L2 - Geomorphology and Sedimentary Processes

Introduction to Weathering and Erosion

• Weathering: The process of breaking down rocks chemically and physically at Earth's surface in situ.

• Erosion: The movement of these grain particles across terrestrial landscapes, which continues the generation of sediment.

• Importance of understanding these processes for studying sedimentary rock composition.

Geomorphology

• Definition: The study of landscapes to infer and quantify natural processes.

• Provides insights into the modern context of geological processes based on historical records.

Significance of Geomorphology

• Helps to identify and contrast different landscapes, leading to insight into atmospheric and environmental hazards.

• Example: The debris flow in China after the Wenchuan earthquake illustrates how geomorphic processes can create hazards downstream by obstructing river flows.

• Geomorphology is linked to nutrient cycling vital for ecosystems, particularly plant life, through the processes of weathering and erosion.

Concepts

• Sediment Routing System: Pathways through which sediment is transported from source to sink areas (e.g., rivers to oceans).

• Denudation: The process of land surface lowering through erosion and weathering, quantified through measures of mass transfer.

• Elements of Landscapes:

  • Hill Slopes: Loose erodible material produced by weathering of intact bedrock.

  • River Channels: Carriers of sediment that cut through and shape landscapes. Driven by potential energy.

Sources and Sinks Concept

• Source Area: Regions where sediment is produced (upper landscape).

• Sink Area: Locations of deposition (valleys, ocean floors).

• The process involves gravitational forces driving water and sediment movements, influenced by climate and tectonic activities.

Erosion and Sediment Transport

• Not all particles move through these systems uniformly; factors such as fluid dynamics and sediment sorting play significant roles.

• Importance of sediment transport in the context of geological records and preservation potential.

High-Elevation Case Study: Taiwan

• Taiwan exemplifies rapid mass transfer with its steep topography alongside the ocean.

• Major processes involved include clastic sediment movement (solid grains) and chemical denudation (dissolved materials).

Measurement of Denudation

• Methods to quantify denudation include river sampling and long-term sediment measurement in reservoirs.

• Can be done through direct measurements (mass of sediment passing a point over time) or indirect (mass of sediment accumulated in a basin over a period of time).

• Observations show a variation of denudation rates across different geographies, with mountainous areas experiencing substantially higher rates.

Relationships Between Erosion and Climate

• High rainfall and steep mountains correlate with enhanced denudation rates.

• Geographical distributions of sediment loads reveal complex patterns influenced by terrain, climate, and river basin characteristics.

Hill Slopes vs River Channels

• Depending on the rate of both hillslope and river channel elements, the landscape can evolve based into either a system with shallow slopes (where hill slope lowering is quicker than river incision), or steep slopes (where river incision is faster than hill slope lowering).

• Hillslope: Loose erodible material (regolith) which is produced through weathering of intact bedrock.

• River Channel: Sediment supplied by hillslope transorted by flowing water (rivers/glaviers), which can also incise into solid bedrock.

Coupling of Systems

• Incision: When rivers transport away hill slope sediment faster than it is supplied thus leading to direct erosion of bedrock.

  • Thinner regolith on landscape, where bedrock is near the surface building steep slopes which eventually fail structurally.

• Alluviation: When rivers are filled with sediment they cannot transport, leading to further geomorphic changes.

  • Produces abundant regolith on the landscape leading to convex up hillslopes.

Tectonic and Climatic Interactions

• Changes in bedrock uplift rates significantly affect the geomorphic processes at play, influencing landscape evolution over time.

• Given the dynamic nature of landscapes, their forms will shift depending on tectonic activity and climatic influences, leading to variations between dense sedimentation and smoother, diffusive landscapes.

• Increased levels of uplift due to tectonic collision can result in high landscapes with regolith cover thus leading to landslides. The point at which they collapse is called the threshold hillslope.

Conclusions and Key Takeaways

• Weathering generates regolith for sediment transport; rivers facilitate landscape change via incision.

• Modern erosion predominantly occurs in mountainous regions due to active tectonic forces.

• Continuous interplay between erosional processes shapes our understanding of geological effects on landscapes.

Next Steps

• Future lectures will dive deeper into grain scale insights and sedimentary rock formation.