Tectonic Hazards Practice Test

Magnetic reversals leave a visible record in the ocean floor. As new magma erupts at a mid-ocean ridge and cools, the iron minerals align with Earth’s current magnetic field. When the field reverses, the newly formed rock records the opposite polarity. Over time, this creates alternating bands of normal and reversed magnetization on either side of the ridge. Because the crust spreads outward in both directions, the stripes are mirror images on opposite sides, providing strong evidence that reversals happen globally and that seafloor spreading moves rocks apart from the ridge. The other options don’t capture this mirrored magnetic pattern: fossil distributions relate to continental drift, not magnetic reversals; seafloor age away from ridges shows spreading but not polarity reversals; and constant volcanic activity along ridges isn’t about magnetic records.