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Located a short walk from the South Coast Road on the Samoan island of Savai'i is a site of worldwide geological rarity - the Mu Pagoa Waterfall. Its rarity stems from its location, in that it cascades directly into the sea. Various sources put the total number of such waterfalls in the world at around 10, so this waterfall being as accessible as it is, is indeed a special place to visit.
GEOLOGY OF SAMOAN ISLANDS
A possible model for the formation of the volcanic Samoa island chain is explained by the Samoa hotspot situated at the east end of the Samoa Islands. In theory, the Samoa hotspot is a result of the Pacific Tectonic Plate moving over a 'fixed' deep and narrow mantle plume spewing up through the Earth's crust.
The Samoa islands generally lie in a straight line, east to west, in the same direction the Plate is moving. In the classic hotspot model, primarily based on studies of the Hawaii hotspot, the volcanic islands and seamounts further away from the Samoa hotspot should be progressively older. However, Savai'i, the most western of the Samoa island chain, and Ta'u Island, the most eastern of the Samoa islands, both erupted in the last century, data which is an enigma for scientists.
Another discrepancy in the data from the Samoa islands is that subaerial rock samples from Savai'i, the most western of the islands, were too young by several million years to fit the classic hotspot model of age progression in an island chain, raising arguments among scientists that the Samoa islands does not have a plume origin. The nearness of the island chain to the Tonga Trench at the south became a possible explanation for these discrepancies as well as the possibility that the islands were formed by magma seeping through cracks in stressed fracture zones.
In 2005, an international team gathered further submarine samples from the deep flanks and rifts of Savai'i. Tests on these later samples showed much older ages, about five million years old, that fit the hotspot model. (Wikipedia)
The most recent significant volcanic activity in the Samoan islands was the eruption of the Matavanu crater, which produced lava that flowed down to the north east coast of Savai'i, extending into the sea and covering three villages, from 1905 to 1911.
GEOLOGICAL SETTING OF THE WATERFALL
The islands of Samoa are volcanic in origin, and virtually all of the bedrock on the islands is volcanic in nature, particularly lava flows from eruptions. This is evident in many areas of the islands, particularly along the coast lines where the rock is exposed and attacked by the forces of wind and sea.
The Mu Pagoa Waterfall is located at a section of the coast where the lava forming the coast line ends in a short vertical cliff into the sea. The river has slightly eroded the surface of the lava, forming a number of shallow channels where the water cascades over the edge. The falls, however, dramatically change in size between times of the year when there is more or less flow of water down the river.
THIS EARTHCACHE
To log a find on this earthcache you must visit the Mu Pagoa Waterfall and perform a site investigation to answer the questions below, the answers to which must be sent to the cache owner before logging the cache. Note that most of the land in Samoa is held under customary ownership and as such you may be charged a small fee by the locals in order to cross land to access the waterfall. We were charged 2 tala each (less than $1US each).
1. Describe the rock and give four characteristics (colour, texture, formation/structure and surface features in the surrounding area) that identify it as originating as a lava flow.
2. Explain your theory as to how the waterfall propogates upstream.
3. Typically, waterfalls drop into a plunge pool that is eroded by the force of the falling water. Describe in detail what is at the base of this waterfall and explain why you think it is the way it is. (Is it different to other waterfalls and if so, why?)
4. Estimate the height of the waterfall (in metres).
5. (Optional) Please also take a photo (including your GPS if possible) to document the water flow over the falls at the time of your visit. Do not give away answers to the questions above, particularly for question 3. This photo should be upploaded with your log online on the geocaching website.
Hint: Questions 2 and 3 may or may not be related.
You can log this cache straight away after you have emailed your answers to the cache owner, no need to wait for confirmation. Please include the name of this earthcache in the email - you'd be surprised how many people forget. Also, when contacting us with answers, if you want a reply, please include your email address. Any problems with your answers we'll be in touch.
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