| This exceptional example of columnar basalt pillars is along
US Highway 26 about midway between the Painted Hills Unit and the Sheep
Rock Unit of John Day Fossil Beds National Monument. Columnar pillars form
when lava cools fairly evenly. They are particularly common in basaltic lava flows. Large fissure eruption that form these kind of basalt flows release very hot lava (hotter than most lavas). This very fluid and
can flow over large areas. As the lava crystallizes, the volume of the cooling
lava shrinks and the newly formed rock cracks under the tensional forces
generated by shrinkage during cooling. The crack typically form in the most efficient geometric
pattern, hexagonal pillars, which tend to be fairly uniform in shape and size
throughout the lava flow (Spry, 1962). Here a stream has cut through the lava flow,
revealing the polygon-shaped pillars of an ancient lava flow. |
