4.3 Magnetic anomalies and paleomagnetic evidence

Magnetic anomalies and paleomagnetic evidence are key to understanding plate tectonics. These patterns in oceanic crust reveal the history of seafloor spreading and Earth's magnetic field reversals.

By studying these magnetic signatures, scientists can map the age of the seafloor and reconstruct past plate movements. This data supports the theory of plate tectonics and helps explain how our planet's surface has changed over time.

Magnetic Anomalies and Plate Tectonics

Understanding Magnetic Anomalies

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• Magnetic anomalies represent variations in Earth's magnetic field strength and direction deviating from the expected dipole field
• Differences in magnetic properties of crustal rocks cause these anomalies
  • Remnant magnetization in oceanic basalts plays a significant role
• Linear patterns parallel to mid-ocean ridges form due to seafloor spreading and new oceanic crust creation
• Magnetometers on ships, aircraft, or satellites measure magnetic anomalies
  • Produce detailed maps of oceanic crustal magnetization
• Factors influencing strength and polarity of magnetic anomalies include
  • Intensity of Earth's magnetic field at rock formation time
  • Subsequent tectonic processes (rifting, subduction)

Significance in Plate Tectonics

• Magnetic anomalies provide crucial evidence for plate tectonic theory
• Allow scientists to map age and spreading history of oceanic crust
  • Reveal patterns of seafloor formation and movement
• Support the concept of seafloor spreading
  • Symmetric patterns on either side of mid-ocean ridges
• Enable reconstruction of past plate configurations
  • Help understand evolution of ocean basins (Atlantic Ocean)
• Contribute to global tectonic models
  • Identify major plate boundaries and their characteristics

Paleomagnetic Reversals in Oceanic Crust

Paleomagnetic Reversal Process

• Paleomagnetic reversals involve global-scale changes in Earth's magnetic field polarity
  • North and south magnetic poles switch positions
• Occur at irregular intervals, typically every few hundred thousand to million years
• Complex processes in Earth's outer core cause these reversals
  • Convection currents in liquid iron affect magnetic field generation
• Curie temperature effect preserves paleomagnetic reversals in oceanic crust
  • Magnetic minerals retain orientation once cooled below this temperature ($\approx 580°C$ for magnetite)

Recording Reversals in Oceanic Crust

• New oceanic crust forms at mid-ocean ridges
• Magnetic minerals in cooling basalt align with Earth's magnetic field
  • Record polarity at time of formation
• Creates alternating normal and reversed polarity bands in oceanic crust
  • Parallel to mid-ocean ridge axis
• Width of magnetic bands proportional to
  • Spreading rate of oceanic crust
  • Duration of each polarity interval
• Provides time scale for dating seafloor rocks
  • Helps reconstruct plate tectonic histories
• Examples of major reversals
  • Brunhes-Matuyama reversal (780,000 years ago)
  • Jaramillo reversal (1.07 million years ago)

Magnetic Anomaly Patterns and Crust Age

Interpreting Magnetic Anomaly Patterns

• Symmetric stripes form on either side of mid-ocean ridges
  • Reflect seafloor spreading process
• Width and spacing of magnetic stripes indicate spreading rates
  • Wider stripes suggest faster spreading (East Pacific Rise)
  • Narrower stripes indicate slower spreading (Mid-Atlantic Ridge)
• Correlate observed patterns with established geomagnetic polarity time scale
  • Assign absolute ages to different parts of oceanic crust
• Youngest crust located closest to mid-ocean ridge
  • Age increases with distance from ridge axis

Advanced Analysis Techniques

• Vector magnetic anomaly analysis provides additional information
  • Orientation and intensity of past magnetic fields
• Variations in spreading rates reveal complex tectonic processes
  • Ridge jumps (Galapagos Spreading Center)
  • Microplate rotations (Easter Microplate)
• Asymmetries in magnetic anomaly patterns indicate
  • Differential spreading rates
  • Ridge propagation events
• Integration with other geophysical data enhances interpretation
  • Gravity anomalies
  • Seismic profiles

Paleomagnetism and Seafloor Spreading

Paleomagnetic Evidence Supporting Seafloor Spreading

• Crucial role in development and acceptance of seafloor spreading hypothesis
  • Proposed by Harry Hess in early 1960s
• Symmetric magnetic anomaly patterns on either side of mid-ocean ridges
  • Strong support for continuous creation of new oceanic crust at these boundaries
• Paleomagnetic studies of oceanic basalts confirm
  • Rocks retain record of Earth's magnetic field at formation time
  • Validates use of magnetic anomalies as dating tool
• Correlation between magnetic anomaly ages and radiometric dating of seafloor basalts
  • Strengthens seafloor spreading model
• Global consistency of paleomagnetic patterns across ocean basins
  • Supports unified theory of plate tectonics

Integration with Other Geological Evidence

• Paleomagnetic data combined with other observations provides comprehensive understanding
  • Heat flow measurements near mid-ocean ridges
  • Seismic studies of crustal structure
• Allows reconstruction of past plate configurations
  • Calculate spreading rates over geological time
• Quantitative framework for understanding plate tectonic processes
  • Rates of plate movement (cm/year)
  • Direction of plate motion
• Explains distribution of earthquakes and volcanoes along plate boundaries
  • Ring of Fire in Pacific Ocean
• Supports continental drift theory
  • Matching paleomagnetic orientations in now-separated continents (South America and Africa)