Plate Boundaries & Motion
Divergent, Convergent & Transform · Subduction Zones (O–C, O–O, C–C) · Plate Speeds & Heat Engine (28 Micro-Realms)
The Three Tectonic Margins & Velocity Spectrum
Lithospheric plates interact along three distinct boundary types: 1) Divergent Boundaries (Spreading Sites) where new crust is generated as plates pull apart, 2) Convergent Boundaries (Subduction Zones) where crust is destroyed in three ways (Ocean–Continent, Ocean–Ocean, Continent–Continent), and 3) Transform Boundaries where plates slide horizontally past each other at speeds ranging from less than 2.5 cm/yr (Arctic Ridge) to over 15 cm/yr (East Pacific Rise).
I. Divergent Boundaries: Constructive Margins & Spreading Sites
3 Boundary Types
- Divergent (Constructive)
- Convergent (Destructive)
- Transform (Conservative)
Divergent Boundaries
- Plates pull away from each other
- Tensional extensional stress
- Constructive plate margin
Spreading Sites (NCERT)
- Sites where plates move apart
- Basaltic magma wells up
- Generates new oceanic crust
Mid-Atlantic Ridge (NCERT)
- Best-known divergent example
- American Plates separate from
- Eurasian & African Plates
East African Rift Valley
- Continental divergence stage
- Nubian & Somalian blocks split
- Chain of deep rift lakes
Red Sea Proto-Ocean
- Arabian plate rifts from Africa
- Seawater floods axial graben
- Youngest linear ocean basin
Iceland Ridge Exposure
- MOR rises above sea level
- Hotspot + Mid-Atlantic Ridge
- Fissure basalt volcanism
II. Convergent Boundaries: Destructive Margins & 3 Convergence Ways
Convergent Boundaries
- Crust is destroyed as plate dives
- One plate sinks under another
- Compressional destructive margin
The Subduction Zone
- Location where sinking occurs
- Wadati–Benioff deep quake plane
- 3 distinct ways of convergence
I. Ocean–Continent (O–C)
- Dense SIMA subducts under SIAL
- Forms trench + continental arc
- Nazca + S. America $\to$ Andes
II. Ocean–Ocean (O–O)
- Older/denser ocean slab subducts
- Deepest trenches (Mariana)
- Volcanic Island Arcs (Japan)
III. Continent–Continent (C–C)
- Neither buoyant SIAL plate sinks
- Intense crustal doubling & folding
- India + Eurasia $\to$ Himalayas
Suture Zone & Ophiolites
- Weld line of vanished ocean
- Indus–Tsangpo Suture Zone
- Trapped Tethyan ophiolites
Accretionary Prism
- Scraped-off ocean sediments
- Bulldozed at trench inner wall
- Andaman-Nicobar outer ridge
III. Transform Boundaries: Conservative Margins & Strike-Slip Faults
Transform Boundaries
- Plates slide horizontally past
- Along transform strike-slip faults
- No vertical subduction
Conservative Nature
NOR DESTROYED
- Crust is neither produced
- Nor destroyed (NCERT)
- Surface area stays constant
Perpendicular MOR Offsets
- Generally perpendicular to MORs
- Offsets spreading ridge segments
- Eruptions differ along crest
San Andreas Fault
- Classic continental transform
- Pacific vs N. American plate
- 1,300-km California shear zone
Rotation & Coriolis Effect
- Earth's rotation on its axis
- Affects separated plate portions
- Differential motion away from axis
Transform Seismicity
- Frequent shallow earthquakes
- Virtually zero volcanism
- North Anatolian & Alpine faults
Triple Junctions (Afar)
- Where 3 plates meet at a point
- Afar Triangle: Red Sea + Aden
- + East African Rift (R–R–R)
IV. Rates of Plate Movement & Force for Plate Movement (NCERT)
Measuring Plate Rates
- Strips of normal & reverse field
- Parallel to mid-oceanic ridges
- Determines rate of movement
Slowest: Arctic Ridge
- Slowest rate on Earth (NCERT)
- Less than 2.5 cm/yr
- Gakkel Ridge in Arctic Ocean
Fastest: East Pacific Rise
- Fastest rate on Earth (NCERT)
- More than 15 cm/yr
- Near Easter Island (~3,400 km W of Chile)
Indian & Atlantic Rates
- Mid-Atlantic: ~2.5 cm/yr
- Indian Plate today: ~5 cm/yr
- Peak India sprint: 15–20 cm/yr
Two Internal Heat Sources
- 1. Radioactive decay (U, Th, K)
- 2. Residual primordial heat
- Drives convective mantle flow
Convection Cell Loop
- Heated material rises to surface
- Spreads, cools & sinks deeper
- Slow-moving mantle engine
Crustal Area Equilibrium
- Global divergent addition
- Equals convergent destruction
- Constant planetary surface area
Core Distinctions
• Divergent (Constructive): New crust is generated as plates pull away (Spreading sites; Mid-Atlantic Ridge).
• Convergent (Destructive): Crust is destroyed as one plate dives under another (Subduction zone; 3 ways: Ocean–Continent, Ocean–Ocean, Continent–Continent).
• Transform (Conservative): Crust is neither produced nor destroyed as plates slide horizontally past each other.
• Slowest Rate: Arctic Ridge (less than 2.5 cm/yr).
• Fastest Rate: East Pacific Rise near Easter Island, in the South Pacific about 3,400 km west of Chile (more than 15 cm/yr).
The heat that drives slow movement of the hot, softened mantle beneath the rigid plates comes from two main sources: 1) Radioactive decay and 2) Residual heat.
Boundary & Rate Matrix
High-Yield UPSC Prelims Reference