Here is my subduction zone and it is going to be subducting from left to right in this figure. I am drawing the crust here. Here is the other side. This is the overriding plate. This is the lithospheric mantle. This is the sinking slab so this is all cold. It is going this way. This stuff is cold, too. At a certain depth, and let us say that depth is right here, all the water is going to be driven out of the slab that is going down and it is going to percolate upwards into this overlying mantle which is hot. The water gets driven off here and you end up forming a melt and the melt rises. I am certainly oversimplifying the process of where the water is driven off and how the melt actually rises to the surface, but one thing that is accurate about this drawing is that the volcanoes on the overriding plate will happen directly above the place where the water was driven off. They are not going to happen over here, or over here, or on this plate. They are going to happen right above the point at which the slab is deep enough that the pressure is high enough that the volatiles are squeezed out of this and percolate into here, thus lowering the melting point of this overriding rock and creating some magma. So here is your volcanic arc. The cool thing about this observation is that you can look at a map without any kind of cross-section and you can measure the position of the trench because that is easy to see bathymetrically. It is deep. There is the trench. And you can see where the volcanoes are. Here are the volcanoes. And you can measure the distance in between them. That distance is going to tell you something about how steeply that slab is descending. This depth at which the pressure is high enough to get rid of the volatiles is more or less the same everywhere around the world. If you have a slab that is heading down at a very steep angle, then the volcanoes will be a lot closer to the trench. And if the slab is descending at a much shallower angle, taking a longer horizontal distance to get to this depth, then the volcanoes will be farther inland. I can demonstrate that right here. Here is a subduction zone where the angle is steep. Here is one where it is much more shallow. And let us say that the depth is the same everywhere. In this case the volcano is going to happen right above this depth. Over here the volcano is going to happen here, farther away from the trench. The distance between the trench and the volcano is this distance here. For this subduction zone, the distance is much farther. So you can actually look at a map and you can tell the angle of subduction for all the subduction zones around the world just by seeing where the trenches are and where the volcanoes are.