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SEA LEVEL CHANGE

SEA-LEVEL CHANGE

Sea-level change has been a feature on Earth for billions of years, and it has important implications for coastal processes and both erosional and depositional features. There are three primary mechanisms of sea-level change, as described below.

Eustatic sea-level changes are global sea-level changes related either to changes in the volume of glacial ice on land or to changes in the shape of the sea floor caused by plate tectonic processes. For example, changes in the rate of mid-ocean spreading will change the shape of the sea floor near the ridges, and this affects sea level.

Over the past 20,000 years, there have been approximately 125 m of eustatic sea-level rise due to glacial melting. Most of that took place between 15,000 and 7,500 years ago during the significant melting phase of the North American and Eurasian Ice Sheets. At around 7,500 years ago, the rate of glacial melting and sea-level rise decreased dramatically, and since that time, the average rate has been in the order of 0.7 mm/year. Anthropogenic climate change led to accelerating sea-level rise starting around 1870. Since that time, the average rate has been 1.1 mm/year, but it has been gradually increasing. Since 1992, the average rate has been 3.2 mm/year.

Isostatic sea-level changes are local changes caused by subsidence or uplift of the crust related either to changes in the amount of ice on the land or to growth or erosion of mountains.

Almost all of Canada and parts of the northern United States were covered in thick ice sheets at the peak of the last glaciation. Following the melting of this ice, there has been an isostatic rebound of continental crust in many areas. This ranges from several hundred meters of rebound in the central part of the Laurentide Ice Sheet (around Hudson Bay) to 100 m to 200 m in the peripheral parts of the Laurentide and Cordilleran Ice Sheets — in places such as Vancouver Island and the mainland coast of B.C. In other words, although global sea level was about 130 m lower during the last glaciation, the glaciated regions were depressed at least that much in most places, and more than that in places where the ice was thickest.

Tectonic sea-level changes are local changes caused by tectonic processes. The subduction of the Juan de Fuca Plate beneath British Columbia is creating tectonic uplift (about 1 mm/year) along the western edge of Vancouver Island, although much of this uplift is likely to be reversed when the next sizeable subduction-zone earthquake strikes.

Estuaries and fiords commonly characterize coastlines in areas where there has been a net sea-level rise in the geologically recent past. This valley was filled with ice during the last glaciation, and there has been a net rise in sea level here since that time. Coastlines in areas where there has been a net sea-level drop in the geologically recent past are characterized by uplifted wave-cut platforms or stream valleys. Uplifted beach lines are another product of relative sea-level drop, although these are difficult to recognize in areas with vigorous vegetation.

EMERGENT AND SUBMERGENT COASTS

Coastlines that have a relative fall in sea level, either caused by tectonics or sea level change, are called emergent. Where the shoreline is rocky, perhaps with a sea cliff, waves refracting around headlands attack the rocks behind the point of the headland.

They may cut out the rock at the base forming a sea arch which may collapse to isolate the point as a stack. Rocks behind the stack may be eroded, and sand eroded from the point collects behind it forming a tombolo, a sand strip that connects the stack to the shoreline. Where sand supply is low, wave energy may erode a wave cut platform across the surf zone, exposed as bare rock with tidal pools at low tide. Wave energy expended at the base of a sea cliff may cut a wave notch.

Sea cliffs tend to be persistent features as the waves cut away at their base and higher rocks calve off by mass wasting. If the coast is emergent, these erosional features may be elevated relative to the wave zone. Wave-cut platforms become marine terraces, perhaps with remnant sea cliffs inland from them.

Tectonic subsidence or sea level rise produces a submergent coast. Features associated with submergent coasts include estuaries, bays and river mouths flooded by the higher water. Fjords are ancient glacial valleys now flooded by post Ice Age sea level rise. Elongated bodies of sand called barrier islands form parallel to the shoreline from the old beach sands, often isolated from the mainland by lagoons behind them. The formation of barrier islands is controversial; some workers believe as above that barrier islands were formed by rising sea level as the ice sheets melted after the last ice age. Accumulation of spits and far offshore bar formations are also mentioned as possible formation hypotheses for barrier islands.

Tidal flats or mudflats form where tides alternately flood and expose low areas along the coast. Combinations of symmetrical ripple marks, asymmetrical ripple marks from tidal currents, and mud cracks from drying form on these tidal flats. An example of ancient tidal flat deposits is exposed in the Precambrian strata found in the central part of the Wasatch Mountains of Utah. These ancient deposits provide an example of applying Hutton’s Uniformity Principle. The presence of features common on modern tidal flats prompts the interpretation that these ancient deposits were formed in a similar environment. There were shorelines, tides, and shoreline processes acting at that time, yet the age of the ancient rocks indicates that there were no land plants to hold products of mechanical weathering in place so rates of erosion would have been different.  The Uniformity Principle must be applied with some knowledge of the context of the application.

Typically tidal flats are broken into three different sections, which may be abundant or absent in each tidal flat. Barren zones are areas with strong, flowing water and coarser sediment, with ripples and cross bedding common. Marshes are vegetated with natural sand and mud. Salt pans are the finest-grained parts of the tidal flats, with silty sediment, mud cracks, and is less often submerged.

Lagoons are locations where spits, barrier islands, or other features have partially cut off a body of water from the ocean. Estuaries are a (typically vegetated) type of lagoon where fresh water is flowing into the area as well, making the water brackish (between salt and fresh water). However, terms like a lagoon, estuary, and even bay are often loosely used in place of one another. Lagoons and estuaries are indeed transitional between terrestrial and marine geologic environment, where littoral, lacustrine, and fluvial processes can overlap.

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Introduction to Physical Geography by R. Adam Dastrup is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

Kategoria: Moje artykuły | Dodał: kolo (2019-04-09)
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