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Place 04 · The Ridge and the Trench · Chapter 2 of 7
The Seafloor Is Young
Some rocks on the continents are close to 4 billion years old; almost nothing on the open ocean floor is older than about 180 million.
About 15 minutes · 1 video · 1 interactive map · 2 quick activities
Start here
Wegener was right, and nobody believed him
In 1912 a German meteorologist named Alfred Wegener argued that the continents had once been joined and had drifted apart. He had matching fossils, matching rocks and coastlines that fit like puzzle pieces, but he could not explain what force could move a continent, so most geologists threw the idea out. He died on the Greenland ice in 1930, decades before the seafloor proved him right.
Watch this
See it move
Keith Meldahl — Magnetic Reversals and Sea Floor Spreading. Three minutes from MiraCosta College geologist Keith Meldahl, drawing the stripes faster than you can say “paleomagnetism.”
While you watch, look for
- What happens to Earth’s magnetic field during a reversal
- Why the stripes on one side of the ridge match the stripes on the other side
- Something you did not expect. Hold onto it.
The short version
Three things going on here
The compass in the rock
Rocks remember north
When lava cools into basalt, tiny grains of the mineral magnetite line up with Earth’s magnetic field, like little compass needles frozen in place. That record is called paleomagnetism. And the field does not sit still: every so often it flips, so a compass would point south, and the last flip was about 780,000 years ago.
The stripes
A barcode on the seafloor
In the 1950s ships towed magnetometers across the ocean and found long stripes of stronger and weaker magnetism in the seafloor rock. In 1963 Fred Vine and Drummond Matthews, and separately Lawrence Morley, explained them: new crust forming at a ridge records the field of its day, then moves aside. So every flip of the field leaves matching stripes on both sides of the ridge, like a mirror image.
The conveyor
Made at the ridge, young everywhere
Seafloor spreading means ocean floor is built at the ridges and carried away on both sides, less than 2 cm a year per side in parts of the Atlantic and approaching 10 cm a year per side in parts of the Pacific. The rock is youngest at the ridge, gets older the farther away you go, and wears a thicker blanket of mud. The oldest open ocean floor is only about 180 million years old.
Photos: Саша Алалыкин, Errol Briant, Matteo Sassola / Pexels
Play with it
Color the ocean floor by age
This map colors the seafloor by the age of its rock. Check the legend, then find the Mid-Atlantic Ridge and follow the colors east toward Africa and west toward the Americas.
Map: “Seafloor Crustal Age” web map by Esri, ArcGIS Online.
Two questions to answer while you are in there
- Where is the youngest seafloor: in the middle of the oceans or next to the continents? Do the colors match on both sides of the ridge?
- The band of young seafloor is much wider along the East Pacific Rise than along the Mid-Atlantic Ridge. Which ridge is spreading faster?
Try it yourself
Check yourself
Try it
The Evidence, in Order
Pick the one true statement in each set, and the case for seafloor spreading builds itself step by step.
Try it · interactive
Magnetic Stripes
Run a spreading ridge through time while Earth’s magnetic field flips: matching stripes of normal and reversed rock march away on both sides. Then pick a stripe, measure its distance from the ridge and work out its age from the spreading rate.
Open Magnetic Stripes in its own window (opens in a new tab).
Words you will hear
Four terms, that is all
Continental drift
Wegener's idea that continents move over time
Seafloor spreading
New seafloor forms at ridges and moves apart
Paleomagnetism
Ancient magnetic field direction recorded in rock
Magnetic reversal
Earth's magnetic north and south swapping places
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: The Seafloor Is Young
Eight questions covering the video, the three cards, the four terms and the interactive above.
If you want more
Go deeper
- Webb 4.1 — Alfred Wegener and the Theory of Plate Tectonics — LibreTexts · 8 minutes
- Webb 4.5 — Divergent Plate Boundaries — LibreTexts · 12 minutes
- USGS This Dynamic Earth — Developing the Theory — U.S. Geological Survey · 10 minutes
Photo credits (Pexels): Dương Nhân, Zelch Csaba, Саша Алалыкин, Errol Briant, Matteo Sassola.