Earth’s Magnetic Field and Rock Magnetization: Key Concepts
Earth’s Magnetic Field and Rock Magnetization
Types of Rock Magnetization
a) Sedimentary Rock (Detrital Remanent Magnetism)
Origin: Magnetic minerals align with the Earth’s magnetic field during sediment deposition. This alignment is locked in when the sediment lithifies into rock.
Example: Magnetite in sandstone aligns during deposition and retains the Earth’s magnetic field direction.
b) Metamorphic Rock (Thermoremanent Magnetism)
Origin: During metamorphism, rocks heat up and lose their magnetization. As they cool below the Curie temperature, magnetic minerals re-align with the Earth’s magnetic field.
Example: In metamorphosed basalt, magnetite re-aligns as the rock cools.
c) Igneous Rock (Thermoremanent Magnetism)
Origin: As molten magma or lava cools, magnetic minerals align with the Earth’s magnetic field, preserving the alignment when the rock solidifies.
Example: In basalt, magnetite aligns with the Earth’s magnetic field as the lava cools and solidifies.
Closure Temperature in Age Determination
Closure temperature: The temperature below which a mineral becomes a closed system for a specific isotopic system. Below this temperature, isotopic diffusion ceases, and the isotopic ratios start reflecting the time elapsed since the mineral cooled through this temperature, determining the absolute age (RM).
Importance in Age Determination:
- Locks Isotopic System: The isotopic ratios represent the radiometric age.
- Provides Cooling History: Indicates the cooling rate and thermal history of the rock or mineral.
- Correlation with Geological Events: Helps correlate with geological events.
Factors Affecting Closure Temperature
- Mineral Type: Different minerals have varying closure temperatures due to their crystal structure and bonding.
- Diffusion Rates: Minerals with lower diffusion rates have higher closure temperatures.
- Cooling Rate: Faster cooling results in a higher closure temperature, as isotopic systems have less time to equilibrate.
Limitations of Hooke’s Law in Deep Earth Deformation
Hooke’s Law cannot explain deformation in the deeper Earth because it assumes elastic, reversible behavior, which only applies to shallow, low-pressure conditions. At greater depths, high pressure and temperature cause rocks to deform plastically or ductilely, meaning the deformation is permanent and not proportional to stress. In the mantle and core, rocks undergo slow, long-term creep or plastic flow, which Hooke’s Law does not account for. Additionally, materials may experience failure or fracture under extreme stress, further deviating from Hooke’s Law.
Key Factors:
- Non-Elastic Behavior at High Pressure and Temperature
- Viscoelastic and Plastic Deformation
- Long-Term Deformation
- Failure Under High Stress
Isotherms and Topography
Isotherms: These are lines of constant temperature in the subsurface.
Effect of Topography: Isotherms bend upward under valleys, indicating cooler regions extending deeper. Isotherms bend downward under peaks, where heat tends to accumulate closer to the surface.
Components of Earth’s Magnetic Field
Magnetic Declination (DDD):
- The angle between the geographic north and the magnetic north, measured in the horizontal plane.
- Positive when magnetic north is east of geographic north, and negative when it is west.
Magnetic Inclination (III):
- Also called the dip, it is the angle between the Earth’s magnetic field and the horizontal plane.
- Positive in the northern hemisphere (field dips downward) and negative in the southern hemisphere.
Horizontal Component (HHH):
- The projection of the Earth’s magnetic field vector onto the horizontal plane.
Vertical Component (ZZZ):
- The projection of the Earth’s magnetic field vector onto the vertical axis.
Total Magnetic Field (FFF):
- The vector sum of the horizontal and vertical components, representing the total strength of the Earth’s magnetic field.