Earth’s Structure, Forces, and Plate Tectonics

Posted on Jan 13, 2025 in Geology

Earth’s Composition and Structure

Geology: The science of Earth.

  • Continental Shelf (C-Shelf): Extends past the shoreline.
  • Continental Crust (C-Crust): 35-40km average thickness, granite composition (light-grey).
  • Oceanic Crust (O-Crust): 7km average thickness, basalt composition (dark), 46% O, 28% Si, 3gm/cm3.
  • Mantle: 35% Fe, 30% O, 15% Si, 5g/cm3, extends to 2900km depth, upper (olivine), lower (same composition, higher pressure minerals).
  • Core: 85% Iron (Fe), 11% Nickel (Ni), 4% Sulfur (S), 11gm/cm3 density, outer core (molten) to 5150km depth, inner core (solid) to 6700km depth.

Earth’s Layers and Dynamics

  • Lithosphere (L-sphere): Strong plate, includes the crust and the uppermost mantle.
  • Asthenosphere (A-sphere): Soft, weak, hotter layer that flows under pressure, allowing the lithosphere to move, 80-150km thick.
  • Isostasy: Thicker, less dense, hotter crust floats higher, resulting in regional elevation and mountains.

Forces Shaping Earth

Forces that affect Earth: gravity, water, wind, gravitational pull (Sun/Moon), electromagnetic energy from the Sun, radioactive decay.

Rocks and the Rock Cycle

  • Sediment: Loose material.
  • Sedimentary Rocks: Form on Earth’s surface, mostly from loose sediment.
  • Igneous Rocks: Formed from cooled and solidified magma.
  • Metamorphic Rocks: Rocks changed by temperatures, pressures, or deformation. Metamorphism can alter sedimentary or igneous rocks.
  • Hydrothermal Rocks: Rocks that precipitate directly from hot water.

Rock Cycle (any variation): weathering, erosion & transport, deposition, burial & lithification, deformation & metamorphism, melting, solidification, uplift.

Earth’s Spheres and Surface Features

Four Spheres: Atmosphere, biosphere, hydrosphere, lithosphere. Ocean covers 71% of Earth’s surface, with seven continents.

  • Island Arcs: Volcanic sites.
  • Seamounts: Underwater mountains.
  • Abyssal Plains: Most of the ocean floor, smooth surface, 3-5km deep.
  • Mid-Ocean Ridges: Symmetrical ridges 2-3km higher than the seafloor.
  • Fracture Zones: (Definition needed).
  • Oceanic Plateaus: (Definition needed).

Plate Tectonics and Earthquakes

Earthquakes: Most occur at plate boundaries, mid-ocean ridges (shallow quakes), trenches, and island arcs.

Volcanoes: Found at island arcs, trenches, mid-ocean ridges, and belts.

  • Divergent Boundaries: Spreading centers, continental rifts, mid-ocean ridges, with much volcanism and small to moderate earthquakes.
  • Convergent Boundaries: Subduction zones, mountain ranges (ocean/continent), deep trenches (ocean/ocean), huge mountain ranges (continent/continent), with big earthquakes and volcanoes.
  • Transform Boundaries: Shear movement, zig-zag pattern, perpendicular to mid-ocean ridges and trenches.
  • Continental Rift: Plate boundaries above land, with normal faults.
  • Subduction: One plate slides beneath another.

Driving Forces: Mantle convection, slab pull, ridge push, mantle convection. Plate movement is 1-15 cm/year, measured by GPS, lasers, and other methods.

Paleomagnetism: Magnetic direction preserved in volcanic/intrusive rock, striping helps determine the rate of spreading. The crust is youngest at the mid-ocean ridges.

Continental Drift and Plate Movement

Continental drift: 200 million years ago, fossils and rock types matched, ancient climates, geomagnetic patterns.

Accretionary prism. Island arcs = subduction, while seamounts = hot spots.

Gondwana. The oldest seafloor is at trenches.

Earthquake Mechanics and Seismic Waves

Most earthquakes occur due to movement along faults. Mechanical energy is transmitted through rocks as vibrations called seismic waves.

  • Hypocenter: The place where an earthquake is generated underground (most are less than 100km deep, though subduction zones can be 700km deep).
  • Epicenter: Earth’s surface directly above the hypocenter.
  • Normal Faults: Divergent slip (divergent plates, spreading centers, continental rifts).
  • Reverse & Thrust Faults: Convergence (subduction zones, continent/continent collisions).
  • Strike-Slip Faults: Shear movement (large quakes, transform boundaries).

Volcanic Earthquakes: Eruptions cause waves. Landslides, explosions, and dam failures can also cause earthquakes.

Elastic Rebound: Rocks regain their shape after faulting stress subsides.

Scarp: A break in the Earth’s surface from faulting.

Earthquake Cycle: Repeated process of stick/slip. 90% occur at subduction zones (shallow to deep).

Seismic Waves and Measurement

  • Body Waves: Travel inside the Earth.
    • P-waves: Compressive, travel through solids and liquids, fastest (6-14km/s).
    • S-waves: Up and down motion, perpendicular to movement, slower (3.6km/s), cannot travel through liquids.
  • Surface Waves: Travel along the surface of the Earth (horizontal and vertical motion).

Seismometer: Measures earthquakes. Spikes indicate P-wave (small), S-wave (bigger), and surface wave (biggest).

P-S Interval: Helps locate the epicenter.

  • Magnitude: Measure of energy, Richter scale.
  • Intensity: Shaking felt, Modified Mercalli scale (1-12).

Secondary Effects: Fires, floods, structural damage (most deaths), tsunamis.