The letters following each Review Question refer to the corresponding Learning Objective from the Chapter Opener. How does the lithosphere beneath a passive margin differ from that beneath an abyssal plain? (B)

Passive margins are the regions where continents transition into oceanic lithosphere, but without any significant tectonic activity such as plate boundaries. These areas typically feature a broad continental shelf, a continental slope, and a rise.

An abyssal plain is an extensive, relatively flat area of the deep ocean floor, found at depths between 3000 and 6000 meters. These plains are the result of the accumulation of sediment carried by ocean currents and deposited on the ocean floor.

The lithosphere beneath a passive margin consists of both continental and oceanic crust. The continental crust is typically older and thicker, composed mainly of granitic rock, and extends from the landmass to the edge of the continental shelf. The oceanic crust begins at the base of the continental slope and extends outwards towards the ocean. It is mostly composed of basaltic rock, which is relatively thinner and denser than the continental crust.

The lithosphere beneath an abyssal plain is entirely composed of oceanic crust. It is formed by the solidification of magma at the mid-ocean ridges (spreading centers) and moves outward as new crust forms. The older, denser oceanic crust eventually sinks back into the mantle at subduction zones. The top layer of the oceanic crust is covered with a layer of sediment, which accumulates over millions of years.

The lithosphere beneath a passive margin differs from that beneath an abyssal plain in several ways: 1. A passive margin contains both continental and oceanic crust, whereas an abyssal plain consists only of oceanic crust. 2. The continental crust in a passive margin is generally thicker and less dense than the oceanic crust found in both the passive margin and abyssal plain. 3. The sedimentary layers deposited on the oceanic crust in abyssal plains are usually thicker and more continuous compared to those found on the passive margins. By understanding these differences between the lithospheres beneath passive margins and abyssal plains, we can better comprehend the geological processes that shape our planet's surface.