A fjord is a long, narrow and usually very deep inlet of the sea between steep walls, made when a glacier carved a valley below sea level and the sea flooded in after the ice melted. Fjords are found on mountainous coasts that were covered by ice sheets in the last ice age: Norway, Greenland, Iceland, Chile, New Zealand, British Columbia, Alaska and Antarctica. Norway's Sognefjord runs 205 km inland and reaches 1,308 m deep. The deepest known fjord, Skelton Inlet in Antarctica, is about 1,930 m.
The word is Norwegian, from the Old Norse fjǫrðr, which is related to the English "ford" and "ferry". It meant a stretch of water you travelled across or along. In Norway it can name almost any long inlet, but geographers keep it for the glacial kind.
How a Glacier Digs Below Sea Level
Rivers can't cut much below the level of the sea they flow into. Glaciers can. A valley glacier can be a kilometre or more thick, and at its base it drags rock fragments frozen into the ice across the bedrock. Where the ice is thickest and flows fastest, often where a valley narrows or where several glaciers join, it grinds the valley floor down far below sea level. It also widens the old V-shaped river valley into the steep-sided U-shape that marks a glaciated valley.
When the ice sheets melted at the end of the last glacial period, roughly 20,000 to 10,000 years ago, sea level rose by about 120 m and the sea flooded these deepened valleys. That is why a fjord is often deeper than the open sea outside it. The Sognefjord's deepest point is far below any part of the North Sea, which averages about 95 m.
The land has been rising ever since. Freed from the weight of the ice, Scandinavia is still rebounding by several millimetres a year, and old beaches can be seen as raised terraces high above the present shoreline. The ice ages that shaped these coasts are explained in our guide to what an ice age is.
The Threshold at the Mouth
Most fjords are shallowest at the mouth. Near the coast the glacier began to float, or spread out and thin, so it eroded less, and it often left a ridge of rock or moraine called a sill, or threshold. A fjord can be more than 1,000 m deep inside and only a few tens of metres deep across its entrance.
The sill controls how the fjord works. Fresh water from rivers and melting snow spreads over the top as a light surface layer, while the heavy salt water in the deep basin behind the sill can stay trapped for months or years. In some fjords the deep water loses nearly all its oxygen before a storm or a cold winter pushes dense new seawater over the sill and replaces it. Layered, oxygen-poor mud like this is valued by climate scientists because it preserves a year-by-year record.
Norway's Fjord Coast
Norway is the classic fjord country. Its mainland coastline measures about 2,650 km in a straight line, but more than 25,000 km once every fjord and bay is followed. The Sognefjord is its longest and deepest, with valley walls rising more than 1,000 m straight out of the water. Our guide to the Sognefjord follows it from the coast to its inner arms. Further south, the 179 km Hardangerfjord is the second-longest, with orchards of apples and cherries on the sheltered slopes above the water.
Two of its narrow side branches, the Geirangerfjord and the Nærøyfjord, were listed together as a UNESCO World Heritage Site in 2005 as among the finest fjord landscapes on Earth. The Nærøyfjord is only about 250 m wide at its narrowest. The Geirangerfjord is known for waterfalls such as the Seven Sisters, which fall directly into the sea.
Fjords divide western Norway into steep-sided sections, and much of its infrastructure exists to cross them. The ferry network is among the densest in Europe. The Lærdal Tunnel, opened in 2000, runs 24.5 km under the mountains between two branches of the Sognefjord and is the world's longest road tunnel. Norway is also slowly building the E39, a project to remove every ferry crossing on the coastal highway from Kristiansand to Trondheim.
Fjords Beyond Norway
Greenland's Scoresby Sund, about 350 km long, is usually counted as the largest fjord system in the world. New Zealand's Fiordland has fourteen fjords, including Milford Sound and Doubtful Sound. They were called "sounds" by early European sailors, and the name stuck even though geologically they are fjords. Milford Sound, named after Milford Haven in Wales, has walls that rise almost 1,700 m to the summit of Mitre Peak.
On the Pacific coast of the Americas, glaciers carved long fjords into British Columbia and south-east Alaska, and the Chilean coast south of Puerto Montt breaks into a maze of fjords and channels that continues almost to Cape Horn. Some fjords still end at an active glacier. In Alaska's Glacier Bay and in Greenland, ice still breaks off into the head of the fjord. Greenland's Ilulissat Icefjord, a UNESCO site since 2004, is fed by Sermeq Kujalleq, one of the fastest-moving glaciers in the world.
Fjords Closer to Home in Britain, Ireland and Iceland
Fjords are not limited to remote coasts. Many of Scotland's sea lochs on the west coast are fjords, including Loch Etive, Loch Hourn and Loch Nevis. Each is a glacially deepened valley with a shallow sill at the mouth, and some of them are deeper inside than the sea beyond. Killary Harbour, on the border of Counties Galway and Mayo, runs about 16 km inland between steep mountains and is often called Ireland's only true fjord, although Carlingford Lough and Lough Swilly are sometimes counted too.
Iceland's Westfjords peninsula has a coastline broken into so many fjords that the road around it is several times longer than the peninsula is wide. Villages sit at the heads of fjords, separated from each other by high plateaus. Several of them were connected only when tunnels were drilled through the ridges between the fjords, a smaller version of Norway's approach to the same problem.
Fjord, Ria, Fjard and Sound
A ria is a river valley drowned by rising sea level, without glaciation. It gets gradually deeper and wider towards the sea, as in north-west Spain's Rías Baixas or Cornwall's Fal estuary. A fjard is a glacial inlet in low, rocky ground, typical of the Swedish and Finnish coasts. It is shallow and has none of a fjord's high walls. Sound has no geological meaning. It is a name for a wide channel or inlet, which is how both a glacial fjord in New Zealand and Puget Sound in Washington can carry the same word.
Life in Deep, Still Water
The layered water of a fjord supports unusual life. Cold, dark, stable deep water lets animals normally found far offshore live close to the coast. In Norway's Trondheimsfjord, reefs of the cold-water coral Desmophyllum pertusum (long known as Lophelia) grow in water as shallow as about 40 m, where divers can reach them. The same shelter makes fjords ideal for fish farming. Norway is the world's largest producer of farmed Atlantic salmon, most of it raised in net pens in fjords. That has led to a long dispute over sea lice and escaped farm fish affecting wild salmon returning up the same fjords to spawn.
Cruise Ships in the World Heritage Fjords
The Geirangerfjord and Nærøyfjord attract hundreds of cruise calls a year, and in narrow, windless fjords exhaust can hang between the walls. Norway has responded by legislating that ships in its World Heritage fjords must become zero-emission, starting with smaller vessels in 2026 and extending to large cruise ships in 2032. Small villages such as Geiranger, with a few hundred residents, can receive thousands of cruise passengers in a single day in summer, which is why the rules are closely watched by other fjord destinations.
Rockfalls and Tsunamis
Steep walls above deep, narrow water are dangerous. When rock falls into a fjord, the confined water turns it into a tsunami. In Norway's Tafjord in 1934, a rockslide of about two million cubic metres sent waves up to 62 m high along the shore and killed 40 people. Rockslides into Lake Loen, a fjord-like lake, killed 61 people in 1905 and 74 in 1936. Today the unstable slope at Åkneset above the Storfjord, parts of which creep downhill by several centimetres a year, is monitored continuously, with warning sirens in Geiranger and Hellesylt.
The most extreme example is in Alaska. In 1958 an earthquake shook about 30 million cubic metres of rock into Lituya Bay, and the water it displaced washed 524 m up the opposite slope, the highest wave ever recorded. More recently, in September 2023 a landslide into Dickson Fjord in east Greenland set off a wave that sloshed back and forth for nine days. Seismometers around the world detected it as a mysterious signal repeating every 92 seconds before scientists traced it to the fjord.