A subduction zone has an unusually shallow dip angle — the descending slab descends nearly horizontally beneath the overriding plate for hundreds of kilometers before steepening. What would you predict about volcanic arc activity above this zone compared to a steeply dipping subduction zone?
AVolcanic activity would be more intense directly above the trench, since the shallow slab brings water closer to the surface
BVolcanic arc activity would be suppressed or displaced far inland, because the flat slab slides beneath the overriding plate without descending into the hot mantle wedge needed to generate magmas
CVolcanic activity would be unchanged — arc volcanism depends only on convergence rate, not slab geometry
DA shallow slab produces more megathrust earthquakes but has no effect on volcanism
Volcanic arcs form when water released from the descending slab lowers the melting point of the mantle wedge above, generating magmas that rise to produce volcanism. This dehydration occurs when the slab reaches sufficient temperature and pressure — typically at depths of 100-150 km directly above the slab. If the slab descends nearly horizontally, it underplates the overriding plate without sinking into hot mantle, so dehydration-induced melting is suppressed. Flat subduction beneath parts of South America is associated with reduced arc volcanism and the inland thickening of the overriding crust that produced the Sierras Pampeanas.
Question 2 Multiple Choice
Which process best explains why earthquakes occur within subducting slabs at intermediate depths (70–300 km), where temperatures and pressures would normally prevent brittle failure?
ARidge push forces from the mid-ocean ridge exceed the slab's tensile strength at these depths
BDehydration embrittlement: minerals in the oceanic crust release water as they break down under increasing pressure and temperature, and this water weakens the surrounding rock enough to allow brittle failure
CThe slab is too cold to undergo any plastic deformation, so it remains brittle at all depths
DSeismic tomography artifacts create the false appearance of deep earthquakes within the slab
Under normal mantle conditions, rocks at 70-300 km depth are hot enough to deform plastically — they flow rather than fracture. But the subducting slab carries hydrated minerals (serpentinite, amphiboles) that are metastable under increasing pressure and temperature. When these minerals break down, they release water. This fluid raises the pore pressure in the surrounding rock, reducing effective normal stress and allowing brittle failure even at depths and temperatures that would otherwise prohibit it. The connection between dehydration reactions and intermediate-depth seismicity is one of the key insights linking mineralogy to seismic hazard in subduction zones.
Question 3 True / False
Slab pull — the gravitational force exerted by the cold, dense descending slab — is thought to be a larger driver of plate motion than ridge push at mid-ocean ridges.
TTrue
FFalse
Answer: True
Ridge push arises from the elevation difference between spreading ridges and older, cooler ocean floor — a relatively modest gravitational force. Slab pull arises from the negative buoyancy of the cold, dense oceanic slab as it sinks into the less-dense mantle — a much larger force acting along the full length of the descending slab. Evidence includes the observation that plates attached to subducting slabs generally move faster than plates that are not. Slab pull is currently considered the dominant driving force in plate tectonics, which reframes the mid-ocean ridge from a 'spreading engine' to a passive response to slab descent.
Question 4 True / False
Megathrust earthquakes — the largest earthquakes on Earth — occur within the body of the subducting slab itself, caused by brittle failure as the dense slab pulls downward.
TTrue
FFalse
Answer: False
Megathrust earthquakes occur on the interface between the subducting and overriding plates — the shallow, locked contact zone where the two plates are coupled together by friction. When stress builds up faster than the plates can creep past each other, the locked section ruptures suddenly, causing the overriding plate to snap upward and generate massive tsunamis (as in the 2004 Indian Ocean and 2011 Tohoku events). Earthquakes within the slab itself (intraslab events) are a different phenomenon, typically smaller and caused by bending stresses or dehydration embrittlement at depth.
Question 5 Short Answer
Why do subducting slabs appear as high-velocity anomalies in seismic tomography, and how does this connect to the slab's role in the mantle convection system?
Think about your answer, then reveal below.
Model answer: Seismic wave velocity in rock increases with decreasing temperature and increasing rigidity. Subducting slabs are 500-700 K colder than the surrounding mantle (they haven't had time to equilibrate thermally), so seismic waves travel faster through them than through the adjacent warmer mantle. Tomography maps these velocity contrasts to image slab geometry. In the mantle convection system, slabs are the cold, dense downwelling limb — the return flow that completes the convective cycle initiated at mid-ocean ridges (hot upwellings). Their thermal anomaly is both what makes them detectable (high velocity) and what drives them downward (negative buoyancy).
This connection illustrates how different geophysical tools (seismology, thermodynamics, fluid dynamics) converge on the same physical reality. The slab's coldness is simultaneously the cause of its high seismic velocity, the source of its negative buoyancy (slab pull), and the reason it generates seismicity through dehydration reactions as it warms during descent. Understanding subduction zones requires integrating all of these perspectives — which is why it sits at the center of modern geodynamics.