Outpourings of magma can occur as a fluid-like lava or as fiery clouds of ash and rock fragments. The ability of lava to flow depends on its viscosity or "stickiness". The viscosity of magma depends on temperature, silica content, and incorporated gases.  Basaltic magma has approximately 50% SiO², the smallest amount of incorporated gas and readily flows across the surface. Andesitic magma contains 60% SiO² and has a substantial content of incorporated gas, giving it a moderate viscosity. Rhyolytic magma is 70% SiO² and contains the largest amount of gas. Rhyolytic magma produces the highest viscosity lava and is responsible for violent eruptions.

 Figure 16.3 Hydrocholoric acid steam rises from the ocean as a Pu`u `O`o lava flow reaches sea water. Photo credit: M. Ritter

Volcanic eruptions, especially explosive eruptions, eject fragments of volcanic rocks and lava collectively called tephra .  Ground-hugging pyroclastic flows of fast-moving gas and fragments of rock having temperatures of 500 ^oC can rush down the flanks of a volcano at speeds reaching 100 km/hr, carbonizing all in their path.

Volcanic eruptions are accompanied by the release of noxious and sometimes lethal gases. Steam makes up the majority of gas that is released by a volcano followed by carbon dioxide, sulfur dioxide, and hydrogen chloride. When lava spills into the ocean, the intense heating of salt water produces a noxious steam cloud of hydrochloric acid. Sulfur dioxide combines with water in the atmosphere to produce acid rain. Naturally produced acid deposition around the top of Mauna Loa has devastated the local vegetation. Watch USGS researchers collect lava sample near Pu`u `O`o in "Volcano Lava Sample" below.

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