IB Geography D.1.1 Mechanisms of Plate Movement Notes

Earth's Internal Heat Sustains Plate Tectonics

1. Heat transfer moves energy upward by conduction through rigid lithosphere and by slow material circulation within the mantle.
2. Buoyancy develops because hotter mantle is slightly less dense than cooler mantle, creating broad zones of upwelling and downwelling without making the mantle liquid.
3. Mechanical contrast matters because the strong lithosphere can fracture into plates while the weaker asthenosphere deforms and permits their movement.
4. Heat left from Earth's formation and heat released by radioactive decay maintain a hot interior.
5. The mantle is mostly solid rock, but high temperature and pressure allow it to deform slowly over geological time.
6. Lithosphere means the rigid crust and uppermost mantle that are divided into moving tectonic plates.
7. Asthenosphere means the weaker upper-mantle zone beneath the lithosphere that can flow slowly.

1. Oceanic lithosphere is thinner and denser than continental lithosphere, so old oceanic plates can sink at subduction zones.

Rigid plates move above a slowly deforming mantle rather than floating on a liquid layer.

Slab Pull, Ridge Push and Mantle Flow Move Plates

1. Force balance differs between plates because a plate attached to a long, cold subducting slab experiences more gravitational pull than one without an active subduction zone.
2. Basal interaction between a plate and the flowing mantle can assist or resist motion, so mantle flow should not be presented as a conveyor belt that simply carries every plate.
3. Measured motion from satellite geodesy confirms that plates move in different directions and at rates of a few centimetres per year, consistent with several forces acting together.
4. Slab pull occurs when cold, dense oceanic lithosphere sinks into the mantle and pulls the rest of its plate toward a trench.
5. Slab pull is often the strongest force on plates that contain long subducting margins.
6. Ridge push is the gravitational movement of lithosphere away from the elevated crest of a mid-ocean ridge.
7. Hot mantle rises beneath ridges, then cools, thickens and becomes denser as the new oceanic plate moves away.
8. Mantle convection redistributes heat and helps organise the upwelling and downwelling linked to plate motion.
9. The three mechanisms operate together, so a simple circular convection cell is only a model of a more complex system.

Plate Margins Concentrate Earthquakes and Volcanism

1. Ocean–ocean convergence commonly produces a trench, a dipping earthquake zone and a volcanic island arc as one oceanic plate descends beneath another.
2. Ocean–continent convergence produces a continental volcanic arc because dense oceanic lithosphere subducts while the more buoyant continent remains above it.
3. Continent–continent collision thickens and folds crust to build mountains but produces little subduction-related volcanism once the intervening ocean has closed.
4. Divergent margins form where plates separate and decompression melting produces magma and new crust.
5. Convergent margins form where plates move together through subduction or continental collision.
6. Transform margins form where plates slide laterally past one another without creating or destroying crust.
7. Friction and rock failure produce earthquakes at all three margin types, while widespread volcanism is most closely linked to divergent margins and subduction zones.

Most geophysical activity follows plate margins because that is where relative plate motion concentrates strain and melting.

Subduction Recycles Dense Oceanic Lithosphere

1. Slab dehydration releases water from altered oceanic crust and sediments as pressure and temperature rise during descent.
2. Flux melting occurs when this water lowers the melting point of the mantle wedge, producing magma that is less dense than the surrounding rock.
3. Margin coupling determines earthquake potential because a strongly locked plate interface stores more elastic strain before rupture than a freely slipping segment.
4. Subduction begins when dense oceanic lithosphere bends and descends beneath another plate at a convergent margin.
5. The bending plate forms a deep ocean trench and experiences earthquakes from shallow to deep focus along the descending slab.
6. Water released from the slab lowers the melting temperature of the overlying mantle and helps generate magma.
7. Buoyant magma rises through the overriding plate and can form a volcanic island arc or a continental volcanic arc.
8. An oceanic plate cannot subduct beneath a less dense continental plate in the opposite direction because density controls which plate sinks.

A subduction margin links an ocean trench, a dipping earthquake zone and a volcanic arc through one descending slab.

Rifting Can Develop into Seafloor Spreading

1. Fault sequence begins with crustal stretching, normal faulting and subsiding grabens before sustained magma supply creates new oceanic crust.
2. Rift shoulders rise where heated lithosphere expands and fault blocks are uplifted, producing steep relief around the central valley.
3. Failed rifts show that extension does not always open an ocean because stretching may stop before continental crust separates completely.
4. Extension stretches and thins continental lithosphere while normal faults lower blocks to form a rift valley.
5. Pressure falls as mantle rises beneath the thinning crust, so decompression melting can feed basaltic volcanism.
6. Continued separation may allow a narrow sea and then an ocean basin to form as new oceanic crust is created.
7. The East African Rift shows active continental rifting, while the Mid-Atlantic Ridge shows established seafloor spreading.

A rift valley records crustal extension, but only sustained rifting produces a new ocean basin.

The Mid-Atlantic Ridge creates oceanic crust as plates separate on either side of the spreading centre.

Mantle Plumes Explain Some Intraplate Volcano Chains

1. Age progression allows the direction and approximate speed of plate movement to be inferred when volcanoes become systematically older away from the active hotspot.
2. Changing magma supply explains why individual islands grow, subside and erode as they move away from the plume source.
3. Model limitation matters because some intraplate volcanism may also reflect small-scale mantle convection or reactivated lithospheric weaknesses rather than a deep fixed plume.
4. Mantle plume describes a buoyant upwelling of unusually hot mantle that can generate magma beneath a plate.
5. A relatively fixed hotspot can build a line of volcanoes as the overlying plate moves across it.
6. The youngest active volcanoes sit above the magma source, while older extinct volcanoes record the plate's previous positions.
7. The Hawaiian chain records Pacific Plate movement even though it lies far from a plate margin.

The age sequence of the Hawaiian islands records plate movement across a persistent source of magma.