The oceanic crust that's under the mantle has a lot of seawater in its cracks and pores. When these rocks reach a certain depth in the mantle, they melt and form magma. This magma, made lighter by the water it holds, rises to the surface and creates volcanic eruptions. These eruptions can produce lava or ash, depending on how much gas and water vapor are in the magma. This process is how volcanoes like Popocatépetl and Colima were formed, and they often line up in a belt called a volcanic arc.
Subduction also causes changes in how quickly and at what angle the oceanic plate goes into the mantle. When the speed slows down and the angle gets steeper, the continental crust becomes stretched and thinner, creating big valleys behind the volcanic arc. If the stretching continues, a new ocean can form between the arc and the mainland. A good example of this is Japan, which has a volcanic arc and the Sea of Japan that separates it from Asia.
When the tilt of the oceanic plate going into the mantle decreases and the speed increases, the mantle "pushes" against the continental plate, causing high pressure that crumples the crust. This creates mountain chains by deforming the crust, which is called orogenic folding chains, on the back side of the volcanic arc. One example of this is the Sierra Madre Oriental. Another current example is the Andes, where there are both volcanoes and big mountains created by folding.
To sum up, four things happen at convergent boundaries because of subduction:
- the making of volcanic mountains or belts,
- the creation of valleys or oceans behind the volcanic belt on the continent, and
- the formation of high mountain ranges through the "wrinkling" of the crust (orogenic folding chains).
Over the past 200 million years, Mexico has experienced these four events, with some happening constantly (1 and 2) and others happening one after the other (3 and 4). These events have shaped the country's landforms and created its minerals and energy resources.
INAH, México, grandeza y diversidad, 2021, pp.41-42.