A convergent plate boundary (otherwise known as destructive) involves 2 plates moving towards each other. The Earth is made up of 7 major plates and many minor ones. Plates sit on the top of the Earth’s surface and are known as sections of crust or lithosphere. They are classed as oceanic or continental in nature. Oceanic sits beneath the sea or ocean typically and a continental crust has land, continents and countries lying on top.
Oceanic-oceanic Convergent Plate Boundaries
At an oceanic-oceanic convergent plate boundary, 2 plates will approach each other. This movement is caused by a process beneath the surface of the crust called convection. The layer beneath the plates is called the mantle (or asthenosphere), it is made of very hot, liquid rock and is up to 2900km thick. The very high temperatures cause convection currents to form. This is when hot, molten rock rises close to the surface, moves laterally, and then as it cools it will begin to descend back towards the core of the Earth.
Convection currents occurring in the mantle drag the plates above and cause them to converge. The more dense plate will move beneath the less dense plate in a process called subduction. This is due to a process called slab-pull. As this subduction occurs there is a lot of compression acting on the heavier plate. This force causes the plate to buckle and become deformed, the point of subduction is marked by a deep ocean trench.
Earthquakes occur at these margins, with shallow earthquakes occurring close to the trench and deeper ones found further from the subduction zone. This is known as a sloping plane of earthquakes called the Wadati-Benioff zone. As the plate is subducted, friction increases as it grinds against the other plate. Stress and tension build until it is released as seismic waves during an earthquake.
The crust which is subducted into the mantle begins to melt when it comes into contact with hotter material – this is known as subduction melting. This melting occurs at a depth of around 600km. Hydration melting also occurs at this plate boundary, where seawater is carried down with the subducting plate and lowers the melting temperature as it mixes with the molten material. This melting occurs at a depth of 80km. The melting processes produce magma which rises to the surface as it is less dense than the surrounding rock. This magma will push through weaknesses in the crust and erupt over the ocean floor, this process repeats over and over and eventually create volcanoes in an island-arc which will lie parallel to the deep-ocean trench. Over a long period of time, continued eruptions make the volcanoes build up into substantial landmasses.
Let’s look at a real example of an oceanic-oceanic convergent plate boundary:
The Pacific Plate and the Indo-Australian Plate are both oceanic plates. The Pacific Plate is more dense than the Indo-Australian and so it is the plate that is subducted. The deep-ocean trench that marks the subduction zone is called the Tonga Trench and is around 1,375 km long. The Tonga Islands have formed from the recycling of melted crustal material and run parallel to the Tonga Trench, there are around 150 islands in total.
Oceanic-continental Convergent Plate Boundaries
The exact same processes occur at a oceanic-continental plate boundary. The oceanic plate is always the one which is subducted as oceanic crust is more dense than continental. The processes of melting occur in the same manner and material is recycled. Volcanic activity will occur along with buckling of the edge of the continental plate. The continental material thickens due to the compressional forces. Crustal material and sediment between the two plates will buckle upwards. This creates a different landform than at oceanic-oceanic margins. In this case a range of fold mountains will form.
Let’s look at an example of an oceanic-continental convergent boundary:
The Nazca Plate (an oceanic plate) is subducted beneath the continental South American Plate due to slab-pull. This has resulted in volcanic activity on the west coast of South America and formed mountains such as Mt Tacora in Chile and Nevado del Ruiz in Colombia. The Andes Mountains are the range of fold mountains formed by the buckling upwards of material between the plates.
Take a look at the helpful Venn-diagram below. It highlights the similarities and differences at oceanic-oceanic and oceanic-continental convergent plate boundaries: