Sea Shorts: The birth of an Isthmus

Welcome to Bruny Island's Neck!

This long, sandy stretch sure is beautiful. It's known as an isthmus (which can be pronounced with or without the 'th') - which basically means a narrow piece of land in the water which connects two pieces of land together. In Bruny Islands case, The Neck (as it is known) connects the northern and southern parts of Bruny Island together. It stretches for a fair way - around 2 kilometres, but at some places reaches just 100 meters in width. But just how was this isthmus created?

 Bruny Island is located off the coast of Tasmania, Australia. Credit Samantha Andrews/Ocean Oculus

Bruny Island is located off the coast of Tasmania, Australia. Credit Samantha Andrews/Ocean Oculus

Bruny Island is located just off the coast of Tasmania, Australia. Here, Bruny Island is treated to the famous Roaring Forties - strong winds that come from the west between 40 and 50 degrees latitude. These ferocious winds are particularly strong during the winter and, like all winds, can generate waves. In the summer months, the prevailing wind direction changes. Gentler summer breezes come from the east, also generating waves. Both sets of waves carry with them sand, stones, and pebbles. Over time - a long, long time - these sediments from the two different directions are pushed together and build up to eventually form a special kind of isthmus called a tombolo - which basically means its made of sand.

But that's not the whole story. Why would the sand and stones and pebbles end up pushed together where they do? For the answer, we need to go back many thousands of years to the end of the last ice age.

During this time, sea levels began to rise - a lot. Tasmania used to be connected to Australia but 50 meters of sea level rise later, Tasmania - and the people, other animals, and plants that lived there found themselves cut off from 'the mainland'. Other parts of Tasmania were also swallowed by the sea. Outcrops and islands were formed. New underwater mounds and banks were born from previously dry land. Whilst this was going on, the waves brought the sand, stones, and pebbles from the west and from the east. It was the newly created shallow sea floor that allowed the sand, stones, and pebbles to collect where they did.

Not all isthmus are created like this. The Panama Canal, for example, is an isthmus that seems to have been created by the movement of the Earth's plates millions of years ago.