What this lesson teaches
I can explain how magnetic striping on the seafloor supports plate tectonic theory.
Syllabus K329, Topic 4.1, Key Question 3. How does magnetic striping support the plat tectonic theory?
Make a guess
Earth's magnetic field flips while a ridge keeps making new basalt. What happens to the older rocks?
- They flip to match the new field over time.
- They lose their magnetism as the field weakens.
- They keep the direction recorded when they cooled.
- They melt and form again at the ridge.
Show the answer
They keep the direction recorded when they cooled.
Cooling basalt locks in the field's direction at that time. New basalt records the new direction, so stripes form.
Cooling basalt records Earth's magnetic field, and matching stripes on both sides of a ridge prove seafloor spreading.
Ocean crust is mostly basalt, a rock formed from cooled lava. Basalt contains iron-rich minerals. As it cools, they line up with Earth's magnetic field and lock that direction in.
Earth's magnetic field has flipped many times. Normal polarity means it points the way it does today. Reversed polarity means it points the opposite way. Only the field flips, not the Earth.
New basalt at a ridge records the field at that time. Spreading carries it away. Later basalt may record the opposite direction. So stripes of normal and reversed rock form. The same pattern appears on both sides of the ridge, like a mirror. The stripes are measured with instruments. They are not visible.
- Basalt
- Rock from cooled lava; most ocean crust.
- Normal and reversed
- Field points as today, or the opposite way.
- Mirror pattern
- Matching stripes on both sides of a ridge.
Step by step
Record the field
Cooling basalt locks in the field's direction.
Move away
Spreading carries the rock from the ridge.
Record the next period
New basalt records the next direction, making stripes.
Real examples to use
1950s ocean surveys (USGS)
From the 1950s, scientists towed magnetometers, adapted from devices built in the Second World War to find submarines, across the ocean floor. They found stripes of normal and reversed rock in rows on both sides of the mid-ocean ridges.
Worked example: A pattern with a cause
If stripes read normal, reversed, normal moving away on one side, the other side shows the same order. Both sides recorded the same flips while spreading apart.
Watch out for this
When the field flips, all old rocks flip too.
Cooled rocks keep their old record. Only newly cooling basalt records the new direction.
Check your understanding
Earth's magnetic field reverses while a ridge keeps making basalt. What happens?
- New basalt records the new direction; old stripes keep theirs.
- The existing stripes switch to the new direction as the field changes.
- The plates reverse direction and move back towards the ridge.
Show the answer
New basalt records the new direction; old stripes keep theirs.
Yes. That is how stripes form.
Check your understanding
Why do the magnetic stripes on the seafloor match on both sides of a ridge?
- Earth's magnetic field is the same on both sides of every ridge.
- New crust records the field as it forms, then spreads both ways.
- Ocean currents laid down magnetic sediment in matching bands.
Show the answer
New crust records the field as it forms, then spreads both ways.
The matching pattern supports seafloor spreading.