Transcript The Coast
Section 6 Part II Isostasy “weighing the same” Buoyant properties of layers of rocks that float on other layers (due to density and thickness) Crust ‘floats’ on denser, underlying layer Continental shelf Continental shelf – gently sloping region surrounding continents (sea is relatively shallow) Covered in sediments (erosion of continents) Part of continent was above sea level during ice age (sea level was lower then) Water depth over continental shelf depends on 2 factors: Isostatic changes in land height Changes in sea level Principle of Isostasy Crust is higher where it is thicker and less dense Lower = thinner and denser Layer Density = g per cm3 Continental Crust 2.7 – 3.0 Sediments on continental shelf Oceanic crust 2.4 Mantle 3.3 – 5.7 3.0 – 3.3 Highest density Continental crust – mostly made of granite (less dense rock) Granite: igneous rock made with ~20% quartz Oceanic crust – basalt (more dense rock) Basalt: igneous rock less than 20% quartz, mostly volcanic So, continental plates ride on the denser oceanic plates Littoral Zone Land meets sea Nature of shore factors: Geology of adjacent land Exposure to erosion by sea Area of coast between high water mark and lowest part of submerged shore Rocky Shore Rock exposed to sea erosion Variety sized boulders, stones and pebbles Granite = resistant to weathering Sandstone = easily broken down Rocky Shore Most exposed type of shore Most resistant to erosion Largest boulders left at the top of the shore due to pounding waves Very steep cliffs to horizontal flat rocks with a wavecut platform Limiting Factors Rocky Shore Support wide range of organisms Stable substrate Large rocks and stones – firm surface Algae Mollusks and cnidarians (and sea anemones) Rock pools retain water when tide recedes Rocky Shore Environmental Factors that influence communities: Desiccation (extreme drying) Species near top of shore exposed to air for longer periods Temperature Wave action Light intensity Aspect Slope Nature of substrate Sandy Shores Erosion of sandstone Deposition of sand by the sea Silica and other minerals Slope gradually toward sea Sandy Shores Unstable Fine particles are easily moved by winds and tides Not a suitable substrate Sea weeds No shelter for organisms at surface Under: burrowing organisms Ghost crabs, bivalve mollusks, annelid worms (ragworms and lugworms) If sand mixed with muddy deposits, more stable and supportive Muddy Shores Least exposed to erosion silt particles can settle Very fine mineral sediments Organic remains Little slope Can form mud flats Estuary Semi-enclosed body of water Freshwater meets sea water Muddy substrate – slow flow of water allows suspended particles to settle Delta (∆) Get their name from the Greek letter River carrying suspended sediments reaches large body of water (lake or ocean) River increase in width = flow rate decreases Suspended sediments settle Accumulate into a fan-shaped structure over time River divides to form distributary channels 2 examples (next page) Nile River Delta ∆ Mississippi River Delta ∆ Diameters of Mineral Particles Particle Type Diameter (mm) Silt 0.002 to 0.02 Fine Sand 0.02 to 0.2 Coarse Sand 0.2 to 2.0 Gravel (small stones) > 2.0 Mangroves (Swamp, Forest) Trees and shrubs in tropical/subtropical saline coastal habitats Between 25⁰N and 25 ⁰S Form woodland or shrub land habitat (coastal/estuarine conditions where sedimentation of silts occur) Specifically adapted to: Wide range of salinity Low oxygen concentrations in sediments Mangroves Pneumatophores - specifically adapted root-like structures that obtain oxygen directly from air Extensive root systems: Trap particles suspended in water Reduce water flow Increases deposition of sediments Dissipates wave energy Protects coastal area from erosion Habitats for: Algae, oysters, crabs, barnacles and other crustaceans, sponges, fish