Blow Holes are fairly common features along the Australian Coast, ranging from the massively spectacular to the smaller but interesting locations like this one on King Island. Blowholes are likely to occur in areas where there are crevices, faults, lines of weakness or similar structural formations in the rock formations along the coast. The development of Blow Holes is therefore related to the susceptibility of the coastal rock to erosion. Littoral(sea)caves can be formed in limestone (karst) landscapes. Limestone is a sedimentary rock with a high concentration of Calcium Carbonate and is formed mainly by the deposition of decomposed sea creatures and corals. The block structure of the limestone creates vertical and horizontal lines of weakness that can be easily eroded by sea water and wave action. These caves can also be formed in igneous rock where intrusions such as fault lines, dykes and sills are eroded causing tubular erosion in the otherwise very hard and durable rock. In time, the littoral cave (in either rock type) enlarges, growing inland and vertically through weak joints in the parent material. As weathering continues the roof of the cave is exposed, and the blowhole continues to enlarge. Eventually the roof of the littoral cave is weakened and collapses. As powerful waves hit the coast, water rushes into these crevices and bursts out in a high-pressured release. It is often accompanied by a loud noise and wide spray, and for this reason, blowholes are often sites of tourism. A blowhole system always contains three main features: a catchment entrance, a compression cavern and an expelling port. The arrangement, angle and size of these three features determine the force of the air to water ratio that is ejected from the port. The blowhole feature tends to occur in the rear section of a littoral cave. As their name suggests, blowholes have the ability to move air rapidly. Strong reverse draughts in response to pressure changes in a connecting littoral cave can send wind speeds upwards of 70 km/h. The smaller King Island blowhole is found in a small section of a particular form of rock which is found in several locations on the Island but creates unique features in an otherwise “normal’ coastal region. As it is a small example you will experience a special moment if you are there when the hole is actually blowing.
Blow hole formation – Source ABC Science
Please send your answers to the following question to the CO and enjoy your short walk to this interesting location.
At the Posted Coordinates (WP1)
Q1 Observe the rocks you are standing on and decide if they are limestone or igneous rocks explaining why you have come to that decision.
Q2. Face north and draw or describe the features of the blow hole structure.
At WP2
Q3 (a) What is the angle of the layering of the beds and in which direction do they run?
Q3 (b) What evidence is there that these rocks are the ones that you nominated in Q1.
Q4. Given what you have now discovered describe how these particular blow holes were formed.
Q5. What has caused the jagged features show in a number of places on the top of the rocks?
It would be great to see a photo of the area, particular if you are one of the lucky ones to see the holes blowing.