Download 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