Final answer:
There are two arcuate ridges above the Mariana Subduction Zone because there are two subducting plates involved in the subduction process. The depth and angle of subduction can be determined using earthquake data and cross-sections of the subduction zone.
Step-by-step explanation:
The reason there are two arcuate ridges above the Mariana Subduction Zone instead of one is because there are two subducting plates involved in the subduction process. The Mariana Trench is formed where the Pacific Plate is subducting beneath the Philippine Sea Plate and the Mariana Plate is subducting beneath the Pacific Plate. These two subduction zones result in the formation of two parallel arcuate ridges above the Mariana Trench.
When the subducting plates reach a certain depth, partial melting of the asthenosphere occurs, leading to the formation of volcanoes. The closer the volcanoes are to the coastline, the deeper the subducting plate is and the further it has traveled beneath the overriding plate.
By examining earthquake data and cross-sections of the subduction zone, scientists can determine the depth and angle of subduction at different locations. This information helps explain why there are two arcuate ridges above the Mariana Subduction Zone rather than one.