A blowhole or a sea cave ?
- The weathering of the rocky headland has resulted in the feature known as the Seal Rocks Blowhole, which is located on the northern side of the Sugarloaf Point headland.
- It is a visually interesting feature and example of coastal erosion around the headland.
- Different maps have named the formation either a blowhole or a sea cave.
- This earthcache will discuss the erosion at the coastline and how erosion occurs to form the landforms.
- The geology observed will help you decide!
- This area at Sugarloaf Point is part of the traditional Country of the Worimi people.
- Several Worimi elders in the community retain knowledge of mythological stories relating to the Point. One Myth relates to the Blowhole and the other to the first set of rocks off Sugarloaf Point.
- This coastline has been lashed by strong winds and many ships have been wrecked here leading to the stationing of a Lighthouse in 1875.
Coastal Erosion
- The sea shapes the coastal landscape. Coastal erosion is the wearing away and breaking up of rock along the coast.
- Destructive waves erode the coastline in a number of ways:
- Abrasion or Corrasion = when rocks and other materials carried by the sea are picked up by strong waves and thrown against the coastline causing more material to be broken off and carried away by the sea.
- Hydraulic Action = sheer force of water crashing against the coastline causing material to be dislodged and carried away by the sea.
- Attrition = when material such as rocks and stones carried by waves hit and knock against each other wearing them down. As these materials are worn down sand and rounded beach pebbles are formed.
- Solution of Corrosion = when certain types of cliff erode as a result of weak acids in the sea.
Sea Cave Formation
- The waves erode along lines of weakness in the headland by hydraulic action, abrasion and solution.
- Lines of weakness get enlarged and form small sea caves.
- The caves are deepened and widened on both sides of the headland.
- Eventually the sea cuts through the headland forming an arch.
- Continued erosion erodes the rock at the top of the arch which makes it unsupported as the arch is enlarged and eventually collapses to form a stack.
- The stack gets eroded until only a stump remains.
- Over time the stump will disappear.
Blowhole Formation
- a blowhole or marine geyser is formed as seacaves grow landwards and upwards into vertical shafts and expose themselves towards the surface, which can result in hydraulic compression of sea water that is released through a port from the top of the blowhole.
- A blowhole is actually the end product of a long geological process that can take thousands of years, if not longer!
- At first, the hydraulic action of the waves of the ocean crashing against land causes small fractures in the surface.
- The constant movement of waves at the bottom of cliffs erodes away at the area around the crack and renders it weak enough to create sea caves at the bottom.
- The movement of waves forces air into these small fissures, which are formed on the surface.
- As a result, the rock is placed under ever-increasing stress.
- During high tide, as more faults in the rock are created, more of the surface is eroded away and more air is forced into these fissures.
To log the Earthcache
1. Is the "Seal Rocks Blowhole" the correct terminology for its name? Justify your answer.
2. What is the next stage in the process of erosion that this geological formation seen at GZ will become?
3. Observing the area of erosion at GZ, what do you think were the methods of erosion that occurred here and why?
4. Take a photo of your GPSr or personal item with the lighthouse in the background.
Please send answers to CO within good timeframes (not more than 2 weeks of visiting or risk logs being deleted). 🌏