eduKate Learning Manual
Science | Edge Cases Science | Earth, Water, Atmosphere & Celestial World
Understand → Observe → Explain → Test → Transfer → Go Deeper
Brinicles
How Sea Ice Grows a Hollow Ice Tube Downward
Wait, What? Sea Ice Can Grow a Hollow Finger of Ice Down Into the Ocean
Icicles usually grow from roofs because liquid water drips downward through cold air and freezes.
A brinicle grows underwater, from the underside of sea ice, through seawater.
It forms because freezing sea ice rejects much of its salt. The concentrated brine left behind can become extremely cold, remain liquid because salt lowers its freezing point, and then drain downward because it is denser than the surrounding seawater.
freezing sea ice → salt-rich cold brine → downward plume → surrounding seawater freezes → hollow ice tube.
The scientific job claimed here is specific: brinicles own tubular ice growth around descending super-cold brine released from sea ice. This does not duplicate ordinary icicles, the Thermocline article, Ocean Acidification, or general sea-ice formation.
Big Question: How can liquid brine remain below the normal freezing point of seawater and freeze the water around it while flowing downward?
Quick Answer
When seawater freezes, the growing ice crystal lattice incorporates relatively little salt. Much of the salt becomes concentrated in liquid brine channels and pockets within the sea ice. Because salty water has a lower freezing point, this brine can remain liquid at temperatures far below the freezing point of ordinary seawater.
The brine is also denser than surrounding seawater. When it drains from the underside of sea ice, gravity pulls it downward. The brine can be cold enough to freeze neighbouring seawater. Ice forms around the descending plume, building a hollow tube whose interior continues to carry brine downward.
Laboratory experiments published in The Cryosphere in 2024 reproduced this tubular growth and studied how brine flow controls the structure.
The Cryosphere — Experimental modelling of tubular ice brinicles →
What You Will Learn
- Why sea ice does not simply trap all the salt in seawater.
- What brine rejection means.
- Why concentrated salt lowers freezing point.
- Why very cold brine can remain liquid.
- Why brine sinks.
- How seawater freezes around a descending plume.
- Why the resulting structure is hollow.
- How brinicles differ from ordinary icicles.
- How convection and flow rate affect tube growth.
- Why brinicles matter for salt and heat transport.
- What field and laboratory observations show.
- Why “underwater icicle” is useful visually but incomplete scientifically.
Part 1 — Sea Ice Begins With Water Molecules, Not Salt Crystals
Pure ice is built from water molecules arranged in a crystal lattice. Dissolved sodium and chloride ions do not fit easily into that structure.
As seawater freezes, much of the salt is excluded from the growing ice and concentrated in remaining liquid regions.
more ice forms → remaining liquid becomes saltier.
Part 2 — Brine Channels Form Inside Sea Ice
Sea ice is not simply a solid slab of pure frozen water. It contains a network of liquid brine inclusions and channels whose size and connectivity depend on temperature, salinity and ice history.
As the ice cools, more water freezes out of the brine. The residual liquid becomes even more saline.
Eventually, dense brine can drain through connected pathways toward the ocean below.
Part 3 — Salt Lowers the Freezing Point
Dissolved salt disrupts the equilibrium between liquid water and ice. More cooling is required before the salty solution freezes.
This is freezing-point depression.
Ordinary seawater near ocean salinity freezes around −1.8 °C, but highly concentrated brine can remain liquid at much lower temperatures. Laboratory and field descriptions of brinicle systems report brines far colder than the surrounding seawater while still liquid.
Part 4 — Why the Brine Sinks
Adding dissolved salt generally increases water density. The concentrated brine produced inside sea ice is denser than the underlying seawater.
Once a pathway opens, gravity drives the brine downward.
high salinity + low temperature → dense brine → sinking plume.
Part 5 — The Brine Freezes Water Around It
The surrounding seawater is close to its own freezing point. The descending brine can be much colder.
Heat flows from the relatively warmer seawater toward the colder brine. As neighbouring seawater loses enough heat, some of it freezes.
The ice forms around the moving brine stream rather than simply blocking it.
Part 6 — Why the Tube Is Hollow
The centre of the structure remains a flowing channel of liquid brine. Freezing occurs mainly at the interface between the cold plume and the surrounding seawater.
That geometry naturally makes a tube:
liquid brine inside → newly frozen seawater wall outside.
As long as brine supply continues and conditions remain suitable, the tube can extend downward.
Part 7 — Why It Grows Downward
The brine plume is pulled by gravity. The coldest region therefore advances downward with the flowing liquid.
New ice forms around the advancing plume tip. This produces finger-like downward growth from the underside of the sea ice.
A brinicle can extend metres through the water column under suitable polar conditions.
Part 8 — Why It Is Not Just an Icicle Underwater
An ordinary icicle is built mainly by water flowing over an ice surface and freezing as heat is lost to cold air.
A brinicle is built because an extremely cold saline liquid flows through water with a higher freezing point and freezes that surrounding water into a tube.
| Ordinary icicle | Brinicle |
|---|---|
| Grows in air | Grows underwater |
| Liquid water freezes onto ice | Cold brine freezes surrounding seawater |
| Usually solid or layered | Hollow brine-flow tube |
| Driven by dripping and air cooling | Driven by salt rejection, density and brine convection |
Part 9 — Flow Rate Matters
If brine flow is too weak, the tube may freeze shut or fail to advance. If it is too strong, the plume may outrun orderly tube formation or create unstable structures.
Laboratory work shows that brinicle width, tip shape and growth speed depend on flow conditions and geometry.
This makes a brinicle a fluid-dynamics problem as well as a freezing problem.
Part 10 — Convection Inside the Tube
The brine inside the tube does not necessarily descend as a perfectly smooth plug. Density differences and cooling can drive internal convection and instabilities.
These flows help determine how quickly heat is removed and where fresh ice forms.
A visible ice tube is therefore the solid boundary of a moving liquid system.
Part 11 — Brinicles Move Salt Through the Ocean-Ice Boundary
When sea ice rejects salt, the ocean beneath can become saltier and denser.
Brine drainage is one pathway that transports this salt downward. At larger scales, sea-ice formation contributes to dense-water production and polar ocean circulation.
A brinicle is therefore a striking local structure connected to much larger processes of ocean salinity, density and circulation.
Part 12 — What Happens If a Brinicle Reaches the Seafloor?
If the water is shallow enough and the brinicle persists long enough, it can reach the seabed.
Cold brine may then spread along the bottom. Local freezing can occur around the contact region.
Popular descriptions sometimes call this an “ice finger of death” because very cold brine and ice can affect slow-moving seafloor organisms. That phrase is dramatic, but the scientifically useful mechanism is temperature, salinity and freezing—not an active predatory structure.
Part 13 — Brinicles as Self-Organising Tubes
Brinicles resemble a broader family of self-organising tubular structures, including chemical gardens.
In each case, a flowing solution interacts with its surroundings and continuously builds a solid or semi-solid wall around itself.
The similarity does not mean the chemistry is identical. It reveals a shared pattern:
flow + reaction or phase change + boundary formation → growing tube.
Part 14 — Follow One Brinicle From Sea Ice to Ocean
- Surface seawater begins freezing.
- Ice crystals preferentially exclude dissolved salt.
- Remaining liquid becomes concentrated brine.
- Brine collects in channels and pockets inside sea ice.
- A connected pathway reaches the underside of the ice.
- Dense brine drains into the ocean.
- Gravity pulls the plume downward.
- The plume is colder than nearby seawater.
- Heat flows from seawater into the plume.
- Surrounding seawater freezes around the plume.
- A hollow ice wall forms.
- The continuing brine flow extends the tube downward.
Think Like a Scientist: How Do We Test the Mechanism?
- Measure brine salinity and temperature.
- Measure surrounding seawater temperature.
- Track brine flow speed.
- Record tube radius and growth rate.
- Change brine flow experimentally and observe geometry.
- Use dye or tracers to visualise internal convection.
- Compare laboratory tubes with field observations under sea ice.
- Model heat transfer, salinity and buoyancy together.
Observation vs Inference
- Observation: concentrated cold brine exits sea ice and sinks.
- Observation: ice forms around the descending flow.
- Measurement: brine is colder and saltier than surrounding seawater.
- Inference: freezing-point depression allows the brine to remain liquid while it freezes surrounding water.
- Boundary: natural brinicle dimensions depend strongly on local sea-ice and ocean conditions.
Common Misconceptions and Better Models
| Misconception | Better model |
|---|---|
| A brinicle is just a normal icicle underwater. | It forms around a descending plume of very cold brine. |
| Salt makes water freeze more easily. | Dissolved salt lowers the freezing point. |
| The brine freezes first because it is coldest. | Its high salinity lets it remain liquid while less-salty seawater freezes around it. |
| Sea ice contains no liquid water. | Sea ice can contain brine channels and inclusions. |
| The tube grows because ice sinks. | The brine sinks; ice forms around its path. |
| Brinicles kill everything nearby. | Effects are local and depend on temperature, salinity, contact and organism mobility. |
Checkpoint Questions
- Why does freezing sea ice concentrate salt?
- What is freezing-point depression?
- Why can brine remain liquid below −1.8 °C?
- Why does concentrated brine sink?
- What water actually forms the brinicle wall?
- Why is the structure hollow?
- How does a brinicle differ from an icicle?
- Why does flow rate matter?
- How can brinicles affect ocean salinity transport?
- What evidence would distinguish brine-driven freezing from ordinary icicle growth?
Answer Key
Open after attempting the questions
- The ice lattice excludes much of the dissolved salt.
- Dissolved solute lowers the temperature at which liquid and ice coexist.
- Its high salinity depresses its freezing point.
- High salinity raises its density relative to surrounding seawater.
- Mainly the surrounding seawater at the plume boundary.
- Liquid brine continues flowing through the centre.
- It is an underwater hollow tube formed around cold brine, not a drip-frozen structure in air.
- It controls heat transfer, stability, wall thickness and tip advance.
- It transports salt-rich water away from sea ice into the ocean.
- Measure a cold, dense, saline downward flow inside a hollow tube while surrounding seawater freezes.
Primary Science Bridge
- water freezes when enough heat is removed;
- dissolved substances change material properties;
- denser fluids can sink;
- heat moves from warmer to colder regions;
- one visible structure can be produced by several linked processes.
Secondary and JC Bridge
| Core idea | Higher-resolution route |
|---|---|
| Freezing | Phase equilibrium and latent heat |
| Solutions | Freezing-point depression |
| Density | Buoyancy-driven convection |
| Heat | Coupled conduction and advection |
| Sea ice | Brine channel networks |
| Ocean | Salt flux and dense-water formation |
Deep Science Window — Phase Diagrams of Salt Water
The equilibrium freezing temperature depends on salinity. As ice forms and excludes salt, the residual brine moves along a different path through temperature–composition space. This is why the liquid fraction and salinity inside sea ice change strongly with temperature.
Deep Science Window — Coupled Transport
A brinicle cannot be explained by heat transfer alone. Salinity controls freezing temperature and density; density controls flow; flow transports cold brine; that flow changes where freezing occurs. It is a coupled heat–mass–fluid system.
Evidence Boundaries
- Brinicle ≠ ordinary underwater icicle.
- Cold brine ≠ frozen brine. Salinity can keep it liquid.
- Ice tube ≠ downward-moving ice. The brine moves; the wall grows around it.
- One laboratory geometry ≠ every natural brinicle.
- Local brine drainage ≠ whole-ocean circulation by itself.
- Dramatic imagery ≠ universal ecological destruction.
Manual Summary — KNOW → CONNECT → EXPLAIN → APPLY → CHECK
KNOW: brine rejection, freezing-point depression, density, convection, heat transfer, brinicle.
CONNECT: sea-ice freezing to salt concentration, salt concentration to density, density to sinking, and sinking cold brine to tubular freezing.
EXPLAIN: why an underwater ice tube can grow around a liquid that is colder than the water it freezes.
APPLY: predict what happens if brine becomes less salty, warmer or flows more slowly.
CHECK: identify which water freezes and which liquid keeps flowing.
Teaching Guide for Parents, Tutors and Teachers
Begin with the apparent contradiction: the coldest liquid stays liquid while the warmer surrounding water freezes. That creates a natural reason to learn freezing-point depression.
- Review freezing and salt solutions.
- Explain salt rejection during sea-ice formation.
- Build the dense-brine sinking model.
- Track heat from seawater to brine.
- Explain why freezing occurs around the flow.
- Finish with the hollow-tube geometry and ocean connections.
Safety boundary: do not attempt extreme sub-zero brine demonstrations with improvised cryogenic mixtures. Use diagrams, safe salt-and-ice observations, or published laboratory videos and data.