Overview
Mount Suswa is a unique, double-caldera volcano located in Kenya’s Gregory Rift Valley, part of the East African Rift System. Known for its rare concentric calderas and extensive trachytic volcanism, Suswa has played a significant role in understanding rift-related magmatism and caldera collapse processes. Its evolution spans over several stages, including at least two major caldera-forming events, and features both effusive and explosive volcanic activity.
📍 Location and Geological Setting
- Coordinates: ~1.18°S, 36.37°E
- Elevation: ~2,356 meters (7,730 ft) above sea level
- Tectonic Context: Central Kenya Rift, within the East African Rift System
- Nearby Features: Mt. Longonot, Olorgesailie, Lake Magadi, Lake Naivasha
Suswa is part of a chain of Quaternary volcanoes aligned along the axis of the Gregory Rift. The region is characterized by normal faulting, crustal thinning, and active magmatism.
🔥 Evolutionary Stages of Mt. Suswa
Geologists divide Suswa’s evolution into eight main stages (S1–S8), based on stratigraphic, structural, and compositional evidence.
Stage S1: Pre-Caldera I Volcanism – Angat Kitet Formation
- Age: >400,000 years BP (approximate)
- Key Rock Type: Undersaturated trachyte
- Structures: Lava flows, lava domes, scoria cones (e.g., Soitamrut)
- Deposits: Vesicular lava, pumice lapilli tuffs
- Entity: Angat Kitet Formation
- This stage saw the building of a broad shield-like volcanic edifice, with thick trachytic lavas erupting from fissure systems and forming multiple lava domes.
Stages S2–S5: Syn-Caldera I Activity – Formation of the First Caldera
These stages represent the complex collapse and associated eruptive phases during the formation of Caldera I, the larger and older of the two calderas.
S2 – Olgumi Formation
- Deposits:
- Globule ignimbrites: Unique, trachyte-rich pyroclastic flows with vesicular globules and columnar jointing.
- Carbonate-trachyte tuffs: Suggest water involvement.
- Trachybasaltic ash and spatter beds
- Interpretation: Likely the result of phreatomagmatic eruptions (interaction of magma with water).
- Entity: Ring Feeder Zone (RFZ) – a ring-shaped fracture system around the caldera.
S3 – Oloolwa Formation
- Deposits: Trachyte lava flows with flow folding, vapour-phase cavities, and vesicular textures
- Notes: The flows erupted from the RFZ, confirming activity during caldera subsidence.
S4 – Esinoni Formation
- Deposits: Bedded pumice lapilli tuffs, likely air-fall tephra from explosive eruptions.
- Notes: Overlies and truncates earlier formations, indicating episodic eruption and collapse.
S5 – Enkorika Formation
- Features: Agglutinate flows and localized lava structures within the RFZ.
- Interpretation: Possibly late-syn or early-post-caldera I.
Caldera I formed via incremental subsidence, not a single collapse event, resulting in the current broad depression (approx. 12 km in diameter).
Stage S6: Post-Caldera I Lavas – Entarakua Formation
- Lavas: Phonolitic
- Processes: Re-filling and overtopping of parts of the caldera rim
- Entity: Entarakua lava flows
- Marks a transition from trachytic to phonolitic magmatism.
Stage S7: Syn-Caldera II – Ol Doinyo Onyoke Cone and Collapse
- Event: Growth of a central cone (Ol Doinyo Onyoke) within Caldera I
- Collapse: Withdrawal of magma led to the formation of Caldera II (the inner caldera or ring trench)
- Deposits: Lava flows, breccias from cone collapse, and phonolitic dome structures
This stage reflects a resurgent caldera system, similar to well-known structures like Crater Lake or Santorini.
Stage S8: Post-Caldera II Volcanism – Eululu Formation
- Recent lavas from the SE ring trench and southern flanks
- Possibly within the last 1,000 years
- Composition: Phonolitic, consistent with evolved magmas in late-stage rift volcanoes
Though there is no recorded historic eruption, fumarolic activity and warm ground suggest that Mt. Suswa is still thermally active.
🧬 Key Geological Entities and Features
| Entity | Description |
|---|---|
| Caldera I | Large, outer caldera (~12 km diameter), formed by progressive collapse |
| Caldera II | Inner ring trench (resurgent collapse), ~5 km across |
| Ring Feeder Zone (RFZ) | Ring-shaped fault and vent system feeding syn-caldera eruptions |
| Angat Kitet Formation (S1) | Oldest trachytic lavas, shield-building phase |
| Olgumi Formation (S2) | Globule ignimbrites and carbonate-rich pyroclastics |
| Oloolwa, Esinoni, Enkorika (S3–S5) | Lava flows, airfall tephra, and agglutinates from collapse-related vents |
| Ol Doinyo Onyoke Cone | A central cone formed then collapsed, initiating Caldera II |
| Eululu Formation (S8) | Latest lava flows, possible recent eruptive activity |
🧪 Special Geological Features
- Globule Ignimbrites: Pyroclastic flows with unique spherical ash globules, likely formed through magma-water interaction.
- Volatile Blisters: Gas expansion features in ignimbrites, indicating post-emplacement gas release.
- Vapour-phase Crystallization: Formation of minerals like anorthoclase in cooling, gas-rich environments.
- Liquid Immiscibility: Evidence of magmatic separation into carbonate-rich and silicate-rich phases, rare in continental settings.
📊 Summary Timeline
| Stage | Formation | Main Event |
|---|---|---|
| S1 | Angat Kitet | Pre-caldera shield building |
| S2–S5 | Olgumi–Enkorika | Explosive activity and Caldera I formation |
| S6 | Entarakua | Post-collapse lava flows (phonolite) |
| S7 | Ol Doinyo Onyoke | Second cone and Caldera II collapse |
| S8 | Eululu | Most recent eruptions |
🌍 Why Suswa Matters
- It provides a rare double-caldera structure within a rift valley.
- A prime example of explosive caldera collapse in an intraplate setting.
- Offers insight into magma evolution from trachyte to phonolite.
- A site of potential ongoing volcanic hazards due to its dormant (not extinct) status.
🔚 Final Notes
Mt. Suswa’s geological story is one of complexity, transition, and violence, but also of scientific richness. Its eruptive deposits tell tales of quiet lava domes, violent explosions, water-magma battles, and collapse events that shaped the Rift landscape.
Today, Mt. Suswa remains a powerful geological landmark—an open-air textbook for geologists, and a quiet giant still capable of waking.