Chapter 4: Dynamics of Convergence Zones

Active margins are characterized by significant seismic activity.
The most devastating earthquakes originate within the continental plate.
Earthquakes also occur at much greater depths, exhibiting a characteristic distribution.
These earthquakes are located within a plane known as the Wadati-Benioff zone.
The Wadati-Benioff zone is approximately 100 km thick and has a varying inclination.
It corresponds to the rigid and brittle oceanic plate subducting into the more ductile asthenosphere.
Seismic tomography reveals a cold material (positive anomalies) coinciding with the Benioff-Wadati zone, surrounded by hotter material (negative anomalies).
The oceanic plate sinking into the asthenosphere is termed the subducting plate.
The plate above which the oceanic plate subducts is termed the overriding plate.
These plates are driven against each other in a convergent motion, defining active margins as subduction zones.
Subduction can also occur beneath another oceanic plate.

Eruptions at active oceanic margins are typically explosive.
Magmas in subduction zones are rich in silica ( $SiO_2$ ).
High silica content results in high viscosity, impeding magma flow.
Magma tends to crystallize within the volcano's conduit, forming a plug.
Gases released during magma decompression accumulate beneath the plug.
Increasing pressure eventually shatters the plug, leading to explosive eruptions.
These eruptions can produce volcanic plumes several kilometers high and pyroclastic flows.
Pyroclastic flows consist of ash, gases (water vapor, carbon dioxide), and debris at high temperatures.
These flows can travel down the volcano's flanks at speeds up to 600 km/h, causing widespread destruction.

Magmatic rocks found in subduction zones are of two types: volcanic and plutonic, with plutonic rocks being more abundant.
Volcanic rocks exhibit a microlitic texture, indicative of rapid magma cooling during eruptions; these are mainly andesites and rhyolites.
Plutonic rocks have a granular texture, indicative of slow cooling within large magma chambers that do not reach the surface but form plutons after complete cooling.
Their presence at the surface is due to erosion; these are diorites and granites.
All these rocks originate from the same type of initial magma that has evolved.
Minerals crystallize at different rates during magma cooling: fractional crystallization.
The first minerals to form are poorer in silica, so the remaining liquid becomes enriched in silica and depleted in elements incorporated into the early-formed minerals.
The magma's chemical composition can also be enriched by elements from the melting of continental crustal rocks in contact with magma chambers: crustal contamination.
Continental crustal rocks (granite, gneiss) are rich in quartz, which is rich in silica.
Fractional crystallization and crustal contamination cause the diversity of magmatic rocks and the explosive nature of subduction zone volcanoes due to magma enrichment in silica.

Laboratory studies indicate that only the peridotite of the lithospheric mantle of the overriding plate can partially melt to generate the initial magma, but only if it is hydrated.
Metamorphism of crustal rocks in the subducting plate releases water that percolates upwards into the mantle peridotite of the overriding plate.
This peridotite descends with the subducting plate.
At depths of 80 km, the temperature and pressure conditions allow it to partially melt and generate magma, which rises towards the continental crust due to its lower density.

As the plate subducts, its crustal rocks experience increasing pressure with relatively little temperature change.
Minerals transform, and new mineral associations appear.
During the initial 35 km of subduction, chemical elements from plagioclase, actinolite, and chlorite recombine to form glaucophane, which is less hydroxylated.
Metagabbros of the greenschist facies progressively transform into blueschists, which are less hydrated.
Beyond this depth, glaucophane and residual plagioclase progressively disappear in favor of garnet and jadeite, which contain no hydroxyl groups.
The resulting eclogite is a completely dehydrated rock.
This metamorphism also increases the density of the crustal rocks, weighing down the plate and pulling it deeper.
This increase in density significantly contributes to the plate's movement.