That viscosity prevents gas within it from readily escaping as the magma rises to Earth's surface through a break in its crust. When lava can no longer contain the increasing gas pressure within, cataclysmic rhyolitic eruptions occur. Basaltic eruptions are more gentle because gas bubbles to the surface before it generates explosive pressure. Boiling water does not cast large volumes of liquid into the air in violent fits and starts like, for example, a boiling vat of mud. The high viscosity of the mud causes it to behave like rhyolitic lava.
Basaltic lavas are runnier, like maple syrup, while rhyolitic lavas are more like molasses.
At Craters this evidence has since been entirely hidden by the more recent, gentler, low-silica basaltic eruptions. All the rock you see at Craters is basalt. Cooler lava that crept downhill and was slowly turned, twisted, and ground up into irregular chunks became aa. As an attention grabber and to introduce volcanism and the effects of gas and pressure, prepare the following experiment. Drop an Alka-seltzer into the canister and put the lid on and place a second, empty canister on top of the first.