Mount Kilimanjaro, Africa’s highest peak, is a prominent freestanding mountain. Rising from the plains of northern Tanzania, it is recognized globally for its snow-capped summit. Its towering form results from distinct geological processes over millions of years, setting it apart from typical mountain ranges.
Geological Context
Kilimanjaro’s formation links to the East African Rift Valley, a vast geological feature across eastern Africa. This rift system represents a divergent plate boundary where the African Plate is gradually splitting into two smaller plates: the Nubian and Somali Plates. This ongoing separation, occurring at a rate of a few millimeters per year, creates immense tension within the Earth’s crust.
As the crust thins and stretches, it develops deep fractures and weaknesses. These fissures provide pathways for molten rock, known as magma, to rise from deep within the Earth’s mantle towards the surface. The East African Rift Valley is characterized by significant volcanic activity, with numerous volcanoes dotting its length. The rifting process creates conditions for volcanic eruptions by providing conduits for magma. Kilimanjaro emerged from this setting as a singular, massive volcanic structure from magma upwelling through crustal weaknesses.
The Volcanic Process
Mount Kilimanjaro was built through successive volcanic eruptions beginning approximately 2.5 million years ago. Magma, generated from the partial melting of the mantle due to the rifting process, ascended through the Earth’s crust. As this molten material reached the surface, it erupted as lava flows, ash, and other volcanic debris.
Kilimanjaro is classified as a stratovolcano, also known as a composite volcano. This volcano type has a conical shape, formed by alternating layers of hardened lava, ash, and tephra (fragments of rock and volcanic glass). The lava from stratovolcanoes is viscous, meaning it is thick and sticky. This high viscosity prevents the lava from flowing far, causing it to solidify relatively close to the vent and build up steep sides.
Hundreds of these eruptions, both effusive (lava flows) and explosive (ash and debris), gradually deposited material layer upon layer. This continuous accretion of volcanic material led to Kilimanjaro’s immense size.
Formation of the Three Peaks
Kilimanjaro is not a single volcano but an aggregation of three distinct volcanic cones: Shira, Mawenzi, and Kibo. Each cone represents a different phase in the mountain’s long geological evolution.
Shira is the oldest, with activity commencing around 2.5 million years ago and its last significant phase about 1.9 million years ago. This ancient cone eventually collapsed, forming a large caldera that is now visible as the Shira Plateau.
Following Shira’s activity, Mawenzi began to form, estimated to be around 1 million years ago. Located east of Kibo, Mawenzi has rugged, deeply eroded peaks, a testament to prolonged weathering and glacial carving.
Kibo, the youngest and highest of the cones, started forming around 460,000 years ago, slightly west of Mawenzi. Continuous eruptions from Kibo built it to its current height, making it the dominant and tallest peak.
A Dormant Stratovolcano
Today, Mount Kilimanjaro is classified as a dormant stratovolcano. This classification means that while it is not currently erupting, it retains the potential for future activity, distinguishing it from an extinct volcano. The last major eruption of Kibo, the highest peak, occurred approximately 360,000 years ago, though some sources suggest activity within the last 10,000 years.
Despite its dormant status, evidence of underlying geothermal energy is present. Fumaroles, vents emitting steam and gases like sulfur, can be observed within Kibo’s crater, particularly around the Ash Pit. The Ash Pit, within the Reusch Crater at Kibo’s summit, is a deep, inverted cone formed during past volcanic activity. The presence of glaciers, particularly on Kibo’s summit, is a visible characteristic, though their extent has diminished. These features serve as reminders of Kilimanjaro’s volcanic origins and ongoing geological story.