The top 100 metres of Mount Tomah consists of a layer of volcanic rock called basalt. This is a remnant of a large lava flow, which 18 million years ago, covered a large area of the Blue Mountains.
Basalt Outcrop
As the basalt flow cooled, the lava solidified into rock at the surface and began to shrink. When cooling rates are fairly uniform, this shrinkage causes stresses in the rock, until the stresses are relieved by the formation of a crack, also known as a contraction joint. These cracks develop at right angles to the cooling surfaces. During the cooling process, the entire rock layer splits apart so that the crack tips advance through to the hot lava below.
As the lava underneath continues to uniformly cool allowing the heat to escape at regular intervals, the solidifying rock contracts and fractures fairly evenly, relieving the shrinkage stresses layer by layer. This leads to tall, well-developed, generally hexagonal basalt columns called columnar jointing.
Over time, basalt weathers to form a rich acid clay loam which is significantly more fertile than most Australian soils.
Columnar Jointing
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.
The columns can vary from three meters to a few centimeters in diameter, and can be as much as 30 meters tall. When the lava cools in a uniform fashion, the columns are typically parallel and straight, but can also be curved and vary in diameter when uneven cooling occurs. Whereas an array of regular and straight columns is called a colonnade, an irregular array is termed an entablature.
The number of sides of the individual columns can vary from three to eight, with six sides being the most common. That said, the cooling rates of developing columnar basalt are often far from uniform, though. In many cases, the lava cools too rapidly, and smaller, irregular, glassy columns crystallize out, with pentagonal shapes, hexagonal shapes, and sometimes heptagonal shapes all precipitating.
Please note: This earthcache is located within the grounds of the Blue Mountains Botanic Garden Mount Tomah and is open every day except Christmas Day. Opening times are Monday to Friday 9:00am-5:30pm, Saturday, Sunday and Public holidays 9:30am-5:30pm.
Earthcache requirements:
In order to log this earthcache you must answer the following questions. You do not have to wait for a reply before logging your find. I attempt to reply to all e-mails submitting answers and, if there are any problems I will contact you to obtain more information.
- On average how many sides do the columnar jointing at Mount Tomah have? What does this number tell you about the cooling rate of the lava?
- What is the approximate size (in centimetres) of the columnar jointing at Mount Tomah? From your observations and reading of the above earthcache lesson, would you call this stand of columnar jointing a colonade or an entablature?
The answers to these questions should not be included in your log, nor should your log include spoiler photos. Please send the answers to me via e-mail. You may choose to use the new message centre if you wish.
Optional (but not required): please take a photograph of your GPS at or near the Earthcache and post this in your log. This is not required, but would be appreciated.