Speleothems inside of Avaiki Cave
This EarthCache shows you the spectacular Avaiki cave which is located 400m north of Makefu village.
This cave was the landing place of the first canoe bringing Polynesian settlers to Niue.
The cave of Avaiki in Niue is filled with a vast array of limestone formations - great clusters of stalactites and stalagmites, huge pillar formations where they have joined and giant stone curtains draped from the ceiling
What is speleothem?
In geology, any secondary mineral deposit in caves is called a speleothem or cave mineral; which can also arise in other moist cavities such as mines and tunnels. The most common speleothems are cave sinter or stalactites.
Speleothem is a generic name given to the large variety of cave features created by calcite precipitation. Speleothems form when percolating water that is highly charged with dissolved calcium carbonate finds its way into an airfilled chamber.
When entering an open space, some of the carbon dioxide in the water is released by degassing.
The decrease in CO2 content reduces the water’s ability to hold all its solute material, and calcium carbonate is precipitated out of solution
What are stalactites and stalagmites?
The words stalactite and stalagmite can be traced back to the Greek word "stalassein“, which means "to drip."
This is fitting because it describes how they are formed in nature. The slow drip drip drip of ground water leaving behind deposits. Although they look lifelike and a little creepy, stalactites and stalagmites grow simply because of water running over and through inorganic material leave behind trace deposits which build up over time.
How were the stalactites and stalagmites in Avaiki Cave formed?
Avaiki Cave is a limestone cave. Stalactites and stalagmites inside of this are features formed by precipitation on the roof and floor of caves by dripping water.
Limestone caves, where most stalactites and stalagmites are found, are mainly composed of calcite, a common mineral found in sedimentary rocks. Calcite molecules are made of calcium and carbonate ions, and are referred to as CaCO3 (calcium carbonate).
When rainwater flows down into a cave and trickles through rocks, it picks up carbon dioxide and minerals from limestone.
If we add water, carbon dioxide and calcium carbonate together, we get the constituent parts needed to leave deposits: This is exactly what’s happening here as the rain water drips down into this cave.
During my visit here at Avaiki Cave (outside the rainy season) I counted the number of drips per minute. It was exactly 7 drips/min. During rainy season it drips a lot more. The logical consequence: the more droplets drip down the stalactites, the faster they are built up
How can you distinguish stalactites and stalagmites?
If the water which drops tot he floor oft he cave still hast some dissolved calcite in it, it can deposit more dissolved calcite there, forming a stalagmite. A stalagmite may form directly below a stalagtite as water drips from the ceiling of Avaiki Cave on to the floor.
These may join up over time to form pillars.
Expressed in simple words: a stalactite grow downwards and the stalagmite grow upwards (until they eventually meet). But both of them – stalactites and stalacmites – grow extremly slow. The rate of growth is related to the concentration of the mineral and the amount of water that flows, but may only average just a tenth of a millimetre per year.
What is the geological peculiarity of this place?
Here inside this cave, stalactites and stalagmites are so abundant that groups resembling organ pipes are common. Many have pure internal crystal structures and emit resonant notes are if they are gently tapped. There are also fairly abundant helictites, literally ‘spiral’ stalactites which actually grow in all directions.
Passing the cave on the way down to the sea you will see lots of broken stalactites litter cave floors in many places as a result of vandalism, although some may also have been broken from their original position as the floor of a cave subsides through the roof of a younger cave below.If water flows down the side of a cave wall or along the floor then calcite precipitation gives smooth flowstones.
Source references:
www.wondermondo.com/wonders-of-niue/
www.researchgate.net/publication/266478080_Geomorphic_features_of_Niue_Island_chasm_caves_and_other_karst_varieties#pf14
General information: a visit to the Avaiki Caves is free of any costs and possible at any time. If you’re planning an afternoon visit, I recommend taking a flashlight with you and don’t walk alone (even if it is safe to enter the cave). You will see a parking area close to the road.
Hopefully you have enough information to answer the questions below.
Tasks:
1. Please describe the rock that forms the cave ceiling. How are the speleothems formed here?
2. During you visit, do you see drops of water dripping from the cave ceiling? How many drops per minute can you count? Please compare the current rate to it and see if it's dripping faster or slower than average at the time I visited (7 drips/min). Do you conclude that the stalagtites are growing slowly or faster?
3. Estimate how long the longest stalactites are and approximately how old would you estimate them to be? Please justify your answer. (Yes, it's a little math).
4. Draw or paint or sketch some of the stalactites you see here and attach your drawing to your log entry.
5. A photo picture of your team or your GPS is mandatory.
You are welcome to log your answers straight away but please message or Email me with your answers. I'll let you know if anything is unclear. Please don't forget: the answers must be received within 10-14 days or the log may be deleted.
Enjoy your stay.