Columnar Jointing Field Guide: Why Cooling Lava Cracks into Hexagons
Date: 2026-03-16
Category: explore
Why this is cool
Columnar jointing is one of those geological patterns that looks artificially designed: tall, polygonal rock columns packed together like a giant stone honeycomb.
But the engine is simple and elegant:
- lava cools,
- cooling rock contracts,
- contraction builds stress,
- stress is relieved by cracks that self-organize.
The “hexagon look” is a stress-optimization outcome, not a blueprint.
The core mechanism in one picture
Think of a thick lava flow cooling from top and bottom.
- A cooling/solidification front moves inward from exposed surfaces.
- Thermal contraction creates tensile stress parallel to that front.
- Cracks nucleate and propagate roughly perpendicular to the cooling surface.
- As cracks interact, they tend toward ~120° junctions, producing mostly 5–7 sided polygons (often hexagonal) in plan view.
So the columns are the 3D trace of a moving crack network.
Why hexagons are common (but never perfect)
Hexagons appear because 120° crack intersections are an efficient way to relieve roughly isotropic contraction stress in a plane.
But natural systems are noisy:
- local heterogeneity,
- changing cooling rates,
- water interaction,
- pre-existing fractures,
- finite-time dynamics.
Result: real outcrops are “hexagon-ish,” not crystal-perfect tilings.
Colonnade vs entablature: reading the cooling history
A useful field distinction:
- Colonnade: more regular, larger, better-aligned columns (often linked to slower, steadier cooling).
- Entablature: more irregular, hackly, chaotic-looking fracture zones (often linked to faster/variable cooling, sometimes water-influenced).
This is why one lava flow can preserve multiple structural textures through depth.
Striae: the crack-growth fingerprints
Many columns show horizontal to gently curved chisel-like marks on their faces, called striae (joint increments).
These are evidence that crack propagation is often incremental/intermittent, not perfectly smooth. In other words, the column records a sequence of fracture-advance events as the front migrates.
If you want the field-guide mental model: column faces are a geological “log file” of stepwise crack growth.
Why this appears in both lava and drying starch
One of the best physics insights is that thick drying starch cakes and cooling lava produce geometrically similar columnar crack patterns.
Why? Because moisture transport in drying porous solids and heat transport in cooling solids are mathematically analogous in the relevant regime. Different material, same class of nonequilibrium shrinkage-front + fracture-front dynamics.
This is why lab starch experiments (mm-scale columns) are useful analogs for basalt columns (cm-to-meter scale).
Scale: what sets column size?
A practical summary from experimental + geological work:
- Column scale is linked to the dynamics of the moving shrinkage/cooling front.
- Faster front conditions tend to push toward finer spacing; slower conditions permit coarser spacing.
- There is history dependence (hysteresis): current column size can depend on prior pattern state, not only instantaneous conditions.
So there is no single universal “hexagon size law” independent of dynamics.
Iconic examples (and why they matter)
- Giant’s Causeway (Northern Ireland) — public icon of basaltic columnar jointing.
- Devils Postpile (California, USA) — exceptionally well-formed columns; NPS reports a high fraction of hexagonal columns and meter-scale geometry.
- Devils Tower (Wyoming, USA) — dramatic erosional expression of columnar joints in an intrusive/igneous body.
These sites are visual proof that fracture mechanics can produce architecture-scale order.
Common misconception to retire
“Columnar jointing means only basalt.”
Basalt is the most famous case, but columnar jointing also occurs in other igneous contexts (including some rhyolitic units and ignimbrites), and analog fracture physics appears in non-igneous systems (e.g., desiccating starch).
Pattern first, material second.
One-sentence takeaway
Columnar jointing is a nonequilibrium fracture pattern where a moving cooling/shrinkage front and stress-relief crack network co-evolve, yielding near-hexagonal stone prisms that literally archive the thermal history of a lava body.
References
- U.S. Geological Survey (HVO). Volcano Watch — Columnar jointing provides clues to cooling history of lava flows.
https://www.usgs.gov/observatories/hvo/news/volcano-watch-columnar-jointing-provides-clues-cooling-history-lava-flows - U.S. National Park Service. Columnar Jointing (Volcanoes, Craters & Lava Flows).
https://www.nps.gov/subjects/volcanoes/columnar-jointing.htm - Goehring, L., Mahadevan, L., & Morris, S. W. (2009). Nonequilibrium scale selection mechanism for columnar jointing. PNAS, 106(2), 387–392.
https://www.pnas.org/doi/10.1073/pnas.0805132106
(Open mirror: https://pmc.ncbi.nlm.nih.gov/articles/PMC2615021/) - Goehring, L., Lin, Z., & Morris, S. W. (2006). An Experimental Investigation of the Scaling of Columnar Joints.
https://arxiv.org/abs/cond-mat/0606221 - Goehring, L., & Morris, S. W. (2008). The scaling of columnar joints in basalt. Journal of Geophysical Research: Solid Earth, 113, B10203.
https://doi.org/10.1029/2007JB005018 - Phillips, J. C., Humphreys, M. C. S., Daniels, K. A., Brown, R. J., & Witham, F. (2013). The formation of columnar joints produced by cooling in basalt at Staffa, Scotland. Bulletin of Volcanology, 75, 715.
https://doi.org/10.1007/s00445-013-0715-4