Matching landform to margin: the classic confusions

Overview

Which margin produces each of the five major landforms, how a rift valley is told from a trench, why island arcs and fold mountains differ, and the margin that builds nothing.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   At which margin type does each of the five major tectonic landforms occur?

   Model answer

   Ocean ridges form at constructive margins. Rift valleys form at constructive margins, both as the axial trough along a slow-spreading ocean ridge and as a continental rift where a landmass is being stretched. Deep sea trenches form at destructive margins. Island arcs form at destructive margins where one oceanic plate subducts beneath another. Fold mountains form at collision margins and also at destructive margins where oceanic crust subducts beneath a continent. Conservative margins produce none of these five.

2. 2

   Why is it wrong to describe an ocean ridge as a landform of a destructive margin?

   Model answer

   Because an ocean ridge is built by the creation of new crust, not its destruction. It marks the line where two plates separate and basaltic magma rises to fill the gap, so the crust there is newly formed and youngest at the axis. A destructive margin consumes crust and produces a trench, which is a depression. Confusing the two reverses the direction of plate movement, since one requires plates moving apart and the other requires them moving together.

3. 3

   How can a rift valley and a deep sea trench be told apart, given that both are long, steep-sided depressions?

   Model answer

   By what produced them and where. A rift valley is an extensional feature: the crust is being pulled apart and a central block has dropped down between two parallel normal faults, so the sides are roughly symmetrical and the floor is flat and faulted. A deep sea trench is a compressional feature on the ocean floor: it is the flexural bend where a plate starts to descend, so it is strongly asymmetric, with a gentler outer wall and a steeper inner wall, and it is far deeper. One is where plates separate, the other where they converge.

4. 4

   Are island arcs and fold mountains produced by the same process?

   Model answer

   They share a driving process but not an outcome. Both occur where plates converge, but an island arc is a chain of volcanoes built on oceanic crust by magma from a descending slab, and it appears only where one oceanic plate subducts beneath another. Fold mountains are built by the compression, folding, thrusting and thickening of continental crust, and appear where oceanic crust subducts beneath a continent or where two continents collide. An island arc is constructed by volcanism, a fold mountain range by deformation.

5. 5

   Why do the Andes and the Himalayas look similar but form differently?

   Model answer

   Both are high, linear fold mountain belts built by convergence and compression, so they share folding, thrust faulting, crustal thickening and shallow earthquakes. The Andes, however, lie above an active subduction zone, with an oceanic plate descending beneath South America, so they carry a chain of active explosive volcanoes and have a deep sea trench offshore and a Benioff zone beneath. The Himalayas result from two continents colliding after the intervening ocean closed, so there is no slab, no trench and almost no volcanic activity.

6. 6

   Which margin type produces no distinctive large-scale landform, and why?

   Model answer

   The conservative margin. Plates slide past one another horizontally, so no crust is created, none is destroyed and none is shortened. Landform building requires one of those three, so all a conservative margin can produce is a fault trace with sideways offsets of rivers, ridges and other features, plus small ridges and basins where the fault bends. It is a major producer of earthquakes but not of relief.

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