background

volcanoes often have an undeserved reputation for being fast, active, and dangerous. most of the time, however, they are just sitting around quietly not doing much at all except slowly eroding away. most of the action is going on deep below the surface. beneath the crust and lithosphere, the convecting mantle is constantly in slow moving action. this blog is mostly dedicated the dynamic earth and the slower processes that lead to the  volcanic eruption. 

Big broad question
1) why do volcanoes erupt? (how does mantle and lithospheric movement relate to volcanoes?)

Short answer
the plates move because the earth is hot. the earth is hot because it is radioactive. volcanoes erupt because the earth is hot and the plates are moving. a lot more is going on, however. 

Longer answer
the driving force of plate tectonics and volcanic eruptions is the earth's heat, but the vast majority of the lava that erupts at volcanoes did not ultimately melt because of an increase in temperature. most lavas melt either because of decreases in pressure caused by the moving plates, or by changes in mantle chemistry also brought on by the moving plates.       

Three ways to melt the mantle (asthenosphere).
1) increase heat (12%). this occurs at hot spots. hot spots are not well understood but most geologist think they are related to deep plumes of heat that originate near the core-mantle boundary. others don't think deep plumes exist at all, but they tend to be in the minority.

2) decrease pressure (73%). this accounts for the majority of volcanoes on earth and predominately occurs where tectonic plates are diverging away from each other. as the lithosphere spreads apart, the underlying asthenosphere rises to fill the space. As it rises, the pressure decreases, causing the rock to melt. Here is an animation of this process. 

3) add water (15%). adding water and other components like potassium and sodium, allows the asthenosphere to melt at a lower temperature. the most common way that water is added to the asthenosphere is at subduction zones, where oceanic crust and sediments are dragged deep into the earth. subduction of cold oceanic crust actually makes the asthenosphere cooler, so the melting that occurs here demonstrates the power of chemistry.

The percentages I included here are not exact and subject to best guessing.

The moving lithospheric plates generate most of the Earth's magmatism. But what makes the plates move?

Four ways to move the plates
1. Slab pull: subduction of dense, oceanic lithosphere pulls the plate across the surface of the earth. 
2. Ridge push: warm, newly formed oceanic lithosphere sits atop warm, buoyant asthenosphere creating elevated topography that collapses laterally pushing the plate along.
3. Plume push: warm buoyant mantle plumes push the overriding lithosphere upward, which then collapses laterally pushing the plates along.
4. Slab suction: the descent of thick piles of subducted oceanic lithosphere drag the mantle mantle on either side downward as well, creating asthenospheric convection that drags the overriding lithosphere laterally. This has been suggested to explain the most extreme topography on earth.    

Summary: radioactive decay in the mantle keeps the earth hot. the escaping heat causes the plates to move. the moving plates cause changes in pressure and chemistry that result in volcanoes. 


The majority of this blog is dedicated to the controversies and questions involved in applying these basic principles to the realities and observations on our planet.