Questions: Geomagnetic Reversal Chronology and Magnetostratigraphy

Question 1

A geologist finds a sedimentary section in Spain containing no dateable volcanic ash and no index fossils. The section shows a clear pattern of normal and reversed magnetic polarity zones. Can the section be dated, and if so, how?

• A. No — without fossils or radiometric material, no age can be assigned to the section
• B. Yes — the polarity sequence can be compared to the GPTS and, if a unique match is found, numerical ages can be assigned to polarity boundaries
• C. Yes, but only approximately, because reversals are too irregular to constrain age to better than ±10 million years
• D. No — sedimentary rocks do not reliably record magnetic polarity and cannot be used for magnetostratigraphy

Answer: (B)

This is magnetostratigraphy in action. Because geomagnetic reversals are globally synchronous, the pattern of polarity zones in any section — regardless of its lithology or fossil content — must match the same GPTS as every other section of the same age. By comparing the local polarity column (normal/reversed sequence and their thicknesses, weighted by estimated sedimentation rates) to the GPTS, a geologist can identify the most likely correlation and assign ages to boundaries. The method works independently of fossils, which is its great power — it can date sections where biostratigraphy fails.

Question 2

The Cretaceous Normal Superchron was a ~40 million year period with no geomagnetic reversals. What does this imply for magnetostratigraphy applied to sediments deposited during this interval?

• A. Sediments from the Cretaceous Normal Superchron cannot be dated by any method because they record only one polarity
• B. Magnetostratigraphy cannot subdivide this interval because there are no polarity boundaries to correlate with the GPTS — other dating methods must be used for internal chronology
• C. The superchron appears in the GPTS as a long reversed-polarity interval and is straightforward to identify
• D. Sediments from this period show reversed polarity and are easily correlated across basins

Answer: (B)

Magnetostratigraphy works by correlating polarity boundaries — transitions between normal and reversed zones — to the GPTS. When there are no reversals for 40 million years, there are no polarity boundaries to match. A section recording the CNS appears as an undifferentiated normal-polarity interval; you can identify it as CNS, but you cannot subdivide its internal chronology using magnetostratigraphy. This is a genuine limitation of the method: biostratigraphy, cyclostratigraphy, or radiometric dating must provide internal age control within superchrons. The CNS appears as normal polarity (not reversed) in the GPTS — option C is also wrong.

Question 3

Geomagnetic reversals occur at different times in different regions of the Earth, which is why the magnetic anomaly patterns on the seafloor differ between ocean basins.

• T. True
• F. False

Answer: False

False — geomagnetic reversals are globally synchronous. When the dynamo reverses, the field changes polarity everywhere on Earth at essentially the same time (on geological timescales). The magnetic anomaly patterns on different seafloors differ because spreading rates differ: a fast-spreading ridge (like the East Pacific Rise) creates wider anomaly stripes for the same reversal interval than a slow-spreading ridge (like the Mid-Atlantic Ridge). The timing of the reversals is the same; only the width of the stripes recording those reversals varies with spreading rate.

Question 4

The geomagnetic polarity time scale was extended back through the Cretaceous primarily by using marine magnetic anomalies combined with estimates of seafloor spreading rates.

• T. True
• F. False

Answer: True

True. Dateable volcanic rocks on land (calibrated by ⁴⁰Ar/³⁹Ar dating) established the GPTS for the past ~5 million years. To extend the scale further back in time, geophysicists exploited the symmetric patterns of magnetic anomalies flanking mid-ocean ridges. If the spreading rate is known or can be estimated, the width of each anomaly stripe converts directly to a duration. By measuring anomaly patterns across many ridge systems and using spreading rates constrained by other data, the GPTS was extended through the Cretaceous and into the Jurassic — far beyond the reach of continental lavas.

Question 5

Explain why the global synchroneity of geomagnetic reversals makes magnetostratigraphy a powerful correlation tool, and identify its key limitation.

Model answer: Because reversals happen simultaneously everywhere on Earth, the same polarity boundary in rocks from Italy and the Pacific represents exactly the same moment in time. This allows correlation of sedimentary sections across any distance without relying on shared fossils or lithologies. The key limitation is resolution: magnetostratigraphy can only place age constraints at polarity boundaries. Within a long normal or reversed zone — especially during superchrons — the method provides no internal chronology, and other dating methods must be used.

The synchroneity is what distinguishes magnetostratigraphy from, say, biostratigraphy: a fossil zone may appear at different times in different regions due to facies controls or migration lags, but a polarity reversal is isochronous globally. This makes it a true global correlation tool. The limitation matters practically: the Cretaceous Normal Superchron (no reversals for ~40 Ma) is a blank interval for magnetostratigraphy, requiring cyclostratigraphy or radiometric dating to establish internal chronology.