Water and other gas-producing compounds recycled in subduction zones. Subduction zone magmas are rich in volatiles. Pyroclastic materials Small bits of debris which cool relatively quickly. Ash and dust Tuff - the rock product of consolidated ash and dust. Pumice - material full of small vesicles.
Eruptions consist of lava fountains up to a several hundred feet high which feed the lava flows.
Can continue unabated for many days. Gentle slopes of a few degrees. Common on oceanic crust. As shield volcanoes age, flank eruptions become more prevalent, and magmas may increase in viscosity, perhaps due to assimilation. Rarely higher than a few hundred meters.