Questions: Plate Boundary Types and Tectonic Processes

Question 1

Geologists find a continental mountain range with no active volcanism, thrust faults, and crustal thickness nearly double the global average. What plate interaction most likely created this setting?

• A. Divergent boundary: continental rifting that thickened the crust
• B. Oceanic-continental convergence: subduction forming a volcanic arc
• C. Continental-continental collision: neither plate subducts easily, so crust crumples and thickens
• D. Transform boundary: lateral slip building topography over time

Answer: (C)

When two continental plates collide, neither subducts easily because continental crust is too buoyant. Instead, the crust crumples and thickens, creating high mountain ranges like the Himalayas. The absence of volcanism is diagnostic: no subducting slab means no water release into the mantle and no flux melting. Oceanic-continental convergence would produce both a volcanic arc and a subduction trench.

Question 2

Why do transform boundaries typically lack volcanism, while both mid-ocean ridges and subduction zones produce it?

• A. Transform boundaries are too cold for magma to form because plates slide without generating heat
• B. At transform boundaries, plates move laterally with no decompression melting and no fluid release into the mantle
• C. Transform boundaries only occur deep underwater where pressure prevents eruption
• D. The lithosphere at transform boundaries is too thin to allow magma ascent

Answer: (B)

Volcanism requires a mechanism to generate melt. At divergent boundaries, rising mantle decompresses and partially melts. At convergent boundaries, water from the subducting slab lowers the mantle's melting point. At transform boundaries, plates simply slide past each other — no mantle upwelling, no subducting slab, no melt-generating mechanism. Hence no volcanism, but frequent shallow earthquakes.

Question 3

The deepest earthquakes on Earth occur at subduction zones because the cold, brittle oceanic slab fractures as it descends into the mantle.

• T. True
• F. False

Answer: True

Earthquakes require brittle fracture, which occurs in cold, rigid material. Subducting oceanic slabs are cold and brittle relative to the surrounding hot mantle, allowing seismic rupture down to ~700 km depth. At mid-ocean ridges and transform boundaries, earthquakes are shallow (tens of kilometers) because the seismogenic zone is limited to the cooler, brittle upper lithosphere.

Question 4

When two continental plates collide at a convergent boundary, the denser plate typically subducts beneath the other, forming a deep ocean trench.

• T. True
• F. False

Answer: False

Continental crust is too buoyant to subduct easily — it has lower density than oceanic crust or the underlying mantle. When two continental plates collide, neither sinks efficiently. Instead, the crust crumples and thickens, building mountain ranges. The Himalayas formed this way when India collided with Eurasia — no subduction trench, no deep-focus earthquakes below ~70 km, no arc volcanism.

Question 5

Explain why subduction zones and mid-ocean ridges both produce volcanism, even though the mechanism generating magma is completely different at each.

Model answer: At mid-ocean ridges (divergent boundaries), hot mantle rock rises to fill the gap left by separating plates. As it ascends, pressure decreases without the rock losing heat — decompression melting causes the mantle to partially melt and erupt as basaltic lava, even though no external heat is added. At subduction zones, the descending oceanic slab carries water locked in hydrous minerals. As the slab heats under pressure, it releases this water into the overlying mantle wedge, lowering the melting point and triggering flux melting. The resulting magma rises to form volcanic arcs. Both produce volcanism, but through opposite mechanisms: pressure decrease at ridges versus water addition at subduction zones.

This distinction explains why magma compositions differ (basalt at ridges vs. andesite/rhyolite at arcs) and why arc volcanoes are more explosive (water-rich magma traps more dissolved gas). Knowing the boundary type lets geologists predict both the presence and style of volcanism.