Geomorphology II: Weathering and Denudation, the Geomorphic Cycle of Davis and Penck, Slope Development and Geomorphic Hazards

William Morris Davis set out the geographical cycle in 1899. Landscape, he wrote, is a function of structure, process and stage (often summarised as "stage is time"). His model begins with a rapid uplift of a land mass, followed by a long stillstand during which rivers cut down and slopes are worn back. The landscape passes through youth (deep, narrow V-shaped valleys, broad undissected interfluves, rapids and waterfalls), maturity (maximum relief, valleys widened, interfluves reduced to ridges, a fully integrated drainage net) and old age (broad floodplains, meanders, gentle slopes), ending in a low, gently undulating peneplain above which only a few residual hills, monadnocks, stand. Renewed uplift rejuvenates the cycle, leaving knick points, incised meanders and terraces. Davis later adapted the scheme to arid (1905) and glacial landscapes.

Walther Penck's Die morphologische Analyse, published posthumously in 1924, rejected the separation of uplift and erosion. For Penck, uplift and denudation go on together, and landforms express the ratio between the rate of uplift and the rate of degradation. When uplift accelerates and outpaces erosion (aufsteigende Entwicklung, waxing development), rivers incise faster and faster and slopes become convex. When the two rates are equal (gleichförmige Entwicklung, uniform development), slopes are straight at a constant angle. When uplift slows and erosion gains (absteigende Entwicklung, waning development), slopes become concave. Penck's low starting surface of slow uplift is the Primärrumpf, his final low surface the Endrumpf, and he read stepped surfaces around rising domes as piedmont benchlands (Piedmonttreppen).

Beyond Davis and Penck, three further positions recur. G.K. Gilbert (1909) explained the convexity of hilltops by soil creep: the amount of creeping soil increases downslope from the crest, so the slope must steepen to carry it. Alan Wood (1942) identified four slope elements — the waxing slope (convex crest), the free face (bare rock scarp), the debris slope (talus at the angle of rest) and the waning slope (concave foot). Lester C. King (1953) built on this a model for semi-arid and savanna landscapes: scarps retreat parallel to themselves, leaving at their foot an expanding, gently concave pediment; pediments coalesce into a pediplain above which isolated steep-sided inselbergs survive. King regarded pediplanation, not peneplanation, as the world's normal cycle. J.T. Hack (1960), reviving Gilbert, argued for dynamic equilibrium: when slopes and channels are adjusted to the rate of uplift and the resistance of the rock, form stays constant through time, so landforms are time-independent.

Slope processes are of two families. Mass movement includes creep, solifluction, slides and falls; slope wash includes rainsplash, unconcentrated sheetwash and rilling. Where weathering is fast and removal slow, slopes are transport-limited and covered by a thick soil, usually convex; where removal outpaces weathering, they are weathering-limited, with bare rock and straight or steep profiles. The nine-unit landsurface model of Dalrymple, Blong and Conacher (1968) — interfluve, seepage slope, convex creep slope, fall face, transportational midslope, colluvial footslope, alluvial toeslope, channel wall and channel bed — ties each unit of a slope to its dominant process.

A hazard is a potentially damaging natural event; it becomes a disaster when it meets vulnerable people and assets. The usual expression is risk = hazard × vulnerability, reduced by coping capacity. The four geomorphic hazards the document names occur where the landforms that produce them occur. Earthquakes cluster on plate boundaries — the Himalayan arc, the Andaman–Sumatra subduction zone — and in intraplate zones of old faults, such as Latur (1993) and Bhuj (2001); filling a large reservoir can also trigger them (reservoir-induced seismicity, as at Koyna in 1967). Volcanic hazards depend on magma: silica-rich, viscous, gas-charged magma at subduction zones erupts explosively, with pyroclastic flows, ash falls and lahars (volcanic mudflows — the one from Nevado del Ruiz buried Armero in 1985), while basaltic magma at hotspots and ridges flows quietly.

Landslides need a slope and a trigger. The preparatory causes are steep slopes, weak or fractured rock dipping out of the slope, deep weathering, the removal of vegetation and the undercutting of slopes by rivers or road construction; the triggers are intense or prolonged rainfall, which raises pore-water pressure and cuts the soil's shear strength, and earthquakes. In India the Himalaya, the Western Ghats and the north-eastern hills are the landslide-prone zones — Malin in the Western Ghats (2014) and the Kedarnath debris flows of June 2013 are instances. Avalanches are rapid flows of snow. Loose-snow avalanches start at a point and fan out; slab avalanches, the most dangerous, occur when a cohesive slab breaks away over a buried weak layer. They are commonest on slopes of about 30° to 45°, after heavy fresh snowfall, strong wind loading or rapid warming.