Coastlines of submergence

Overview

Coastline of submergence, ria and how does it form and shape and depth characteristics of a ria.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is a coastline of submergence?

   Model answer

   A coastline where relative sea level has risen, so the sea has flooded landforms that were created above sea level. It is produced either by a eustatic rise in the volume of ocean water, typically the return of meltwater at the end of a glacial period, or by an isostatic sinking of the land, or by a combination of the two. Its characteristic signature is a deeply indented, irregular coast in which the sea occupies valleys that were clearly cut by rivers or by ice, with the former interfluves left as headlands, peninsulas and islands.

2. 2

   What is a ria and how does it form?

   Model answer

   A ria is a drowned river valley. During a glacial period, sea level was much lower, so rivers cut their valleys down to that lower base level, extending across what is now the sea bed. When the ice melted and eustatic sea level rose, the sea flooded the lower reaches of those valleys. The result is a long, winding inlet that follows the course of the former river, and because the tributary valleys were drowned as well, the inlet has a branching, dendritic plan resembling the original river network. Rias are typical of upland coasts that were not themselves glaciated but were affected by the global rise in sea level.

3. 3

   Describe the shape and depth characteristics of a ria.

   Model answer

   In cross section a ria is V-shaped with relatively gentle, smoothly sloping sides, because it is a fluvial valley that has simply been flooded. In long section it is deepest at its mouth and becomes progressively shallower inland, and it narrows inland as well, following the natural downstream increase in size of a river valley. Depths are modest, often only a few tens of metres. There is no barrier at the mouth. The upper reaches are typically filled with mudflats and salt marsh, because the sheltered water allows fine sediment brought down by the river to settle.

4. 4

   What is a fjord and how does it form?

   Model answer

   A fjord is a drowned glacial trough. A valley glacier flowing to the sea eroded a deep U-shaped valley by abrasion and plucking, and because ice can erode below sea level where it is thick and moving, it overdeepened the valley floor to well below the present sea surface. When the ice retreated and eustatic sea level rose with the returning meltwater, the sea flooded the trough. The result is a long, deep, steep-sided inlet, generally straighter than a ria because ice cut off the interlocking spurs, and running far inland.

5. 5

   Describe the shape and depth characteristics of a fjord, and explain the threshold at its mouth.

   Model answer

   In cross section a fjord is U-shaped with very steep, often near-vertical rock walls, and it is extremely deep, commonly several hundred metres and in places over a kilometre. In long section its floor is irregular, with rock basins and, crucially, a shallow rock threshold or sill across the mouth where the water may be only a few tens of metres deep. The sill exists because the glacier lost thickness, and therefore erosive power, as it approached the sea and began to float, so it could no longer scour the bed there; in some cases the sill is a terminal moraine. Hanging valleys with waterfalls enter high up the walls, marking tributary glaciers that could not cut down as deeply as the main trunk.

6. 6

   Distinguish between a fjord and a ria.

   Model answer

   Both are drowned valleys produced by a rise in relative sea level, but they differ in what cut the valley and therefore in their form. A fjord was cut by ice, so it is U-shaped in cross section with steep rock walls, very deep, relatively straight, has hanging valleys entering it and has a shallow rock threshold at its mouth so that it is deeper inside than at the entrance. A ria was cut by a river, so it is V-shaped with gentler sides, comparatively shallow, sinuous and dendritic in plan, has no threshold, and is deepest at its mouth and shallows steadily inland. Fjords occur on high-latitude coasts that were glaciated; rias occur on coasts that were not.

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