Volcanoes and Volcanic Hazards Violence or explosiveness of eruption determined by o Composition of the magma o Temperature of the magma o Dissolved gases in the magma All three control the viscosity Viscosity controls the nature of an eruption Look at Table 5 1 Factors affecting viscosity o Temperature Hotter magma less viscous o Composition silica content More silica felsic more viscous Lower silica mafic lower viscosity fluid like o Dissolved gases Volatiles mostly water As a given magma loses gases it becomes more viscous Why Volcanoes Erupt o Hawaiian type calmer eruptions Magma moves up and pours out o Explosive eruptions Gases expand within a magma as it nears Earth s surface due to The violence of an eruption is related to how easily gases decreasing pressure escape Fluid basaltic lavas generally produce quiet eruptions Highly viscous lavas rhylotie or andesite produce more explosive eruptions What comes out of volcanoes o Lava o Gas o Pyroclastic materials Lava Flows o AA blocky sharp spiny o Pahoehoe fluid ropey form at higher temps o Former lava conduits Lava tubes Block lavas Pillow lavas o Andesitic and rhyolitic short flows smooth surfaces o Underwater formation outer skin congeals but then it broken through stacked appearance toothpaste squeeze Gases o Natural air pollution o Create passage to surface Pyroclastic debris o Ash and dust fine glassy fragments o Lapilli walnut size material o Particles larger than lapilli Blocks hardened or cooled lava Bombs ejected as hot lava streamlined shape o Pyroclastic materials Pumice Scoria lava Light colored porous rock from frothy felsic andesitic lava Black to reddish brown vesicular rock from basaltic