Why Ocean Floor Is Young and Continents Are Old

Lab Brief #3

🎯 Your mission

Learn the rule. Then catch nature using it.

⚡ The twist

Weather isn't random — it's just complicated.

What you'll learn

No sea floor anywhere on Earth is older than about 200 million years, while the oldest continental rocks approach 4 billion. The reason is density: oceanic crust is basalt, dense enough to subduct and be recycled, whereas continental crust is granite, too buoyant to sink, so it simply accumulates. The age map of the sea floor — youngest at every mid-ocean ridge and symmetrically older either side — is the single clearest record that the floor is spreading.

Key words

oceanic crust
Dense basaltic crust that forms at ridges and is recycled at subduction zones within about 200 million years.
sea-floor spreading
The formation of new oceanic crust at a mid-ocean ridge, pushing older crust symmetrically outward.

Make a hypothesis first

Before you observe: what shape will it leave?

Say your guess out loud — then find out if you were right.

Step 1 of 4 · Read

Think about what you just learned: say what "oceanic crust" and "sea-floor spreading" mean in your own words before you start.

Step 2 of 4 · Do

Colour a sea-floor age map by age band and measure the distance from the Mid-Atlantic Ridge to the 100-million-year line on both sides. Divide distance by age to calculate the spreading rate in cm per year and compare the two sides.

Step 3 of 4 · Observe

Record what actually happened, including anything that did not match what you expected.

Step 4 of 4 · Record

Write a conclusion that answers the question this lesson opened with, and state your uncertainty.

Question 1 of 3

The oldest ocean floor is about:

How sure are you?😐 maybe

Question 2 of 3

Continental crust survives far longer than oceanic crust because it is:

How sure are you?😐 maybe

Question 3 of 3

Sea-floor age patterns either side of a mid-ocean ridge are:

How sure are you?😐 maybe

Mind = blown

🌊 The deepest part of the ocean is deeper than Mount Everest is tall.

Nice work!

You got 0 of 3 right.

Next: The Atmosphere's Temperature Profile Reverses Twice All lessons

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✨ More to explore

🌍 Where you see this in real life: Today's weather, tomorrow's beach, next century's coastline — all this.

🏆 Stretch challenge (try it this week): Look at the sky for 5 minutes and try to predict tomorrow's weather.

👨‍👩‍👧 For the dinner table: “What's the slowest change you can think of?”

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Why Ocean Floor Is Young and Continents Are Old

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