BLOG # 2 - Twelve Apostles

The southern coast of Australia is layers of limestone sedimentary rock which you can see as the lighter and darker layers in the first image below.  Sedimentary rocks contain joints at the different layers because each layer has different levels of hardness, allowing weathering forces to take advantage of the weaknesses of the rock.  These joints are responsible for the development of the Twelve Apostles along the Great Ocean Road in southern Australia.

Photo Credit: Janet Julia Kennedy

Ten to twenty-five million years ago, the coastline of Australia was where the stacks exist in the middle of the ocean, and has since eroded from the relentless power of the Southern Ocean waves crashing into the limestone. The erosion is a form of chemical weathering called dissolution which the rock is  completely dissolved by water. The distance between the stacks and the mainland is how far the ocean has eroded the coastline of Australia. With each subsequent wave, the softer rock is broken down and washed away into the Southern Ocean.   Over millions of years, the waves eroded the base enough to create caves, which you can see in the image below.

Photo Credit: Janet Julia Kennedy, photo of informational sign at Port Campbell National Park: tourist viewing area of the Twelve Apostles

Photo Credit: Dr. Casey Allen's February 16, 2012 class lecture, edited for accuracy

As the caves grow, the denser parts of the limestone are left behind creating arches between the pillars/stacks and the mainland. As the arches thin, they will ultimately collapse into the Ocean. When the arches collapse they leave behind only stacks of rock that are, as of today, about 450 feet tall.  

As discussed in the video, new stacks are constantly being formed because wave erosion is creating arches from the existing stacks.  You can see the waves breaking through one of the stacks about one minute into the video.  Eventually the arch in the picture below will collapse, creating two stacks out of one.  Over the next couple million years, all of the current stacks will collapse, but new ones will form out of the mainland.

Photo Credit: Janet Julia Kennedy

The Twelve Apostles have been an incredible tourist attraction for Victoria, Australia, making up a substantial portion of the $800 million tourist revenue that the Great Ocean Road provides annually.

http://articles.cnn.com/2005-07-04/world/australia.apostles_1_australian-state-stacks-sea?_s=PM:WORLD

http://www.vnc.qld.edu.au/enviro/landform/landf-su.htm