Recently surfers and photographers that frequent the famous big wave surf slab at Shipstern Bluff in Tasmania noticed that the cliff looked a little different. Below one photographer compared shots from 2015 and 17 Jan 2017 which clearly showed that a massive chunk of the cliff has recently collapsed.
Whether the collapse and any future deterioration will impact the famous break or access to it is still to be determined. However when considering the natural process of coastal erosion this collapse should not come as a surprise.
Coastal erosion is an ongoing process that has shaped and will continue to shape and carve out new coastlines until the planet ceases to exist. There are four main methods in which coastal erosion can occur; hydraulic action, corrasion/abrasion, attrition and corrosion.
When a wave breaks on a cliff face, the air between the water and cliff is put under immense pressure. This pressure pushes the air into small cracks and imperfections in the cliffs face slowly widening them overtime. These cracks will get to a size at which they destabilise the surrounding rock causing it to collapse. This method of erosion is known as hydraulic action.
Corrasion/Abrasion occur when there is sand, shingle and other materials present in the wave when it breaks on the cliff. These materials have a sandpaper like effect on the cliff face over time sanding it back.
When rocks, sand and other materials rub and knock together in the water, they are smoothed, rounded and broken down into smaller and smaller particles over time. This can reduce the stability of a rock slab.
The final method of erosion is corrosion. When carbon dioxide from the atmosphere comes into contact with the oceans surface, minute amounts of carbon dioxide are absorbed into the water. If enough is absorbed it will slightly increase the acidity of the water, the more absorbed the more acidic. Several minerals (such as Limestone) are vulnerable to this acidity and will slowly corrode when they come into contact with the water.
The portions of a cliff that erode faster will depend on the type and frequency of the beds within the cliff. Beds are the differing layers of rock types within a rock or cliff that have built up due to numerous conditions over the life of that piece of rock. Some cliffs will have few differing layers with rocks of similar densities and hardness whilst others will have numerous layers made of a large array of materials differing in density and hardness.
As Shipstern Bluff is constantly bombarded by massive storms and swells it is clear that it is a prime candidate for erosion and reshaping. This can be seen clearly in the layer bedding (different colours = different layers of rock) in the cliff. Before the collapse you can see right in the middle there is a band of lighter rock that is more eroded and cut in than the surrounding beds. As well as this you can see the very base of the cliff is also heavily eroded which is to be expected as this takes the brunt of the oceans power.
Now the final factor that impacts upon a section of coastline and how erosion will affect it over time is the lithology of the rocks that make up the coastline. Lithology is the description of a rocks contents and the properties it possessws. For the sake of simplicity, rocks will be described here as hard or soft. Hard rocks are solid and can hold up to the beating of the ocean longer while soft rocks fall apart much easier.
If you took a birds eye view of the rock make up of a coastline you would typically see two types of layouts, concordant and discordant. Concordant coastlines (pictured above) have layers of hard and soft rock that run parallel to the coast creating a hard barrier such as long sections of cliffs. However once a small section of hard rock is broken and the soft rock is exposed, it can eventuate in the creation of coves. McWay Cove (pictured below) and Lulworth Cove are good examples of this.
In contrast to concordant coastlines you have discordant coastlines (pictured below). These coastlines have rock formations that run perpendicular to the ocean.
This allows the waves easy access to the soft rock which is quickly eroded away creating headlands and bays (which is always good, who doesn't love a good point break) such as Pacifica (title picture), Bells Beach & Raoul Bay which is where Shipstern Bluff is located.
This formation is what allows surfers to take on the monstrous swells that hit the Bluff multiple times a year. The deeper water on the bay side of the break allows jetskis and surfers access to the wave without having to tackle it head on. It is unlikely that the recent collapse will effect this, however, now that a new section has been exposed it will be interesting to see where and when the next collapse will occur and what impact that will have on the break.
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