Exercises: External geological processes

Differentiate between physical weathering and chemical weathering, and give a specific example of each.

The diagram summarizes the sequence of external processes, from a rock outcropping in the mountain to a new sedimentary rock. Name the process corresponding to each numbered arrow (1–5), choosing from: diagenesis, erosion, weathering, deposition and transport.

What is pedogenesis and which factors are involved in soil formation?

What is erosion and how does it differ from weathering? Explain why both processes usually act together on the same outcrop.

Name the four agents of geological transport (gravity, water, wind, and ice) and one characteristic type of deposit for each.

What is sedimentation and in what type of environments does it occur? Name two continental and two marine sedimentary environments.

What is biological weathering? Describe two mechanisms, such as root pressure or the acids from organic matter, with an example of each.

Order these processes in the formation chain of a sedimentary rock: cementation, erosion, deposition, weathering, transport, and compaction.

Freeze-thaw weathering is very effective in high mountains and thermal fracturing in deserts with large temperature ranges. Explain the physical mechanism of each and why each climate favors it.

A river sharply decreases its velocity upon entering a plain. Explain, using the concept of transport capacity, why it deposits pebbles first, then sand, and finally silt and clay.

The oxidation of iron-bearing minerals gives reddish tones to many soils. Describe this chemical weathering process and name another characteristic chemical process that acts on limestone rocks.

In jointed granites, alteration advances along the fractures and gradually rounds the blocks until boulders form (spheroidal weathering). Explain why edges and corners are altered before flat faces.

What is solifluction and under what conditions of cold climate and saturated soil does it occur? What appearance do its deposits take on hillsides?

A river transports its load in three ways: dissolved, in suspension and as bed load. Explain what each one consists of and how the proportion between them changes during a flood.

Hydrolysis alters the feldspars of granites and transforms them into clay minerals. Describe this chemical weathering process and explain why granites disintegrate into coarse sands (grus) in humid climates.

Compare the intensity of chemical weathering in a humid tropical climate and in a cold arid climate. Which factor weighs more in each case, temperature or water availability, and why?

A development team proposes building a groin perpendicular to the coast and artificially regenerating a sandy beach at the foot of a flysch cliff on an exposed stretch of the Cantabrian coast. In their report, they claim that once the sand is dumped, sedimentary dynamics will create a stable, accumulative system similar to the wide beaches and sand spits of the Gulf of Cádiz.

1. a) Compare the oceanographic conditions (wave energy, tidal regime, and longshore drift) and morphological features of both coastlines to reasonably refute the feasibility of this claim.
2. b) Predict the evolution of the artificial beach's equilibrium profile after the first winter storms, and interpret what will happen on both sides of the projected groin due to longshore drift dynamics.
3. c) Justify why the lithology of flysch promotes cliff retreat through basal undercutting, and explain the fate of the detached materials.

In a borehole drilled at the bottom of a U-shaped Pyrenean trough valley, two superimposed sedimentary levels were extracted:

• Level A (upper, 8 m): chaotic, unstratified deposit formed by an abundant clayey-silty matrix enclosing highly heterometric clasts (from gravels to angular boulders), some of them with polished facets and parallel striations.

• Level B (lower, 12 m): well-rounded, imbricated gravels with cross-stratified sand intercalations, a very low proportion of clay matrix, and good grain-size sorting.

1. a) Identify the geological agent and the physical transport and deposition mechanism responsible for each level, justifying your deduction based on the observed textural and sedimentological differences.
2. b) Reconstruct the glacial evolution (advance or retreat) that explains why level A is stratigraphically situated above level B.
3. c) If a dam is planned to be founded in this valley, predict two geotechnical or hydrogeological issues that level A will present compared to level B regarding permeability and load stability.

Along a 350 km Iberian river basin that originates in a granitic massif with quartz and quartzite veins and empties into an open marine shelf, bed sediment samples are analyzed at three points:

• Section 1 (10 km from the headwaters): angular blocks and gravels of fresh granite, quartz, and potassium feldspar; low sphericity and poor sorting.

• Section 2 (at 180 km): rounded pebbles dominated by quartz and quartzite, coarse sands with feldspars partially altered to kaolinite and degraded micas.

• Section 3 (coastal alluvial plain, at 340 km): medium to fine sands composed of 96% well-rounded quartz, with an almost complete absence of feldspars and micas.

1. a) Justify the changes observed in the size, roundness, sphericity, and mineralogical composition of the sediments along the course, relating them to mechanical and chemical weathering mechanisms.
2. b) If in section 3 a layer with 15 cm angular pebbles of fresh granite chaotically mixed with the fine sands were suddenly detected, formulate two grounded geomorphological hypotheses to explain this anomaly.
3. c) Predict which types of detrital sedimentary rocks will form from the sediments of section 1 and section 3 after diagenetic processes, classifying them according to their textural and compositional maturity.

The Ebro Delta experiences relative sinking due to natural subsidence and sediment compaction of between 1.5 and 3.0 mm/year, to which is added an average sea level rise of about 3.5 mm/year. Before the 1960s, the river transported around 25-30 million tons of sediment per year to the delta; currently, due to the Mequinenza and Ribarroja reservoirs, the suspended and bed load reaching the mouth is less than 0.15 million tons/year.

1. a) Interpret these data in terms of coastal sediment budget and predict which deltaic coastal morphologies (sand spits such as El Fangar or El Trabucador, barrier bars, and coastal lagoons) will be most affected in the short and medium term.
2. b) The construction of a frontal concrete sea wall along the entire deltaic coastline has been proposed to halt the advance of the sea. Critically evaluate this proposal, predicting the consequences it would have on longshore drift transport and on the sediment deficit of adjacent beaches and bays.
3. c) Design and compare two management alternatives based on sedimentological principles (for example, sediment flushing or bypassing trapped behind dams versus mechanical sand nourishment), assessing their long-term geological sustainability.

In the lower reach of a river emptying into the peninsular Atlantic coast, two prominent geomorphological features are recognized:

1. Four levels of hanging and stepped river terraces located at relative elevations of +55 m ($T_1$), +35 m ($T_2$), +18 m ($T_3$), and +5 m ($T_4$) above the present riverbed.

2. In the terminal estuary, geotechnical boreholes reveal a river paleochannel deeply incised into the bedrock down to -40 m below present sea level, backfilled at its base by fluvial gravels and covered by a thick sequence of estuarine muds and fossilized beach sands.

1. a) Explain the formation of the stepped terraces by integrating base-level changes associated with Quaternary glacial and interglacial cycles with the alternation between linear incision and alluvial sedimentation phases.
2. b) Interpret the origin of the paleovalley incised to -40 m and its subsequent estuarine sedimentary fill, relating them chronologically to the Last Glacial Maximum (around 20 ka ago) and the subsequent Holocene marine transgression.
3. c) Reasonably predict how the river's equilibrium profile will respond if a local drop in base level occurs, caused by the coseismic uplift of a fault crossing the channel in its middle reach, indicating which erosional or depositional landforms will be generated upstream and downstream of the fault.