Grades 3–5 · Earth & Space
Why can cooling volcanic rock crack into columns?
A cliff of basalt can look like a bundle of giant stone pencils. The columns were not carved one by one. They formed when a thick body of hot volcanic rock cooled, shrank, and cracked. Basalt is one kind of rock in which these column patterns are common.
In the diagramCooling contraction can crack rock into columnsColumn axisCooling surfaceJoints outline columns; real shapes and directions vary.Columns lie roughly normal to their cooling surface.
Cooling removes thermal energy. Once the material is solid enough, its continued contraction can build stress because neighboring parts cannot shrink freely in every direction. Fractures relieve some of that stress. A fracture is a crack through the rock.
Cracks can advance inward from cooling surfaces as more of the body cools. This often leaves long columns roughly perpendicular to those surfaces. A top view shows the polygon shapes of their ends, while a side view shows the long joints between columns.
Six-sided ends are common, but nature does not make every column a perfect hexagon. Cooling history, existing cracks, and local conditions can produce different sizes, shapes, and directions. The illustration uses a regular pattern to make the three-dimensional structure clear.
About this explanation
Starting knowledge: Understands cooling, solid rock, cracks, and top versus side views of a solid shape.
Sources
- U.S. National Park Service — Columnar Jointing
- U.S. Geological Survey — Volcano Watch — Columnar jointing provides clues to cooling history of lava flows