High Energy Coastlines GG3025 Lecture 3/4

Shore Platforms

Shore Platforms Coastal erosion platform in the leucogranites near Le Croisic

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

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

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

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)

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

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

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?

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

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

Shore Platforms

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

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 http://www.wooster.edu/geology/Bioerosion/Bioerosion.html

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

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.

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.

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.