Detailed notes and diagrams explaining weathering and erosion, the Hjulstrom Curve, sedimentary rock descriptors, sedimentary structures, key facies in sedimentary, fluvial and marine environments, and the process of diagenesis
Sedimentary Rocks and Processes
Weathering and Erosion
• Weathering – the physical breaking up of rock into smaller pieces
• Erosion – the removal of material
Weathering Processes
Weathering
Phys Bio Chem Definition
Process
Rainwater containing carbon dioxide from the air and soil reacts
Carbonation ✓
with carbonates to form soluble hydrogen carbonates
Water expands by 9% on freezing so fractures filled by water are
Freeze-thaw ✓
prized further apart
Heating and cooling of rocks in desert regions causes minerals to
Insolation ✓
expand and contract at different rates until the rock fractures
The addition of oxygen to the atomic structures of minerals or a
Oxidation ✓
similar chemical change
The addition of water molecules to the atomic structures of
Hydrolysis ✓
minerals
Root Growth ✓ Plant roots growing in fractures
Solution ✓ Soluble materials dissolve and are carried away in solution
Deeply buried rocks are compressed by the pressure of the rocks
Pressure
✓ above. When the overburden is removed, the rock expands to
Release
produce stress release fractures
Climate
Climate, rainfall and temperature play an important role in governing
what type of weathering dominates an area and the rate at which it
occurs:
Erosional Processes
Agents of erosion – things that move material
• Water
• Wind
• Ice
Hjulstrom Curve
• Experimentally derived and helps geologists to determine what velocity is required to erode,
transport and deposit different sized clasts
• Based upon the findings of Filip Hjulstrom’s investigations
• The graph is based on the results from an experiment with flowing water (1m depth) over flat-lying
unconsolidated sediment
, Positives of the Hjulstrom Curve Negatives of the Hjulstrom Curve
• Uses a logarithmic scale to show a wide • Only 1m depth shown on graph
range of clast sizes and velocities • Only applies to rivers
• It describes the relationship between • Loose sediment on bottom of river only
velocity and grain size • Doesn’t take density, shape or angularity of
• Uniformitarianism – present is the key to sediment into consideration
the past
Transportation
Processes that modify shape during transportation:
• Abrasion – particles grind down the bedrock
• Attrition – the particles grind against each other
Processes of Transportation
Process Grain Size Affected
1 Suspension Silts and clays
Ions dissolved from bedrock and
2 Solution
held in solution
3 Saltation Sand
4 Traction Large Gravel
5 Sliding Large Gravel
The Way in Which Each Agent of Erosion Transports Sediment
Agent of Erosion Saltation Bedload Traction Suspension Solution
Ice X ✓ ✓ ?
Water ✓ ✓ ✓ ✓
wind ✓ X ✓ X
As sediment moves downhill it can change in several ways:
1) Size
2) Shape
3) Composition
Size:
• The further a grain moves from its source, the smaller the grains become
• Size can be categorised using the Wentworth Scale:
Exam Board Category on the Wentworth Scale:
• Coarse
• Medium
• Fine
Shape and Roundness:
• The further a sediment grain moves from its source the more rounded it becomes
• Roundness – described using six different classes from very angular to well rounded
• Shape – can also change and is influenced by the planes of weakness within the rock
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