3 Shore PlatformsCoastal erosion platform in the leucogranites near Le Croisic
4 Shore PlatformsGentle rock slope that extends from high tide to low tideThe remnant of erosion of headlands, because erosion occurs at and above the water levelAbrasion and water-level weathering have a planing effect as the shoreline diurnally transgresses and regresses over the platformThe platform geometry reflects an equilibrium between wave energy and rock resistanceBecause 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 PlatformsShore platforms are associated with high energy environmentsWave-dominated coasts – erosion (although tides and deposition still active)Wave energy, weathering and erosionBiological activitiesReview of Shore Platform literature by Alan Trenhaille (1988) – University of Windsor, Hamilton, Ontario, Canada
6 Shore PlatformsResearch shows glacial-interglacial links to sea-levelsMore recently established links between shore platform morphology (geometry) and morphogenetic environments:wave heightswave climateexposure indextidal range
7 Shore PlatformsSome 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 PlatformsProblem is to determine what processes are active in the shore platform environmentObservation of platform morphology and characteristics of the rock debris on the platformWhat are these?Mechanical wave erosion – quarrying and abrasionWeathering – water layer weathering and water solutionBio-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, layeringDisaggreation rather than disintegrationDriven 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 platformTransit of debris (that can be moved)Swash and BackwashSmooth surfaces protect against quarryingNote also the ‘passive’ geology of the coastlineEnd result of the two processes active: sloping platform – equilibrium erosional landform?
11 Shore Platforms Weathering LowerSmoothLevel surfaceContribute 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 crystalsWetting and dryingNotch at back of platform or undercut around a platform rock poolChemical ‘reactions’ between minerals and seawaterEvidence in the form of honeycombing, pitting, flutingMicro-features – would not survive high wave energy environments
14 Shore Platforms Processes are favoured by: Sea-Water Solution Lime-rich rocksHigh 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 heightsSea-Water SolutionFavoured by lime-rich rocksOccurs along the edge of platform rock poolsLinked 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 poolsLevel but benched platform
15 Shore Platforms Bio-Erosion Marine organisms resident on shore platformAcid secretions on ‘holdfasts’ breaking up rock cementBut protection on lower parts of platforms by kelp (seaweed) leading to outer rims or ramps at the lower edge
17 Shore PlatformsSeveral distinct habitats exist in rocky shores, each with its own survival challenges for plants and animals living there.PlatformsThese 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.