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Blowholes can occur in areas where there are crevices, such as lava tubes, in rock along the coast. These areas are often located along fault lines. 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.
These blowholes were formed as sea caves grow landwards and upwards into vertical shafts and expose themselves towards the surface, which can result in blasts of water from the top of the blowhole if the geometry of the cave and blowhole and state of the weather are appropriate.
Located 75 kilometres north of Carnarvon, all by bitumen road, this natural phenomenon is an awe inspiring sight. Powerful ocean swells force water through sea caves and up out of narrow holes in the rocks, jets of water errupt into the air, sometimes up to a height of 20 metres, creating a spectacular sight.
The rocky shelf looks rough and sharp. Looking at the geological map of the area (called Quobba) it says the rock is calcarenite which is the equivalent of sandsone but composed of limestone fragments such as shells and corals. A section of uplifted Pleistocene and Tertiary rocks begins just south of Point Quobba and stretches to north West Cape at the tip of Exmouth Gulf. The Birdrong Formation forms a homocline dipping gently to the west-northwest. This is interrupted by regional anticlines in the areas between Giralia and Quobba.
Quobba with it's shallow intermittent reef, high exposure, beach rock and dunes above high tide level makes it suseptible to sediment from the Gascoyne River to the south, exposed beaches, low frontal dune vegetation cover and some mobile dunes.
Between Point Quobba and North West Cape, water level, waves and currents interact with outcrops of coastal limestone and coral reef to modify the inshore processes, including sediment transport and water circulation patterns which can cause sea caves.
Subtidal terraces occur as well along the shore between Point Quobba and Gnaraloo Bay where low bluffs, beachrock ramps and inshore pavements are prevalent. The susceptibility of mixed sandy and rocky coast to changing metocean conditions is dependent on the geometry of the rocky topography and the location and availability of unconsolidated sediment in relation to it.
Blowholes have the capacity to change the topography near their locations. The Blowholes salient is an unstable local feature, which can potentially retreat or migrate in response to extreme events, sea level rise and the change in inshore wave climate associated with sea level rise. These changes in environmental forces at the Blowholes could also cause a reduction in beach width and potentially erode the foredunes. Blowholes can eventually erode the area surrounding the crevices to form larger sea caves. In some instances, the cave itself may collapse. This event may create shallow pools along the coast.
The Blowholes site is at the northern extent of a primary compartment and sediment cell. The beach, foredunes, foredune plains and dunes are influenced by the rate of sediment supply from the south. If the supply was reduced, whether by reduced supply from the strandplain or increased loss to blowouts, there would be narrowing of the beach and potential foredune erosion. If the supply was increased, by mobilisation of sediment from the strandplain under higher sea levels, this could lead to accretion, foredune growth, increase in beach width and increased wind-blown sediment transport through the settlement.
Q1. How were these Blowholes formed?
Q2. What type of rock has been eroded to form the sea caves?
Q3. Jets of water may erupt into the air up to what height? What environmental factors will effect the height of the eruption?
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