How does plate tectonics theory explain the combination of low-temperature but high-pressure minerals found in a blueschist? (F)

Plate tectonics theory is a unifying scientific theory that describes the movement of Earth's lithosphere's large sections (tectonic plates) over the asthenosphere. This movement is driven by the heat generated within Earth's mantle, causing the rise of relatively hot but rigid material from deep down and the subduction (sinking) of the colder and denser material from the surface.

Within plate tectonics, two main types of plates are distinguished: oceanic and continental plates. The oceanic plates lie beneath the ocean floor and are mainly composed of denser igneous rocks such as basalt. The continental plates form Earth's landmasses, consisting of lighter and less dense materials such as granite. When these two plates collide, the denser oceanic plate subducts beneath the less dense continental plate.

Metamorphic rocks form due to the change in the mineral composition, texture, and structure of pre-existing rocks (protolith) through the processes of heat and pressure. This can occur at convergent plate boundaries, where tectonic plates collide with each other, leading to the subduction of one plate beneath the other. The pressure from the collision and the heat generated from the subduction process can cause the protolith to change into metamorphic rocks.

When the denser oceanic plate subducts beneath a continental plate, it experiences increasing pressure while still at a relatively low temperature. The oceanic crust, rich in basaltic material, interacts with water carried down by the subduction process. The combination of this high pressure and low temperature induces metamorphism, producing a specific type of mineral assemblage called a blueschist.

A blueschist is a unique metamorphic rock characterized by the presence of blue, high-pressure, and low-temperature minerals, such as glaucophane and lawsonite. These minerals have a stable form under specific pressure and temperature ranges that are typical of subduction zones (pressure from 6-12 kbar and temperature ranging from 200-500°C). The high-pressure environment reflects the deep subduction and the effect of the weight of the overlying rocks, while the low temperature is attributed to the cold subducted oceanic slab and the limit in temperature increase due to subduction. In summary, the plate tectonics theory explains the presence of low-temperature but high-pressure minerals in a blueschist as a result of the subduction process of an oceanic plate beneath a continental plate. This unique environment allows for the formation of specific minerals that make up blueschist metamorphic rocks.