Travel Reference
In-Depth Information
Basalt columns at Overhanging Cliff dwarf passing cars.
2.1/16.2 Overhanging Cliff [GEO.12], with limited parking along the road. The road excava-
tion has undercut a fine example of columnar basalt that is 2.2 million years old. As you might
expect, landslides here have been a major problem for road builders.
Lava Columns
Columns in lava are formed when the lava flowed, stopped, cooled enough to become sol-
id, and then continued to cool and shrink. A solidified lava flow can easily shrink in a ver-
tical direction and still remain homogeneous and unfractured; it simply becomes a thinner
lava flow. Cooling rock, however, shrinks in all directions, not just vertically. As it shrinks
in the horizontal direction, cracks must form. The cracks extend and intersect with oth-
ers, making columns that are polygons, with space between the columns. The process that
makes the columns is very much like the formation of mud cracks. Just as the sun shining
on a mud puddle causes the mud to dry, shrink, and crack, basalt shrinks into columns,
but it's due to cooling rather than drying.
Not all lava flows exhibit columns; they are most commonly seen in basalt lavas, but
also occur in rhyolite lavas. The columns in basalt lavas are more closely spaced and regu-
lar in form than those in rhyolite lava. There are examples of both in the park.
The area near Tower has some clear examples of basalt lavas that look like stockade
fences. You can see them at the roadside at Overhanging Cliff as well as across the canyon,
where two lava flows run horizontally for miles. All of these are fine examples of columnar
jointing, that is, structures that look like closely packed, vertical logs of lava.
 
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