THE FORMATION OF THE ISLAND
Lake Victoria, being 400,000 years old, is relatively young on a geological scale. Its formation is due to the East Africa rift system (EARS) which is a roughly north-south alignment of rift basins in East Africa and defines the boundary between the Somalian and African tectonic plates.
Despite its youthful age, through studies of the Lake floor it has been possible to determine that it has completely dried up three times during its existence, demonstrating its fragility. The lake last dried out about 17,300 years ago, refilling 14,700 years ago. These episodes of desiccation were due to periods of global glaciation, where a majority of the earths freshwater was stored within vast glaciers in the northern and southern hemispheres with little global precipitation. Today 85% of the water into the lake and 15% of the water outflow is down to rainfall and evaporation instead of great rivers.
Bulago Islands geology consists of the Buganda-Toro (or Rwenzori Fold Belt) group of geological units, dating to 2500-1600 ma. They are Palaeoproterozoic in age and formed during a time when the land masses of earth formed the supercontinent “Rhodinia”.
These “young” units rest upon the north western extent of the Tanzanian Craton, a 3 billion year of part of stable ancient continental lithosphere. The rocks of this age are generally metamorphosed granitic complexes, with one example (not in situ) found just outside of One Minute South.
The first thing of note when you arrive on the island is a hard, nodular and red layer that seems to “cap” the island, which is Secondary Laterite (Ironstone).
The origin of ironstones also is not well understood, but most appear to be derived from the erosion and re-deposition of lateritic (iron-rich) strata. For these original laterites to form a parent rock they have to be rich in iron, such as volcanic rocks associated with the rift valley system. A possible representative, as found on the island, could be metamorphosed basalt. Its green colouration is due to the low-grade (low temp-low pressure) metamorphism that forms green minerals such as chlorite, epidote and actinolite.
This basaltic parent rock is then placed under physiochemical conditions, such as a well-drained terrain, abundant moisture for hydrolysis, high oxidation potential and persistence of these conditions for thousands of years. Eventually all elements vulnerable to these conditions, like silica, are weathered out leaving an iron-rich bi-product.
The “Hard-Cap” of this rock found all over Bulago Island has been reworked and re-cemented through water based transportation. Evidence of this are clasts of other rocks, generally rounded by water transportation, found within the mass.
These Ironstone beds sit uncomfortably on older sandstone beds which contain white veins of quartz.
The islands elevation in regards to the rest of the Lake Basin, in the authors opinion, comes down to the influence of granitic intrusions that form the cores for the hills on the island. On the summit of the hill with the pagoda, outcropping on the floor, are graphic textured granites abundant in quartz and feldspar.
- Thanks to Tom Pascoe; geologist and sweet piece of dude
