Plate Boundaries – Detailed Study Notes

Describe and distinguish the three fundamental plate-boundary types (divergent, convergent, transform).
Match each major lithospheric plate with its dominant boundary type and associated geologic structures.
Relate each plate-boundary type to the principal rock stresses (tension, compression, shearing).
Explain the relative motion of interacting plates, the forces that drive or resist that motion, and the surface features produced.

Alfred Wegener – proposed Continental-Drift Theory; super-continent “Pangaea”.
Robert S. Dietz – advanced Seafloor-Spreading Theory.
Plate-Tectonics Theory – lithosphere is fragmented into rigid plates that move over a plastic asthenosphere.
Asthenosphere – ductile, partially molten upper-mantle layer on which plates “float”.
Plates – large blocks of oceanic and/or continental lithosphere.
Seafloor-Spreading – creation of new oceanic crust along mid-ocean ridges as plates diverge.

Motion: plates move away from each other (tensional stress).
Asthenospheric magma rises, cools, and forms new crust.
Oceanic–Oceanic divergence → mid-ocean ridges (e.g., Mid-Atlantic Ridge).
Continental–Continental divergence → rift valleys (e.g., East African Rift).
Key surface clues: linear ridges, volcanic activity along rifts, shallow earthquakes.

Motion: plates move toward each other (compressional stress or confining stress).
Crust is destroyed or deformed; one plate may override, fold, or subduct under the other.

Denser basaltic oceanic crust subducts beneath lighter granitic continental crust.
Generates volcanic arcs on continental margin (e.g., Andes Mountains).
Produces deep-ocean trenches parallel to the arc.

Motion: plates grind past one another laterally (shearing stress).
Crust neither created nor destroyed.
Characterized by long strike-slip faults (e.g., San Andreas Fault).
Sudden release of built-up strain → strong shallow earthquakes, offset linear features.

Slab-Pull – weight of subducting slab drags the rest of the plate downward.
Slab-Suction – convection set up by a sinking slab pulls both plates toward the trench.
Ridge-Push – gravitational sliding of newly formed, elevated ridge crust away from ridge axis.

Slab Resistance – friction on the top of the descending slab.
Collisional Resistance – thickened crust resists further shortening at continent–continent zones.
Transform-Fault Resistance – friction along strike-slip segments that offset spreading ridges.
Drag Force – general basal friction between plate base and upper asthenosphere.

Average plate speed ≈ $1\text{–}2\,\text{inches/year} \; (\text{≈} 2\text{–}3\,\text{cm/year})$ .
Boundaries rarely move uniformly; long periods of locking followed by sudden slips (earthquakes).

Pacific – mostly divergent on E–S (East Pacific Rise) but convergent with N-American & Philippine.
North American – transform (San Andreas) vs. Pacific; convergent vs. Juan de Fuca; divergent vs. Eurasian across Atlantic.
South American – divergent vs. African; convergent vs. Nazca.
Eurasian – divergent vs. North American (N. Atlantic); convergent vs. African & Indo-Australian.
African – divergent vs. South American & Eurasian; passive margins elsewhere.
Indo-Australian – convergent vs. Eurasian (Himalayas); divergent vs. Antarctic.
Antarctic – divergent with surrounding plates.

Philippines sits mainly on the Philippine Sea Plate; boundaries are largely convergent.
Subduction zones: Philippine Trench (E), Manila & Negros Trenches (W) → frequent volcanoes & earthquakes.
Presence of Philippine Fault – major left-lateral transform component across the archipelago.

Basaltic oceanic crust subducts beneath granitic continental crust because it is denser.
Divergent vs. Convergent: divergence creates new crust; convergence destroys or shortens old crust.
A chain of volcanoes on a continental margin facing an oceanic plate generally signals a convergent boundary.
Mid-ocean ridges formed between Pacific & Farallon plates are under tensional stress.
Compression between Farallon & North American plates caused by Pacific plate pushing Farallon eastward.

Classification of boundaries is based on movement, not on location or rock type alone.
Convergent boundaries can be ocean-ocean, ocean-continent, or continent-continent; only the first two involve subduction.
Divergent boundaries often lie beneath oceans but can also split continents (continental rifts).

Floating leaves in water: bowl represents asthenosphere; leaves represent plates; water movement mimics convection driving plate drift.
Homemade dough plates: simulate divergence (stretch & depression), convergence (collision & uplift), transform (lateral sliding, offsets).

Understanding plate interactions aids in hazard assessment: locating quake-prone faults, volcanic belts, tsunami-generating trenches.
Urban planning (e.g., building codes near transform faults) relies on awareness of plate-boundary stresses.

Average ridge-axis depth increase due to cooling: $\approx 2.5\,\text{km}$ below spreading center over $80\,\text{Ma}$ (general trend).
Subduction-related trenches may reach depths of $\approx 11\,\text{km}$ (Mariana Trench).
Typical transform displacement rates along San Andreas ≈ $50\,\text{mm/yr}$ .

Divergent → Tension → Mid-ocean ridge / Rift valley / Normal faults.
Convergent → Compression (or confining) → Trench, volcanic arc, fold-thrust mountains.
Transform → Shearing → Strike-slip faults, linear valleys, offset streams.

Draw diagrams for each boundary type showing relative motion arrows and resulting landforms.
Practice classifying real-world plate edges on a blank tectonic map.
Relate quiz statements to a specific stress, force, or geologic feature to quickly narrow choices.