Australia: The Land Where Time Began

A biography of the Australian continent 

Coast, Shore & Beach

Shore

According to the authors1 the part of the landmass that is close to the sea and has been modified by the action of the sea. The words shore and coast mean the same thing. The movement of land over time, sea level changes, erosion and deposition cause the shoreline (coast) to move with time. It is known from the sedimentary record that over time there have been a number of marine intrusions onto the continents and subsequent retreats of the oceans that have occurred periodically, indicating times when the land was below and above sea level. The sea levels have varied by up to 120 m between the glacials and interglacials. The rock type forming the coast is a direct determinant of the resistance of the coast to erosion, granitic coasts eroding only slowly, while sands are easily redistributed by ocean currents along the coast. The dynamical relationship between the ocean and the solid surface often change dramatically when there is a combination of sea level change and hydrological forces associated with an estuary.

Beach

A beach is at the seaward limit of a landmass, extending approximately from the highest to the lowest tide levels, a zone composed of  unconsolidated particles. The limit of the beach on the landward side can be vegetation, permanent sand dunes, or increasingly human constructions. A the point where the depth of the ocean at low tide is about10 m where the sand is not moved onto or off the beach.

Coasts

According to the authors1 there are many ways in which coasts can be classified, examples are active or passive margins on the long timescales of plate tectonics. Active margins have associated with them active volcanism, faulting and folding, and are rising like many in the Pacific Ocean. Margins in the Atlantic Ocean are passive, being pushed by spreading seafloor, accumulating thick sediment wedges and generally falling.

Coasts can also be referred to as erosional or depositional, according to whether sediment is being added or removed, though on shorter timescales both erosion and deposition can be occurring. On longer timescales, such as over thousands of years, mean sea level changes can result in sediment being either added or removed. Waves and currents attack erosional coasts, and fine material is carried by both that assists in the erosion of the coast. The abraded material from the coastline is carried along the coast and out sea by the alongshore currents and the rip currents that are produced by the waves, where it can be added to by sediments from deltas and the discharge from rivers. High-energy coasts with large waves erode the coast most rapidly than along low-energy coasts with wave fields that are generally weak. The erosion resistance of the rocks determines the speed of erosion. Characteristic features on coastlines, such as sea cliffs and sea caves, and an alternation between bays and headlands result from variations in the hardness of the rocks.

When sediment, most commonly sand, is deposited in suitable places have been transported there, often in quiet bays between headlands away from high surf activity. When new sand is deposited after older sand has been scoured by the waves the beach is in equilibrium, sand being deposited at a similar rate to that washed away from the waves. The evidence for this process is suggested by the authors1 to be seen in the accumulation of sand around a new object being placed on a beach and the loss of sand from a beach when a breakwater is built up-current from it. Seasonal and interannual wind variations influence currents that remove sand from a beach in a season of high wind speed that produce high waves and replaced by different currents associated with winds of low speed that produce lower waves in a different season. 

A number of factors such as the total amount of water in an ocean, changes that affect the containment volume of the oceans of the world and changes in the temperature/salinity characteristics that change its density, which in turn changes the amount of expansion or contraction of the volume of water, all have an effect on the height of the sea level that affects the coast. The volume of ice in ice sheets and glaciers on land are the main determinant of the volume of water in the ocean. The volume of the oceans is not affected by sea ice volume changes in the arctic or the The Antarctic, as ice floats in water. Volume changes result from tectonic, the slow rebound of continents that is still occurring at the present following the melting of the land-based glaciers following the last ice melting, rebounding due to the continued melting of glacier ice. Seawater expands as its het content increases and contracts when it cools.

Between the years 1870 and 2003 there was a rise of  20 cm in the sea level, and included in this was a 3 cm rise in the last 10 years, and of the 3 cm rise 1.60cm has been ascribed to thermal expansion, 0.4 cm to the melting of the ice sheets of Greenland and Antarctica, 0.8 cm to the melting of other glaciers, with a 0.3 cm residual. Over the next 100 years sea levels have been projected to rise by 30 ± 10 cm due to ocean warming, absorbing most of the anthropogenic heat production in the climatic system of the Earth. See Bindoff et al., 2007 in the 4 assessment report of the Intergovernmental Panel on Climate Change).

Sources & Further reading

  1. Emery, William J., Pickard, George L., Tally, Lynne D., & Swift, James H., 2011, Descriptive Physical Oceanography, an Introduction, Academic Press.

Links

Website for source 1

 

Author: M. H. Monroe
Email:  admin@austhrutime.com
Last Updated 16/04/2012

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading