Exercises: Landform shaping

Match each agent with the landform it produces: a river, wind, a glacier, and the sea, with dunes, a river valley, a cliff, and a glacial corrie.

What is weathering? Give an example related to the action of frost on rocks (freeze-thaw weathering).

Order the stages of landscape shaping — deposition, erosion, weathering, and transport — and define each with a single sentence.

Match each landform with the agent that shapes it: a sea stack, a river canyon, a dune field, and a moraine, with the sea, a river, the wind, and a glacier.

Explain the difference between erosion and transport using the work of a river as an example.

What is deposition? Give an example of a place where a river deposits the materials it transports.

Name two coastal landforms produced by the action of the sea and briefly explain how one of them forms.

State whether each of these processes is weathering, erosion, or deposition: a rock cracks due to the action of ice; a river wears away its channel; the wind deposits sand and forms a dune.

The diagram shows a river from its source in the mountains to its mouth at the sea. State in which of the labelled zones (A, B, or C) erosion, transport, and deposition predominate, and justify your answers.

The biosphere also shapes landforms. Explain two examples: the action of tree roots on rocks and the building of reefs by corals.

In the upper course of a mountain river, erosion predominates, whereas at its mouth, deposition predominates. Relate this change to the gradient and the speed of the water.

Deltas form at the mouth of some rivers. Explain why the river deposits the materials it transports there and cite one cause that can make a delta retreat.

On a rocky coast, isolated sea stacks, rock arches, and cracks in the cliff walls can be seen. Order the stages this shaping goes through and cite the agent responsible.

Weathering can also be biological. Explain what it consists of and give two examples, one related to roots and one to lichens.

The terraces of traditional market gardens slow down erosion on hillsides. Explain how they act on runoff and what happens when they are abandoned.

A glacier drags blocks of rock along its base and deposits them as it melts, forming moraines. Explain how the same agent carries out erosion, transport, and deposition at the same time.

A construction company excavates the base of a clay slope with an inclination of 35° to widen a road. A geological report warns that, after heavy rains, there is a very high risk of mass wasting onto the road. The construction company claims that planting a superficial layer of grass will be enough to hold the ground.

1. a) Refute the construction company's claim: justify what happens to clay when it becomes saturated with water and why the shallow roots of grass will not prevent a deep-seated slope failure.
2. b) Design and describe two different structural geotechnical measures (such as retaining walls with drainage, deep anchors, or slope reduction through benching/terracing) to stabilize the slope, justifying the physical principle by which each would prevent collapse.

The following data are recorded at a river gauging station:

• Spring: discharge of $40\ \text{m}^3/\text{s}$ and water velocity of $1,8\ \text{m}/\text{s}$.

• Summer: discharge of $5\ \text{m}^3/\text{s}$ and water velocity of $0,2\ \text{m}/\text{s}$.

A student claims that 'in summer, the river completely stops transporting materials because it loses all its strength and, therefore, the riverbed remains unchanged until the following spring'.

1. a) Refute this statement using geological arguments: explain which types of transport (solution, suspension, saltation, traction/rolling) can remain active in summer and which particles (clays, silts, sands, or pebbles) will continue to move.
2. b) Predict what will happen to the coarse sediment load that the river was carrying in spring when the velocity drops to $0,2\ \text{m}/\text{s}$, and interpret in which parts of the riverbed (fast-flowing sections or slackwater/pool areas) they will tend to be deposited.

In a mountain range, a main valley is observed with a wide floor and a U-shaped cross-section, featuring polished vertical walls and striations on the rock. However, on the bedrock floor of that same valley, a present-day river has carved a narrow, deep gorge with a V-shaped profile.

1. a) Interpret the geological history of this landform: deduce which two external geological agents have acted successively over time to produce this combination of shapes.
2. b) Justify what interaction between the atmosphere (climate change), the hydrosphere, and the geosphere explains why the first shaping agent disappeared from the area and was replaced by the action of the second.

Before the construction of a dam, a river contributed $120\,000\ \text{t}/\text{year}$ of sediment to its delta. After the dam was built, the input was reduced to $30\,000\ \text{t}/\text{year}$. At the same time, coastal marine dynamics (waves and longshore currents) remove $75\,000\ \text{t}$ of sand each year from the delta's coast.

1. a) Calculate the net annual sediment balance of the deltaic coast (sediments supplied minus sediments removed) before and after the dam was built.
2. b) Based on the result, predict what will happen in the medium term to the coastline and the adjacent dune ecosystems.
3. c) The local city council proposes dredging sand from the nearby seabed and periodically dumping it onto the beach. Critically evaluate this proposal: justify why it does not solve the root problem and what unexpected negative impact it could cause on the marine seabed.

The beaches of a coast are fed by the sand carried by the longshore current. If a port is built that interrupts this current, predict what will happen to the beaches located on either side of the port and justify your answer.

Two identical $1\ \text{m}^3$ sandstone cubic blocks are left exposed to weathering for a century in two different environments: block A, atop a coastal cliff battered by direct waves and salt-laden sea breeze; block B, on a continental desert plateau with frequent winds laden with dust and sand and a daily temperature range of $30\ ^\circ\text{C}$.

1. a) Predict which specific weathering and erosion processes each block will undergo (distinguishing mechanical processes such as thermal stress weathering, haloclasty/salt weathering, or abrasion, from chemical processes).
2. b) Justify which of the two blocks will show a faster loss of volume and what distinctive surface microfeatures (such as cavities/honeycombs or faceted faces) each one will display after 100 years.

In a stratigraphic section excavated in the floodplain of the lower course of a river (where usually only silts and fine clays settle), an intermediate layer $40\ \text{cm}$ thick composed of coarse gravels and rounded pebbles is discovered between two strata of very fine sediments.

1. a) Interpret what extreme hydrological event must have occurred for the river to be able to transport and deposit those coarse pebbles in a stretch with such a gentle slope.
2. b) Justify how the kinetic energy and transport capacity of the water varied from the beginning to the end of this event to produce this stratigraphic sequence of three superimposed layers.
3. c) Predict what differences you would expect to find in the degree of roundness of the pebbles in this extraordinary layer compared to pebbles of the same size found in the upper course of the river, and explain the geological reason.

In a laboratory, two rock samples of identical mass and volume are subjected to a continuous flow of water with dissolved carbon dioxide (slightly acidic), simulating natural rain for six months: sample A is granite (quartz, feldspar, and mica) and sample B is limestone (calcite).

1. a) Predict what type of chemical weathering each sample will experience (complete dissolution versus selective mineral hydrolysis) and justify which of the two will lose more soluble mass during the test.
2. b) Apply these results to nature: justify why in El Torcal de Antequera (limestone) karst processes produce dolines, limestone pavements (lapies), and sinkholes through dissolution, whereas in La Pedriza (granite) water breaks down feldspars into sand-sized debris and exploits joints to leave large rounded boulders ('berrocales' and tors).

Topic Challenge. Imagine that you work as an environmental consultant and the city council asks you to evaluate the vulnerability of the local terrain. Prepare a brief technical proposal by completing these phases:

1. Climate data and landforms: Check the precipitation values from the nearest station on the AEMET website (note the monthly record in $\text{l/m}^2$, for example $55,4\text{ l/m}^2$). Observe a nearby slope or embankment in your surroundings and identify the visible landforms (gullies, rills) and the dominant geological agent.

2. Soil study: Collect a small surface sample ($50\text{ g}$ to $100\text{ g}$) in a park or open field. Determine its texture by feel (sandy, silty, or clayey), detect the presence of organic debris, and infer whether it is an incipient or mature soil, considering its weathering and the profile visible on the embankment (thickness and horizons).

3. Reasoned proposal: Write a one-page report linking how the recorded climate, slope, and soil properties interact in shaping the landscape. Design two measures based on native revegetation to prevent soil loss.

Evaluation criteria:

• Accuracy in obtaining data from AEMET and in describing the relief.

• Well-founded analysis of the sample and the stages of soil formation.

• Scientific coherence in linking weathering with fluvial or pluvial landform processes.

• Feasibility and relevance of the proposed corrective measures.