Plate Kinematics and Motion

Study Notes

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.

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.

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.

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.

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.

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.

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.

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).

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).

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.

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.

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

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.

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.