Questions: Rock Magnetism: Domains, Hysteresis, and Saturation

Question 1

Two rock samples both contain magnetite. Sample A has Mᵣ/Mₛ ≈ 0.48 and a broad, square hysteresis loop. Sample B has Mᵣ/Mₛ ≈ 0.05 and a thin, narrow loop. Which sample is a more reliable recorder of ancient magnetic field directions, and why?

• A. Sample B, because low remanent magnetization means less contamination from secondary magnetic overprinting
• B. Sample A, because high Mᵣ/Mₛ indicates single-domain grains with high coercivity that resist remagnetization
• C. Sample B, because narrow hysteresis loops indicate purer mineral chemistry with fewer impurities
• D. Sample A, because broad loops indicate large multi-domain grains with more total magnetic material

Answer: (B)

High Mᵣ/Mₛ (≈0.5) is the signature of single-domain behavior: the entire grain is uniformly magnetized, domain-wall movement is impossible, and high coercivity means a large reverse field is needed to flip the magnetization. This makes single-domain grains resistant to later overprinting and capable of preserving paleomagnetic signals for billions of years. Multi-domain grains (low Mᵣ/Mₛ, thin loops) have easily displaced domain walls and low coercivity — their magnetization is easily reset by later fields, making them unreliable paleomagnetic recorders.

Question 2

What is the physical meaning of COERCIVITY (Hc) on a rock magnetism hysteresis curve?

• A. The maximum magnetization a sample achieves when all magnetic moments are aligned by a strong applied field
• B. The magnetization remaining in a sample after an applied field is reduced to zero
• C. The magnitude of the reverse field required to reduce the net magnetization of the sample to zero
• D. The temperature at which a ferromagnetic mineral loses its magnetic order entirely

Answer: (C)

Coercivity is the reverse field intensity needed to cancel out the net magnetization — it measures resistance to demagnetization. High coercivity means a strong opposing field is required to flip the magnetic moments, indicating a stable, hard magnetic material. This is directly relevant to paleomagnetics: high-coercivity minerals retain their magnetization against later ambient fields (which are typically much weaker than Hc for good recorder minerals like fine-grained magnetite or hematite).

Question 3

A magnetite grain smaller than approximately 80 nm is classified as single-domain because its entire volume is magnetized uniformly in one direction, making domain-wall formation energetically unfavorable.

• T. True
• F. False

Answer: True

Domain walls have an energy cost (exchange energy and magnetostatic energy). For small grains, the energy cost of forming a domain wall exceeds the magnetostatic energy saved by splitting into multiple domains. Below the critical single-domain size (~80 nm for magnetite), the grain is more stable as a uniformly magnetized unit — a single domain. These grains have the highest Mᵣ/Mₛ, the highest coercivity, and are the most stable paleomagnetic recorders.

Question 4

Multi-domain magnetite grains are better paleomagnetic recorders than single-domain grains because they contain more total magnetic material and therefore produce a stronger magnetic signal.

• T. True
• F. False

Answer: False

Despite their larger size and greater total magnetic material, multi-domain grains are poor paleomagnetic recorders. Their many magnetic domains are oriented in different directions and largely cancel each other, resulting in a low net remanent magnetization (low Mᵣ/Mₛ). More importantly, their low coercivity means domain walls move easily in response to later ambient fields, allowing the original remanence to be overprinted. Single-domain grains, though tiny, each carry a stable, high-coercivity magnetization aligned with the ancient field — making them far superior recorders despite their smaller individual moments.

Question 5

Explain why the ratio Mᵣ/Mₛ distinguishes single-domain from multi-domain grains, and what a high ratio tells you about a rock's reliability as a paleomagnetic recorder.

Model answer: In a single-domain grain, the entire grain is uniformly magnetized — there are no domain walls to move, so when the external field is removed, the grain retains nearly half of its saturation magnetization (Mᵣ/Mₛ ≈ 0.5). In a multi-domain grain, domain walls rearrange freely when the field is removed, allowing domains to partially cancel, leaving only a small fraction of Mₛ as remanence (low Mᵣ/Mₛ). High Mᵣ/Mₛ therefore signals single-domain behavior, high coercivity, and resistance to later field overprinting — all the properties needed for stable, long-lived paleomagnetic recording.

The hysteresis loop shape is the primary field test for whether a rock sample is worth pursuing for paleomagnetic study. A thin, narrow loop (low Mᵣ/Mₛ, low Hc) condemns the sample as an unreliable recorder before any expensive demagnetization experiments are undertaken.