The Shaping of Shark Bay
Shark Bay is one of the most dynamic landscapes in Australia. Its peninsulas, prongs, islands, sea cliffs, bays, claypans, dunes and beaches are the result of winds, waves, sea level changes and even earthquakes. Shark Bay’s seascape is no less complex, with seagrass meadows creating channels, banks and areas of super-saline seawater. This ongoing and interlinked geological process is of World Heritage significance.
Landscape: shaped by ice ages and floods
The shaping of Shark Bay began in the last two million years (the Quaternary period). This period has been characterised by cold (glacial) phases and two warm (interglacial) phases. It was during the most recent glacial phase, or ‘ice age’, that much of Shark Bay’s surface geology was formed.
About 125,000 years ago, much of the Earth’s water was locked up in ice sheets. The sea level was much lower than it is today, and the Shark Bay coastline was further west. Then about 10,000–8,000 years ago the Earth entered its most recent interglacial period.
Melting ice caused sea level rises which flooded the depressions on either side of Peron Peninsula, and filled the valleys between Edel Land’s giant, wind-shaped sand dunes.
- Dirk Hartog, Bernier and Dorre Islands became isolated from the mainland.
- Bernier and Dorre Islands separated from each other 3,000–6,000 years ago.
- Shark Bay’s coastline is more than 1,500 km long – greater than the distance between Brisbane and Melbourne, or London and Rome!
10 000 Years Ago
Shark Bay as it appeared some 10,000 years ago. The coastline was up to 100km further west than it is today. Orange areas are land, white is the approximate coastline at the time and blue is ocean.
About 8,000 years ago rising sea levels flooded the region, creating Shark Bay's characteristic double-basin shape we see today.
Seascape: shaped by plants and animals
Shark Bay’s modern-day seascape owes much to seagrass. The plants thrive in the shallow, sun-warmed waters of the flooded bays, forming huge underwater meadows. Apart from providing an abundance of food and shelter for a myriad of marine life they also trap and bind other sediments moving on tides and currents.
- Over the past 5,000 years, these accumulated sediments have transformed Shark Bay’s double basin structure into a maze of channels, sills and banks.
- One such bank is the Fauré Sill, near Fauré Island southeast of Monkey Mia. This sill has turned Hamelin Pool into a partially landlocked basin separated from the rest of Shark Bay.
Shark Bay’s warm and windy climate means evaporation rates are high. This, combined with the seagrass banks, has created an unusually high salt content of the seawater.
Faure Sill is a shallow sand bank formed over thousands of years by seagrass trapping sediment and other organic material.
Super salty water
The seawater in much of Shark Bay is highly saline. Different parts of Shark Bay have distinct salt concentrations. Most of Shark Bay’s waters are metahaline, or up to 1.5 times as salty as the open ocean. Water in L’haridon Bight and Hamelin Pool are hypersaline, or twice as salty as the open ocean. Shark Bay has one of the few hypersaline marine environments in the world.
The hypersaline waters of Hamelin Pool are critical for the survival of stromatolites. These rock-like structures are built by microbes. Few predators and competitors can survive the super-salty conditions, allowing the microbes to flourish and form stromatolites much as they did billions of years ago. Other salt-tolerant life forms also flourish in Hamelin Pool and L’haridon Bight, such as the Hamelin cockle ( Fragum erugatum). Further inland, compacted shell deposits have cemented together to form a rock called coquina. Thus modern-day animals have contributed to Shark Bay’s geology much as their ancestors did, thousands of years ago.
These diagrams show how Shark Bay’s seagrass banks correspond with the different salinity gradients. The salt concentrations in turn affect marine life. The influence of seagrass on the geology, chemistry and biology of Shark Bay is a World Heritage value.
For a printable fact sheet on Shark Bay's hypersaline waters click here!