Magnetic evidence for ocean floor spreading

Until the 1950s, scientists thought the ocean floor had remained essentially unchanged for millions of years. That opening makes me expect a correction, and the next sentence supplies it: new discoveries made the old view unsupportable. Now I want to know what was discovered and how it shows change.

The first discovery involves odd magnetic variations. Basalt makes up much of the ocean floor, and scientists already knew it contains magnetic material called magnetite. So unusual compass readings aren't completely surprising. But how does magnetism show that the geology has changed? We haven't connected those ideas yet.

Some basalt has normal polarity, matching Earth's current magnetic field; other basalt has the opposite alignment. The explanation is timing. While the rock is molten, magnetite grains align with Earth's magnetic field like little compass needles. Once the magma hardens, that alignment stays fixed. Earth can reverse its field later without the old rock changing along with it.

Think of the rock as keeping a magnetic record. Its alignment tells us the direction of Earth's field when it cooled. Normal and reversed aren't two different abilities to follow the field: both kinds followed it when they formed. They preserve different past orientations. That's useful information, but I still want the connection to a changing ocean floor.

The second paragraph gives us a pattern, not just scattered magnetic oddities. Along either side of an underwater mountain range, the mid-ocean ridge, the rock forms alternating stripes of normal and reversed polarity. These stripes point to formation at different times, under different magnetic conditions. Now we're getting somewhere.

Scientists propose that the floor is pulling apart along the ridge. At first, pulling apart may sound like it should create a gap, not mountains. The next step explains it: magma rises through those weak zones and erupts, creating new crust. Repeated over millions of years, this builds the ridge. The new rock forms at the spreading center while older rock moves outward. Magnetism records this process; it isn't the force pulling the floor apart.

Then come the checks on that explanation. Rocks near the ridge crest are young, and those farther away are progressively older. That fits new crust forming in the middle. The youngest rocks also have normal polarity, matching the current field.

The final sentence is long, so separate its steps. Scientists dated volcanic rocks on land and used their magnetic orientations to date Earth's magnetic reversals. That gave them a timeline to compare with the ocean's stripes. If the floor spreads outward at several centimeters per year, the stripe pattern closely matches that timeline. The spatial pattern and the sequence of magnetic changes fit together under the spreading explanation.

So our rough main point is that discoveries involving magnetic variations and the mid-ocean ridge changed scientists' view: the ocean floor evolves rather than remaining static. The last paragraph isn't a separate lesson about magnets. Its age evidence and magnetic comparison help establish the geological change we were waiting to understand.