Question

What happens to slabs of subducted lithosphere?

Short answer

When two plates converge, the denser oceanic plate (the subducting plate) slides beneath the less dense continental plate (the overriding plate). As the subducted lithosphere goes deeper into the Earth's interior, it undergoes deformation, compaction, metamorphism, and partial melting, generating magma. This magma can rise and form volcanic arcs at the Earth's surface. The subduction process also leads to earthquake activity, as pressure build-up results in abrupt shifts between the plates. Over geological time, the subducted lithosphere is further incorporated into the mantle through mantle convection, maintaining the chemical balance between the crust, mantle, and atmosphere, and eventually gets recycled into new oceanic crust via processes like seafloor spreading.

Step-by-step solution

Introduction to Plate Tectonics and Subduction

Plate tectonics is the theory that Earth's lithosphere is divided into several plates, which move and interact with one another due to the movement of the underlying mantle. When two plates converge, one plate can be forced to slide below the other, which is referred to as subduction. The plate that slides beneath the other is called the subducting plate, typically an oceanic plate, while the other plate, usually a continental plate, is called the overriding plate.

Physical Changes During Subduction

As the denser oceanic plate sinks below the less dense continental plate, it moves deeper into the Earth's interior. Due to the increased pressure and temperature, the subducted lithosphere undergoes several physical changes: 1. Deformation and compaction: The slab bends and compresses as it dives into the mantle. 2. Metamorphism: The high pressure and temperature cause rocks in the subducting plate to undergo chemical and mineralogical changes, transforming into denser, more stable forms. 3. Partial melting: Some minerals within the subducting plate begin to melt, generating magma that can rise through the mantle and crust.

Effects on Earth's Surface: Formation of Volcanic Arcs

The magma resulting from the partial melting of the subducted lithosphere rises due to buoyancy and can reach Earth's surface, creating volcanoes. This volcanic activity typically occurs in a line parallel to the subduction zone, forming a volcanic arc. Examples of volcanic arcs include the Andes in South America and the Aleutian Islands in Alaska.

Effects on Earth's Surface: Earthquakes

As the subducting plate descends, pressure builds up at the interface between the subducting and the overriding plate. Movement along the subduction zone eventually results in abrupt shifts, causing earthquakes. These earthquakes can be deep or shallow, depending on the depth of the subducted plate.

Recycling of Earth's Lithosphere

Over geological time, the subducted lithosphere gets further incorporated into the mantle through mantle convection. This process is relevant for Earth's long-term geological cycles and helps to maintain the chemical balance between the crust, mantle, and the atmosphere. Eventually, material from the subducted plate can be recycled and returned to the surface in the form of new oceanic crust through processes like seafloor spreading.