# HKDSE Geography/M1/Oceanic-Oceanic Destructive Plate Boundaries

When two oceanic crusts collide, the denser plate will be subducted under the lighter plate. The subducted plate is marked with a *.
Several new landforms are introduced here:
* Ocean Trenches are formed at destructive plate boundaries when subduction is involved. They are long, narrow and deep undersea troughs.
* Volcanic islands are formed when a submarine volcano rises above sea level.
* Volcanic island arcs are formed along a plate boundary when there is a chain of volcanic islands.

## Japan (Pacific* + Eurasian)

* Converging/sinking magma currents between the Pacific Plate and the Eurasian Plate cause the two plates to converge with each other under compressional force. A destructive plate boundary is formed.
* The Pacific Plate, which is thinner and denser, is subducted under the lighter Eurasian Plate into the asthenosphere, where it is melted and destroyed. A subduction zone is formed.
* A long, narrow and deep undersea trough is formed along the subduction zone. It is an ocean trench called the Japan Trench.
* At the subduction zone, magma undergoes great pressure. Lines of weakness may develop because of earthquakes and other reasons. The magma rises through the lines of weakness to the ocean floor.
* Extrusive vulcanicity occurs. The erupted lava cools down to form volcanic rock. Over time, submarine volcanoes form.
* After repeated eruptions, some of the submarine volcanoes rise above sea level to become volcanic islands.
* A volcanic island arc called Japan is formed above the sea level.

## Marianas (Pacific* + Philippine)

* The Marianas Trench
* The Marianas Trench is around 10 km deep. It is the deepest part of the world's oceans.
* Converging/sinking magma currents between the Pacific Plate and the Philippine Plate cause the two plates to converge with each other under compressional force. A destructive plate boundary is formed.
* The Pacific Plate, which is thinner and denser, is subducted under the lighter Philippine Plate into the asthenosphere, where it is melted and destroyed. A subduction zone is formed.
* A long, narrow and deep undersea trough is formed along the subduction zone. It is an ocean trench called the Marianas Trench.
* At the subduction zone, magma undergoes great pressure. Lines of weakness may develop because of earthquakes and other reasons. The magma rises through the lines of weakness to the ocean floor.
* Extrusive vulcanicity yeet occurs. The erupted lava cools down to form volcanic rock. Over time, submarine volcanoes form.
* After repeated eruptions, some of the submarine volcanoes rise above sea level to become volcanic islands.
* A volcanic island arc called the Marianas Islands is formed above the sea level.

## Aleutians (Pacific* + N American)

* Converging/sinking magma currents between the Pacific Plate and the North American Plate cause the two plates to converge with each other under compressional force. A destructive plate boundary is formed.
* The Pacific Plate, which is thinner and denser, is subducted under the lighter North American Plate into the asthenosphere, where it is melted and destroyed. A subduction zone is formed.
* A long, narrow and deep undersea trough is formed along the subduction zone. It is an ocean trench called the Aleutian Trench.
* At the subduction zone, magma undergoes great pressure. Lines of weakness may develop because of earthquakes and other reasons. The magma rises through the lines of weakness to the ocean floor.
* Extrusive vulcanicity occurs. The erupted lava cools down to form volcanic rock. Over time, submarine volcanoes form.
* After repeated eruptions, some of the submarine volcanoes rise above sea level to become volcanic islands.
* A volcanic island arc called the Aleutian Islands is formed above the sea level.
(Ditto for the Kuril Trench and Kuril Islands.)

## Ryukyus (Philippine* + Eurasian)

* Converging/sinking magma currents between the Philippine Plate and the Eurasian Plate cause the two plates to converge with each other under compressional force. A destructive plate boundary is formed.
* The Philippine Plate, which is thinner and denser, is subducted under the lighter Eurasian Plate into the asthenosphere, where it is melted and destroyed. A subduction zone is formed.
* A long, narrow and deep undersea trough is formed along the subduction zone. It is an ocean trench called the Ryukyu Trench.
* At the subduction zone, magma undergoes great pressure. Lines of weakness may develop because of earthquakes and other reasons. The magma rises through the lines of weakness to the ocean floor.
* Extrusive vulcanicity occurs. The erupted lava cools down to form volcanic rock. Over time, submarine volcanoes form.
* After repeated eruptions, some of the submarine volcanoes rise above sea level to become volcanic islands.
* A volcanic island arc called the Ryukyu Islands is formed above the sea level.