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Ice at one end, warmth at the other: how does a whole ocean start to move?
Ocean Currents lab · Last updated: 16 October 2026
You put ice at one end of an ocean and warmth at the other, then watch the water begin to move on its own. They find out how cold, dense water sinks and warm water flows in to replace it. The same engine drives currents round the whole planet and shapes the weather of entire continents.
What you'll discover
- Cold water is denser than warm water, so it sinks, while warmer water stays near the surface.
- Sinking cold water at one end and rising water elsewhere set up a slow loop that turns the whole ocean.
- Ocean currents carry heat from the tropics towards the poles, warming some coastlines.
The science in plain words
Water gets denser as it cools, and salty water is denser than fresh water. Near the poles, seawater is chilled by icy air, and when sea ice forms it leaves salt behind in the water below. This cold, salty water becomes heavy enough to sink to the ocean floor.
The sinking water spreads slowly along the bottom of the ocean towards warmer regions. To replace it, warmer surface water flows towards the poles. Elsewhere, deep water gradually rises back to the surface. The result is a giant loop, sometimes called the global conveyor belt. Because it is driven by differences in temperature and saltiness, scientists call it the thermohaline circulation. Water may take around a thousand years to complete the whole journey.
Not all currents are made this way. Steady winds blowing over the sea drag the surface water along, creating fast surface currents such as the Gulf Stream. Earth's rotation turns these currents so they form large circling loops called gyres. Together, wind-driven and density-driven currents move enormous amounts of heat. They help explain why western Europe has milder winters than parts of Canada at the same latitude, and they carry nutrients that feed fish and other sea life.
Where it fits
- India, NCERT: Ocean currents are not part of the NCERT science syllabus for classes 6 to 10; they are taught in NCERT geography. The lab builds on Class 7 Heat Transfer in Nature.
- United States: Grades 6 to 8
- Next Generation Science Standards: MS-ESS2-6
Common questions
What causes ocean currents?
Ocean currents are driven mainly by wind and by differences in water density. Winds push surface water along, while cold, salty water near the poles sinks and spreads along the sea floor. Earth's rotation and the shapes of the continents steer the currents into great loops.
Why does cold water sink?
As water cools, its particles move less and pack more closely, so the same amount of water takes up less space and becomes denser. Denser water is heavier than the warmer water around it, so it sinks. Extra salt makes seawater denser still.
How do ocean currents affect climate?
Currents carry warm water from the tropics towards the poles and cold water back towards the equator. The Gulf Stream and its extension, for example, help keep western Europe milder than other places at the same latitude. Changes in currents can shift rainfall and temperatures.
Related labs
- Heat Transfer: Bridge hot water and cold with a metal bar. Which metal carries heat best?
- Weather Patterns: Why does wind swirl round a storm instead of blowing straight in?
- Ocean Tides: Why does the sea come in and go out twice a day?
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Try it yourself
The Ocean Currents lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.