How does the rock cycle operate? How do rocks metamorphose at destructive plate boundary?
The rock cycle demonstrates how a rock can start out as one type and change into another. It is a cycle of transformation which affects all rocks and involves all parts of the crust. The formation and changing of rock go on and on.
All rocks on earth were originally igneous. During vulcanicity, when cracks develop in the crust and reach the magma, the pressure of magma is released. Magma rises along the cracks to the earth’s surface. On its way, it cools and solidifies. During the cooling process, crystallization takes place in magma or lava to form minerals. The aggregate of minerals forms igneous rocks, as shown in Figure 1.
When igneous rocks on the surface are subject to erosion, the eroded materials will be carried away. Rocks are broken down into small fragments and removed downhill and into rivers by moving agents like rainwater or by gravity. Sedimentation or deposition occurs. Layers of sediment pile up and compaction followed by cementation take place. Both compaction and cementation finally transform loose sediment into solid rocks. Now, the original igneous rocks have transformed into sedimentary rocks.
In places where vulcanicity and crustal movement occur, any existing rock may be subject to high temperatures and high pressure. As a result, rocks will change from their original form into a completely new rock. For instance, igneous rocks can be re-melted and re-crystallized into other rocks, while sedimentary rocks can be fused by heat and pressure into much harder rocks. In this way, both sedimentary and igneous rocks are now turned into metamorphic rocks.
Finally, the melting of metamorphic rocks in the upper mantle during subduction turn the metamorphic rocks back into igneous rocks. By now, the original igneous rocks are, after going through different processes, turned back into igneous rocks again.
The metamorphism of rocks occurs in two instances. First, internal stress accumulated along a fault or in a fold facilitates the metamorphism of rocks. High pressure can make the mineral crystals in rocks realign to form metamorphic rocks. This is called dynamic or regional metamorphism, which is usually associated with large-scale mountain building processes at destructive plate boundary. When two plates collide, whether they are both or one of them is continental or oceanic plates, sedimentary rock layers between the two plates are compressed to rise above sea level. The sedimentary rocks are flattened and the minerals inside are aligned towards the plane of flattening. This creates the foliated structure in the resultant rock, which is called metamorphic rock.
The second type of metamorphism is called contact or thermal metamorphism, which is associated with the rising magma intruding into rock layers. At destructive plate boundary, when two oceanic plates collide with one another, one of them is subducted beneath the other. The melting of plate during subduction produces magma. This increases the pressure of magma currents and magma intrudes into the earth crust. Since high temperature can make the rock crystals melt and re-crystallize into new rock, contact or thermal metamorphism is localized and produced by heat from magma intrusion. The zone where the intrusion touches the existing rock is called an aureole, which refers to the zone of metamorphism. Therefore, rocks metamorphose at destructive plate boundary where ocean-ocean collision occurs.
To conclude, the transformation of rocks is dynamic within the rock cycle. Every single type of rock can exist in different forms due to crustal changes, in which rocks metamorphose at destructive plate boundary is one step of the entire rock cycle.