1.4 Fundamentals of Geomagnetism

Case Study 1: South Atlantic Anomaly and Magnetic Field Weakening

• Models/Theories:
  • Dynamo Theory (explains Earth's magnetic field generation in the outer core)
  • Magnetic Reversal Theory
• Recent Data (2023):
  • The South Atlantic Anomaly (SAA), an area of weakening magnetic field intensity, has expanded over the past decade. ESA’s Swarm satellites recorded a field strength drop of ~8% between 1970 and 2023.
  • Magnetic north is drifting ~55–60 km per year toward Siberia.
• Spatial Variation:
  • The SAA spans parts of South America to southern Africa, showing greatest magnetic field weakness over Brazil and the South Atlantic.
• Temporal Variation:
  • The Earth’s magnetic field strength has decreased by nearly 9% over the past 200 years, with localized weakening accelerating recently.
• Source:
  • ESA (2023), Nature Geoscience, and DownToEarth ("Geomagnetic Changes and Their Risks", Sept 2023)
• Insight:
  • The SAA illustrates a real-time weakening of Earth’s geomagnetic field, potentially linked to pole reversals. It supports the Dynamo Theory and indicates active changes in the outer core dynamics.

Case Study 2: Magnetic Striping at Mid-Ocean Ridges

• Models/Theories:
  • Sea-Floor Spreading Model (Vine-Matthews Hypothesis)
  • Dynamo Theory
• Recent Data (2023):
  • Magnetic surveys in the Mid-Atlantic Ridge show symmetrical striping of normal and reversed magnetic polarity on both sides of the ridge, correlating with known geomagnetic reversals.
• Spatial Variation:
  • Patterns of striping are found along all active mid-ocean ridges—Mid-Atlantic Ridge, East Pacific Rise, Carlsberg Ridge in the Indian Ocean.
• Temporal Variation:
  • These magnetic anomalies represent over 170 magnetic reversals over the last 180 million years, with a frequency of ~1 reversal every 200,000–300,000 years.
• Source:
  • Science Journal, USGS (2023); National Geographic coverage on seafloor spreading and magnetic field behavior.
• Insight:
  • Magnetic striping validates the Dynamo Theory and Plate Tectonics, revealing Earth’s internal magnetism imprinted on the ocean floor.