Which mechanism is now understood to be the dominant driver of plate motion?
ARidge push — hot magma upwelling at mid-ocean ridges forces plates apart
BSlab pull — dense, cold oceanic lithosphere sinking at subduction zones drags the plate behind it
CMantle convection currents directly pushing the base of plates like a conveyor belt
DTidal forces from the Moon stretching the lithosphere
Slab pull is estimated to contribute roughly three times more force to plate motion than ridge push. Cold oceanic lithosphere is denser than the underlying asthenosphere, so when it subducts, gravity pulls the sinking slab downward and the rest of the plate follows. Ridge push is real but secondary — the elevated ridge does create a gravitational slide force, but it is weaker. Mantle convection is important but acts as a lubricating background rather than the primary driver.
Question 2 True / False
The mantle is liquid rock that flows like a slow-moving fluid, similar to thick lava, which is why convection can occur within it.
TTrue
FFalse
Answer: False
The mantle is composed of solid rock under most conditions. It behaves like a fluid only over geological timescales (millions of years) through solid-state creep — atoms diffuse through crystal lattices and minerals deform plastically under sustained stress. This is fundamentally different from a liquid: seismic S-waves (which cannot travel through liquids) propagate through the mantle, confirming it is solid. The asthenosphere is partially molten in some regions but is still largely solid.
Question 3 Short Answer
Why are the world's major earthquake belts and volcanic zones concentrated at plate boundaries rather than distributed randomly across continents and ocean floors?
Think about your answer, then reveal below.
Model answer: Plate boundaries are where relative motion between plates occurs, generating stress. At convergent boundaries, subducting slabs generate deep earthquakes and volcanic arcs. At divergent boundaries, rifting produces shallow earthquakes and mid-ocean ridge volcanism. At transform boundaries, plates slide past each other causing shallow strike-slip earthquakes. The interiors of plates are relatively stable because they move as rigid units with little internal deformation.
The key insight is that earthquakes and volcanism require either brittle fracture (stress exceeding rock strength) or decompression/fluid-induced melting — both of which occur at boundaries where plates interact. Plate interiors do experience intraplate events from hotspots or ancient weaknesses, but these are minor compared to boundary activity.