Geomorphology: Landforms, Processes, and Earth’s Evolution
What is Geomorphology?
Geomorphology is the study of landforms, the processes that create and alter them, and the landscape evolution of the Earth. It examines how physical, chemical, and biological processes shape the Earth’s surface, resulting in various materials and landforms. These landforms and materials serve as a record of Earth’s surface history.
Geomorphology is a broad, interdisciplinary field encompassing geography, geology, and earth and environmental sciences. It includes many subdisciplines, where the processes are similar but the controls differ. Subdisciplines include fluvial, hillslope, glacial, tectonic, coastal, and desert geomorphology.
Driving Mechanisms of Geomorphology
The driving mechanisms of geomorphology include:
- Climate/Atmosphere (e.g., type of vegetation)
- Tectonics
- Gravity (e.g., hillslope processes)
- Biotic factors (e.g., vegetation and how roots control erosion)
Physical changes in the landscape caused by river or glacial action can be explained by interactions between moving water or ice, sediment transport, and river or landscape surface form. These interactions are controlled by the setting (climatic, geologic, and biotic). The setting is a key factor in geomorphology, and biophysical factors affect the processes and their rates.
Key Geomorphology Definitions
Process: A set of actions or mechanisms that operate in a specific order to produce a phenomenon. It is an occurrence in response to a driving force. For example, the process of meandering rivers results from water flow, sediment transport, and bank vegetation. Processes occur on forms, and forms are the result of processes. It is an action produced as a result of a force of change.
For example, wind is a process that occurs in response to atmospheric pressure differences and is affected by the landscape. Wind can lead to other processes, such as trees being blown down. Wind itself can change the landscape, and the changed landscape can further alter wind processes.
Form: A landform over which processes occur and is the result of processes.
Catastrophism vs. Uniformitarianism
Catastrophism: Proposed by Georges Cuvier, this theory suggests that landscape changes are due to sudden, large-scale impactful events over shorter periods, such as floods, earthquakes, and eruptions (e.g., Sudbury Basin, Impact Crater).
Uniformitarianism: A key principle in geomorphology, proposed by James Hutton, this theory suggests that changes are due to continuous, smaller-scale actions and processes. These processes remain the same over time, and landforms develop over long periods (e.g., erosion, deposition, compaction). The principle is summarized as “the present is the key to the past.”
Gradualism: Proposed by Charles Lyell, this theory provides finer details of uniformitarianism, stating that change is slow and steady (e.g., erosion and deposition).
These concepts are not mutually exclusive. For example, the Grand Canyon was carved by gradual erosion via water and major floods. The Sudbury Basin went from a shallow sea to a mountain to flat land. Volcanic eruptions can be both explosive (catastrophism) and effusive (gradualism with periods of catastrophism).
Davis’ Cycle of Erosion
Davis Concept/Cycle of Erosion: Proposed by William Morris Davis, this concept suggests that erosion is based on form (youth, mature, old age, peneplane, rejuvenation). It analyzes the form and infers the process.
However, this concept has problems: it oversimplifies complex processes of landscape evolution, doesn’t account for tectonic forces and climatic variations, assumes a steady-state, and assumes erosion and deposition will balance over time. It also starts with the answers and jumps to conclusions.
Quantitative Geomorphology
Quantitative: Proposed by Horton/Strahler, this process-based approach, unlike older form-based theories, measures landform characteristics and the processes that form them to describe landscapes. It is the current standard and considers the driving forces (e.g., meandering streams measured by radius of curvature and meander wavelength, channel geometry measured by gradient, channel width, and drainage). It also considers the inner-connectivity between processes.
Scales are important in process-based geomorphology. Processes operate on variable time and space (size) scales (e.g., wind operates in the background of longer time-scale processes like climate).
Real-World Applications
Geomorphology has real-world applications in:
- Hazard assessment and predictive modeling
- Water quality and availability (glacial landforms as aquifers)
- Planetary studies of landforms
- Agriculture
Systems in Geomorphology
Systems: A set of interrelated objects and the processes relating these objects, linked by flows of matter (mass that takes up space within the system) and energy (capacity to change the motion of or do work on matter).
Types of Systems
- Isolated: No substantial interchange of matter with its surroundings. Energy cannot cross system boundaries (e.g., the Universe).
- Closed: No substantial interchange of matter with its surroundings, but energy can enter and leave the system (e.g., Earth).
- Open: Inputs and outputs interchange with surroundings. Both matter and energy can cross system boundaries. Most natural settings on Earth are open (e.g., rivers).
Lakes can be both open and closed, depending on the scale. For fish, it’s closed; for water molecules, it’s open.
Feedbacks
Feedbacks: As systems operate, they generate outputs that influence aspects of the system or interactions within the system. Feedbacks can help maintain equilibrium.
- Negative feedback: Discourages change in the system, acting to self-regulate and stabilize the system (e.g., deposition of coarse sediment in a channel section results in a decrease in river gradient, leading to a decrease in erosion).
- Positive feedback: Encourages change in the system, creating a snowball effect (e.g., heavy rain lowers infiltration rate, resulting in runoff and thinner soil layers due to soil erosion, which further reduces soil infiltration).
Equilibrium
Equilibrium: The idea that material, process, and geometry form a self-correcting balance. It is a process-response balance between opposing forces. Changes in controlling or independent variables result in responses in the dependent variables to maintain overall system equilibrium. Dynamic equilibrium means that landscapes adjust rapidly in response to changes in the processes operating on them, involving continual change-response adjustments and short time-scale adjustments.
Thresholds
Thresholds: The critical limit of force, change, magnitude, or intensity that must be exceeded to trigger a certain reaction or result. Thresholds result from imbalances within the system and can be either extrinsic (external, crossed due to a change in an external variable, e.g., a storm event triggering flood discharge) or intrinsic (internal, crossed due to a change within a landform, e.g., sinkhole collapse). For example, if the threshold for hillslope water moisture is exceeded, the system may respond with slope failure or landslides.
Rate of Change and Driving/Resisting Forces
The rate of change is determined by the energy being put into the system (driving forces) and the resistance of the material being changed.
- Increased driving force + decreased resisting force = increased rate
- Decreased driving force + increased resisting force = decreased rate
For example, if the driving force (slope) is no longer high enough to move sediments, deposition occurs.
Drivers: Climate, gravity, internal heat/tectonics. Energy sources of driving forces can be either exogenic (forces acting on the Earth’s surface, e.g., weathering-erosional processes, solar radiation, Earth’s tilt/rotation, gravity, climate) or endogenic (processes and forces acting internally, e.g., tectonics and mountain building, volcanism).
Resistance: Material/substrate (lithology and rock types), vegetation, and structure (e.g., friction).
Picture example: mass balance: exogenic vs endogenic
For example, the rate of landscape change = uplift – erosion.
Time and Energy
Time: Essential in any geologic process. Variable levels of time are required for desired products of change (e.g., time scale variation between slow steady-state soil creep vs. instantaneous slope failure). Geologic processes are by nature cyclic and repetitive over time.
Energy: The capacity to do work (potential energy: energy of position, heat/thermal energy: molecular kinetic energy). Work = Fs (F=force, S=distance). Work occurs when mass is displaced (e.g., geomorphic work = transport of sediment/rock material, alteration of landforms on significant time scales).
Driving Force: The application of energy in the context of performing work on Earth materials (e.g., hydraulic force + particles = erosion).
Recurrence Interval and Recovery Time
Recurrence Interval: The amount of time it takes, on average, for an event of a given magnitude to occur. It doesn’t mean it occurs every x years but rather has a 1/x chance of occurring in any given year.
For example, the chances of getting two 100-year floods in two consecutive years are 1/100 * 1/100 = 1/10,000 (0.0001% chance).
Recovery Time: The amount of time it takes for a system to stop feeling the effects of a particular event.
Magnitude and Frequency
Frequency events of a given process are low magnitude. High-magnitude events occur much less frequently and do more work but might not occur frequently enough to make a big difference.
The general theory is that the event that does the most work is not the most frequent or the strongest but somewhere in the middle.
Denudation
Denudation: Any process that wears away landforms and results in decreasing the elevation and relief of a landscape. It counteracts processes that increase elevation and relief (uplift, faulting, folding). Denudation processes include weathering, mass movement, and erosion/transport.
Weathering and Erosion
Weathering: In-situ decay, breakdown, and alteration of surface materials. It is an exogenic process (physical, chemical, biological) that alters the physical and chemical state of materials at or near the surface.
Erosion: Relies on transporting agents (wind, water, ice) and the downward movement of materials via gravity. It carries weathered products away from the source area (displacement).
Weathering rates are not always higher immediately after exposure to elements and decline with time. It depends on the setting. Sometimes exposed surfaces stay exposed. Higher rates following exposure are related to the length of exposure. For example, lava flows that are quickly buried by subsequent lava flows are less likely to be weathered than a flow that remains exposed to the elements for a long period of time.
Physical vs. Chemical Weathering
Physical Weathering: Mechanically breaks down rock into smaller pieces via rock fracturing, freeze-thaw, etc. It acts primarily on preexisting fractures and pore spaces and depends on the parent material.
Chemical Weathering: Chemical alteration of the mineral composition of a rock through chemical reactions between the minerals, air, and water, including biochemical processes.
Both act simultaneously and affect the nature and rate of one another. Disintegration increases the rock surface area, while changes in strength occur with changes in composition.
Physical Weathering Processes
- Pressure Stress Release (fracturing and jointing): Overburden covering buried rock is gradually removed by erosion, reducing confining pressure. The rock expands and fractures, often concentrically and parallel to the rock surface, resulting in exfoliation/sheeting (removal of slabs of rock along these joints) or jointing. It is more common in massive rocks (granites, non-sedimentary rocks). An exfoliation dome is a landform produced when sheeting occurs in concentric shells, common in intrusive igneous rocks like granite.
Jointed rock examples
Exfoliation dome
- Thermal Expansion and Contraction (insolation weathering): Driven by changes in the temperature of the parent material due to insolation or fire. The surface temperature of dark rock with lower albedo can absorb more energy, varying between 0-50°C between day and night. Since rocks, especially jointed rocks, have low thermal conductivity, differential stresses of expansion and contraction cause separation of concentric shallow layers. Jointing occurs (called spalling or spheroid weathering when it affects boulders).
- Salt Weathering (growth of foreign crystals): Both physical and chemical weathering. Hydrate and dehydrate repeatedly, causing stress in fractures and between grain boundaries in permeable rock. The removal of salt with incoming water leaves larger gaps, allowing for more physical weathering to occur.
- Hydration (slaking)
- Freeze-thaw
- Plants
Chemical Weathering
Chemical weathering breaks down rock by chemical alteration through reactions between minerals and water and/or air.
- Oxidation (air, reversible): Loss of electrons from one element to oxygen, changing and weakening the mineral structure. It is immediate. The water table is a key boundary between oxidizing and reducing environments. Iron is the most commonly oxidized mineral element. Oxidized material can weather and erode more. For example, iron reacts with oxygen to create hematite (iron oxide). Pyrite + oxygen + water = sulphuric acid (hematite). Runoff from these areas can further cause weathering due to increased acidity (acid rock drainage).
- Hydration (water, reversible): Incorporation of water molecules into the crystal structure of minerals, changing the structure of the mineral but not forming new compounds. Hydration/dehydration cycles cause mineral grains to separate from each other as they expand/contract, causing granulation and clay production. The resulting mineral has a greater volume than the original. For example, Anhydrite (CaSO4) -> Gypsum (CaSO4 x 2H2O).
- Hydrolysis (water, irreversible): A chemical reaction where water loosens the chemical bonds in a mineral. It is different from hydration as it produces new minerals and ions. It is a key process for breaking down silicates and creates clay minerals. The most common weathering reaction on Earth is the dissolution of feldspars into clay (kaolinite), common in granite. Quartz is the only mineral not affected by hydrolysis, which is why it and clay are the two most common minerals in sedimentary rocks. Decomposition of minerals in water as hydrogen ions replace cations in minerals forms new compounds and is not reversible. Pure water is a poor H+ donor, but when CO2 dissolves in water, it forms carbonic acid (where does the CO2 come from? = decay of plant material).
- Carbonation: Similar to hydrolysis but produces ions, not minerals. It is reversible if the solvent is removed. The most common example is the dissolution of calcite in limestone in water with carbonic acid (calcite -> calcium and bicarbonate ions). The removal of calcite results in sinkholes, caves, and karst formations.
- Cation Exchange: Substitution of mineral cations in solutions for those held by mineral grains and crystals. It is most effective in clay-textured sediments as cations adhere to the surface of negatively charged clay minerals.
- Chelation: Biochemical weathering. Chelating agents are produced by the alteration of humus in plants acids and excreted by lichens. H+ released during chelation from organic molecules is available for hydrolysis. Plants contribute to the decomposition of soil and rock waste at depths to the base of the root zone.
Without weathering, rock would not break down into smaller pieces in the silt and clay size range.
Differential weathering: Without both physical and chemical breakdown of rocks and sediments, soil formation would not be possible.
Soil
Soil: A naturally occurring body or layer that consists of generally unconsolidated materials formed in situ from the product of chemical and physical weathering.
Bedrock = consolidated rock. Regolith = loose layer of material covering solid bedrock. Parent Material = consolidated or unconsolidated material from which regolith or soils develop.
Soil Forming Factors = CLORPT (Climate, Organism, Relief, Parent Material, Time).
Cohesion: The attraction of water molecules to other water molecules. Adhesion: The attraction of water molecules to solid surfaces. Capillarity: The ability for liquid to flow without the assistance of gravity or to remain in place in opposition to the force of gravity. It is the primary force that enables soils to retain water and help regulate movement.
Soil properties like grain size, soil texture, porosity, and water-holding capacity can dictate the thresholds of landscape change.
Mass Movement
Mass Movement: Any movement of material primarily under the influence of gravity with little to no transporting agent. It is a collective term for all gravitational downslope movement of rock, weathered rock debris, or soil with little or no transporting agent (water plays an important role but is not a transporting agent). Whether or not material moves depends on the balance between shear stress on the material and the shear strength of the material (affected by water content, based on friction and cohesion stickiness, related to electrochemical attraction between soil particles). Shear strength is a function of the angle of internal friction (interlocking between particles and the surface roughness of the material), the angle of repose (the minimum angle which causes movement), normal stress, and cohesion (attraction between particles of the same type, origin, and nature, high in clay soils, silts, and finer soils).
Erosion
Erosion: Denudation by flowing water, wind, or ice, which dislodges, dissolves, or removes surface material.
Factors affecting slope stability include slope angle, substrate material, water content (shear strength, fills pore space and makes material heavier, can reduce friction along a sliding surface, can change the internal friction of material but depends on the amount of water), precipitation, weathering and erosion, and vegetation.
Damp sand can have a steeper angle of repose than dry sand, but too much water means the grains no longer touch, so there’s no friction, and it flows like a liquid.
The influence of vegetation on slope stability includes interception of rainfall, uptake of groundwater by roots, root reinforcement (anchor), and soil development (cohesion).
Strain
Strain: Any deformation or change in shape or volume of a material caused by the application of stress.
Elastic behavior: Removing the stress always results in recovery. Plastic behavior: When material can no longer recover from stress. Viscous behavior: When material is fluid and flows in response to stress. Newtonian fluids flow at rates proportional to applied stress and undergo greater rates of deformation with increasing stress.
Mass Movement Classifications
Mass movement occurs when resisting forces holding material in place are overcome by gravitational (driving) forces, generally when resisting forces are reduced due to water pressure.
Classifications are based on:
- Type of material and type of movement
- Rate of movement (rapid vs. hidden)
- Water content
Heaves (soil creep, solifluction, gelifluction – both induced by freeze-thaw), falls (rock falls/topples – fast, typically dry, common in mountainous regions and areas with freeze-thaw), slides (translational/rotational), and flows.
Earth flow: Since it involves finer material, it is a much slower-moving flow, short-lived, and occurs on shallow slopes. It has an hourglass shape with deposition at the bottom.
Debris flow: A flow of everything, fast, moderately wet, and mixed soil and debris. Inner grain fast saturated muddy mud. Usually occurs on higher-angled slopes. Finer material is tailed on the top, and coarser material is at the bottom.
Mud: Fine materials, fast-moving, and wettest. Often triggered by intense rainfall events.
Lahars: Only in volcanic settings. A mixture of water and ash or ash deposits. Can move really fast but solidifies as soon as it stops. Movement downwards of fine grain.
The ultimate source of sediment is mountains of high elevations, and the ultimate sink is the ocean floor. A region of high wind where erosion occurs is a source, and a region of low wind (deposition) is a sink.
What type of dune is likely depicted here? = Parabolic