The magnetic field stops reversing
For nearly 40 million years the compass never flips — the longest steady interval known.
- When
- —
- Duration
- 38 million years
- Known to
- ± 2 million years
- Era
- Cretaceous, Mesozoic
Earth's magnetic field normally reverses polarity every few hundred thousand years, but through the mid-Cretaceous it simply stops, holding normal polarity from about 121 to 83 million years ago. The Cretaceous Normal Superchron is the longest such interval in the well-resolved record, and it coincides with unusually vigorous mantle activity: the great oceanic plateaus erupt inside it, and fast seafloor spreading pushes sea level to its Phanerozoic high. Whether the quiet field causes the volcanism, follows from it, or shares a deeper cause in the core-mantle boundary is unresolved.