Plate Kinematics and Motion

Plate Kinematics

• Plate motion is described in terms of rates and directions on the surface of the Earth, using spherical geometry.
• Three assumptions are made:
  • The Earth is a sphere.
  • The Earth's circumference remains constant over time.
  • Plates are internally rigid, with motion occurring only at plate boundaries.
• Two types of reference frames:
  • Absolute reference frame: describes plate motion relative to a fixed point in the Earth's interior.
  • Relative reference frame: describes the motion of one plate relative to another.

Absolute Motion

• Plate motion relative to a fixed reference point in the mantle (e.g., a hotspot).
• Hotspot volcanoes form above a mantle plume and are used as "fixed" reference points to calculate absolute plate velocities.

Hotspot

• Isolated volcanoes on Earth that give a reasonable approximation of absolute plate velocity.
• Characteristics:
  • Age of volcanoes increases progressively away from the plume.
  • Orientation of the hotspot track gives the direction of plate motion.
  • Rate of change in the age of volcanic rocks along the track represents the velocity of the plate.

Examples of Hotspots

• Hawaii Emperor chain in the Pacific:
  • 6000 km-long track runs towards WNW and then bends abruptly NNW.
  • Consists of extinct volcanoes of increasing age to the NW.
  • Oldest remaining volcanoes in the chain are older than 80 Ma.

Mantle Plume

• A hot, buoyant layer at the base of the mantle forms due to heat from the core.
• Characteristics:
  • Plume ascends because it is hotter and less dense than the surrounding mantle.
  • Fixed position correlates with its origin near the core boundary.
  • Unlike mid-ocean ridges, which have sources in the shallow mantle.
• Structure:
  • Cylindrical conduit with a bulbous head.
  • Forms a broad mushroom-like head when it reaches the bottom of the lithosphere.

Effects of Mantle Plume

• Generates surface uplift and bulging of the lithosphere.
• Causes huge amounts of partial melting and magma generation near the base of the lithosphere.
• Associated with breaking up continents and supercontinents.

Hotspot Tracks on Continents

• Less noticeable and usually don't form clear chains of volcanoes.
• Reasons:
  • Thick continental lithosphere makes it hard for magma to break through.
  • Disruptions of magma paths and changes in magma composition make detection more difficult.
• Example: The track of the Mesozoic Great Meteor Plume.

Flood Basalts (LIP)

• Characterized by layered basaltic deposits several kilometers thick.
• Form huge plateaus on land and below sea level.
• Characteristics:
  • Massive.
  • Formed highest single mountain on Earth.
  • Form huge flood basalt fields.
  • Silent but deadly, with several episodes of mass extinctions.
  • Thick lava flows cover huge areas (>100,000 km2).
• Short duration but high lava volume (> 1 million km3 during an interval of only one or several million years).

Examples of Flood Basalts

• The Central Atlantic Magmatic Province (CAMP):
  • Formed 201 my ago.
  • Covered 10 million km2.
  • Occurred at the boundary of the two main landmasses: Laurasia and Gondwana.
  • Split the supercontinent Pangea.
• The Siberian Trap:
  • The largest volcanic event in the last 500 my.
  • 250 my ago.
  • Eruptions lasted 2 my.
  • Covered 7 million km2.
  • Erupted 4 million km3 of lava.
  • The great Dying-PT mass extinction (95% of all species wiped out).

Relative Motion

• The motion of any plate with respect to another can be defined by imagining that the position of one of the plates is fixed.
• Euler pole: the intersection between the imaginary rotation axis and the surface of the Earth.
• Characteristics:
  • Rotation on Euler pole.
  • Displacement follows small circles.
  • Transforms parallel to small circle segments.

Plate Interaction

• Several different geometries/configurations:
  • Transform connect two segments of growing plate boundaries (R-R transform fault).
  • One growing and one subducting plate boundary (R-T transform fault).
  • Two subducting plate boundaries (T-T transform fault).
• Triple junctions:
  • Where three plates are in contact.
  • Can be stable or unstable.
  • Examples:
    • Ridge-trench-transform (RTF) triple junction.
    • Ridge-ridge-ridge (RRR) triple junction.

Plate-Driving Forces

• The movement of the Earth's plates is partly the result of heat convection cells in the mantle.
• Convection cells carry hot molten material from deep in the earth to the surface at the divergent boundary.
• Material cools, loses heat, and then descends back into the earth at the convergent boundary.
• Types of forces:
  • Slab-pull forces: negative buoyancy of the slab.
  • Ridge-push forces: topographic spreading.
  • Drag forces/resistive forces: under the moving plates.

Ridge Push

• Ridge-push force is the outward-directed force that pushes plates away from the axis of a mid-ocean ridge.
• Causes seafloor spreading.

Slab Pull

• Slab-pull force is the force that pulls lithosphere into a convergent margin.
• Due to the subducting cold slab of lithosphere being denser than the surrounding warmer asthenosphere.

Basal Drag Forces

• The flow of asthenosphere due to convection creates some basal drag at the base of plates.
• Can assist or retard motion.
• Asthenosphere flows:
  • In the same direction as the plate motion - basal drag accelerates motion.
  • In the opposite direction - basal drag retards motion.
  • At an angle to the plate motion - basal drag changes the direction of motion.

Quiz about plate motion, rates, and directions on the Earth's surface, using spherical geometry and different reference frames.