HKDSE Geography/M1/Continental-Oceanic Destructive Plate Boundaries

When a continental crust collides with an oceanic crust, the denser oceanic crust always subducts into the mantle. Sedimentary rocks on the ocean floor are compressed, folded and pushed up to form a mountain belt parallel to the plate boundary, called a fold mountain range. An oceanic trench is also formed.

Peru-Chile Trench (Nazca* + South American)

In a nutshell:

• A trench
• A fold mountain range
• Some volcanoes

1. Converging/sinking magma currents between the Nazca Plate and the South American Plate cause the two plates to converge with each other under compressional force. A destructive plate boundary is formed.
2. The Nazca Plate, which is thinner, denser and oceanic, is subducted under the lighter, thicker, continental South American Plate into the asthenosphere, where it is melted and destroyed. A subduction zone is formed.
3. A long, narrow and deep undersea trough is formed along the subduction zone. It is an ocean trench called the Peru-Chile Trench.
4. The ocean trench is filled with sediments.
5. As the two plates push together, the sediments are compacted into sedimentary rock.
6. The sedimentary rock is eventually compressed, bended and folded up to form the fold mountain range called the Andes.
7. The subduction of the Nazca Plate causes earthquakes and leaves lines of weakness in the crust. The edge of the Nazca Plate is heated up, melted and reabsorbed into the mantle.
8. At the subduction zone, magma undergoes great pressure because the melted crustal materials have lower pressure. It rises through the ocean floor or the fold mountains along the lines of weakness.
9. Extrusive vulcanicity occurs. The erupted lava cools down to form volcanic rock. Over time, volcanoes form, e.g. Mt Chimborazo in Ecuador.

Rockies (Pacific* + North American)

In a nutshell

• A fold mountain range
• Some volcanoes
• (These used to be a trench there, but it has now disappeared, so let's forget it.)

1. Converging/sinking magma currents between the Nazca Plate and the South American Plate cause the two plates to converge with each other under compressional force. A destructive plate boundary is formed.
2. The Pacific Plate, which is thinner, denser and oceanic, is subducted under the lighter, thicker, continental North American Plate into the asthenosphere, where it is melted and destroyed. A subduction zone is formed.
3. The sedimentary rock between the two plates is compressed, bended and folded up to form the fold mountain range called the Rockies.
4. The subduction of the Nazca Plate causes earthquakes and leaves lines of weakness in the crust. The edge of the Nazca Plate is heated up, melted and reabsorbed into the mantle.
5. At the subduction zone, magma undergoes great pressure because magma has higher temperature and pressure than the melted crustal materials. It rises to the ocean floor or the fold mountains along the lines of weakness.
6. Extrusive vulcanicity occurs.