Final answer:
Step F, which refers to the subduction of ocean lithosphere under continental lithosphere, leads to volcanic activity near coastlines. This process happens when the more dense oceanic plate subducts under the less dense continental plate, causing partial melting that triggers volcanism.
Step-by-step explanation:
The reason why Step F occurs is because ocean lithosphere descends under continental lithosphere. When two tectonic plates converge, if one of the plates has an oceanic crust, this plate subducts under the other due to its higher density. This process creates a subduction zone, typically marked by the presence of an ocean trench, such as the Japan trench along the coast of Asia. The oceanic lithosphere, when subducted, is forced into high pressure and temperature regions inside the Earth, leading to partial melting of the asthenosphere. This partial melting in turn often results in volcanism and the formation of volcanic arcs near the coastline.
Volcanoes are often closer to the coastline in subduction zones where the oceanic lithosphere subducts under continental lithosphere. Near the subduction zones, the descending oceanic plate melts and induces volcanic activity that forms parallel to the coastline and on the overlying continental plate, as seen with the Andes mountains in South America. The oceanic crust is hydrated, which means it contains minerals with water in their crystal structure. As the crust descends, these minerals release water, aiding in the melting and contributing to magma production, which rises to form volcanoes.