as the square of its thickness divided by its viscosity. This implies that a rhyolite flow would have to be about a thousand times thicker than a basalt flow to flow at a similar speed. Temperature The temperature of most types of molten lava ranges from about to . depending on the lava's chemical composition. This temperature range is similar to the hottest temperatures achievable with a forced air charcoal forge. Lava is most fluid when first erupted, becoming much more viscous as its temperature drops. Lava flows quickly develop an insulating crust of solid rock as a result of radiative loss of heat. Thereafter the lava cools by very slow conduction of heat through the rocky crust. For instance, geologists of the United States Geological Survey regularly drilled into the Kilauea Iki lava lake, formed in an eruption in 1959. After three years, the solid surface crust, whose base was at a temperature of , was still only thick, even though the lake was about deep. Residual liquid was still present at depths of around nineteen years after the eruption. A cooling lava flow shrinks, and this fractures the flow. Basalt flows show a characteristic pattern of fractures. The uppermost parts of the flow show irregular downward-splaying fractures, while the lower part of the flow shows a very regular pattern of fractures that break the flow into five- or six-sided columns. The irregular upper part of the solidified flow is called the entablature, while the lower part that shows columnar jointing is called the colonnade. (The terms are borrowed from Greek temple architecture.) Likewise, regular vertical patterns on the sides of columns, produced by cooling with periodic fracturing, are described as chisel marks. Despite their names, these are natural features produced by cooling, thermal contraction, and fracturing. As lava cools, crystallizing inwards from its edges, it expels gases to form vesicles at the lower and upper boundaries. These are described as pipe-stem vesicles or pipe-stem amygdales. Liquids expelled from the cooling crystal mush rise upwards into the still-fluid center of the cooling flow and produce vertical vesicle cylinders. Where these merge towards the top of the flow, they form sheets of vesicular basalt and are sometimes capped with gas cavities that sometimes fill with secondary minerals. The beautiful amethyst geodes found in the flood basalts of South America formed in this manner. Flood basalts typically crystallize little before they