1. High Energy Coastlines
GG3025
Lecture 3/4

2. Shore Platforms

3. Shore Platforms
Coastal erosion platform in the leucogranites near Le Croisic

4. Shore Platforms
Gentle rock slope that extends from high tide to low tide
The remnant of erosion of headlands, because erosion occurs at and above the water level
Abrasion and water-level weathering have a planing effect as the shoreline diurnally transgresses and regresses over the platform
The platform geometry reflects an equilibrium between wave energy and rock resistance
Because the platform slopes, the sea cliff becomes progressively lower and eventually is replaced by a long shore platform, given enough time and a stable sea level

5. Shore Platforms
Shore platforms are associated with high energy environments
Wave-dominated coasts – erosion (although tides and deposition still active)
Wave energy, weathering and erosion
Biological activities
Review of Shore Platform literature by Alan Trenhaille (1988) – University of Windsor, Hamilton, Ontario, Canada

6. Shore Platforms
Research shows glacial-interglacial links to sea-levels
More recently established links between shore platform morphology (geometry) and morphogenetic environments:
wave heights
wave climate
exposure index
tidal range

7. Shore Platforms
Some work has examined rates of change on the shore platform (rock pool enlargement and in thinly bedded sedimentary rocks, rates of min-scarp retreat)
Two ‘camps’ of research:
Sub-aerial weathering (Southern Hemisphere)
Mechanical wave erosion (Northern Hemisphere)

8. Shore Platforms
Problem is to determine what processes are active in the shore platform environment
Observation of platform morphology and characteristics of the rock debris on the platform
What are these?
Mechanical wave erosion – quarrying and abrasion
Weathering – water layer weathering and water solution
Bio-Erosion – marine/intertidal organisms living on/in the platform

9. Shore Platforms Mechanical Wave Erosion
Wave quarrying: loosening and shifting of blocks of rock on the platform (prepared by weathering)
Aided by joints in rock, layering
Disaggreation rather than disintegration
Driven by pressure-release mechanisms caused by a breaking wave (trapping and releasing air)
Evidence for erosion: quarried surfaces and debris

10. Shore Platforms Wave Abrasion
Wear and Tear on the platform
Transit of debris (that can be moved)
Swash and Backwash
Smooth surfaces protect against quarrying
Note also the ‘passive’ geology of the coastline
End result of the two processes active: sloping platform – equilibrium erosional landform?

11. Shore Platforms Weathering
Lower
Smooth
Level surface
Contribute to evolution and formation of shore platform created by mechanical wave processes

12. Shore Platforms Water Layer Weathering: Wave splash and spray
Formation of salt crystals
Wetting and drying
Notch at back of platform or undercut around a platform rock pool
Chemical ‘reactions’ between minerals and seawater
Evidence in the form of honeycombing, pitting, fluting
Micro-features – would not survive high wave energy environments

13. Shore Platforms

14. Shore Platforms Processes are favoured by: Sea-Water Solution
Lime-rich rocks
High temperatures + insolation (effective evaporation)
Level surfaces (accumulation of spray and salt-water)
Spray and strong wave activity will also extend such activities above the tidal heights
Sea-Water Solution
Favoured by lime-rich rocks
Occurs along the edge of platform rock pools
Linked to changing pH values (life style of rock pool populations)
Slicing off of thin layers of bedrock by pool enlargement, and capture of adjacent rock pools
Level but benched platform

15. Shore Platforms Bio-Erosion
Marine organisms resident on shore platform
Acid secretions on ‘holdfasts’ breaking up rock cement
But protection on lower parts of platforms by kelp (seaweed) leading to outer rims or ramps at the lower edge

16. Shore Platforms Common Marine Bioeroding Organisms:
Mollusks (especially bivalves)
Polychaete Worms
Sponges
Crustaceans (especially barnacles)
Echinoids
Fish

17. Shore Platforms
Several distinct habitats exist in rocky shores, each with its own survival challenges for plants and animals living there.
Platforms
These are formed when waves, wind and rain carve rock into flat platforms. Often, the back of the rock (the bit which hasn’t been eroded yet), forms a cliff, while the ocean edge of the platform steps down into the water. This means one rock platform can support many different kinds of plants and animals, because some sections are almost always under water, while other parts are usually dry.

18. Shore Platforms Rock pools
These are usually formed when a boulder lodges in a depression in the rock and grinds a hollow as it rolls around in the waves. After some time, the depression becomes deep enough to hold water during low tide. If the boulder stays in the pool it will gradually grind it deeper, but sometimes, a big wave washes the boulder out and the rock pool stays shallow.
Because pools trap grit, stones and boulders, only certain plants and animals can survive in them. The grit smothers some organisms, while stones and boulders rolling around in storms can smash delicate creatures.

19. Shore Platforms Boulder fields
Groups of boulders are found where little wave action occurs, and the shore’s not too steep.
During storms, the boulders roll around and flip over, smashing any animals living on their underside or the rock bottom. Animals and plants which were on top of the boulder may find themselves having to cope on the bottom, in the dark and permanently under water. And anything which lived on the bottom will be exposed to air, sunlight and heat.
Since sand gathers inside boulder fields, abrasion increases, smothering some plants and grinding others.