Nature / Earth / Earth processes
Why cooling basalt breaks into columns
Those regular stone pillars began as cracks in cooling rock. Their shapes tell a different story from the erosion that later exposed them.
The striking order in a wall of basalt columns begins with cracks. As a body of volcanic rock cools and contracts, fractures divide it into adjoining prisms. Later erosion exposes that structure, sometimes leaving a cliff that looks almost built.
Two questions help separate the processes: what divided the rock, and what uncovered it? The first concerns cooling. The second concerns the landscape’s subsequent history.
The rock has solidified, but it is still cooling
The National Park Service’s explanation of columnar jointing places the fracturing after solidification, while the rock remains hot. Contraction creates stress; cracking relieves it. The joints develop inward from cooling surfaces.
Basalt is a familiar setting for this pattern, but the NPS records columnar jointing in other volcanic rocks and deposits too. A column is therefore a structural clue, not a complete identification of the rock’s composition.
Why the columns often stand upright
Columns tend to align perpendicular to the main cooling surfaces. If a lava flow cools from its upper and lower boundaries, the resulting arrangement can be broadly vertical. It need not stay that way everywhere: curved or tilted columns can reflect different cooling geometry.
The USGS account of Devils Postpile describes lava ponding in a valley and losing heat both to the air above and to underlying rock. Cracks advanced from those boundaries toward the interior. That is a more useful picture than imagining separate stone pillars growing upward from the ground.
Why six sides are common, rather than compulsory
NPS explains that three fractures meeting near 120 degrees can relieve contractional stress efficiently and produce a predominantly hexagonal network. Real outcrops contain variation: its overview describes columns with three to seven sides.
USGS reports that about 55% of the columns at Devils Postpile have six sides. That figure belongs to this particular outcrop. It is neither a universal percentage nor a test that an unfamiliar rock face must pass.
When looking at a photograph, count the edges you can actually see. A slanting view may hide part of a polygon. An apparently four-sided face in the picture may be an incomplete view rather than a four-sided column.
Making columns and revealing columns are different events
At Devils Postpile, USGS describes later glaciation polishing and grooving the upper surfaces. It also describes fallen column fragments accumulating below, with earthquakes and freezing and thawing contributing to breakage. These later changes help explain the exposed surface and rubble without explaining the original polygonal pattern.
A useful reading sequence is joint pattern → orientation → later damage. First identify the cracks that separate the columns. Then consider their direction. Finally distinguish broken edges and loose fragments from the structure still connected to the outcrop. This is an observation guide, not a way to calculate an eruption date from a photograph.
The same separation of material and process is useful when asking why volcanic ash is fragmented rock.
References
- National Park Service. (2023, April 18). Columnar Jointing. https://www.nps.gov/subjects/volcanoes/columnar-jointing.htm
- U.S. Geological Survey. (2023, November 19). Vertical columns of volcanic rock at Devils Postpile National Monument. https://www.usgs.gov/volcanoes/long-valley-caldera/science/vertical-columns-volcanic-rock-devils-postpile-national
Reading notes: These NPS and USGS interpretive resources agree on cooling contraction. Their rounded ages for Devils Postpile differ, and their cooling-rate descriptions operate at different scales. This explanation does not select an eruption age or assign one cooling speed to every columnar outcrop. No field measurements or source photographs are presented as our own.