Tectonic Plate Motion
The Earth’s size has remained relatively the same since shortly after its formation 4.6 billion years ago. This implies that the Earth’s crust must be destroyed at the same rate it is being created. The destruction or recycling of the Earth’s crust takes place at convergent boundaries. Convergent boundaries are the zones where two tectonic plates moving towards each other come together at their plate boundaries. At convergent boundaries tectonic plates collide and crust is destroyed as one plate is pulled beneath the other, the type of collision depends on the types of plates involved. These can be categorised as:
- Oceanic – Continental Plate Convergence
- Oceanic – Oceanic Plate Convergence
- Continental – Continental Plate Convergence
Oceanic-Continental Plate Convergence
When an oceanic plate collides with a continental plate the crust forming the oceanic plate gets bent and pulled under the continental crust. This is because the rocks that form the oceanic crust are denser and thinner than the rocks that form the continental crust. The process by which the oceanic crust is pulled under the continental crust is called subduction and the zone at which this occurs at the plate boundaries is called the subduction zone. At the subduction zone where the oceanic crust sinks beneath the continental crust a deep oceanic trench or valley is created. These can be many of thousands of kilometres long and 8 to 10 kilometres deep making them the deepest part of the ocean floor.
Oceanic and continental plate collisions result in mountain and volcano formation. Mountains are formed as the overriding continental plate gets lifted up over the oceanic plate. An example of this is found at the boundary between the South American continental plate and the Nazca oceanic plate. The South American continental plate is being lifted over the subducted Nazca oceanic plate creating the Andes mountain range.
The formation of volcanoes is a result of the subduction of the oceanic crust. As the oceanic crust gets forced deeper into the Earth the increasing heat and pressure releases trapped water and gases, the combing effect of which is the crust melting to form magma. This magma rises and accumulates in magma chambers which then feeds and creates volcanoes on the overriding continental plate.
Earthquakes are also common at oceanic and continental plate boundaries. These occur when the deepest part of the subducting oceanic plate break into smaller pieces. Some of the pieces become locked in place for long periods and when they are suddenly released they generate large earthquakes.
Oceanic-Oceanic Plate Convergence
This is similar to oceanic-continental convergence in that one of the plates is subducted under the other during the collision. At the subduction zone a very deep trench is formed in the ocean floor.
Oceanic and oceanic plate convergence result in the formation of volcano chains. The crust that is pulled under or subducted melts to form magma. This magma rises to the top of the overriding oceanic plates and erupts on the ocean floor. Over millions of years, the lava and debris from the volcanic eruptions pile up on the ocean floor until the volcano rises above sea level to form a volcanic island. These types of islands are usually formed as chains called island arcs, which run parallel to the trench at the subduction zone.
Oceanic-oceanic plate convergence experience similar powerful earthquakes to oceanic-continental convergences.
Continental – Continental Plate Convergence (Collisions)
When two continental plates collide they resist downwards motion or subduction because the continental rocks that make them up are of similar density. Instead, the crust tends to buckle and gets pushed upwards and sideways. Thus, when two continental plates collide mountains are formed.
Approximately 200 million years ago when Pangaea broke apart the Indian plate began its journey northwards. About 50 million years ago the Indian plate collided with the Eurasian plate. This collision caused the crust to buckle upwards and the Eurasian plate to crumple up over the Indian plate. Over millions of years the continuous collisions between the two plates has resulted in the formation of the Himalayan mountain range and the Tibetan Plateau. The Himalayas are the highest continental mountains in the world holding many peaks at 8000 meters above sea-level, the highest being Mount Everest at 8848 meters. Since the Indian Plate is continuing in its northward movement into the Eurasian plate the Himalayas continue to grow at a rate of approximately 5mm to 20mm a year.