet tablecloth sliding down and bringing the other side with it)
   - 2x more effective than ridge push
   - Bigger plates move faster than smaller (more push)
4. Trench suction
   - Dipping subducting slab pulls overlying plate towards it (plate tries to fill the hole)
   - Continents not moved directly by slab pull (continental cannot be subducted)
   - Likely a minor force, but may be combined with ridge push to move continental blocks

Question 28

hot spots/mantle plumes

Answer 28

• Hawaii doesn’t fit the model – it’s in the middle of a plate
• John Tuzo Wilson suggested that Hawaii was created as the Pacific Plate moved over a fixed hot spot
• Smaller, less active islands have moved off of the hot spot
• Hawaii formed due to magma plumes
• Develop at 2900km -> heat causes mantle rock to become buoyant, rise up and impact base of lithosphere
• Evidence of this can be found in rocks (Xenoliths)