What Causes Volcanoes to Form?
Volcanoes form when hot magma rises from the mantle and is placed into the earth’s crust. So what causes the magma to form in the first place? There are three primary reasons that magma forms and rises to the earth’s surface.
These processes are:
What Causes Volcanoes to Erupt?
What actually causes a volcano to erupt can be fairly complex. All volcanoes are different, and for most, there are many factors that likely affect whether the volcano will or won’t erupt.
But if we are to be simplistic, most volcanoes erupt when the pressure in the magma chamber becomes greater than the strength of the overlying rock holding that pressure inside.
Subduction is the process of one tectonic plate sliding beneath another tectonic plate. This happens at a very slow rate, but influences many aspects of Earth as we know it. Subduction is the direct cause of some of the world’s most powerful earthquakes, and is often the primary cause for the world’s largest Tsunami’s.
The primary reason why subduction causes volcanoes to form is surprisingly related to water. Water, which saturates the plate sliding beneath another tectonic plate, causes rock to melt at significantly lower temperatures. Being melted at such a low temperature compared to the surrounding rock causes gigantic blobs (known as diapirs) to slowly rise through the earth until it reaches the crust. When this rock reaches the crust, it often will accumulate to form magma chambers. From here, the chamber will often and grow build until it the pressure becomes too great, resulting in eruptions onto the surface. And henceforth, a volcano is created.
Rifting refers to the spreading out of the top layer of crust. It might seem strange that spreading crust could result in volcanoes, but it makes sense when you understand the dynamics of rocks and pressure.
Put simply, rifting lowers the pressure in the rock layers in the mantle and crust below where the spreading is occurring. Rock melts into magma at a much lower temperature when the pressure is lowered by a great deal. When you lower the pressure enough with significant enough rifting, you end up with magma generation, resulting in volcanoes.
Rifting is a less common process than subduction for forming land-based volcanoes, but it sometimes will co-occur with subduction-based volcanoes. In other words, a volcano may form as a result of subduction, but some minor rifting processes may aid magma production at a subduction-based volcano.
Another point about rifting that is worth mentioning is that rifting can also occur as a result of magma pushing its way to the surface. This is what is currently occurring in the famous African Rift system (see video below for a great in-depth explanation of what is going on there).
Hot spots are thought to be caused by deep plumes in the earth’s mantle, which are extra hot compared to the normal surrounding mantle rock. Whereas rifting and subduction need to lower the pressure, or add a catalyst (water) to get rock to melt, a Hot spot is very simple in that the rock is just super-hot, which causes it to melt even at a very high pressure. This creates volcanoes, or volcano chains like we see in Hawaii.