Cooling basalt records Earth's magnetic field. Matching bands on opposite sides of a ridge support seafloor spreading.
Basalt forms when lava cools and is common in oceanic crust. Its iron-bearing minerals record the direction of Earth's magnetic field as the rock cools. The record remains even when the field later changes.
Earth's magnetic field has reversed many times. Normal polarity means it points broadly as it does today. Reversed polarity means it points the opposite way. The magnetic field reverses; Earth itself does not flip over.
New basalt at a ridge records the polarity at that time. Spreading carries it away, and later basalt may record the opposite polarity. This creates alternating magnetic bands, with a corresponding sequence on the other side of the ridge.
The stripes are measured magnetic patterns, not visible painted bands. Their widths vary with the time between reversals and the spreading rate.
Step by step
Record the field
Cooling basalt records the magnetic polarity at that time.
Move away
Spreading carries the rock away from the ridge.
Record the next period
New basalt records later polarity, creating matching sequences on both sides.
Worked example: A pattern with a cause
If the bands on one side read normal, reversed, normal as you move away from the ridge, expect the same outward sequence on the other side. Both sides preserve the same history of magnetic reversals while spreading.
Watch out for this
All the old rocks change polarity when Earth's magnetic field reverses.
Cooled rocks retain their earlier magnetic record. Newly cooling basalt records the new field direction, creating another band.
Check your understanding
Earth's magnetic field reverses while a ridge continues producing basalt. What happens?
- Every existing stripe switches to the new polarity.
- Newly cooling basalt records the new polarity; older bands keep their record.
- The plates reverse their direction of movement.