Some of the most spectacular blowholes in the South Pacific can be found along the terraced coastline of south west Tongatapu. These blowholes ('Mapu'a 'a Vaca' means 'Chief's Whistles') stretch for 5km (3mi) along the south shore of Tongatapu.
The western islands of Tonga are formed from coral reefs. Waves wash against the coral, the pressure from it wears a hole, causing the water to shoot up. They are best viewed on a windy day at high (but not too high) tide, when the maximum amount of water is forced up through natural vents in the coral limestone, forming geyser-like fountains of seawater.
Here you'll find not one blowhole, but hundreds of them along this three mile stretch of wild windswept coastline.
If you do not have access to a car (or taxi) , they are accessible a km walk past the inland village of Houma. Buses to Houma from Nuku'alofa take about 30 minutes.
It is reputed that one of the ancient Chiefs of Tonga spent his hours meditating over the blowholes and listening to the whistling of the waves - hence their name - 'Mapu'a 'a Vaca' meaning 'Chief's Whistles'.
There is very little history about these blowholes and most of the Tongan history has never been documented. There is a basic stub about the blowholes on the Wiki at http://en.wikipedia.org/wiki/Mapu_a_Vaea
More Details
In geology, a blowhole is formed as sea caves grow landwards and upwards into vertical shafts and expose themselves towards the surface, which can result in quite spectacular blasts of water from the top of the blowhole if the geometry of the cave and blowhole and state of the weather are appropriate.
Geology
Geologists believe that blowholes where created by volcanic flow, shaping a cavity which was then eroded by sea water. Another theory claims that a blowhole is a stage in the erosion of cavaties originally created by freshwater once above sea level. Sea waves, according to this theory, entering a cavity will splash upwards on the cavity walls, and together with the strong air currents entering and leaving the cavity will erode the rock and create a blowhole in stages. First an elongated cavity will be created, then upward shafts are added to the cavity. When the erosion creates a cavity in the upper end of the existing cavity the blowhole phenomenon is received. Finally a crack is created releasing more of the air at lower pressure, loosing the blowhole effect, until the cavity roof is completely eroded and opened first into an arch and then into a stack.
Physics
The basic physics which are thought to explain the blowhole phenomenon are a large cavity with an opening under water and a 'spout' - a small opening to the air at the end of a shaft. When entering the cavity, water compresses the air, causing a secondary wave which leaves through the 'spout' followed by a burst of water and air. When leaving the cavity, the water is delayed by pressure at sea on the one hand and low air pressure in the cavity on the other, because the spout cannot let air in at the speed of the outbound water. The air filled from the 'spout' creates another secondary wave, this time outwards bound, which then pushes the water out in an action similar to the spurts of water when emptying a bottle, after air had been permitted in..
No known research has ever been done modeling or excavating blowholes.
In order to log this cache, you need to
a) Have a look at the ground you are standing on and note the colour and texture of the ground. Can you explain what this ground is made of and how old you think it is?
b) Note the height above sea level from where you are standing and the height above sea level of the blowholes. Can you explain why there is such a vast difference in heights and what you think created this difference?
c) There have been some cases of people jumping onto blowholes with the thought of waiting for the water to come through the vent and push them high into the air, but in reality this doesn't happen. It's been shown it's not safe to do this. In fact it's extremely dangerous. Can you explain why ?
d) Optional : Take a photo of you with the blowholes in the background.
e) Enjoy the scenery.
Please do not post your answers with your log. Email the answers to us first and once you have approval you can log your find.