As we noted in § 30.3, there are three types of volcanic flows:

Volcanic flows at mid-ocean spreading centers are passive upwellings that fill the void created as the lithospheric slabs on either side are pulled away. Due to the parabolic shape of the cooling slabs, the upwelling speed is greatest, the motion is closest to adiabatic and the temperature is hottest beneath the spreading center. As can be seen from Figure 7.3, the mantle adiabat, when extrapolated to the top of the mantle, is greater than the melting temperature. This means that the upwelling material becomes partially molten. The viscosity of molten silicates is far smaller than the viscosity characterizing sub-solidus creep and the density of melt is less than that of the solidified remainder. It follows that the molten silicate (now called magma), percolates upward, collecting in magma chambers. The molten material in these chambers wells upward to the surface, where it forms basaltic pillow lava. Since this flow occurs in an expanding environment, the magma is not confined and out-gassing of volatiles is a quiet affair.

The situation regarding out-gassing is quite different for the andesitic volcanism that occurs above subduction zones. This volcanism originates at the top of the subducting slabs due to a combination of viscous-dissipative heating and the presence of water. Most andesitic volcanoes occur on the rim of the Pacific Ocean, forming the so-called ring of fire. As the lithospheric slabs trundle their way from spreading center to subduction zone, minerals in the top of the slab become hydrated; that is, they incorporate water into their crystalline structure. After subduction, these hydrated minerals break down, releasing the water from its crystalline cage. This liberated water does two things. First, it lubricates the top of the slab, permitting it to slip beneath the adjoining crustal material.