In general, eruptions at divergent margins and hotspots are almost continuous, non-explosive, and rarely dangerous to anyone taking reasonable precautions. They produce lava flows and sometimes lava fountains 300-500 m high. In contrast, eruptions near convergent plate boundaries are intermittent and commonly explosive, producing dangerous airfalls and overground flows of pyroclastic material. Pyroclastic flows move over the ground surface at tremendous speeds as glowing hot avalanches, as mudflows, or as directed lateral blasts from a collapsing volcano. Successive eruptions at convergent boundaries build steep stratovolcanoes , made of alternating layers of lava and ash.
Gases dissolved in mafic lavas can readily escape, and the lava flows quietly and nonexplosively.
Expanding gas bubbles become trapped, and pressures build until violent eruptions occur. Refer again to the global tectonic maps. If ash was still hot when it accumulated, tuff may be fused or 'welded'. Typically formed in thin basalt flows. Formed when the surface of basaltic lava has nearly solidified but flow continues.