Rodrigues - Geology and Geomorphology

Geology and Geomorphology

source: Saddul, 2002 "Mauritius, A Geomorphological Analysis Publisher: Mahatma Gandhi Institute)

Formation of Rodrigues Island (Approximately 3 Million Years Ago)

Rodrigues Island began its formation on the remnants of an ancient guyot's crater at the eastern end of the Rodrigues Ridge. The island's construction kicked off with the accumulation of basalt flows from fissures around Mont Limon. These flows, which were 2 to 10 meters thick and consisted of plagioclase, olivine, and pyroxene, formed a shield approximately 10 km in diameter and 250 meters high.   

Hydrographic Network Development (Approximately 2 Million Years Ago)

After the initial volcanic activity, a calm period ensued for about a million years, allowing for the establishment of Rodrigues Island’s hydrographic network. This period was crucial for the formation of the island's natural drainage and valley systems.

Resumption of Volcanic Activity (1.5 to 1.3 Million Years Ago)

Volcanic activity resumed with the eruption of prismatic basalt flows, rich in gabbro nodules and measuring 5 to 30 meters thick. Preceded by bedded breccias on the western flank of the central crater, these flows exhibited Hawaiian characteristics. They filled the newly formed valleys and contributed to the formation of the island's elliptical plateau measuring 18 x 8 km.

Period of Eruptive Calm and Alteration

Following the prismatic flows, the island underwent significant geological changes during another period of eruptive calm. The volcanic landscape was deeply altered, setting the stage for further unique geological formations.

Phreatomagmatic Eruption 

A violent, localized phreatomagmatic eruption later created a vast explosion crater (or maar) near Mount Malartic, measuring 4 km in diameter. This crater was encircled by a ring of layered tuffs, rich in ash, bombs, and accidental blocks over 20 meters high. The degassed Hawaiian lavas from this eruption formed a lava lake within the crater, which, upon cooling, was exposed through erosion and hydrothermal activities, resulting in a geological plug.

Last Volcanic Episode  

The final phase of volcanic activity on Rodrigues Island involved an explosive phase characterized by ash and scoria, followed by an effusive phase. During this phase, doleritic Hawaiian basalts emerged from concentric fissures around the tuff ring, extensively covering the island. These lavas formed peaks like Mont Limon and Mont Malartic and filled the ancient hydrographic network. However, extensive erosion over time has left only fragments of these formations visible today.

Throughout its geological history, Rodrigues Island has prominently featured tholeiitic basalts, schistose and aligned, still visible to the east of the island, marking the enduring legacy of its volcanic origins.