Exercises: Landform shaping

Define and order the four stages of the cycle of action of an external agent: erosion, weathering, deposition, and transport.

State four external agents of landform shaping and name a characteristic landform for each, indicating whether it is erosional or depositional.

What is karst topography, on what type of rock does it develop, and what chemical process explains it? Give an example of a surface karst feature and a subterranean one.

Distinguish between erosion, transport and sedimentation using the journey of a rock particle from a mountain slope to the bottom of a lake, indicating which agent intervenes at each stage.

Explain what freeze-thaw shattering is, what climatic conditions it needs (alternating frosts and thaws) and in which areas of Spain it is most intense today (high mountains: Picos de Europa, Sierra Nevada, the Pyrenees).

What is a meander? Explain how it grows and shifts laterally (erosion on the concave bank and deposition on the convex one) and what form remains when the river cuts it off and abandons it.

Distinguish between a cliff coast and a beach coast: which lithology and which marine dynamics favour each one, and name a Spanish stretch of each type.

Explain what regolith or weathering cover is, how it forms by in situ weathering, and what relationship it has with the subsequent formation of soil.

Relate each agent to the landform it produces and briefly explain the mechanism: glacier (U-shaped valley), river (meander), wind (dune), and waves (cliff).

Explain step by step how a delta forms at the mouth of a river and indicate what condition must be met for the delta to advance rather than be eroded by the sea.

Differentiate between physical and chemical weathering, describing two processes of each type (for example freeze-thaw weathering, thermal shattering, dissolution, or oxidation) and the climate in which each is most effective.

The granite boulder fields of La Pedriza show stacked, rounded blocks. Explain the spheroidal weathering of granite (alteration guided by the joints, peeling off in layers) and why it produces those "ball" and tor shapes.

A river can leave old floodplains perched at different heights: river terraces. Explain what change causes their formation (incision due to a fall in base level or uplift of the land) and what information they provide about the recent history of the relief.

Explain how the wind shapes the landscape: what deflation is, how sand is transported (saltation) and how a barchan dune forms and advances, indicating what role vegetation plays in fixing it.

Describe the formation of a glacial cirque and of a U-shaped glacial valley, and name two valleys in Spanish mountains where these forms can be seen today even though the ice no longer exists (the Pineta valley, the Ordesa valley).

What is the base level of a river? Explain what happens to the channel when sea level falls (incision) or rises (filling of the mouth) and apply it to the history of the Ebro delta during sea-level changes.

A technical team proposes reforesting the clay slopes of a cabezo (butte) in the Bardenas Reales with fast-growing pine trees to "definitively halt" the advance of gullies and stabilise the upper sandstone cornice.

1. a) Based on the mechanical and physicochemical properties of clays (plasticity, relative impermeability, shrinkage cracking during dry periods), justify why planting trees with dense canopies and deep roots may fail or even induce mass movements during torrential rain events.
2. b) Design a chronological sequence predicting how the relief of the butte will evolve (erosion at the soft base versus the resistant cornice) if torrential precipitation events concentrated into a few hours increase.
3. c) Argue what real role the sparse native steppe vegetation and biological soil crusts play against the direct impact of raindrops (splash effect) and sheet wash.

Profiles A and B in the diagram correspond to cross-sections of two valleys in a mountain massif that hosted Quaternary glaciers.

1. a) A civil engineering project claims that the slopes of profile B are safer against rockfalls than those of profile A because it features a flat trough floor. Technically refute this idea by analysing how glacial overdeepening and postglacial stress release (decompression) affect the mechanical stability of vertical rock walls.
2. b) If a river with a high discharge establishes itself at the bottom of valley B after the complete retreat of the ice, predict and conceptually draw how its cross-profile will transform over tens of thousands of years.
3. c) Deduce what geomorphological and bathymetric differences would exist between both valleys if a marine transgression floods their mouths: which one will give rise to a ria and which one to a fjord? Justify the typical presence of a shallow bedrock threshold (lip) at the mouth of one of them.

Two sectors of the Spanish coast are compared under the climate change scenarios projected for the end of the century:

• Sector A: Low sandy coast with a beach-lagoon-dune system in the Mediterranean, where the river basins that fed it have experienced a 75% reduction in their sediment supply due to dams.

• Sector B: Cliffed coast in Cantabrian flysch with a wide shore platform (abrasion platform) emerging at low tide.

1. a) Apply the concept of sediment budget and predict how the equilibrium profile of the beach in Sector A will respond to the combination of eustatic sea-level rise and an increase in the intensity of sea storms. Will the dune ridge migrate inland or will it collapse?
2. b) For Sector B, analyse whether the retreat of the cliff will remain constant or accelerate with the rise in mean sea level. Justify your deduction based on the water depth over the shore platform and its buffering effect on the energy of basal wave action.
3. c) A municipality in Sector A plans a groin perpendicular to the shoreline to retain sand. Evaluate the geomorphological consequences that this structure will cause on the beaches located downdrift according to the prevailing longshore drift dynamics.

In a sedimentary basin of the Iberian interior, a broad plateau (páramo) lies at an altitude of 880 m, capped by a 12 m thick horizontal layer of limestone resting on 180 m of clays and gypsum. In the vicinity, several limestone-capped buttes (cerros testigo) and small conical hills (antecerros) devoid of limestone are observed.

1. a) A student states: "The basal clays are eroded away by the river at the valley bottom and the upper limestone collapses like a clean vertical knife cut". Correct this statement by detailing the real interaction of processes on the slope (sheet wash, physical weathering of the clays, basal undercutting, and gravitational collapse of cornice blocks).
2. b) Justify why a butte (cerro testigo) maintains practically the same maximum elevation as the plateau (páramo) as long as it retains its limestone caprock, but undergoes a rapid reduction in height as soon as the caprock is completely dismantled.
3. c) If the stratigraphic series were not horizontal, but instead exhibited a constant dip of 10°, would the same tabular morphology form? Predict what asymmetric relief would develop and in which direction the scarp front would migrate over time.

In a karst massif, calcium carbonate dissolution rates four times higher are recorded under a deciduous forest with a thick humic horizon than in an adjacent area of completely bare limestone exposed to the same annual precipitation.

1. a) Justify this difference based on the chemical equilibrium $\text{CaCO}_3 + \text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{Ca}^{2+} + 2\,\text{HCO}_3^-$, explaining the origin of soil $\text{CO}_2$ and predicting how the equilibrium shifts when water percolates into a ventilated underground cavern.
2. b) The route of a motorway detects a straight alignment of sinkholes (dolines) on the surface. Interpret what underlying geological discontinuity this arrangement reveals and predict what immediate geological hazard massive groundwater pumping from the karst aquifer to dewater the foundations would pose.
3. c) A soluble toxic chemical is accidentally spilled at the bottom of a karst pothole (chasm). Justify why the retention, attenuation, and filtration models typical of detrital aquifers (sands or gravels) are not valid for predicting the dispersion of this contaminant within the karst system.

A torrential basin of 35 km² in southeastern Spain features a dry ephemeral stream bed (rambla) for most of the year (discharge of 0,02 m³/s for 350 days). However, during a DANA event (isolated low at high levels / cut-off low), a hydrograph with a peak discharge of 280 m³/s sustained for 4 hours was recorded.

1. a) Drawing on the concepts of discharge, stream capacity, and competence, demonstrate why that single 4-hour event accomplished more erosion and sediment transport work than several decades of ordinary discharge.
2. b) A property developer claims that, since the stream bed has not had continuous water for more than 20 years, it is safe to build on the alluvial fan by channelling the rambla through a narrow concrete culvert. Refute this proposal by geomorphologically justifying the behaviour of flash floods and clogging by bedload accumulation.
3. c) State which stratigraphic and sedimentological criteria (such as grain sorting, presence of imbricated boulders, or ephemeral terraces) would allow a geologist to determine the recurrence of catastrophic floods in the absence of historical meteorological records.

On a slope with a 16 % gradient over a marly substrate, soil loss is measured in three experimental plots subjected to the same rainfall regime (550 mm annually concentrated in autumn storms):

• Plot A (Mediterranean scrubland with kermes oak and esparto grass): 0,3 t/(ha·year).

• Plot B (traditional olive grove on a slope with bare soil maintained by herbicides and continuous tillage): 41,2 t/(ha·year), with visible presence of rills and incipient gullies.

• Plot C (olive grove with strips of cover crop sown between rows and shredded pruning residues on the ground): 2,1 t/(ha·year).

1. a) Interpret the data by explaining the physical mechanisms that differentiate Plot B from the other two: raindrop kinetic energy, splash effect, soil surface sealing, and the formation of concentrated runoff.
2. b) Design a management proposal for Plot B that integrates a structural/topographical measure (such as terraces or contour ridges) and a biological one. Justify which factor of the erosive process each one neutralizes.
3. c) Make a reasoned prediction of the evolution of the soil profile of Plot B over a 25-year horizon if no intervention is made (horizon thickness, water retention capacity, and fertility) and deduce its impact on the hydrological regime of the receiving basin downstream.

The Monte Perdido glacier (Pyrenees) has lost more than 85 % of its volume in recent decades, and climate models predict its complete functional disappearance in the short term.

1. a) During the active deglaciation phase and the immediately subsequent paraglacial period, predict which geomorphological processes and slope hazards will be triggered in the headwater basin (consider the debuttressing/decompression of rock slopes, the degradation of high-mountain permafrost, and the remobilization of unconsolidated moraines by summer storms).
2. b) Mentally visualize and compare the annual hydrograph of the river downstream under the historical snow-and-ice regime versus the future purely nivo-pluvial regime after the extinction of the ice. In which season of the year will the greatest negative impact on streamflows be recorded and why?
3. c) A hydroelectric report claims that the total melting of the ice will release more annual water resources to run through the valley's dams. Refute this claim by distinguishing between the transient "depletion of storage" (reserve expenditure) and the steady-state hydrological balance, also predicting how the entrainment of paraglacial sediments will affect the reservoir's useful life.