5.4 Geomagnetic reversals and magnetic anomalies

Earth's magnetic field isn't as stable as you might think. It flips polarity every so often, swapping north and south poles. These geomagnetic reversals leave their mark in rocks, giving us a window into Earth's magnetic past.

Magnetic anomalies in oceanic crust tell an amazing story of seafloor spreading. As new crust forms at mid-ocean ridges, it records the Earth's magnetic field at that time. This creates a striped pattern that's key evidence for plate tectonics.

Geomagnetic Reversals and Implications

Defining Geomagnetic Reversals

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• Geomagnetic reversals are events where Earth's magnetic field flips polarity, with the magnetic north and south poles exchanging positions
• During a reversal, the magnetic field weakens and the poles may wander across the Earth's surface before settling in their new positions (South Atlantic Anomaly, Matuyama-Brunhes reversal)
• Geomagnetic reversals are recorded in the magnetic minerals of rocks, providing a record of Earth's magnetic field history (magnetite, hematite)
• The frequency of geomagnetic reversals varies over geological time, with some periods characterized by frequent reversals (Jurassic) and others by long periods of stable polarity (Cretaceous Normal Superchron)

Implications of Geomagnetic Reversals

• Geomagnetic reversals have implications for Earth's magnetic field strength, as the field weakens significantly during a reversal (dipole moment)
• Weakened magnetic field during reversals may lead to increased exposure to solar radiation and cosmic rays at Earth's surface (atmospheric ionization)
• Geomagnetic reversals can affect navigation by organisms that rely on the magnetic field for orientation (migratory birds, sea turtles)
• Study of geomagnetic reversals helps in understanding the dynamics of Earth's core and the generation of the geomagnetic field (geodynamo)

Magnetic Anomalies and Seafloor Spreading

Magnetic Anomalies in Oceanic Crust

• Magnetic anomalies are variations in the strength and direction of Earth's magnetic field compared to the expected values based on the global magnetic field model
• Magnetic anomalies are caused by variations in the magnetic properties of rocks, particularly in the oceanic crust (basaltic composition)
• Oceanic crust records the polarity of Earth's magnetic field at the time of its formation at mid-ocean ridges (thermoremanent magnetization)
• The alternating pattern of normal and reversed magnetic polarity in the oceanic crust creates a series of parallel magnetic anomalies on either side of the mid-ocean ridge (magnetic striping)

Seafloor Spreading and Magnetic Anomalies

• Seafloor spreading is the process by which new oceanic crust is formed at mid-ocean ridges and spreads outward from the ridge axis (divergent plate boundaries)
• As new oceanic crust forms at mid-ocean ridges, it records the polarity of Earth's magnetic field at the time of its formation
• The magnetic anomaly patterns provide evidence for seafloor spreading and help to reconstruct the history of plate motions (Vine-Matthews-Morley hypothesis)
• Symmetrical magnetic anomaly patterns on either side of the mid-ocean ridge indicate that seafloor spreading occurs at a relatively constant rate over time (spreading half-rates)

Interpreting Magnetic Anomaly Patterns

Age Determination of Oceanic Crust

• The age of the oceanic crust increases with distance from the mid-ocean ridge axis, as older crust is pushed away from the ridge by the formation of new crust
• The magnetic anomaly patterns can be correlated with the geomagnetic polarity timescale to determine the age of the oceanic crust at a given location (chron boundaries)
• Absolute ages of the oceanic crust can be determined by radiometric dating of basaltic rocks sampled from the ocean floor (40Ar/39Ar dating)
• The oldest oceanic crust is found in the western Pacific Ocean (Jurassic age) and the youngest near the mid-ocean ridges (present day)

Spreading Rates and Anomaly Widths

• The width of individual magnetic anomalies depends on the spreading rate of the oceanic crust and the duration of the corresponding polarity interval
• Faster spreading rates result in wider magnetic anomalies, while slower spreading rates produce narrower anomalies (East Pacific Rise vs. Mid-Atlantic Ridge)
• By measuring the distance between corresponding magnetic anomalies on either side of the mid-ocean ridge and knowing the age difference between them, the average spreading rate can be calculated
• Variations in the width of magnetic anomalies along a mid-ocean ridge can indicate changes in spreading rates over time (ridge jumps, propagating rifts)

Geomagnetic Polarity Timescale and Applications

Constructing the Geomagnetic Polarity Timescale

• The geomagnetic polarity timescale (GPTS) is a record of the reversals in Earth's magnetic field polarity over geological time
• The GPTS is constructed by combining data from magnetic anomalies in the oceanic crust, magnetostratigraphy of sedimentary and volcanic sequences, and radiometric dating of key horizons
• The GPTS is divided into normal and reversed polarity intervals, with each interval assigned a specific age range based on radiometric dating and other geochronological methods (chrons, subchrons)
• The GPTS extends back to the Late Jurassic (approximately 170 million years ago) and is continuously updated as new data becomes available

Applications of the Geomagnetic Polarity Timescale

• The GPTS serves as a global reference for correlating and dating marine and terrestrial sediments, volcanic rocks, and other geological events
• Magnetostratigraphy, the study of the magnetic polarity record in sedimentary and volcanic sequences, allows for the dating and correlation of these sequences using the GPTS (Olduvai subchron, Jaramillo subchron)
• The GPTS has been instrumental in understanding the timing and rates of various geological processes, such as seafloor spreading, plate tectonics, and evolutionary events (mass extinctions, hominid evolution)
• Geomagnetic excursions, short-lived reversals or deviations in the magnetic field that do not result in a full polarity reversal, are also recorded in the GPTS and can provide additional age constraints (Laschamp excursion, Blake excursion)