How Are Fjords Created? The Glacial Process Explained

Fjords are drowned glacial troughs, carved into bedrock by rivers of ice that followed the same routes through mountainous coastlines over millions of years and multiple ice ages. The process is not a single dramatic event but a cumulative one: each glacial cycle sends ice streaming through pre-existing valleys and fault lines, grinding them wider and deeper until they reach depths far below sea level. When the ice finally retreats and the ocean rushes in, the result is the steep-walled, deep-water inlets that define the coastlines of Norway, Greenland, Chile, New Zealand, and western Canada. The story of how they form, though, involves geology, ice physics, and ocean dynamics working together in ways that are more complex than the simple image of a glacier scooping out a valley.

It Starts With the Rock, Not the Ice

One of the most counterintuitive findings in fjord science is that geology, not glaciology, is the primary control on where fjords form and which direction they run. Glaciers are powerful erosive agents, but they do not carve randomly. They follow paths of least resistance through the landscape, exploiting pre-existing weaknesses in the bedrock: fault zones, fracture networks, and softer rock layers that were already in place long before any ice appeared. In the Magellan region of southern Chile, for example, researchers found that fjord orientation is strongly controlled by three major strike-slip fault zones, with underwater mapping revealing direct continuity between on-land tectonic features and the layout of the fjord system beneath the water’s surface.¹

This pattern holds globally. A study comparing fjord systems in Greenland, Antarctica, and the British Isles found that mapped fjord orientations bear a striking similarity to the structural geology of each region, with fjords running preferentially along the same directions as structural lineaments in the bedrock.² Successive glaciations followed the same ice-discharge routes, widening and deepening those pre-existing geological structures with each pass. Think of it like water wearing a groove into pavement along an existing crack: the crack came first, and every rainfall deepens it further. Fjords are the glacial equivalent, built up over repeated ice ages that each exploit the same geological blueprints.

How Glaciers Carve Rock

Once ice is channelled into a valley or fault-guided trough, it erodes the bedrock through two main mechanisms. The first is abrasion: rocks and sediment trapped in the base of the glacier act like sandpaper dragged across the valley floor and walls. Fine silt polishes the surface smooth. Larger stones scratch grooves and striations into the rock. Still larger clasts can pry small fragments loose as they grind past. The second mechanism is plucking, sometimes called quarrying, which is more violent. Meltwater seeps into cracks in the bedrock beneath the glacier, refreezes, and when the ice moves, it rips out large blocks. Plucking does not require debris already embedded in the glacier; instead, it produces new debris, feeding more material into the abrasion process.³

These two processes leave distinctive marks on the landscape. Abrasion creates smooth, streamlined bedforms called whalebacks and polished surfaces scored with fine lines. Plucking creates rough, angular lee-side faces on the downstream side of rock bumps. When you see both textures together on an exposed rock surface, you are looking at the signature of ice that once flowed overhead.

The balance between abrasion and plucking depends on the properties of the local bedrock. Harder rock with widely spaced joints tends to resist plucking and gets mostly abraded. Softer rock with dense fractures gives the ice plenty of purchase for quarrying. This is one reason fjords in different regions can look quite different at the small scale, even though the large-scale process is the same.

Why Fjords Get So Deep

Rivers cannot erode below sea level in any sustained way because water loses its erosive energy at base level. Glaciers face no such constraint, which is why fjords routinely reach depths that would be impossible for a river valley. Sognefjorden in Norway plunges over 1,300 meters below sea level. Scoresby Sund in Greenland is even deeper in places. The process that produces these extreme depths is called overdeepening.

Overdeepening works because ice has a positive surface gradient: as long as the ice surface slopes downward in the direction of flow, the glacier can push ice and meltwater up and over bumps in the bed beneath it. This means the ice can keep eroding the floor of a basin even as that floor drops well below the level of the terrain outside the trough. Sediment gets evacuated by the combination of ice movement and subglacial water flow, preventing the hole from filling back in and allowing the glacier to continue biting deeper into the rock.⁴

Erosion is not evenly distributed along a glacier’s length. Research on glacial hydrology shows that erosion peaks at two locations: near the equilibrium line, where snowfall and melting are in balance, and at lower altitudes in the ablation zone where meltwater is most abundant. The abundance of water matters because high subglacial water pressure reduces the effective pressure holding the glacier to its bed, which promotes faster sliding and more vigorous erosion. This pattern helps explain why glacial erosion can simultaneously limit the height of mountain ranges and carve deep fjords or glacial lakes at lower elevations.⁵

The Sill at the Mouth

If you were to drain the water from a typical fjord, you would not see a smooth U-shaped trench running all the way to the open sea. Instead, you would find a pronounced ridge or shallow area at or near the fjord’s mouth, called a sill or threshold. This sill is often a combination of bedrock and moraine material left behind by the glacier at its farthest advance. At the mouth of Greenland’s Ilulissat Ice Fjord, for instance, acoustic mapping revealed a flat-topped threshold called the Iceberg Bank, which is a basement-rock sill embedded within a massive terminal moraine, separating the deep inner fjord from the continental shelf.⁶

Sills form because glacial erosion is strongest where ice is thickest and most confined, which is typically well upstream of the glacier’s terminus. Near the mouth, the ice spreads and thins as it exits the trough, reducing its erosive power. The glacier also deposits sediment and rock at its snout, building up a moraine that acts as a natural dam. The result is a fjord that is often much deeper in its middle reaches than at its entrance, sometimes dramatically so.

This sill has enormous consequences for the fjord’s later life as a marine environment. It acts as a physical barrier that restricts the exchange of water between the deep fjord basin and the open ocean. Water above the sill depth circulates relatively freely, but water trapped below the sill can remain stagnant for years at a time. During these stagnant periods, biological consumption of dissolved oxygen outpaces the supply of fresh oxygenated water, and the deep basin can become hypoxic or even anoxic.⁷ This is a defining feature of fjord oceanography: the very glacial processes that created the fjord also built in a structural vulnerability to oxygen depletion.⁸

From Ice Trough to Seawater Inlet

A fjord is not technically a fjord until the ocean occupies it. The transition from ice-filled trough to marine inlet happened at different times in different places, depending on how quickly the local glacier retreated and how fast sea level rose. In Fiordland, New Zealand, researchers used sill depths and sea-level curves to reconstruct the timing of marine inundation across the region. They found it was surprisingly spread out: the first fjords flooded as early as roughly 17,000 years ago, while the last ones did not become marine until after about 8,700 years ago, with major flooding episodes coinciding with pulses of rapid global sea-level rise known as Meltwater Pulses 1A and 1B.⁹

The depth of a fjord’s sill largely determined when it flooded. Fjords with shallow sills needed sea level to rise higher before water could overtop the threshold and fill the basin. Deeper-silled fjords flooded earlier. This means a single region can contain fjords of very different ages, even though they were all carved by glaciers operating in the same general time frame. The fjord you see today is a product of both the erosion that shaped it and the particular moment when the rising ocean found its way in.

Not One Glacier, but Many

Fjords are palimpsest features, shaped and reshaped by successive glaciations rather than carved in a single episode. Each ice age sends glaciers back through the same troughs, widening and deepening them further. Between glaciations, sediments partially fill the fjord, but the next advance of ice typically scours most of that material out again. In most cases, the repeated cycle of glacial erosion through multiple glacial and interglacial periods has ensured the removal of older sediment sequences within the fjord.¹⁰ This is why the sediment record in many fjords is surprisingly young relative to the age of the trough itself: the older record has been bulldozed away.

In North and Northeast Greenland, researchers have estimated that fjord erosion drove up to roughly 940 meters of flexural isostatic uplift in the surrounding landscape, meaning the sheer volume of rock removed from the fjords was so great that the unburdened crust rebounded significantly upward.¹¹ This gives a sense of the staggering quantity of material that glacial erosion removes over time. The mountains flanking a fjord are not just walls left behind; they are partially elevated by the very process that carved the valley between them.

What Happens After the Ice Leaves

Once a glacier retreats from a fjord for the last time, sediment begins to accumulate on the fjord floor, delivered by rivers, glacial meltwater, slope processes, and biological productivity. In Norwegian fjord valleys, these post-glacial sediment fills can reach 100 meters or more in thickness, building up in a characteristic sequence. The initial fill comes from glacial sources as the ice retreats. Later, rivers take over as the dominant sediment supplier, building deltas that advance into the fjord from the head.¹² Some fjords that once held seawater have been entirely filled with sediment and are now dry valleys, visible only in the stratigraphic record beneath farmland.

Sedimentation rates in active fjord systems vary widely depending on climate and the proximity of glaciers. In Billefjorden, a subpolar fjord in Svalbard, 20th-century sedimentation rates ranged from about 0.08 to 0.39 centimeters per year. During the preceding cold period known as the Little Ice Age, rates were several times lower, likely because the glaciers were at their maximum extent and locking up sediment rather than releasing it. The rapid acceleration in sedimentation that followed the glacial retreat of the 20th century suggests that warming and ice loss can substantially change how fast a fjord fills up.¹³

Fjords as Climate Archives

The same characteristics that make fjords unusual marine environments also make them valuable scientific instruments. Their sheltered, deep basins trap sediment at rates much higher than the open ocean, and because the sill restricts bottom-water exchange, those sediments accumulate in relatively undisturbed layers. This means fjord sediment cores can offer annual or near-annual resolution records of past climate and environmental change.¹⁴

In the inner Sognefjord region of western Norway, researchers have extracted cores with sedimentation rates approaching a centimeter per year, allowing them to resolve individual years. Peaks in mineral clay particles, organic matter, and freshwater diatom abundance in these cores correlate well with measured regional precipitation, temperature, and cloud cover over the past two decades. The same records also track broader climate signals, including the North Atlantic Oscillation and northern hemisphere temperature trends, over the past 60 years.¹⁵ Fjord sediments, in other words, are natural recording devices that capture both local weather and hemispheric climate patterns in their layered muds.

Carbon Burial Hotspots

Fjords cover a tiny fraction of the global ocean surface, roughly 0.1%, yet they punch far above their weight in terms of carbon storage. Estimates suggest that about 18 million tonnes of organic carbon are buried in fjord sediments each year, accounting for around 11% of all marine carbon burial globally.¹⁶ Per unit area, fjord carbon burial rates are about a hundred times the global ocean average, and fjord sediments contain roughly twice as much organic carbon as the biologically productive sediments beneath ocean upwelling zones.

This disproportionate role exists because fjords receive large inputs of terrestrial organic carbon from rivers and glacial runoff, and their restricted circulation and high sedimentation rates help bury that carbon before it can be broken down and returned to the atmosphere. Research into the reactivity of this buried carbon suggests that fjords function as transitory reservoirs, storing labile organic carbon over timescales of thousands of years, modulated by interactions between organic matter and minerals in the sediment.¹⁷ The implication is that fjords may play a meaningful role in regulating atmospheric carbon on glacial-interglacial timescales, a factor that is only beginning to be incorporated into global carbon cycle models.¹⁸

Unusual Ecosystems Shaped by Fjord Geometry

The steep walls, restricted circulation, and extreme depth gradients of fjords create ecological conditions found almost nowhere else. One striking phenomenon is deep-water emergence: species that normally live in the deep ocean can survive at surprisingly shallow depths in fjords because the cold, dark, low-salinity conditions near the surface of some fjords mimic deep-sea habitats. In New Zealand’s Fiordland, usually deep-living corals grow within diving depth. But these shallow fjord populations face stresses their deep-ocean relatives do not. Temperatures in shallow fjord habitats can vary two to seven times more than in deep-fjord habitats and seasonally reach levels that appear lethal if sustained, interrupting reproductive cycles and stressing organisms adapted to the stability of the deep sea.¹⁹

These accessible shallow populations offer researchers a rare window into how deep-sea species might respond to ocean warming. Studying corals at 10 meters depth in a fjord is vastly easier than studying them at 500 meters on the open continental slope, and the thermal variability they already endure may preview what deep-water populations will face as ocean temperatures rise.

Fjords and Geohazards in a Warming Climate

The same glacial processes that create fjords can set the stage for dangerous events long after the ice retreats. When a glacier thins or withdraws from a valley, it removes the physical buttress that was supporting the adjacent rock walls. This glacial debuttressing can leave steep slopes primed for collapse, sometimes thousands of years after the ice disappears. A study of an Early Holocene event in South Greenland documented a giant rock-ice avalanche triggered by glacial debuttressing during deglaciation, which generated a tsunami when the debris hit a nearby fjord.²⁰

These events are not just ancient history. In September 2023, a rock-ice avalanche of about 25 million cubic meters plunged into Dickson Fjord in East Greenland, triggered by glacial thinning destabilizing a mountainside. The impact generated a tsunami roughly 200 meters high inside the fjord. The resulting seismic signal was so powerful and sustained that it rang through Earth’s crust for nine days, detectable on seismometers worldwide as a monochromatic signal with a period of about 92 seconds.²¹ Events like this are expected to become more frequent as climate change drives continued ice loss in polar and mountain regions, posing real hazards for communities situated along fjord coastlines. The narrow, enclosed geometry that makes fjords so visually dramatic also concentrates tsunami energy, amplifying wave heights in ways that would not occur on an open coast.