Distribution of Rocky Coasts
According to Davis and FitzGerald (2003) 75% of the world's coasts are rocky. Beaches are lacking or occur as to baymouth barriers, pocket beaches, or thin sediment strips at the base of cliffs. Rocky coasts develop where there is a history of erosion exceeding rates of clastic sedimentation, or sediment supplied to the beach is either low or not retained.
Active Margins
- Coasts lie between orogenic mountain belts and deep sea trench.
- Drainage divides are close to the coast. Therefore streams feeding the coasts are relatively small and produce less sand and mud. In addition the sediment is often trapped within local basins before reaching the coast.
- Continental margins are narrow and often cut by canyons that extend to the shore (.e.g. Scripts Canyon). Sediment deposited at the canyon heads by longshore transport is transported to the basin floor.
- Narrow margin are less efficient in dissipating wave energy . Wave hitting the coast are larger.
- Tectonic activity
- Faulting on active margins typically result in rapid uplift or subsidence thereby inhibiting development of long term equilibrium.
Volcanic Islands
- Limited sediment supply and steep offshore topography
- Lack of sediment and warm water promotes reef development on island in the Pacific
Glaciated Coasts - erosional
- Ice sheets and ice caps: Coasts covered by thick ice during glacial maxima are deeply scoured. Where glacial retreat was rapid little material was left behind to make beaches. This is particularly true along the coasts of Maine. In these areas coastal morphology is greatly influence by selective glacial scouring along structurally or climatologically weak zones. Here in Salem the coast is characterized by irregular bedrock hills and basins (knock and lochan topography) which in this case was formed by glacial plucking and scouring of highly jointed igneous rock. Knock and lochan underlie the numerous small headlands and pocket beaches seen along the North Shore of MA. Along the rocky coasts of New England of coastal erosion progresses slowly due to the resistant nature of the igneous and metamorphic rocks composing the coasts.
- Alpine and outlet glaciers: Highly indented fiord coasts are carved by the selective erosion accomplished by these glaciers.
Low Latitude Carbonate Coasts: Reefs and Beach Rock
Shaping of Rocky Coasts
As with most landforms rocky shores reflect the structure and strength of the exposed lithologies, the physical and chemical processes acting on them, and the time they're allowed to do so.
Processes:
Physical Weathering:
The range of physical weathering processes that affect other areas also affect the rocky shores. The main difference is that waves deliver much of the energy to the coast and are responsible for most of the erosion. Waves can tear apart seawalls and move huge blocks from the base of a cliff. Fractures are enlarged by the injection of compressed air and water forced into them by breaking waves. Although waves provide much of the energy that physically attacks the coast, tidal flow governs the vertical distribution of energy along the shore. Where the tidal range is low wave energy is focused at one level, promoting the formation of notches and terraces where permitted by lithology.
Chemical Weathering
Hydrolysis, oxidation and solution all occur along the coasts. Another important process prominent on rocky coasts is salt weathering. Salt weathering occurs where dissolved ions and water penetrate small fractures and mineral grain boundaries where they react with the rock and/or precipitate. Salt formation, hydration and dehydration, and temperature changes all cause volumetric changes that break apart the rock on a microscopic level.
Bioerosion
A variety of algae and organisms are capable of either dissolving layers of rock or tunneling into it. Bioerosion is particularly important in sculpting carbonate coast. For some interesting examples visit Mark Wilson's site on Bioerosion.
Relative Sea Level
When allowed to work on a coast for an indefinite amount of time waves will eventually straighten the coasts through erosion and deposition. The length of time required for this to happen depends on the resistance of coastal lithologies and wave energy. Relative sea level changes, which may be tectonic, eustatic and/or glacio-isostatic in nature, interrupt this process helping to maintain a more rugged and highly indented coast, particularly in areas of resistant bedrock.
Coastal Landforms Along Rocky Coasts
Landforms along rocky coasts include sea cliffs, marine platforms/terraces, arches, stacks and notches. Structure, lithology and Quaternary events, both tectonic and glacial, play important rolls in their development.
Group Exercise: Look at a geologic and topographic map of Maine.