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How does a fjord form?

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Quick answer

A fjord is a deep, narrow sea inlet with steep sides, formed when a glacier erodes a U-shaped valley below sea level. After the ice melts, the ocean floods the valley. The glacier's weight and movement carve the valley far deeper than a river could, creating the dramatic landscapes of Norway, Chile, and New Zealand.

A fjord forms when a glacier carves a deep U-shaped valley below sea level and the sea floods it after the ice retreats. The steep walls and drowned valley create the narrow, deep inlets seen along coasts like Norway.

Step by step

  1. Snow builds up in a mountain valley and compresses into glacial ice.
  2. The ice slowly flows downhill, scraping and plucking rock from the valley floor and walls.
  3. Over thousands of years, the glacier deepens and widens the valley into a U-shape.
  4. When the climate warms, the glacier melts and retreats up the valley.
  5. If the valley floor is below sea level, seawater floods in, forming a fjord.

Key things to understand

  1. Snow accumulates in mountain valleys and compacts into thick, slowly moving ice.
  2. The glacier erodes the valley floor and walls, deepening and widening it into a U-shape.
  3. When the climate warms, the glacier retreats, leaving a deep, steep-sided valley.
  4. If the valley floor lies below sea level, the ocean floods it, forming a fjord.
  5. Fjords often have shallow thresholds at their mouths, left by glacial debris.

Labelled diagram

  1. Glacier

    Thick ice fills the valley, slowly flowing downhill under its own weight.

  2. Erosion

    Ice scrapes and plucks rock, deepening and widening the valley into a U-shape.

  3. Retreat

    Warming climate causes the glacier to melt and retreat up the valley.

  4. Flooding

    Seawater enters the deep valley, creating a narrow, steep-sided fjord.

How a fjord forms from a glacier

Real-world examples

  • Sognefjord in Norway is over 200 kilometres long and more than 1,300 metres deep.
  • Milford Sound in New Zealand is a fjord with sheer cliffs rising over 1,200 metres.
  • Chile's Patagonian coast is cut by hundreds of glacier-carved fjords, many still fed by glaciers today.
  • Greenland's Scoresby Sund is a vast fjord system lined with icebergs.

Common mistake

✗ Myth: Fjords are formed by rivers cutting through mountains.

✓ Fact: Rivers carve V-shaped valleys, but fjords are carved by glaciers. The ice erodes a U-shaped valley that can lie below sea level, which the sea then floods.

People also ask

What is the difference between a fjord and a valley?
A valley is any low area between hills or mountains, often V-shaped from river erosion. A fjord is a specific type of valley: U-shaped, carved by a glacier, and flooded by the sea. Fjords are always coastal and deep, while valleys can be inland and shallow.
How long does it take for a fjord to form?
Fjord formation takes tens of thousands to millions of years. Glaciers advance and retreat over many ice ages, each cycle deepening the valley. The exact time depends on ice thickness, speed, and rock hardness, but it is a very slow geological process.
Why do fjords have steep sides?
Glaciers erode both the floor and the sides of a valley. The ice plucks rock from the walls, leaving steep cliffs. When the ice melts, the unsupported rock walls can collapse, but the overall U-shape remains, creating the dramatic steep sides characteristic of fjords.
Why is fjord water so deep?
Glacial erosion can gouge the valley floor hundreds of metres below sea level. The depth comes from the ice grinding rock over many thousands of years, not from the sea itself.
How are fjords different from river valleys?
River valleys are typically V-shaped and carved by flowing water. Fjords are U-shaped, much deeper, and often have steep cliffs because they were carved by ice, not rivers.
Can fjords form without glaciers?
No. Fjords are specifically glacial landforms. Without a glacier to carve a deep valley below sea level, a coastal inlet would be a ria or a river mouth, not a fjord.
Where are fjords found?
Fjords occur on coasts at high latitudes where glaciers reached the sea, such as Norway, Chile, New Zealand, Canada, Greenland, and Alaska. They are most common in regions once covered by ice sheets.

Part of the guide: Nature & Earth: Living Things and Rocks →