Collision margins and fold mountains

Overview

Collision margin, isostatic uplift and characteristic features of a young fold mountain range.

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

   What is a collision margin?

   Model answer

   A collision margin is where two plates carrying continental crust converge. Because both are of low density, neither can be subducted into the mantle, so instead of one descending the two are driven into each other. The crust caught between them is compressed, folded, thrust-faulted, stacked and thickened, and is forced upwards into the highest mountain ranges on Earth.

2. 2

   Why does subduction stop when a collision margin forms?

   Model answer

   A collision typically begins as an ordinary destructive margin, with ocean floor subducting between two continents that are being carried towards each other. Once all the intervening ocean floor has been consumed, continental crust arrives at the trench. It is too buoyant to sink, so it jams the subduction zone. The pull of the slab may break it off and let it fall away into the mantle, but the surface convergence continues as compression instead of subduction.

3. 3

   Describe how fold mountains are formed at a collision margin.

   Model answer

   Thick sequences of sediment that had accumulated in the ocean between the two continents, together with the continental margins themselves, are squeezed by the converging plates. The rocks buckle into a series of upfolds and downfolds, and where compression exceeds what folding can absorb they break along low-angle thrust faults, so that great sheets of rock are pushed up and over one another. This stacking doubles or more the thickness of the crust, and the surface rises to form a mountain range.

4. 4

   Why do the Himalayas contain marine rocks at their summits?

   Model answer

   The rocks now at the highest altitudes were originally sediments laid down on the floor of the ocean that lay between the two colliding continents. When the ocean closed, these sediments were scraped up, folded and thrust upwards ahead of the advancing continents rather than being carried down with the descending plate. Marine fossils in rock several kilometres above sea level are therefore direct evidence that an ocean once occupied the site of the mountain range.

5. 5

   What is isostatic uplift, and why does it keep collision mountains high?

   Model answer

   Isostasy is the floating balance between the light crust and the denser mantle beneath. Thickened crust has a deep root projecting into the mantle, and just as most of an iceberg lies below the waterline, the mountains stand high because of that root. As erosion strips material from the summits the load is reduced, so the range floats upwards to restore the balance. This is why a collision range continues to rise even while it is being vigorously eroded, and why the roots are only exposed long after mountain building has ceased.

6. 6

   Why are there few volcanoes at a collision margin, when destructive margins are lined with them?

   Model answer

   Volcanoes at a destructive margin depend on water released from a descending oceanic slab lowering the melting point of the mantle above it. At a collision margin the ocean floor has all been consumed and no slab is descending, so that supply of water stops and the flux melting mechanism shuts down. Some melting can still occur within crust that has been thickened and heated, producing granite intrusions at depth, but there is little of the surface volcanic activity typical of subduction.

Other ways to work this

The same content, four other ways.

What is inside Levo

Six ways to work the same material.