Polygonal Jointing
Polygonal jointing refers to the natural fracturing of rock masses into polygonal shapes, most commonly observed in basalt flows. This phenomenon is driven by thermal contraction during the cooling process of volcanic lava.
Mechanism of Formation
The formation of these structures is a direct result of physical processes occurring as hot volcanic rock cools:
- Thermal Contraction: As basaltic lava flows cool, they undergo significant volume reduction due to temperature drop.
- Tensile Stress: The contraction generates tensile stresses within the rock mass.
- Fracture Propagation: To relieve this stress, the rock fractures. The geometry of these fractures tends toward hexagonal or polygonal patterns because this shape allows for the most efficient packing and stress distribution in a contracting plane.
- Column Development: These fractures propagate vertically and horizontally, creating the characteristic columnar structures seen in nature.
Key Factors
- Cooling Rate: Uniform cooling promotes regular polygonal shapes; rapid or uneven cooling may result in irregular jointing.
- Lava Composition: Basalt is the most common host rock due to its viscosity and cooling properties, though similar jointing can occur in other igneous rocks.
- Flow Thickness: Thicker lava flows typically produce taller and more distinct columns.
Notable Examples
- Giant’s Causeway (Northern Ireland)
- Devils Postpile (California)
- Fingal’s Cave (Iceland/Scotland region)