The composition of lava is the most important characteristic in determining its behavior.
the major compositional groups are:
basaltic lava has low silica contents (~50%) and high eruption temperatures, causing them to flow rapidly and have a low viscosity. basalts flow far from volcanic vents and form broad, sheet-like lava flows.
rhyolitic lava has high silica contents (~70%) and low eruption temperatures, giving them high viscosities. rhyolitic lavas mound up and stay near the volcanic vent because of their resistance to flow.
intermediate lavas, such as andesitic lava, have compositions, temperatures, and volatile contents between those of basalt and rhyolite.
the mineral composition of these groups determines each lava’s prevalence. some minerals melt more readily, leading to larger volumes of lava. basalts and rhyolites are thought to form early and late in the melting process, respectively. more than 90% of the total volume of lava on earth is estimated to be basaltic in composition, while rhyolites make up as little as 1% of the total. andesites and other lavas account for most of the rest.
by considering the compositional group and temperature of lava, we can evaluate the lava’s viscosity and eruptive style.