Welcome to the Port Macquarie Coastal Geotrail. Here you will find rare and amazing rocks that tell the story of plate tectonics - how the earth’s crust is formed and how it moves. On this Geotrail you can travel the equivalent of 6000 km across the Pacific Ocean of today in just 4 km from Shelly Beach to Rocky Beach! You’ll see rocks made by volcanoes, by microscopic marine animals, and by underwater gravity currents. Some of these rocks formed at high pressures and temperatures over 100 km below the earth’s surface.
We start the Geotrail by examining the theory of plate tectonics, the most important underlying concept in geology. A technical resource3MB pdf(PDF, 3MB) is also available looking into what geology is and how the Geotrail was formed - a great resource for school research.
STOP 1 Volcanoes under water
STOP 2 Death at the bottom of the Ocean
At Stop 2 in the middle of Shelly Beach, we see rocks called chert part way along their plate tectonic journey. After the ocean crust basalt seen at Stop 1 was formed, it travelled away from the mid ocean ridge and began to collect sediment on top. This sediment was mainly formed from the bodies of tiny marine organisms that live near the surface of the ocean. After death, their skeletons rain down on the ocean floor. In the deeper parts of the ocean, (over 4 km), most skeletons dissolved in the acidic water, except for ones made from silica (SiO2) such as radiolarians. They collected in layers above the basalt, each layer formed as changes in productivity and climate occur in the ocean. These layers formed from variations in the type and amount of marine animal skeletons and the amount of external sediment added. When thick layers of sediment build up, pressure and chemical changes convert the fossil skeletons into the rock we call chert. As the chert is transported towards the trench, it often slumps and slides deforming into sinuous folds. Fossils known as conodonts (jaw structures of eel-like animals that lived in the ocean) can be used to date the chert, which at Port Macquarie is around 460 million years old. To see the chert, walk left around 100 m along Shelly Beach to the NE.
STOP 3 Sediments from land end up in the Ocean
STOP 4 Going down the subduction zone
STOP 5 Rare rocks from deep within the earth
At Flynn’s Beach, we are nearing the end of the Geotrail. The rocks here are descending into the earth down a subduction zone and affecting the mantle rocks around them. The original mantle material surrounding the basalt is transformed into a rock called serpentinite that has a soft greasy feel, and is often bright green. This colour comes from the original dark mantle minerals transforming into new greenish minerals such as lizardite and antigorite by heat and water produced during subduction. The serpentinite was formed at depths of 1-60 km in the earth where pressures are low, but temperatures are relatively high between 350-600°C. Due to stress in the earth or shearing, the serpentinite has a strong aligned texture called cleavage. It also contains unsheared, large dark fragments of the mantle suspended in it, making it look like green fruit cake on cliff faces. To see the serpentinite, walk right around 200 m to the SE to the southern headland of Flynn’s Beach.
To make your walk safe and enjoyable:
- Remember to take plenty of water and sun protection (sunscreen and hats)
- Do not leave valuables in the car
- Keep to the track, beware of traffic when close to the road and keep away from cliff edges
- Please take rubbish with your or use bins provided
This page was last updated on: 11 December 2017
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