Geology of Inselbergs

Study Notes

Inselbergs are isolated residual hills above a level surface, primarily found on volcanic materials like granite and gneiss.
Formation involves removing weathered regolith from a surface experiencing differential weathering.
The two-stage model suggests steep hills arise from deeply weathered materials being stripped away, while continuous weathering and erosion can also occur simultaneously.
Bornhardts, a specific type of inselberg, are monolithic and typically found in African savanna granite plateaus and tropical humid regions, characterized by steep, convex slopes.
Castle kopjes are residual hills resulting from the breakdown of bornhardts and are linked to granite's pressure release forming vertical joints.
Formation theories for bornhardts include:
- Stripping Theory: Increased removal of regolith reveals untouched rock layers beneath.
- Parallel Retreat: Substantial valley side retreat leaves remnant inselbergs.
Notable bornhardts include Mt Hora in Malawi and Mt Abuja in Nigeria.

Weathering profiles can exhibit high variability, traditionally comprising three zones: residual soil, weathered rock (saprolite), and unweathered bedrock.
At least 10% of corestones remain unweathered in the weathered zone, which is permeable and contains varied mineral weathering stages.
The weathering front is often irregular, with significant depth variations from 30 to 60 meters influenced by jointing density and rock composition.

Tors are large, rounded boulders resting on solid bedrock, shaped by weathering which smooths their edges.
Surrounding tors are loose, weathered rocks and soil disassociated from the boulders through erosion.
Most tors arise in strongly joint rock, ranging in height from 20 to 35 meters, with core stones possibly measuring up to 8 meters in diameter.
Formation of tors occurs due to chemical weathering along joints and bedding planes, with joint spacing affecting core stone size: widely spaced joints create larger core stones, while closely spaced ones lead to increased weathering with smaller stones.

Cockpit Karst features hilly terrains marked by deep solution holes, resembling an ‘eggbox’ structure.
Tower Karst results from extensive erosion of limestone above the water table, leaving limestone towers amid sediment-filled cockpits.
In polygonal karst, uniform-height groups of hills can reach 160 meters in Jamaica, originating from complex solution processes.
Development of polygonal karst is linked to:
- High tectonic uplift rates
- Intense river-induced vertical erosion
Hills' spacing might correlate with historic stream networks, where concentrated solution along joints accelerates localized weathering.
Once the water table is encountered, lateral water flow leads to the creation of closed depressions and dolines.

Tower karst displays significant variability in size, with towers from meters to over 150 meters high in regions such as Sarawak and southern China.
Characterized by steep cliffs, overhangs, and caves at their base, these towers arise from complex limestone landscapes, shaped by erosive processes.

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