The Physics of a Waterfall
Svartifoss, or "Black Fall," is a 20-meter (66-foot) high waterfall in Iceland's Vatnajökull National Park. The name comes from the dark basalt columns that frame the cascade. These columns are the result of a geological process called columnar jointing. When a thick flow of basalt lava cools, it contracts and fractures. To relieve the thermal stress most efficiently, the cracks form a hexagonal pattern, similar to the way mud cracks as it dries.
The perfection of these columns depends on the cooling rate. A thick lava flow that cools slowly, insulated by the ground below and rock above, allows stress to distribute evenly. This slow process results in the formation of large, well-defined hexagonal columns known as a colonnade. Faster, uneven cooling, often caused by exposure to air or water, creates smaller, more irregular fractures called entablature. Svartifoss is a colonnade, where the slow solidification of magma created a near-perfect geometric structure. The columns have horizontal grooves, or striations, spaced 5 to 20 cm apart, which mark the successive stages of the fracture's propagation as the lava cooled. At the base of the falls, a pile of sharp-edged rocks consists of column sections that have broken off over time.