Basalt is a hard, black volcanic rock with less than about 52 weight percent silica (SiO2). Because of basalt's low silica content, it has a low viscosity (resistance to flow). Therefore, basaltic lava can flow quickly and easily move more than 20 kilometers from a vent. [More]
As basalt lava erupts from volcanic vents and cools, it shrinks and cracks. Sometimes vertical columns form. Well developed columns result from homogeneous lava cooling at a uniform rate.
Devils Postpile, California:
Formation of Devils Postpile began when basalt lava erupted in the valley of the Middle Fork of the San Joaquin River. As lava flowed from the vent, it filled the valley near the postpile to a depth of 400 feet. Radiometric dating of rocks thought to correspond with this basalt (a dark gray, fine grained rock with feldspar crystals) suggests an age of less than 100,000 years. Surface cracks formed when tensions caused by the shrinkage of the cooling lava were greater than the lava's strength. Each crack branched when it reached a critical length. Together with other cracks it formed a pattern on the surfaces of the flow. Ideal conditions allowed surface cracks to deepen and form long post-like columns. Some 10,000 years ago a glacier flowed down the Middle Fork of the San Joaquin River and overrode the fractured mass of lava. The moving ice quarried away one side of the postpile, exposing a sheer wall of columns 60 feet high. Many fallen columns lie fragmented on the talus slope below. At Devils Postpile the rock columns have from three to seven sides. The top of the postpile shows the columns' geometry and the polishing and scratch marks of glacial ice.
-- Excerpts from:
Volcano Hazards Program Photoglossary, 2001, and U.S. National Park Service Website, Devils Postpile, 2003