A fiord (sometimes spelled fjord) is a long, narrow, deep inlet of the sea flanked by steep cliffs or mountains. This distinctive geological feature results from glacial erosion followed by the inundation of water. A fiord is a submerged glacial valley, defined strictly by its glacial origin.
How Glaciers Carve the Landscape
The creation of a fiord begins when a massive glacier flows through a pre-existing river valley. The glacier’s immense weight and movement exert tremendous erosive force on the bedrock through two primary mechanisms: abrasion and plucking.
Abrasion occurs as rock fragments frozen into the ice act like sandpaper, grinding the valley floor and sides. Plucking involves meltwater penetrating bedrock cracks, freezing, and expanding, which loosens large blocks of rock carried away by the ice. This concentrated action deepens the valley far below sea level, a process known as overdeepening.
The ice sculpts a profound trough well below the elevation of the adjacent sea floor. When the climate warms, the glacier melts and retreats. Rising sea levels then flood this deeply excavated valley, resulting in a long, submerged glacial trough connected to the ocean.
Essential Physical Features
The most recognizable feature of a fiord is its distinctive U-shaped cross-section, a signature of glacial erosion. Unlike the V-shape of river valleys, the U-shape is characterized by a broad, flat bottom and steep, near-vertical side walls that plunge directly into the water. These towering walls can rise hundreds of feet from the water’s surface, creating a confined channel.
Fiords are often surprisingly deep, frequently hundreds of meters below sea level, with some reaching depths greater than 1,200 meters. The deepest part is often found inland, where the erosive power of the glacier was greatest due to thicker ice. A defining structural element is the sill, or threshold, an underwater ridge of bedrock or glacial debris located near the fiord’s mouth.
This sill is shallower than the rest of the fiord. It represents the area where the glacier’s erosive power lessened as it entered the sea, or it is composed of a terminal moraine. This shallow barrier differentiates a fiord from other coastal inlets. The sill can restrict water circulation, leading to unique hydrological and ecological conditions within the deeper basin.
Global Geography and Distribution
Fiords are geographically concentrated in specific, high-latitude coastal regions that were heavily impacted by continental or alpine glaciation during the Pleistocene Ice Ages. Their distribution is a direct map of where massive ice sheets flowed down to the ocean. Consequently, major fiord systems are found in both the Northern and Southern Hemispheres.
In the Northern Hemisphere, extensive fiord coastlines are located in regions like Greenland, Alaska, Iceland, and the western coast of Canada’s British Columbia. Southern Hemisphere fiords are found in coastal areas of Chile, New Zealand’s Fiordland, and parts of the Antarctic Peninsula. Their existence in these widely separated locations is strictly a function of past climate and the presence of glaciers capable of carving deep troughs into the landscape.
Fiords Versus Other Coastal Inlets
Coastal inlets that appear similar to fiords, such as rias, estuaries, and bays, are fundamentally different due to their formation processes. The defining characteristic of a fiord is its formation exclusively through the intense erosion of a valley by a glacier. This glacial origin is responsible for the fiord’s U-shaped cross-section, profound depth, and the presence of a sill at its mouth.
A ria, by contrast, is a submerged river valley that was formed primarily by fluvial (river) erosion and later flooded by rising sea levels. Rias typically feature a V-shaped cross-section and generally increase in depth moving seaward, lacking the deep, over-excavated basins and shallow sills of fiords. Estuaries are often river mouths where fresh and saltwater mix, and while a fiord can also function as an estuary, the term estuary describes the water dynamics and not the dramatic, glacially-sculpted geological structure.