New crust forms at a mid-ocean ridge and moves away. Rocks on both sides therefore become older with distance from the ridge.
New oceanic crust forms where plates separate at a mid-ocean ridge. Hot mantle rises, and falling pressure allows some of it to melt. Magma rises through fractures, then cools and solidifies into crust.
As more crust forms, earlier crust moves away from the ridge. The youngest rocks are therefore at the ridge. Rocks become progressively older farther away on both sides. This pattern supports seafloor spreading.
Sediment builds up on the seafloor over time. Young crust near a ridge has had little time to collect it; older crust generally has more. Oceanic crust is eventually recycled at subduction zones. This helps explain why it is generally younger than continental crust and preserves a limited sediment record compared with Earth's age.
Sediment supply also varies, and some trenches hold thick deposits. Do not assume every trench has little sediment. Combining sediment evidence with rock ages and magnetic patterns gives a stronger explanation.
Step by step
New crust forms
Magma cools at the ridge, producing the youngest basalt.
Older crust moves away
As plates separate, the earlier crust is carried away from the ridge.
The pattern records movement
The teaching transect increases from 0 to 5, 10 and 15 million years away from the ridge.
Worked example: Read evidence, then explain it
"The rocks get older" describes a pattern. A stronger explanation adds where and why: rocks get older with distance from the ridge on both sides because new crust forms at the ridge and is carried away as the plates diverge.
Watch out for this
The oldest oceanic rocks should be at the ridge because that is where the seafloor started.
The ridge keeps forming new crust. Older crust has already moved away, so the ridge contains the youngest rocks.
Check your understanding
At equal distances on opposite sides of a steadily spreading ridge, samples have similar ages. What best explains this?
- Crust formed near the ridge and moved outward on both sides.
- Old crust moves inward and becomes younger at the ridge.
- The two sides must always spread at exactly the same rate.