Plate Tectonics: Boundaries, Mechanisms, and Paleomagnetism

Introduction to Plate Boundaries and Margins

• Terminology and Definitions

  • The terms "plate boundary" and "plate margin" are synonymous and used interchangeably in geological study.
  • Plate motion is defined by the relative motion between two adjacent plates. It is impossible to characterize a boundary by looking at a single plate; one must observe how it interacts with its neighbor.
  • While real-world boundaries often involve complex combinations of motions (e.g., a mix of convergence and transform motion), academic study focuses on pure motions to understand fundamental mechanics.

• The Three Types of Plate Boundaries

  • Divergent Boundaries: Occur where two plates move apart relative to each other along a line of separation.
  • Convergent Boundaries: Occur where two plates collide or move toward one another.
  • Transform Boundaries: Occur where plates slide past each other laterally and horizontally.
  • California is a primary example of a transform boundary.
  • Transform boundaries are considered the rarest of the three boundary types.

Mechanisms of Plate Motion

• The Lithosphere-Asthenosphere Interaction

  • Plate tectonics is driven by the interaction between the brittle lithosphere and the solid but ductile asthenosphere.
  • The asthenosphere moves in a ductile manner, allowing the rigid plates above to shift.

• Convection in the Asthenosphere

  • Convection is a major driver of plate motion.
  • The speed of this motion is extremely slow, often compared to the rate at which human fingernails grow.
  • This movement is so slow it cannot be visually perceived, even if one were looking directly at it; it is slower than the movement of the hour hand on a clock.
  • The slow nature of this movement is a mitigating factor for the intensity of global volcanism and earthquakes; if the asthenosphere moved faster, these events would be significantly more catastrophic.

• Ridge Push

  • This mechanism occurs at divergent margins where mantle material (specifically the asthenosphere) upwells toward the surface.
  • As the asthenosphere rises, it undergoes partial melting just below the lithosphere, creating magma.
  • This magma creates volcanism at the surface, forming a mid-ocean ridge.
  • The constant upwelling and injection of new material into the gap between plates physically pushes the lithosphere away from the ridge center.

• Slab Pull

  • This occurs at convergent boundaries, specifically at subduction zones.
  • Subduction is the process where one lithospheric plate is pushed into the asthenosphere beneath another plate.
  • While a plate is horizontal, gravity pulls on it equally across its surface. However, once a portion of the plate is angled downward into the asthenosphere, gravity can "yank" or pull on that slab more effectively.
  • This gravitational pull on the subducting slab drags the rest of the plate along, contributing significantly to its overall motion.

Global Plate Geography and Features

• Major and Minor Plates

  • There are seven major plates worldwide and several micro-plates (small plates).
  • Specific Plates Mentioned:
  • North American Plate.
  • Pacific Plate.
  • Juan De Fuca Plate: A micro-plate located near the North American and Pacific plates.
  • African Plate: Currently the only site where a continent is being actively split apart by a divergent margin.
  • Indian Plate: Specifically the collision zone where India meets Asia.

• Topographical Indicators of Boundaries

  • Divergent Margins: Mostly found under ocean basins. On maps, they are marked by light blue colors representing shallow water, as the upwelling material creates underwater mountain ranges (ridges).
  • Convergent Margins: Mostly exist as offshore subduction zones.
  • A notable exception is the collision between India and Asia, which created the Himalayas and Mount Everest, the highest mountain range above sea level.
  • Transform Margins: Characterized by lateral motion; these are visible in specific regions like California.

• Map Colorization Keys

  • Red: Very high elevations.
  • Green: Elevations near sea level.
  • Dark Blue: Deep ocean waters.
  • Light Blue: Shallow ocean waters (often indicating ridges at divergent boundaries).

Paleomagnetism and Earth's Magnetic Field

• Concept of Paleomagnetism

  • Paleo means "ancient," and paleomagnetism refers to the study of the Earth's ancient magnetic fields preserved in rocks.
  • This was a critical field for understanding plate motion before the advent of modern satellites and GPS monitoring.

• Earth as a Magnet

  • The Earth functions like a large bar magnet with magnetic lines of force.
  • Magnetic Principles: Opposite charges/poles attract, while like poles repel.
  • Magnetic lines of force exit the North side of a magnet and enter the South side.

• The Magnetic Pole Paradox

  • Although we refer to the northern geographic region as "Magnetic North," scientifically, the South pole of Earth’s internal magnet is located at the geographic North Pole (the Arctic).
  • Conversely, the North pole of Earth’s internal magnet is located at the geographic South Pole (Antarctica).
  • Function of a Compass:
  • A compass needle is a small magnet.
  • The North end of the compass needle points toward the geographic North Pole because it is attracted to the South pole of the Earth's internal magnetic field (following the "opposites attract" rule).
  • We name our poles based on the direction the compass points for navigation, but the literal physical magnetism is inverted relative to those names.