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Image: Subduction melange, Osa Peninsula, near Drake, Costa Rica (© Martin Meschede 2013)
Solid Earth
5.1 Subduction of oceanic lithosphere
Video: Subduction of oceanic lithosphere
Chapter 5.1
Subduction of oceanic lithosphere
Fig. 5.1.1: Global distribution of plate boundaries (Meschede, 2018)
Subduction zones are distributed throughout the Earth. However, they are not evenly distributed. They are particularly common around the Pacific Ocean and are usually characterized by volcanoes above the subduction zone. This gives rise to the term “Pacific Ring of Fire,” which refers to the volcanic activity. This clearly shows that subduction zones are closely linked to volcanism.
Fig. 5.1.2: Animation: Subduction of oceanic lithosphere with formation of a volcanic arc above the subduction zone (Meschede, unpubl., 2025).
A simplified cross-section through a subduction zone shows how the oceanic lithosphere submerges into the Earth’s mantle and sinks downward. At a depth of approximately 100 to 120 km, i.e., already in the asthenosphere beneath the volcanic arc, melting occurs. This is the location where magma is generated, which rises upward and fills magma chambers in the crust from which the volcanoes above the subduction zone are fed. Above the subduction zone, a volcanic arc forms on the overriding plate, becoming increasingly thicker over time. The plate boundary is located in the deep-sea trench in front of the volcanic arc and continues diagonally downward into the subduction zone. The lithosphere behind the volcanic arc, may come under extensional stress, which can lead to the development of a back-arc spreading zone.
Fig. 5.1.3: 3D cross section through the subduction zone under the island of Java, Indonesia (modified after Frisch & Meschede, 2025)
Using the subduction zone beneath the volcanic island of Java as an example, it can be demonstrated what happens to the subducting oceanic lithosphere during subduction. One process in particular is crucial here, and it is responsible for the functioning of plate tectonic movements.
Basaltic rocks (basalt, dolerite, gabbro) from the oceanic crust undergo metamorphic changes when they submerge into the subduction zone. There, they are exposed to high pressure and relatively low temperatures. High-pressure metamorphism is characteristic of the subduction process and, because it occurs exclusively in subduction zones, is also called subduction metamorphism.
Fig. 5.1.4: Pressure-temperature diagram for different types of metamorphism (modified after Frisch & Meschede, 2025)
During high-pressure metamorphism, basaltic rocks are initially transformed into glaucophane schist, as can be seen in the pressure-temperature diagram in Fig. 5.1.4. There, temperature is plotted against pressure, or equivalently, depth. Glaucophane schist metamorphism begins when the oceanic lithosphere has descended approximately 20 to 30 km into the subduction zone. With further subsidence, the originally basaltic rocks enter the eclogite facies metamorphic zone.
Fig. 5.1.5: Alternating layers of blueschist (glaucophane schist) and greenschist. The rock known as prasinite is found at Cap Corse in the northern, alpidic dominated part of Corsica (photo: Meschede, 2013).
Glaucophane schists, also known as blue schists, often exhibit a characteristic blue coloration caused by the mineral glaucophane, a blue variant of the amphibole group.
Fig. 5.1.6: Eclogite of the Middle Cretaceous, As Sifah, Oman (photo: Meschede, 2017)
The transformation of basaltic rocks into eclogite begins at depths of approximately 35 to 40 km. This leads to further transformation of the minerals, resulting in a rock consisting primarily of garnet and omphacite. Omphacite is a greenish-colored pyroxene that, together with the reddish garnet, forms the characteristic red-green speckled eclogite.
The metamorphic transformation of basaltic rocks into eclogite is of fundamental importance for plate tectonics. This transformation is accompanied by a significant increase in the density of the rock. Basaltic rocks are estimated to have an average density of 3.0 g/cm³. This rises to approximately 3.2 g/cm³ for glaucophane schist, and for eclogite, the density is 3.4–3.6 g/cm³. Thus, the density of eclogite is higher than the average density of the lithospheric mantle and asthenophore, which is approximately 3.3 g/cm³. This means that the oceanic lithosphere becomes heavier in the subduction zone and is thus increasingly pulled downward by gravity. This is today considered the primary driving force behind plate tectonic movements on Earth.
Subduction of oceanic lithosphere
Metamorphism in subduction zones
