The Old Government Quarry is located at Kingscote. To visit this earthcache you will need to follow the beach for 800m north from the boat ramp. The quarry contains Jurrassic age basalt. The locatlity correlates with several other outcrops of basalt on Kangaroo Island. The formal name Wisanger Basalt has been assigned to the stone. The naturally cracked basalt was used as ballast stone and road base in the early days of the South Australian colony. It is reported that Port Road used the stone.
The basalt in the quarry is a brown/green stone with fine grain structure. The stone exhibits jointing producing five and six sided coloumns, often tapering to a point. The jointing has produced the "natural" cracking.
During the cooling of a thick lava flow, contractional joints or fractures form in basalt. While a flow can shrink in the vertical dimension without fracturing, it can't easily accommodate shrinking in the horizontal direction unless cracks form. An extensive fracture network results in the formation of columns. Column structures are predominantly hexagonal, 6-sided, in cross-section. If the contraction is not evenly spaced, then 5-sided or 7-sided columns may occur. The size of the columns depends on the rate of cooling. Rapid cooling will result in small columns, while slow cooling will produce large columns. 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.
Examine the cliff face at the listed coordinates. There is no need to climb the cliff. Please send your answers for the following earthscience tasks to the cache owner by message or email.
- What is the average diameter of the columns? What does this tell you about the cooling rate?
- What is the average length of the columns? Why do you think this horizontal cracking has occurred?
