Ned's Beach is a popular swimming beach on Lord Howe Island, renown for the fish that have become fairly tame through coming into the shallows for feeding every day over the past 60 years or so. At either end of the beach (refer to waypoints) are prominent exposed cliffs which you will need to examine to complete this EarthCache. These areas are accessible in all normal tide conditions, please use common sense in particularly adverse weather. Don't forget to bring some $1 coins for the fish food dispenser, or bring some from one of the local shops - the fish feeding is better at a lower tide too.
Ned's Beach is also a great spot to witness the evidence of the birth and development of Lord Howe Island, and see how the island is being consumed by the sea as well.
A volcanic birth
Lord Howe Island is the highly eroded remains of a 7-million-year-old shield volcano, the product of eruptions that lasted for about 500,000 years. It is one of a chain of islands that occur on the western rim of an undersea shelf, the Lord Howe Rise, which is 3,000 km long and 300 km wide extending from New Zealand to the west of New Caledonia and consisting of continental rocks that separated from the Australian plate 60 to 80 million years ago to form a new crust in the deep Tasman Basin. The shelf is part of Zealandia, a microcontinent nearly half the size of Australia that gradually submerged after breaking away from the Gondwanan supercontinent.
Geologists believe that there were two main volcanic episodes in the formation of Lord Howe Island. Most of the volcanic activity took place some 6.9 million years ago, and probably comprised several volcanic vents, ultimately producing a large shield volcano about 30km in diameter with a maximum height above sea level of about 1,000 metres.
The oceans contribute
This volcanic activity has left the mountains of Lord Howe Island - the older, more eroded northern mountains, and the southern - younger and less eroded. In between the mountains are lower lying areas, which, due to the development of surrounding coral reefs, and fluctuating sea levels, have developed due to different geological processes.
Sea levels have fluctuated dramatically over the past million years or so – particularly as global glaciation has repeatedly advanced and retreated. On at least nine occasions in the past 250,000 years the sea level has been 50 metres to 130 metres lower than at present.
At these times of lower sea level, more extensive shallow marine environments existed on the Lord Howe shelf, allowing greater growth of marine organisms such as algae, corals, starfish, urchins, crabs and molluscs. As these many marine organisms die and crumble their calcium carbonate rich fragments are ground up by the pounding action of waves and are transferred onto the island through wind and wave action - forming beaches, dunes and lagoon sediments. Over time, rainwater seeping through the sand has cemented the grains into a type of sandstone known as calcarenite.
Carbon dating of fossils in a number of the dune deposits suggests an age of 20,000 to 40,000 years, but it is believed that the calcarenite beach deposits behind Ned’s Beach were formed in the last interglacial period around 130,000 years ago when sea levels were roughly the same as they are at present.
Image: Calcarenite at Lagoon Beach
Erosion continues
Through the relentless barrage of wave erosion, only 2.5% of the original mass of Lord Howe Island remains above sea level, and it is thought that within 200,000 years that the erosion of the island will be complete and will be completely submerged.
To claim a find on this EarthCache:
1. In your 'Found It' log - please include a photo of yourself or GPS or personal item with the Admiralty Islands in the background (the islands straight off the beach) - as a bonus line up your photo to show daylight through the large seacave on the major island - this is best done from WP2. Please note that the photo is a compulsory part of logging this EarthCache.
2. Provide answers to the questions below from the two waypoints. The answers need to be sent to the CO in a message or email, and not added to your 'Found It' log. The questions included are the same for both locations, to provide a comparison of the two different geological processes that have occurred here.
At WP1 (South): S31 31.101 E159 04.095
2A. Describe the appearance of the cliffs. Please include colours noted, and banding or patterns in layers and texture of the rock.
2B. Out of the two processes discussed in the description above - volcanic or sedimentation of coral sands, which process do you propose has resulted in these cliffs?
2C. Can you observe evidence of erosion in these cliffs? If so - what do you see?
At WP2 (North): S31 31.009 E159 03.806
2D. Describe the appearance of the cliffs. Please include colours noted, and banding or patterns in layers and texture of the rock.
2E. Out of the two processes discussed in the description above - volcanic or sedimentation of coral sands, which process do you propose has resulted in these cliffs?
2F. Can you observe evidence of erosion in these cliffs? If so - what do you see?
We will respond promptly to every submission, you can however log your find before hearing back from us, we will let you know if there are any issues with your answers.
References:
https://lhimuseum.com/learn/geology/
https://kids.kiddle.co/Lord_Howe_Island_Group