Plate Tectonics

• In 1912, climatologist Alfred Wegener began publishing his ideas on the movement of continents and named it ⇒ Continental Drift
• He put together several lines of evidence
  • Glacial deposits in tropical areas
  • Desert sand deposits in moist regions
  • Coal deposits in Antartica
  • Fossil deposits found across different continents
• Wegener proposed Pangea, a single supercontinent
  • And the hypothesis of Continental Drift
• But he struggled to get support for it

Evidence of Plate Tectonics

• Sea floor topography - Ridges, trenches, volcanoes and not just flat sediments
• Paleomagnetism and seafloor spreading - Ferromagnesian (magnetite) minerals in magma align with Earth’s magnetic field upon cooling and formation of seafloor basalts
  • On both sides of the mid-ocean ridges, stripes of varying intensities of magnetism (some stronger, some weak)
  • Due to reversals in polarity of Earth’s magnetic field and spreading of existing seafloor and creation of new seafloor
• Age of ocean floor - dates of basalts are symmetrical across ridges with youngest at ridges and progressively older as you move away from the ridge
  • Seafloor is rarely older than 200 million years
  • Sediments show same trend with thicker and older deposits as you move away from ridges
  • Continental rocks contain much older ocean floor and one can better study the magnetic orientation of those rocks in 3 directions
  • Direction of needle points to magnetic N and dip of needle varies with latitude
• Polar-wander curves - Rocks of different ages on a continent can point to various magnetic pole positions and make it look like the magnetic pole has wandered over the years

Recap:

• Seafloor topography revealed pronounced structure and relief
• Paleomagnetism helped reveal seafloor spreading occurs
• Age of seafloor confirmed spreading must occur
• Polar0wander curves suggest and track the movement of continents

How Plates Move

• Lithosphere - consists of crust and upper mantle; acts brittle and elastic
• Asthenosphere - only mantle; Rocks flow plastically under high temperature and moderate confining pressure
• Below the asthenosphere pressure increases and mantle becomes rigid again

Tectonic Plates & Boundaries

• Volcanic activity is not exactly random
• Earthquake locations are not random either

⇒ They occur at plate boundaries

• Only a few large plates, some smaller ones, and likely more will be found

There are 3 types of plate boundaries

• Divergent
• Convergent
• Transform

Divergent Plate Boundaries

Lithospheric plates move apart (or diverge), causing tensile stress and a rift valley or rift zone forms

• Resulting less pressure allows for melting in astenosphere
• Magma upwells from asthenosphere to surface

Volcanoes and earthquakes are common!

Two types of divergent plate boundaries:

• Oceanic Ridges
• Continental Rifts

Oceanic Ridge

Example: Mid-Atlantic Ridge

Continental Rift

• Most ocean basins likely formed from continental rifting
• Either the pulling apart of plates or rising magma creates the rift valley
• Lava flows into rift floor, creating new crust
• If rifting continues, it will fill with water and form a sea or ocean
• And eventually become a mid-ocean ridge

Afar Triple Junction of

• Red Sea Rift
• Aden Ridge
• East African Rift

Continental rifting and seafloor spreading leading to creation of ocean basins (1-2 cm/year)

African plate was once one whole plate, but now divided into Nubian, Somalian, and Arabian protoplates

In 10 million years, the whole rift will be submerged

Example: The Red Sea

Convergent Plate Boundaries

Ocean-Ocean

Ocean-Continent

Continent-Continent

Transform Plate Boundaries

How do plates move?

We do not know for sure!

Three models exist:

1. Mantle Convection - Some rising magma at spreading centers spreads in upper asthenosphere, dragging overlying lithosphere with it and keeping spreading center open, until the dense oceanic crust sub-ducts. Creates convection cells within mantle.
2. Slab-pull Model - Starts at the subduction zone with the dense oceanic crust sub-ducting and pulling the plate over, keeping spreading center open.
3. Ridge Push Model - Starts at the topographic highs at spreading center with lithosphere sliding off aided by rising magma pushing them off and dragging asthenosphere with it

How Long and How Fast?

• Oceanic crust only tells us up to 2 MYA, but continental rocks show evidence of plate tectonics back to ~3BYA
• Velocities and / or directions measured by
  • Polar-wander curves
  • Seafloor spreading distance and dates
  • Hot spots - stable volcanic area not a plate boundary
    • Show direction (and shifts) and velocity
    • Hawaii is a classic example
• Overall global average of plate movement is…
  • 2-3 cm/year, which over 100 million years would be…
    • 2000 km! ~Here to Florida or Winnipeg