What happens when an oceanic and a continental tectonic plate converge and when two continental plates converge? Why is the outcome different?

The Earth's outer layer, the lithosphere, is broken into several large and small rigid plates called tectonic plates. These plates move and interact with each other due to the movement of the underlying, semi-fluid asthenosphere. There are two types of tectonic plates based on their composition: oceanic plates (made primarily of dense basalt) and continental plates (made primarily of lighter granitic rock). These plates can interact in various ways, such as converging, diverging, or sliding past each other. In this problem, we will focus on convergence – when two plates move towards each other.

When an oceanic plate converges with a continental plate, the denser oceanic plate is forced underneath the less dense continental plate in a process called subduction. As the oceanic plate is pushed deeper into the Earth's mantle, it begins to heat up, and its water and other volatile content start to cause the mantle above to partially melt. This melting process creates magma, which can rise through the overlying continental crust, leading to volcanic activity or eruptions. In some cases, this can result in the formation of volcanic mountain chains near the plate boundary, such as the Andes in South America.

When two continental plates converge, neither plate can be easily subducted because both are relatively light and buoyant compared to the underlying mantle. Instead, the two plates are forced together, causing deformation and uplift along the plate boundary. This process can create vast mountain ranges, such as the Himalayas, which resulted from the convergence of the Indian and Eurasian plates.

The main difference in outcomes between oceanic-continental and continental-continental convergence is the presence or absence of subduction and its associated processes. In oceanic-continental convergence, the subduction of the denser oceanic plate creates volcanic activity, eventually leading to the formation of volcanic mountain chains. On the other hand, in continental-continental convergence, the lack of subduction leads to the compression and uplift of both plates, creating massive mountain ranges without volcanism. In summary, when oceanic and continental plates converge, subduction occurs, leading to volcanic activity and the formation of volcanic mountain chains. In contrast, when two continental plates converge, massive mountain ranges result from the compression and uplift of both plates, without volcanism. The outcomes are different because of the differences in density and composition between oceanic and continental plates, which affect whether subduction takes place.