Geologists sample an ancient volcanic layer and find its magnetic minerals have reversed polarity — pointing toward the current south magnetic pole. What is the most direct interpretation?
AThe volcanic rock was erupted at high southern latitudes where field polarity is reversed
BEarth's magnetic field had opposite polarity when the rock cooled through its Curie temperature
CThe rock's minerals were chemically altered after formation, reversing the recorded direction
DThe rock was physically rotated 180° by tectonic forces after it formed
Thermoremanent magnetization (TRM) locks in the ambient field direction when an igneous rock cools below its Curie temperature (~580°C for magnetite). Reversed polarity in the rock records reversed polarity in Earth's field at that time — not the rock's latitude, not post-formation alteration (which would require specific conditions and usually leaves other geochemical signatures), and not tectonic rotation (which would also rotate other features in a recognizable way). The clean interpretation is that the geodynamo was running in reverse when the rock solidified.
Question 2 Multiple Choice
Paleomagnetic data from a continent show that the apparent polar wander path (APWP) diverges significantly from the modern pole position going back 300 million years. What does this most likely indicate?
AEarth's geographic poles migrated substantially over the past 300 million years
BThe continent moved relative to a relatively stable pole, changing the inclination recorded in rocks
CThe paleofield was much weaker 300 million years ago, giving unreliable inclination data
DThe magnetic field completely reorganized into a non-dipolar configuration 300 million years ago
The APWP traces the apparent position of the pole as reconstructed from the inclination of remanent magnetization in rocks. The field's dipole axis has not dramatically wandered; rather, the continent moved. Since inclination encodes latitude (tan I = 2 tan λ), a rock formed at lower latitude shows shallower inclination. If a continent was near the equator 300 Ma and is now at 50°N, rocks of that age show shallow inclination — the 'pole' appears to have been at low latitude relative to the continent's current position. Two continents that share an APWP were joined; divergent APWPs record separation.
Question 3 True / False
Geomagnetic reversals occur at regular, predictable intervals — roughly most 200,000 years — and can therefore be forecast.
TTrue
FFalse
Answer: False
Polarity reversals are highly irregular. The intervals between reversals range from tens of thousands to tens of millions of years, with no detectable periodicity. The Cretaceous Normal Superchron lasted ~40 million years without a reversal; other intervals have seen many reversals in rapid succession. This irregularity is why the geomagnetic polarity timescale (GPTS) must be calibrated by radiometric dating of individual volcanic horizons rather than extrapolated from a regular clock. Reversals cannot be forecast from the timing of past reversals.
Question 4 True / False
An apparent polar wander path represents the actual movement of Earth's geographic and magnetic poles through geological time.
TTrue
FFalse
Answer: False
The 'apparent' in APWP is crucial — the pole's apparent position relative to the continent changed primarily because the continent moved, not because the pole itself migrated substantially. Earth's dipole axis stays roughly aligned with the rotation axis over long time averages (the geocentric axial dipole hypothesis). When two continents that were once joined show APWPs that converge into a single path going back in time, it is because they shared the same pole position when joined. The continent is the mobile object; the pole is the relatively stable reference.
Question 5 Short Answer
Why can the paleomagnetic inclination recorded in a rock tell you the latitude at which that rock formed?
Think about your answer, then reveal below.
Model answer: Earth's dipole field produces inclination that varies systematically with latitude: the field is horizontal at the equator (inclination = 0°) and vertical at the poles (inclination = 90°). The relationship is tan(I) = 2·tan(λ), where I is inclination and λ is latitude. When magnetic minerals lock in TRM or DRM, they record the inclination of the ambient field. By measuring the remanent inclination in the rock and applying this formula, you can calculate the latitude at which the rock formed. If that paleolatitude differs from the rock's current latitude, the plate has moved.
This is the quantitative basis for plate tectonic reconstruction from paleomagnetism. The geocentric axial dipole hypothesis — that Earth's field averages to a geocentric dipole aligned with the rotation axis over ~10,000-year timescales — is what allows inclination to serve as a paleolatitude indicator. Without this assumption, inclination would not have a predictable relationship to latitude.