SHARK ISLAND
Shark Island is not really an island, but a dangerous rocky reef break about 100 metres off the coast of Cronulla, in Sydney New South Wales, Australia. It comprises a rock ledge or islet, that is a semi-dry lava and sedimentary rock shelf that falls quickly into a deepwater channel on the land side and slopes more or less evenly off into even deeper water on the ocean side. Shark Island is usually fully exposed at low tide. Welcome to one of the world's most feared wave breaks.
The island is a well-known bodyboarding and surfing location. The waves can stand up fast and violently, making it difficult and dangerous. The island is regarded by bodyboarders as producing one of the "heaviest" waves in the world.
It is also known for the Shark Island Challenge Bodyboarding Contest that is held there, as well as the annual Shark Island Swim Challenge held at Cronulla Beach.
Shark Island is also popular with divers and snorkellers who are brave enough to swim out to the reef when the conditions are favourable.
WHAT IS A REEF?
A reef is a bar or ridge of made of rocks, sand, coral or artificial materials lying beneath the surface of water. Reefs may be up to 80m below the surface. Reefs are often quite near to the surface.
While the best-known reefs are the coral reefs (such as the Great Barrier Reef in Queensland) of tropical waters developed through biotic processes dominated by corals and calcareous algae, there are also reefs that result from geologic processes – the deposition of sand to form rocks over millennia, then wave erosion and other natural processes working on it, acting like a plane wearing down rock outcrops.
Reefs such as the coral reefs of tropical waters are formed by biotic (living) processes, dominated by corals and coralline algae. Corals and calcareous algae grow on top of one another and form a three-dimensional framework that is modified in various ways by other organisms and inorganic processes.
Artificial reefs, such as shipwrecks and other man-made underwater structures, may occur intentionally or as the result of an accident. These are sometimes designed to increase the physical complexity of featureless sand bottoms to attract a more diverse range of organisms.
WHAT IS A ROCKY REEF?
A rocky reef, such as Shark Island, are submerged rock outcrops with varying relief, creating refuges for juvenile and smaller fish in addition to surface area for colonisation of algae and invertebrates. Rocky reefs take a variety of forms, each with a different associated biological community. Starting from the shore, rocky intertidal zones are an interface between land and sea. The rocky intertidal is home to plants, invertebrates, and fish during high tides. Crashing waves, daily low tides that strand marine organisms out of the water, and competition for space make life in the rocky intertidal stressful. Nearshore rocky reefs, such as Shark Island, are completely submerged, but still receive enough light for photosynthesis. They are inhabited by algae, invertebrates, and groundfish.
Rocky reefs in deeper water do not receive enough light for photosynthesis and are dominated by sessile invertebrates, deep sea corals, and groundfish. Most rocky reefs are beneficial because of the physical structure they provide to support an ecosystem. Seamounts are particularly unique habitats that are formed by undersea mountains. The steep slopes of the mount force nutrient rich deep waters to rise to the surface, generating food for a variety of fish and other marine fauna.
Ancient reefs buried within stratigraphic sections are of considerable interest to geologists because they provide paleo-environmental information about the location in Earth’s history. In addition, reef structures within a sequence of sedimentary rocks provide a discontinuity which may serve as a trap or conduit for fossil fuels or mineralizing fluids to form petroleum or ore deposits.
WHAT IS LIFE LIKE ON A ROCKY REEF?
Rocky reefs are fascinating habitats that are rich in life. many marine animals and plants need to attach themselves to something solid for their survival. In southern Australia many of the rocky reefs are covered in brightly coloured animals and plants.
Reefs are found where rocks occur above the waterline. As the rocks are eroded by the action of water, cracks and holes appear which increase the availability of shelter for living things. Different types of rock weather in different ways and may have different species.
Rocky reefs that are exposed at low tide are great places to explore as there are many animals and plants that can survive out of water for some time. The presence of rockpools, crevices, and boulders increases the diversity of life because of the shelter provided.
Below the low tide mark, rocky reefs can support extensive communities of marine plants which can form kelp forests. Reefs that are hidden from sunlight in deeper water, or under ledges, often have brightly coloured communities of invertebrates, many of which rival coral reefs for colour and diversity.
Rocky reefs are also important because they provide habitat for many species of fish, and commercially valuable marine animals such as the rock lobster and abalone.
HOW DOES A ROCKY REEF FORM?
Rocky reefs are formed by erosion. The relentless force of water hitting the tock gradually wears it down, creating the maze of cracks and holes that are so attractive to marine wildlife.
Different rock types weather in different ways. Softer rocks, such as limestone or chalk, can hold boring species that drill their own holes into the rock face. As the rocks are eroded by the action of water, cracks and holes appear which increase the availability of shelter for living things. Like most of the rock in the Sydney Basin, Shark Island is made of Hawkesbury Sandstone. Over time the softer rock between Shark Island and the foreshore of Cronulla eroded away to leave Shark Island exposed, and sitting alone 100m off the shoreline. The ongoing force of the ocean waves continues to wear away the surrounding rock.
The constant disturbance by physical factors like wave action, as well as the activity of large, grazing predators like sea urchins, is what makes rocky reefs such dynamic environments. Waves and grazing clear bare patches of rock, triggering a race for new creatures to colonise the valuable free space.
A couple of points to note:
1. Bicycles are not permitted to be ridden along this pathway. But you can ride up to laneway in Roker St nearby.
2. This is not recommended to be done at night. You won't see much of Shark Island.
You may log your visit immediately to track your geocaching progress. However, to complete your EarthCache experience, please send your answers to the logging tasks via email or GC Messenger within 7 to 10 days. Failure to do so may result in your log getting deleted.
Go to the posted coordinates to make your observations. Complete the following tasks.
Logging tasks:
1. What type of reef is Shark Island?
2. Briefly state how Shark Island formed?
3. Estimate the height (in metres) of any waves that may be breaking over Shark Island.
4. Upload a photo of yourself or something with your geocaching name on it, with Shark Island in the background. See the example below. This will serve as proof of your visit to the site.
Reources:
https://www.wildlifetrusts.org/habitats/marine/rocky-reefs
https://www.fisheries.noaa.gov/west-coast/habitat-conservation/rocky-reef-west-coast
https://en.wikipedia.org/wiki/Shark_Island_(Cronulla_Beach)
CONGRATULATIONS: FTF goes to MaxI125. Well done!
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Did you know, New South Wales has a geocaching association?
Geocaching NSW aims to enhance and improve the activity of geocaching and holds regular events where geocachers meet to enjoy their common interests.
Visit the association website
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