TO SUA OCEAN TRENCH
To Sua Ocean Trench is an area that contains a deep swimming hole filled with crystal clear salt water, surrounded by lush jungle hanging down the edges. To Sua is one of the most unique places to swim in on the planet, and going here is an absolute must if you’re visiting Samoa.
The almost perfectly symmetrical swimming hole surrounded by lush vegetation sits in the middle of a lava field near Lotofaga village atop a jutting edge of spectacular coastline on the south coast of Samoa's main island of Upolo.
To Sua literally means ‘Giant Swimming Hole’ in Samoan
The pool is fed by a series of canals and tunnels with water from the ocean pounding just metres away. The second thing to hit you is the strength of the tidal surge. In fact it's so strong, a rope has been suspended midway across the water hole to hold onto so you can avoid being pulled towards underwater tunnels into the pounding surf outside.
The only access into the sparkling emerald green waters of the pool is via a single ladder with a small sitting/viewing platform at its base. Yet despite its amazing depth and size, To Sua is hard to see as you until you are literally on top of it.
Caution should be exercised in this area. There are plenty of signs advising what should or should not be done.
Also, please note that there is a small entrance fee (WST$20).
What is a sinkhole?
Sinkholes can have three main origins:
1 - A dissolution sinkhole occurs where the there is only a thin layer covering soluble rock. The soluble rock erodes away forming a depression. Over time the more rock is dissolved and the depression grows.
2 - Cover-subsidence sinkholes occur where there is a thicker surface layer. A cavity forms and the surface layer erodes into the cavity leaving a hole.
3 - A cover-collapse sinkhole is perhaps the most dramatic. A cavity forms below the surface that grows, then suddenly the surface layer gives way.
What causes the cavity?
There are many reason why a cavity may form below the surface:
- Sinkholes are well known within karst formations. Karst is an area of land made up of limestone, also known as chalk or calcium carbonate. Slightly acidic water dissolves the limestone leaving a small cavity, and this cavity may grow over time.
- If the sub-surface ground is made of small particles (sand, clay), and as water flows through the ground these particles may be taken away with the flowing water. This is a common method in loosely pack sub-surface ground.
- In coastal areas the action of waves against the shore can result in a cavity, that extend inland under the surface.
- During volcanic eruptions the lava may form and flow along tubes. The lava tube may be created by the lava moving through the rock or by an above ground flow sealing over the top. Once in the tube the lava is thermally insulated and may flow quite some distance. The lava in the tube may eventually drain from the tube rather than solidify in place - leaving a long underground cavity.
- Finally, the cavity may be anthropogenic - i.e. human made. The cavity may have been created by mining, or tunnelling for drains, subways, etc. Or a sudden supply of water is introduced when a water main bursts., carrying away subsurface particles.
There may be combination of processes, e.g. a small cavity may form through dissolved rock, this allows water to flow enabling more dissolution or erosion through friction with particles in the water.
What causes the ceiling to collapse?
- When the weight of, and upon, the ceiling is greater that that which the ceiling can support then the ceiling collapses into the cavity.
- Rising sea levels places additional weight upon the ceilings of cavities under the sea bed. Well known ‘blue holes’ may form this way.
- From a human perspective, building onto the ceiling may add sufficient weight to cause it to collapse. For very shallow cavities with thin ceilings the weight of a person may be sufficient.
What about To Sua?
Samoa, including Upolu, is substantially volcanic in origin. A number of volcanic cones are evident across the country, and eruptions are recorded in recent history, most notable in the early 1900s.
One feature of volcanic islands are the formation of lava tubes, which are known to have formed in Samoa.
Following the end of the most recent ice age the sea water level has risen significantly, but so has the islands themselves. Basically the weight of all this extra water puts pressure on the undersea magma, which flows under the islands and pushes the islands up. A process termed hydroisostatic rebound. Emerged shorelines of two to three metres can be traced around Samoa, particularly along Savaii’s south coast where many wave-cut caves have been cut into basalt cliffs and are now well above wave level.
Questions
To log this geocaching think about the information above, and make observations at To Sua Ocean Trench to arrive at a plausible explanation of why To Sua is the way it is.
You will need to go down and have a swim. While doing so examine the walls, and swim under the open ‘tunnel’.
- What evidence can you observe that conforms to the rock surrounding To Sua being volcanic or karst in origin?
- What evidence can you see that suggests that To Sua once had a ceiling, that has collapsed into a cavity?
- What evidence can you see, or deduce, that might explain the formation of the cavity covered by such a ceiling?
One additional requirement, a photo of yourself (or identifier) in the hole, or above looking down with your GPS, to be included with your log.