The ravines of Michoacán, the dunes of Samalayuca in Chihuahua, and the Cacahuamilpa caves in Guerrero look like landscapes of very different natures, but they follow the same logic: an external agent (water, wind, or ice) wears down the rock, carries off the fragments, and deposits them somewhere else. Understanding that logic lets you read a landscape as a record of processes that have been at work for millions of years.
The Phases of Landform Shaping
At the foot of a rocky escarpment there are freshly fallen angular blocks, but tens of kilometers downstream we find only rounded pebbles and fine sands. What happens to the rock from the moment it fragments at the summit until it is deposited on a plain?
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Define and order the four stages of the cycle of action of an external agent: erosion, weathering, deposition, and transport.
Define and order the four stages of the cycle of action of an external agent: erosion, weathering, deposition, and transport.
Differentiate between physical and chemical weathering, describing two processes of each type (for example freeze-thaw weathering, thermal shattering, dissolution, or…
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.
When heavy rain falls on a slope, the water that does not infiltrate runs over the surface: this is runoff. At first it forms a diffuse sheet that carries away the finest particles. If it becomes concentrated, it cuts rills (grooves a few centimeters deep) and, when these deepen and merge, gullies appear: deep trenches with nearly vertical walls that grow upslope. The Mixteca region of Oaxaca is one of the areas of the country with the most extensive gullies.
Clays are very fine-grained rocks and soils with three key properties:
Plasticity: when wet they become moldable and lose strength; when saturated they can flow or slide as a mass down the slope.
Relative impermeability: water infiltrates little, so much of the rainfall runs off and tears away material, and the water that does get in accumulates above the clay layer.
Shrinkage: when they dry out during the dry season they contract and crack; in the next rain those cracks let water rush in all at once down to deep layers.
Vegetation cover protects the soil: the canopy cushions the impact of raindrops, grass and leaf litter slow the runoff, and roots hold the particles in place. But its effect depends on the species, on the weight it adds to the slope, and on how it changes the distribution of water in the ground. That is why reforestation is planned with geological criteria, not just by "planting more trees."
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State four external agents of landform shaping and name a characteristic landform for each, indicating whether it is erosional or depositional.
State four external agents of landform shaping and name a characteristic landform for each, indicating whether it is erosional or depositional.
The Chihuahuan Desert displays a landscape of flat-topped hills (mesas) and gullies on clay. Explain which external agent dominates this landform shaping, why clay…
The Chihuahuan Desert displays a landscape of flat-topped hills (mesas) and gullies on clay. Explain which external agent dominates this landform shaping, why clay erodes so quickly, and what role the sparse vegetation plays.
Seen from the air, lowland rivers, like those of the coastal plain of Tabasco, trace great curves that shift position after every rainy season and leave horseshoe-shaped lagoons behind. Why don't lowland rivers follow straight paths, and how do they redistribute their own sediment load?
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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.
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.
In Mexico, the Yucatán Peninsula is an enormous limestone platform: there, karst shows up in cenotes (flooded sinkholes) and underground rivers. The Cacahuamilpa Caves (Guerrero) are an example of a cave with stalactites and stalagmites.
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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.
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.
A very thick glacier overdeepens the valley: it deepens the floor and leaves very steep, almost vertical walls. While the ice was there, its weight propped up those walls. When it melts, the rock no longer has that support and undergoes unloading: fractures open parallel to the wall. That is why the walls of a freshly deglaciated U-shaped valley are prone to rockfalls, and the flat floor (trough) does not make them any safer. If a river later settles into that floor, it cuts down into the sediments and the rock, and it can undermine the foot of the walls.
Low Coasts and Cliffed Coasts
The sea does not respond the same way everywhere. It depends on the rock and on the sediment budget: the difference between the sediment that reaches the coast (rivers, cliff erosion) and the sediment that waves and currents carry away. If more comes in than goes out, the beach grows; if less comes in, it retreats.
Low sandy coast. It has a gentle slope. A typical system is beach–lagoon–dune ridge: the beach receives the waves; behind it, the barrier ridge (a bar or coastal dune) forms a sand barrier; and behind that lies a lagoon (a coastal lagoon, like the Laguna Madre in Tamaulipas or the Laguna de Términos in Campeche), separated from the sea. The beach has an equilibrium profile: a slope that adjusts to the waves and the available sediment. If the supply falls (for example, because of dams on the rivers) or sea level rises, the beach loses sand, the profile shifts landward, and the dune ridge retreats, so the coast migrates inland.
Cliffed coast. The waves undercut the base of the rock, the wall loses support, and it collapses. As the cliff retreats, it leaves a wave-cut platform at its foot, a flat rock surface that is exposed at low tide. The speed of retreat depends on the hardness of the rock and how fractured it is: alternations of hard and soft layers (such as flysch) erode unevenly. A rise in sea level lets waves reach higher and increases erosion, but the coast does not migrate inland the way a beach does.
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Relate each agent to the landform it produces and briefly explain the mechanism: glacier (U-shaped valley), river (meander), wind (dune), and waves (cliff).
Relate each agent to the landform it produces and briefly explain the mechanism: glacier (U-shaped valley), river (meander), wind (dune), and waves (cliff).
Profiles A and B in the diagram correspond to cross-sections of two valleys in a mountain massif that hosted Quaternary glaciers. a) A civil engineering project claims…
Profiles A and B in the diagram correspond to cross-sections of two valleys in a mountain massif that hosted Quaternary glaciers.
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.
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.
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.
Analyze how climate change can modify landform processes along the Mexican coastline, considering sea-level rise, changes in rainfall patterns, and storm frequency.
Analyze how climate change can modify landform processes along the Mexican coastline, considering sea-level rise, changes in rainfall patterns, and storm frequency.
Back to the beginning: the ravines of Michoacán combine river incision and weathering, the dunes of Samalayuca are the product of wind transport, and Cacahuamilpa was born from the dissolution of limestone: three different landscapes sculpted by the same four phases of landform shaping.
Summary
Let's practice! 3 medium exercises in this section