A Visit to a Remote Hot Spot on Planet Earth- Reunion Island.
Talk about remote! La Reunion is just about the anti-pode of Flagstaff, Arizona. I visited there November 1 to 3, 2005 on a Private Jet Trip called "Jewels of the Indian Ocean". Rising 10,000 feet out of the sea, the volcano is an additional 11,000 feet below sea level. After taking a spectacular helicopter ride over the active part of the island, we landed in a "cirque" near the little town of Cilous. These cirques are spectacular features and the small group who hiked with me heard about my initial musings on their formation.
I first pointed out to our small group to notice the volcanic layering that could be seen in the walls of rock around Cilous. It is always better to begin geology talks with something that everyone can see because eventually geologists must talk about things that can only be envisioned from the past. The layering of rocks leads to the story of how the island grew by successive eruptions of lava. I then mentioned that Reunion is a mountain that is over 6000 meters high if you include the underwater portions of the volcano that are not visible to us.
The next thing to discuss was how the volcano behaved through time. Beneath the vent, a large magma chamber exists and this is where the lava originates. As eruptions to the surface proceed, the magma chamber becomes emptied and a void is created within the earth directly below the volcano summit. This can cause the top of the volcano to collapse creating a caldera. I then talked about the importance of caldera forming events from many well-known volcanos around the world.
However, the cirques on Reunion have a particular shape to them that suggests that they are not just simply calderas. It looked to me as if groundwater processes were very active in shaping the cirques. It was important to say "shaping" the cirques rather than "creating" them because they may have originated as smaller collapse calderas, that were subsequently enlarged and reshaped by the later groundwater processes. These groundwater processes may include a process known as sapping, whereby water running out from between the layers of lava, undercuts overlying layers. These overhung layers then collapse and the collapsing proceeds up to the top. This semi-circular shape to the cirques looks a lot like sapping has been at work. I refered back to the present climate of Reunion. we heard that this was the place where in March, 1952, a single 24 hour rain storm left 73 inches of rain! With its abundant rainfall, there is lots of groundwater that could run out of the ground starting the sapping process. It's nice to include other topics in geology talks, like climate, weather, and people.
Seeing Weird LImestone Features in Rural Madagascar - Tsingy
I will give you a few words geologically about the tsingy in Madagascar. In other parts of the world this type of erosion is known as karst. That is a region in the old country of Yugoslavia that also has a lot of limestone terrain and the name of that region gives the name to the feature. Karst or tsingy can only form in areas where the limestone is very pure without interbeds of shale or sandstone. The purity of the limestone will allow it to erode this way - if there was shale or sandstone within the limestone, the pillars would not form. Limestone is soluble in rain water, which means that it gets eaten away chemically by water. When rain falls on this limestone, the water reacts with the limestone and creates a weak acid called carbonic acid. This acid water sits in subtle depressions on the rock surface and eats away at it. This causes the depression to become deeper through time. In Madagascar, this vertical deepening is extreme and if you look at the tsingy closely, you'll notice that the pillars of limestone are just those areas in between vertical depressions. I noticed on our boat beach that some of the tsingy pillars had holes in their bases - that would be were the acidic waters ate through connecting two depressions.
The mushroom shaped tsingy on a boat trip we took were formed on their top surfaces the same way as above but the skinny base was formed along the tide line in a different manner. The salt water from the sea played an important role in forming these pedestals. When salt water creeps into the limestone at water line, it grows salt crystals inside the rock. As these salt crystals grow, they physically pry apart the rock components, eroding them much faster in this horizontal zone where the salt water seeps in. That is how the mushroom shaped rocks formed. A trip to Madagascar is amazing!