Geological formation
The south-western tip of Grande Mitsio features a remarkable geological formation: prismatic basalt columns rise from sea level to a height of around thirty metres. These greyish-brown rock structures have a regular hexagonal shape when viewed in cross-section.
The perfect vertical alignment of these prisms and their geometric regularity give the impression of a man-made structure, although they are in fact a natural phenomenon resulting from the cooling of lava. Viewing these formations from the sea allows one to appreciate their monumental scale and the architectural organisation of the basalt formation as a whole.
Formation
The columns develop perpendicular to the cooling surface. The verticality of the prisms in the Nosy Mitsio columns indicates that cooling took place from both the upper and lower surfaces of the lava flow. The transverse fractures that sometimes divide the columns into segments result from secondary stresses that arose during the solidification process.
The erosive action of the Indian Ocean has gradually exposed these geological structures, revealing their spectacular architecture. These formations are among the most remarkable volcanic geological sites in Madagascar, on a par with the basalt columns of Usson or Jaujac in France, or those of the Giant’s Causeway in Northern Ireland and the island of Staffa in Scotland.
Phenomenon
This phenomenon results from a physical process linked to the gradual cooling of basaltic lava. When the lava flow solidifies uniformly and evenly, the rock undergoes thermal contraction. This reduction in volume generates internal stresses that cause the material to fracture into prismatic columns.
Perfect vertical alignment of the columns
The hexagonal geometry of the prisms is explained by the optimal distribution of mechanical stresses within the cooling mass. The hexagon is the geometric configuration that minimises the energy required for cracks to form, whilst allowing the space to be completely filled without any gaps. This natural principle of optimisation is also found in other natural phenomena, such as the structure of honeycombs or desiccation cracks in clay.
The basalt columns of Grande Mitsio can only truly be appreciated from the sea. Nestled at the south-western tip of the island, these monumental columns are inaccessible by land, which is precisely what gives them their wild and unspoilt character. A motorboat or sailing boat allows you to sail as close as possible to the basalt cliff, to appreciate its striking verticality and to grasp the true scale of these prisms that plunge directly into the Indian Ocean. The sea approach also offers the chance to drop anchor and go snorkelling to observe the continuation of the formation beneath the surface.
To organise your trip out to sea, find all our excursions from the archipelago.