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Place 03 · The Galápagos · Chapter 3 of 7
Layers of the Sea
Pour warm water gently onto cold water and the two refuse to mix. The whole Pacific works the same way, and it decides whether the penguins eat.
About 15 minutes · 1 video · 1 simulation · 2 quick activities
Start here
The sea is stacked, not stirred
Divers at the Galápagos know the moment: you sink through warm, clear water and suddenly hit a wall of cold, as if someone opened a fridge. The ocean sorts itself by density, light water floating on heavy water, and the layers barely mix. The undercurrent from Chapter 2 can feed the islands only because that cold layer sits unusually close to the surface here.
Watch this
See it move
Hakai Institute — Long Story Shorts: What is Ocean Stratification? Two minutes on why the sea comes in layers, and why anything that eats should care.
While you watch, look for
- What makes one layer of water heavier than another
- What the layers do to the mixing between the surface and the deep
- Something you did not expect. Hold onto it.
The short version
Three things going on here
The rule
Heavy water sinks
Seawater gets denser, heavier for its size, when it gets colder or saltier. Dense water slides underneath lighter water and stays there. In the open ocean temperature matters most, so warm water floats on top of cold.
The layers
Three layers deep
On top is the mixed layer, stirred by wind and waves until it is all about one temperature. Below it the temperature plunges fast through the thermocline, and the density jumps with it: the pycnocline. Underneath lies the cold, dark deep water that fills most of the ocean.
The lid
A ceiling for nutrients
The thermocline works like a lid: it takes a lot of energy to push light water down or heavy water up through it. Nutrients pile up below it in the dark, while the sunlit layer above runs short. Where the thermocline is shallow, as it usually is at the Galápagos, upwelling taps the rich water easily; push it deep and the surface starves.
Photos: Nancy Zjaba, Luke Miller, Diego F. Parra / Pexels
Play with it
Make things float and sink
This simulation drops blocks into water. On the first screen, change a block’s mass and its size until it floats, then until it sinks, and watch what happens to its density each time.
Simulation: PhET Interactive Simulations, University of Colorado Boulder (CC BY 4.0).
Two questions to answer while you are in there
- What do all the blocks that float have in common, compared with the water?
- Seawater is a little denser than the fresh water in the sim. Would a block that only just sinks in fresh water be more or less likely to float in the sea?
Try it yourself
Check yourself
Try it
Floats on Top or Sinks Below?
Seven water samples meet the sea surface at the Galápagos: decide where each one ends up.
Try it · interactive
Layer Cake
Set the surface temperature, the saltiness and the wind, and watch the water column sort itself into layers, the thermocline rise or sink, and the nutrients either reach the sunlight or stay locked below.
Open the Layer Cake in its own window (opens in a new tab).
Words you will hear
Four terms, that is all
Density
How much mass packs into a space
Stratification
Water stacked in layers that resist mixing
Thermocline
Layer where temperature drops fast with depth
Pycnocline
Layer where density rises fast with depth
Check your understanding
Practice the whole chapter
Everything in this chapter, in eight quick questions. Retry as often as you like. Read the feedback on every answer, right or wrong, because that is where the learning is.
Practice
Practice: Layers of the Sea
Eight questions covering the video, the three cards, the four terms and the interactive above.
If you want more
Go deeper
- Our World Ocean 11.4 — The Ocean Layer Cake — LibreTexts · 8 minutes
- Webb 6.3 — Density — LibreTexts · 10 minutes
- Oceanography 101 (MiraCosta) 7.8 — Cline Curves and the Mixing Zone — LibreTexts · thermocline, halocline, pycnocline · 8 minutes
Photo credits (Pexels): Linda Heyworth, Pia B, Nancy Zjaba, Luke Miller, Diego F. Parra.