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

Blueschist is a type of metamorphic rock that is characterized by blue-hued minerals such as glaucophane and lawsonite. They form under specific conditions of low-temperature (between 200°C and 500°C) and high-pressure (between 5 and 20 kbar), also called blueschist facies metamorphism. These conditions are uncommon in the Earth's crust and provide us with a clue about the geologic environment where blueschist rocks are formed.

Plate tectonics is a geological theory that explains the movement of the Earth's lithosphere (the outermost shell) broken into tectonic plates. These plates are in constant motion, driven by convection currents in the underlying mantle and ultimately lead to the geological events such as earthquakes, volcanoes, and mountain-building. One of the key features of plate tectonics is the subduction zone - this is where one plate dives beneath another due to the difference in their densities, usually at plate boundaries. The denser oceanic plate is forced underneath the less dense continental plate in a process called subduction. This process generates significant heat and pressure, causing the rocks found along these boundaries to experience metamorphism and eventually transform into different rocks like blueschist.

Now, we can combine the concepts of blueschist and plate tectonics to understand why and how they are formed in subduction zones. In a subduction zone, the denser oceanic plate is subducted beneath the lighter, more buoyant continental plate. As the oceanic plate descends, it is subjected to increasing pressure, while the temperature remains relatively low due to the cold oceanic lithosphere that's being subducted. This combination of low-temperature and high-pressure creates the unique conditions required for blueschist facies metamorphism.

In summary, plate tectonics explain the combination of low-temperature and high-pressure minerals found in a blueschist as follows: 1. Plate tectonics result in subduction zones, where an oceanic plate is forced beneath a continental plate. 2. The downgoing oceanic plate is subjected to increasing pressure while the temperature remains relatively low. 3. The unique low-temperature, high-pressure conditions transform the rocks into blueschist. Hence, plate tectonics is responsible for generating the required environment for blueschist rocks to form.