LR

Lecture 3 - The Mobile Crust: Key Terms (VOCABULARY)

Layers of the Earth — Chemical Properties

  • Crust: silicate rocks; thickness 8\ \text{km} \le t \le 75\ \text{km}
  • Mantle: silicate rocks; overall thickness to core boundary around 2900\ \text{km}
  • Core: iron; Outer Core is liquid; Inner Core is solid; Core radius roughly 6370\ \text{km}
  • Layers by chemistry: Crust, Mantle, Core

Layers of the Earth — Physical State

  • Lithosphere: rigid shell that includes the crust and the uppermost mantle
  • Asthenosphere: plastic/partially molten region beneath the lithosphere
  • Mesosphere: solid lower mantle
  • Outer Core: liquid iron
  • Inner Core: solid iron

Plate Tectonics

  • Lithosphere is broken into tectonic plates that move and interact
  • Plate interactions cause earthquakes, volcanism, mountain building

History and Acceptance

  • Plate tectonics was controversial until the 1960s
  • Example of the scientific method in action; multiple lines of evidence led to acceptance

Continental Drift — Wegener

  • Alfred Wegener, 1915: The Origins of Oceans and Continents; proposed Continental Drift
  • Hypothesized a single supercontinent, Pangaea, that broke apart
  • Evidence: fit of continents and geological correlations

Evidence for Continental Drift

  • Glacial deposits near the equator today
  • Tropical coal/swamp deposits in current eastern U.S.
  • Tropical reef fossils in mid/high latitude regions today
  • Same land animal fossils found in South America and Africa
  • Similar mountain belts on opposite sides of the Atlantic

Supercontinents

  • Pangaea split into Gondwanaland (southern) and Laurasia (northern)

Mid-20th Century Evidence

  • Advances in oceanography and geophysics provided new support for plate movements

Ocean Floor Mapping and Bathymetry

  • Post-WWII, seafloor mapping revealed: central elevated ridges in each ocean basin and deep trenches near large underwater mountain ranges
  • Technique: sonar (sound waves) to map depths

Mid-Ocean Ridge and Ocean Basins

  • Ocean floor features: Mid-Ocean Ridge (central axis of spreading), continental margins, abyssal plains, trenches near margins

Seafloor Volcanism

  • Mid-ocean ridges are sites of active volcanism
  • Molten rock erupts and forms new crust (igneous rock)
  • Common seafloor igneous rock: basalt; formation linked to volcanism

Magnetite and Magnetization

  • Basalt is Fe-rich and contains magnetite (Fe(3)O(4))
  • Magnetite aligns with Earth's magnetic field as it cools
  • Curie temperature: T_C \approx 570^\circ\mathrm{C}

What Creates Earth's Magnetic Field?

  • Generated by rotating, convecting liquid iron in the outer core

Magnetism on the Seafloor

  • Magnetometers detect linear belts of normal and reverse magnetization parallel to ridges
  • Normal polarity: magnetization aligned toward north
  • Reverse polarity: magnetization aligned toward south

Magnetic Stripes and Plate Tectonics

  • Stripes record magnetic reversals and support seafloor spreading from ridge centers
  • Stripes run parallel to mid-ocean ridges (normal/reverse alternating)

Magnetic Field Reversals

  • Time-averaged reversal interval: about 4.5\times 10^5\ \text{yr}, but highly variable

Seafloor Age and Stripe Colors

  • Young rocks near ridges; older rocks farther from ridges
  • Age progressions commonly mapped as color stripes: e.g., 0 Mya at ridge, increasing to hundreds of millions of years farther away

Break Up of Pangaea

  • Continents separated along spreading ridges and subduction zones, forming present-day plate boundaries

Magnetic Polar Wander

  • Magnetic north does not coincide with geographic north; the magnetic poles wander over time

Modern Evidence that Plates Move

  • GPS measures plate motion directly; typical rates around 2\ \text{cm/yr}

Seismic Tomography

  • Uses variations in seismic wave speeds to image density changes at depth
  • Visualizes plates and mantle structures: denser vs less dense regions