Sea Ice & Icebergs Explained
Ice shelves are thick floating extensions of glaciers and ice sheets. They form where land ice flows into the ocean, spread outward over seawater and help hold back inland ice that would otherwise move faster toward the sea.
What Are Ice Shelves?
Ice shelves are large, thick platforms of floating glacier ice attached to land. They are common around Antarctica and also occur in parts of the Arctic. Unlike
sea ice, which forms from frozen seawater, ice shelves are made from land-based glacier ice that has flowed into the ocean.
Ice shelves can be hundreds of meters thick and may extend far from the coast. Their fronts often break apart through
glacier calving, creating
icebergs.
How Do Ice Shelves Form?
Ice shelves form when glaciers or ice sheets flow from land into the ocean and remain attached to the coast while floating on seawater. Snowfall inland feeds the ice sheet, gravity pushes the ice outward, and the floating edge spreads into a shelf.
- Snow accumulation: snowfall compresses into glacier ice on land.
- Ice-sheet flow: gravity moves ice slowly toward the coast.
- Grounding line: ice begins to float where it leaves the bedrock and enters the ocean.
- Floating shelf: glacier ice spreads outward over seawater.
- Calving front: the outer edge breaks off periodically to form icebergs.
Why Ice Shelves Matter
Ice shelves act like natural brakes. Because they are attached to the coast and often pinned against islands, seafloor bumps or bay walls, they slow the flow of inland glaciers into the ocean. This stabilizing effect is called buttressing.
When an ice shelf thins, fractures or collapses, the glaciers behind it can accelerate. That matters because the inland ice feeding those glaciers can contribute to sea-level rise.
Ice Shelves vs. Sea Ice vs. Icebergs
Polar ice terms are easy to confuse, but they describe different things.
| Feature | How It Forms | Why It Matters |
|---|---|---|
| Sea ice | Ocean water freezes | Reflects sunlight and affects polar climate |
| Ice shelf | Glacier ice flows from land into the ocean | Buttresses inland glaciers and affects ice-sheet stability |
| Iceberg | Ice breaks from a glacier or ice shelf | Drifts through the ocean and can threaten ships |
What Makes Ice Shelves Thin?
Ice shelves can thin from above and below. Surface melting can create ponds and cracks, while warmer ocean water can melt the underside of the shelf. Basal melting from below is especially important where relatively warm ocean currents reach the ice-shelf base.
- Warm ocean water: melts the underside of the shelf.
- Surface meltwater: fills cracks and can force fractures open.
- Rifting: long cracks weaken the shelf structure.
- Calving: large icebergs break from the shelf front.
- Loss of ice mélange: broken ice in front of the shelf may no longer help resist calving.
Ice Shelf Collapse and Glacier Calving
Ice shelves naturally calve icebergs, but rapid collapse is different. A collapse can fragment large areas of floating ice over a short period, removing buttressing and allowing glaciers behind the shelf to speed up.
Calving is part of the normal life cycle of an ice shelf. Collapse becomes alarming when thinning, cracking and ocean warming cause the shelf to lose stability faster than it can be replenished by glacier flow.
Do Ice Shelves Raise Sea Level When They Melt?
Melting an ice shelf does not directly raise sea level much because the shelf is already floating. The bigger concern is indirect: when an ice shelf weakens or disappears, it can allow land-based glaciers behind it to flow faster into the ocean.
That extra land ice entering the sea can contribute to sea-level rise.
Ice Shelves and Ocean Circulation
Ice shelves are strongly connected to
ocean currents and circulation.
Ocean water can enter cavities beneath ice shelves, melt them from below and carry freshwater away from the ice front.
These interactions connect ice shelves to
ocean temperature,
polar stratification, sea ice formation and global climate feedbacks.
Famous Ice Shelves
Some of the best-known ice shelves are in Antarctica, including the Ross Ice Shelf, Filchner-Ronne Ice Shelf, Larsen Ice Shelf and Amery Ice Shelf. Their size, thickness and stability are closely monitored because they reveal how polar ice systems are responding to ocean and atmospheric change.
Related Polar Ice Topics
Ice shelves connect directly to
Sea Ice & Icebergs Explained,
Icebergs Explained,
Glacier Calving Explained,
Ice Mélange Explained and
Sea Ice Explained.
FAQ: Ice Shelves Explained
What is an ice shelf?
An ice shelf is a thick floating extension of a glacier or ice sheet that remains attached to land while spreading over the ocean.
How do ice shelves form?
Ice shelves form when land-based glacier ice flows into the ocean and begins floating while remaining attached to the coast.
Are ice shelves the same as sea ice?
No. Sea ice forms from frozen seawater, while ice shelves are made from glacier ice that originated as snowfall on land.
Do melting ice shelves raise sea level?
Melting floating ice shelves does not directly raise sea level much, but losing an ice shelf can let inland glaciers flow faster into the ocean, which can raise sea level.
Why are ice shelves important?
Ice shelves buttress inland glaciers, slowing their movement toward the ocean and helping stabilize parts of polar ice sheets.
What causes ice shelf collapse?
Ice shelf collapse can be caused by thinning from warm ocean water, surface melting, fractures, rifting, calving and loss of structural support.
