Fjords are geographical features characterized by steep cliffs that plunge directly into deep seawater. These inlets represent the power of geological forces that sculpted coastlines. Alaska’s coastline is defined by these formations, which carve deep into the mountainous landscape. This article will define the physical characteristics of a fjord and explain the glacial mechanisms responsible for their creation.
Defining the Physical Characteristics of a Fjord
A true fjord is a long, narrow body of water that is a former glacial valley now flooded by the sea. They are distinguishable from other inlets by their unique cross-section, which is a pronounced U-shape with near-vertical walls. This distinct shape contrasts sharply with the V-shaped valleys carved by rivers, providing a clear geological signature of their origin.
The depth of a fjord is another defining characteristic, often plunging far below the level of the adjacent ocean floor. Some of Alaska’s fjords can reach depths exceeding 1,000 feet due to the erosive power of the ice that once filled them. The water within these inlets is stratified, with a layer of fresher meltwater sitting atop deeper, saltier seawater.
At the mouth of a fjord, where it meets the open sea, there is usually a submerged ridge called a sill. This sill is a rocky threshold that is significantly shallower than the rest of the inlet, sometimes covered by only a few meters of water. The formation of this underwater barrier is a result of the glacier’s reduced erosive power at its terminus or the deposition of a terminal moraine.
The Glacial Sculpting Process
The formation of these deep troughs began during past Ice Ages, when continental ice sheets and valley glaciers flowed down pre-existing river valleys toward the coast. A key mechanism in this transformation is known as plucking, where meltwater seeps into fractures in the bedrock at the base of the glacier. When this water freezes, it expands, wedging out large chunks of rock from the valley floor and walls.
These loosened rock fragments then become embedded in the base of the moving ice, transforming the glacier into an abrasive tool. The grinding action of these entrained rocks against the bedrock is called abrasion, which smooths and deepens the valley floor. This combination of plucking and abrasion transforms a river’s V-shaped valley into the characteristic, wide U-shaped glacial trough.
Glacial erosion is most intense where the ice is thickest and where multiple ice flows converge, which is why the inner parts of fjords are often the deepest. As the climate warmed at the end of the last Ice Age, the glaciers began to retreat from the valleys. The land, having been depressed by the weight of the ice, began to slowly rebound upward, a process known as isostatic rebound.
However, the eustatic rise in global sea levels often outpaced this slow upward movement of the land. The deep, glacially carved valleys were subsequently flooded by the ocean, creating the long, narrow seawater inlets we recognize as fjords today.
Alaskan Fjords: Geography and Notable Examples
Alaska’s extensive coastline is one of the world’s regions known for these landforms, with a high concentration found along the Gulf of Alaska. Notable areas for these inlets include the Kenai Peninsula, Prince William Sound, and the Glacier Bay region of southeastern Alaska. Some fjords stretch for many miles inland.
Kenai Fjords National Park protects a landscape dominated by the 700-square-mile Harding Icefield and its outflowing glaciers. Here, the ice continues to actively sculpt the terrain, with several tidewater glaciers carving directly into the deep inlets. This proximity of ice, land, and sea makes the region an area of ongoing geological change.
The environment of these deep inlets creates a rich and diverse ecological habitat. The steep walls offer nesting sites for tens of thousands of seabirds, including puffins and murres. The cold, nutrient-rich waters support a variety of marine life, such as Steller sea lions, sea otters, and various whale species, which thrive within the fjord basins.