Questions: Abyssal Plains and Ocean Trenches: Seafloor Morphology

Question 1

The Pacific Ocean has fewer well-developed abyssal plains than the Atlantic. What is the primary reason?

• A. The Pacific is younger and has not had enough time to accumulate deep sediment
• B. The Pacific's trenches intercept sediment transported down the continental slopes before it can reach the deep ocean floor
• C. Pacific waters are too warm to preserve the organic particles that form pelagic sediment
• D. The Pacific floor is made of younger, rougher basalt that cannot be buried by thin sediment layers

Answer: (B)

Ocean trenches act as sediment traps. In the Pacific, the ring of subduction zones surrounding the basin intercepts turbidites and continental sediment as it flows down the continental slope — the sediment falls into the trench rather than spreading across the deep floor. The Atlantic has passive margins on both sides with no trenches, so sediment transported off the continents by turbidity currents spreads freely across the deep abyssal floor, burying the rough basaltic crust and producing exceptionally flat abyssal plains. This is a direct consequence of tectonic configuration, not age or water chemistry.

Question 2

Ocean trenches are the sites of Earth's most powerful earthquakes. What geometric feature of the subducting slab explains the depth distribution of these earthquakes?

• A. Earthquakes are shallow near the trench and deepen progressively inland along the descending slab — the Wadati-Benioff zone
• B. Earthquakes are deepest directly beneath the trench, where the plate first enters the mantle
• C. All trench-related earthquakes occur at the same depth because the slab descends at a constant angle
• D. Earthquakes are distributed randomly around trenches, reflecting the chaotic nature of subduction

Answer: (A)

The Wadati-Benioff zone is the inclined plane of seismicity that traces the geometry of the descending slab. As the subducting oceanic plate bends into the mantle, it remains cold and brittle enough to store elastic stress and rupture — generating earthquakes. Shallow earthquakes (0-70 km) occur near the trench where the plate is just beginning to descend; intermediate earthquakes (70-300 km) and deep earthquakes (300-700 km) occur progressively further inland as the slab descends. This depth progression is a direct 3D map of the slab's trajectory into the mantle and was crucial historical evidence for the existence of subduction.

Question 3

Turbidites deposited by turbidity currents have a characteristic graded structure — coarse sediment on the bottom, finer material on top — that reflects how particles settle from a turbulent underwater avalanche.

• T. True
• F. False

Answer: True

Turbidity currents are underwater avalanches of sediment-laden water that rush down continental slopes at high speed. As they decelerate and spread across the abyssal floor, the heaviest (coarsest) particles settle first, followed progressively by finer material, producing a graded bed. This graded bedding is the diagnostic signature of turbidites in the sedimentary record. Turbidites can be meters thick and bury the rough basaltic crust under hundreds of meters of sediment near active continental margins, which is the primary mechanism that produces the extreme flatness of abyssal plains.

Question 4

Ocean trenches can form at any type of plate boundary, including divergent (spreading) boundaries where plates move apart.

• T. True
• F. False

Answer: False

Ocean trenches form exclusively at convergent plate boundaries where one plate subducts beneath another. Subduction requires a density contrast: denser oceanic lithosphere sinks beneath either continental lithosphere or younger, less-dense oceanic lithosphere. Divergent boundaries produce mid-ocean ridges, not trenches — plates move apart and new oceanic crust is created by seafloor spreading. Transform boundaries produce strike-slip faults. Trenches are uniquely associated with the compressional stress and downward bending of convergent margins.

Question 5

Explain why abyssal plains are the flattest places on Earth and what process produces that extreme flatness.

Model answer: The basaltic oceanic crust is inherently rough, formed by irregular volcanic seafloor spreading. Abyssal plains are flat because sediment has buried this rough crust. Turbidity currents carry sediment from continental margins and deposit it as graded beds across the deep floor; simultaneously, pelagic sediment (shells, dust, organic particles) slowly rains down from the surface. Over millions of years, these deposits fill in the topographic lows and cover the volcanic irregularities, producing slopes less than 1:1,000 — flatter than any continent.

The key insight is that the flatness is not original — it is created by burial. The underlying basalt has ridges, faults, and volcanic seamounts. What makes an abyssal plain flat is the thickness and evenness of the sediment blanket. Near passive continental margins (like the Atlantic), turbidite deposition is abundant and abyssal plains are well-developed. In the Pacific, trenches intercept the turbidite flow before it reaches the deep floor, so less burial occurs and abyssal plains are less extensive. Flatness is a measure of how much sediment has accumulated, not an intrinsic property of oceanic crust.