Scenery of several basalt flows giving unusual coastial landforms. The Tertiary basalt headland has been eroded by wave action, giving a seas stack on a shore platform with columnar jointing, ultramafic xenoliths (olivine) in boulders on the beach with polygonal jointing, pillow lavas, sea caves and sea potholes. in some places lava from one flow fills cracks in the flow beneath.
Columnar basalt weathers like a Tessellated Pavement, giving rise to high rimmed polygonal pools and depressions.
The Mersey Bluff Lighthouse standing at the mouth of the Mersey River near Devonport is unusual in Australia with its distinctive vertical red striped day mark.
In geology and geomorphology, a tessellated pavement is a relatively flat rock surface that is subdivided into more or less regular rectangles, blocks approaching rectangles, or irregular or regular polygons by fractures, frequently systematic joints, within the rock. This type of rock pavement bears this name because it is fractured into polygonal blocks that resemble tiles of a mosaic floor, or tessellations.
Columnar jointing is a geological structure where sets of intersecting closely spaced fractures, referred to as joints, result in the formation of a regular array of polygonal prisms, or columns. Columnar jointing occurs in many types of igneous rocks and forms as the rock cools and contracts. Columnar jointing can occur in cooling lava flows and ashflow tuffs (ignimbrites), as well as in some shallow intrusions. Columnar jointing also occurs rarely in sedimentary rocks if they have been heated by nearby hot magma.
The columns can vary from 3 meters to a few centimeters in diameter, and can be as much as 30 meters tall. They are typically parallel and straight, but can also be curved and vary in diameter. An array of regular, straight, and larger-diameter columns is called a colonnade; an irregular, less-straight, and smaller-diameter array is termed an entablature. The number of sides of the individual columns can vary from 3 to 8, with 6 sides being the most common.