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)

1. 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.
2. 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.
3. A long, narrow and deep undersea trough is formed along the subduction zone. It is an ocean trench called the Japan Trench.
4. 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.
5. Extrusive vulcanicity occurs. The erupted lava cools down to form volcanic rock. Over time, submarine volcanoes form.
6. After repeated eruptions, some of the submarine volcanoes rise above sea level to become volcanic islands.
7. 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.

1. 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.
2. 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.
3. A long, narrow and deep undersea trough is formed along the subduction zone. It is an ocean trench called the Marianas Trench.
4. 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.
5. Extrusive vulcanicity yeet occurs. The erupted lava cools down to form volcanic rock. Over time, submarine volcanoes form.
6. After repeated eruptions, some of the submarine volcanoes rise above sea level to become volcanic islands.
7. A volcanic island arc called the Marianas Islands is formed above the sea level.

Aleutians (Pacific* + N American)

1. 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.
2. 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.
3. A long, narrow and deep undersea trough is formed along the subduction zone. It is an ocean trench called the Aleutian Trench.
4. 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.
5. Extrusive vulcanicity occurs. The erupted lava cools down to form volcanic rock. Over time, submarine volcanoes form.
6. After repeated eruptions, some of the submarine volcanoes rise above sea level to become volcanic islands.
7. 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)

1. 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.
2. 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.
3. A long, narrow and deep undersea trough is formed along the subduction zone. It is an ocean trench called the Ryukyu Trench.
4. 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.
5. Extrusive vulcanicity occurs. The erupted lava cools down to form volcanic rock. Over time, submarine volcanoes form.
6. After repeated eruptions, some of the submarine volcanoes rise above sea level to become volcanic islands.
7. A volcanic island arc called the Ryukyu Islands is formed above the sea level.