Fjord Formation and the Role of Glaciation
A fjord is a long, narrow, steep-sided inlet carved by a glacier and subsequently flooded by the sea. They are among the most dramatic coastal landforms on Earth — mile-deep troughs of still water flanked by near-vertical rock walls, fed by waterfalls cascading from hanging valleys high above, and plunging to depths that can exceed the height of the surrounding mountains. Fjords exist on the coasts of Norway, Greenland, Iceland, Canada, Alaska, Chile, New Zealand, and Antarctica — wherever glaciers once flowed to the sea and sea levels subsequently rose. Understanding how they form, why some are deeper than others, and what makes fjord environments ecologically distinct begins with understanding the power of glacial erosion.
The Mechanics of Glacial Erosion
A glacier erodes its bed through two principal mechanisms. Abrasion occurs where rock debris embedded in the basal ice — fragments plucked from the bed and entrained by the ice — grinds against the underlying rock surface like a very slow, very heavy piece of sandpaper. The result is polished rock, striated by parallel scratches aligned with the direction of flow, and glacial flour — rock ground to particle sizes below a few micrometres. Plucking (or quarrying) is the second mechanism: the glacier freezes to the bedrock, and as the ice moves, it levers out blocks along existing joints and cracks, incorporating them into the ice and leaving the bed irregularly broken.
The combination of these two processes, operating over thousands to tens of thousands of years, can excavate enormous volumes of rock. A glacier flowing down a pre-existing river valley deepens and widens it dramatically, producing the characteristic U-shaped cross-section of a glaciated valley: steep or vertical walls and a broad, flat floor — in contrast to the V-shaped cross-section of a river-cut valley. Where that U-shaped valley reaches the sea, and sea level subsequently rises at the end of the glacial period, the valley floods to become a fjord.
Why Fjords Are So Deep
The depth of fjords is remarkable. Sognefjorden in Norway, the world's deepest fjord, reaches 1,308 metres at its maximum, and the floors of many other Norwegian fjords exceed 800 metres. This is deeper than most of the open ocean in the surrounding North Sea. Why should glaciers erode so deeply?
The answer lies in the physics of glacier flow under pressure. The thickness of a glacier determines the pressure at its base, and thicker ice means higher basal pressure, which lowers the melting point of ice and allows more effective basal sliding. In a trough where ice is funnelled and thickened, erosion rates are higher, and the trough deepens preferentially. The length of time the glacier occupies the trough also matters — fjords that were repeatedly glaciated through multiple glacial cycles over millions of years are deeper than those glaciated only once or twice.
Many fjords have a shallower sill at their mouth — a bedrock ridge or terminal moraine that was deposited where the glacier floated off the bed as it reached the open sea and its erosive power diminished. The sill separates the deep inner basin from the shallower outer waters. This has significant implications for fjord oceanography: deep, dense, salt-rich oceanic water can only exchange with the water in the inner basin over this sill, creating conditions of restricted circulation and sometimes low oxygen at depth.
The World's Great Fjord Systems
Norway's fjord coast is the archetype: the Sognefjord stretches 204 kilometres inland from the coast to its head at Skjolden, with branches including the photogenic Naeroyfjord (a UNESCO World Heritage Site) and Aurlandsfjord. The Hardangerfjord, Geirangerfjord, and Lysefjord are among the most visited — Lysefjord is home to Preikestolen (the Pulpit Rock), a flat-topped cliff rising 604 metres above the water.
Greenland's fjords are on a comparable or even greater scale. Ilulissat Icefjord, a UNESCO World Heritage Site, receives the output of the Sermeq Kujalleq glacier, one of the fastest-moving glaciers on Earth. Icebergs calved from this glacier travel down the fjord to Disko Bay. The fjord system of Scoresby Sund in eastern Greenland is the world's largest fjord system by area, extending hundreds of kilometres inland from the coast.
Chilean Patagonia contains the world's most extensive southern hemisphere fjord system. The coastline between Puerto Montt and Cape Horn is heavily dissected by fjords, channels, and island groups carved by Pleistocene glaciers. The Fjord of the Cordillera (Canal Beagle) and the fjords of the Bernardo O'Higgins National Park, accessible from Puerto Natales, are among the most dramatic of a coastline that extends for thousands of kilometres.
New Zealand's South Island holds four major fjords in Fiordland National Park, accessible by road or air from Te Anau. Milford Sound (Piopiotahi) — technically a fjord despite its name — is perhaps the most visited fjord in the southern hemisphere. At 16 kilometres long and flanked by walls rising over 1,200 metres, with Mitre Peak as its iconic centrepiece, it receives heavy rainfall and is characterised by a surface layer of fresh water over salt water that creates unusual visibility conditions for diving. Doubtful Sound is larger and more remote, accessible by a combination of boat and overland transport.
Fjord Ecology
The restricted circulation of many deep fjords creates stratified water columns with distinct chemical and biological characteristics at different depths. The fresh-water layer at the surface in high-precipitation fjords (maintained by waterfall input) is low in nutrients relative to the deep salt-water below. Where mixing occurs — at the sill, during storms, or through tidal processes — nutrients are brought to the surface and can drive high biological productivity.
Fjord waters support populations of marine mammals including harbour seals, porpoises, and, in higher-latitude fjords, belugas and narwhals. The cold, clear waters are important habitats for kelp forests and cold-water coral systems in deeper sections.
Explore the glacier map to locate the major fjord coastlines and identify the glaciated regions from which the most dramatic fjord landscapes descend. The ice that carved these landscapes has retreated, in most cases by tens of thousands of years, but the evidence of its work is permanent and magnificent.