4.4 Paleomagnetism and its applications
4 min read•august 14, 2024
studies Earth's ancient magnetic field preserved in rocks. It reveals past field directions and intensities, helping scientists understand , , and behavior.
This powerful tool aids in dating sedimentary rocks, reconstructing past continents, and correlating geological events globally. It's crucial for unraveling Earth's complex history and understanding its magnetic field evolution.
Paleomagnetism and Rock Magnetism
Principles of Paleomagnetism
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- Paleomagnetism studies the Earth's magnetic field preserved in rocks, providing information about the direction and intensity of the Earth's magnetic field at the time the rocks formed or cooled below their Curie temperature
- The primary types of (NRM) include (TRM), (CRM), and (DRM)
- TRM is acquired when magnetic minerals cool below their Curie temperature in the presence of an external magnetic field (igneous rocks, baked sedimentary rocks near igneous intrusions)
- CRM is acquired when new magnetic minerals form or grow in the presence of an external magnetic field (diagenesis, metamorphism)
Natural Remanent Magnetization (NRM)
- NRM is the permanent magnetization acquired by rocks during their formation or subsequent alteration, used to determine the direction and intensity of the Earth's magnetic field at that time
- DRM is acquired when magnetic mineral grains align with the Earth's magnetic field as they settle through water and are deposited in sedimentary layers
- The stability of NRM depends on factors such as the magnetic mineral composition, grain size, and the presence of secondary overprints due to later thermal or chemical events
- Examples of rocks with stable NRM include basalts, volcanic tuffs, and well-preserved sedimentary rocks (red beds, limestones)
Apparent Polar Wander and Plate Tectonics
Apparent Polar Wander (APW)
- APW refers to the observed movement of the Earth's magnetic poles relative to a particular continent or tectonic plate over geological time
- APW paths are constructed by plotting the (ancient magnetic pole positions) determined from paleomagnetic studies of rocks of different ages from a single continent or plate
- The movement of the magnetic poles relative to a continent or plate is due to the movement of the continent or plate relative to the Earth's spin axis, not the actual movement of the poles themselves
- Examples of APW paths include the North American APW path and the European APW path
Plate Tectonics and Paleomagnetism
- APW paths provide evidence for the motion of tectonic plates over geological time, supporting the theory of plate tectonics
- The consistency of APW paths across different continents for the same time periods provides evidence for the existence of past supercontinents (, )
- Differences in APW paths between continents can be used to reconstruct the relative positions and movements of continents through time, aiding in the understanding of plate tectonic processes
- Paleomagnetic data have been used to reconstruct the breakup of Pangaea and the subsequent drift of continents to their present-day positions
Paleomagnetic Data for Reconstruction and Geodynamo Studies
Continental Reconstruction
- Paleomagnetic data can be used to determine the paleolatitude of a continent or tectonic plate at the time the rocks acquired their NRM, based on the inclination of the Earth's magnetic field recorded in the rocks
- The declination of the NRM in rocks provides information about the rotation of a continent or plate relative to the Earth's magnetic north pole
- By comparing the paleolatitudes and declinations of rocks from different continents of the same age, the relative positions of the continents at that time can be reconstructed
- Paleomagnetic data have been used to reconstruct the configurations of past supercontinents (Pangaea, Gondwana, Laurasia)
Geodynamo Studies
- Paleomagnetic data can be used to study the behavior of the Earth's geodynamo, which generates the Earth's magnetic field
- Changes in the intensity and direction of the Earth's magnetic field over geological time, as recorded in rocks, provide insights into the dynamics of the Earth's outer core and the processes that drive the geodynamo
- Paleomagnetic data have revealed the occurrence of magnetic field reversals, where the Earth's magnetic field flips its polarity, with magnetic north becoming magnetic south and vice versa
- The intervals between magnetic field reversals range from tens of thousands to millions of years (, )
Magnetic Stratigraphy for Dating and Correlation
Dating Sedimentary Sequences
- uses magnetic polarity reversals recorded in sedimentary rocks to establish a chronological framework for sedimentary sequences
- During magnetic polarity reversals, the Earth's magnetic field switches between (similar to the present-day field) and , which is recorded in sedimentary rocks as changes in the orientation of magnetic minerals
- The pattern of magnetic polarity reversals in a sedimentary sequence can be compared to the (GPTS), a global record of magnetic polarity reversals dated using radiometric methods
- By identifying the pattern of polarity reversals in a sedimentary sequence and matching it to the GPTS, the age of the sediments can be determined
Correlating Geological Events
- Magnetic stratigraphy allows for the correlation of sedimentary sequences across different basins and even different continents, as the pattern of magnetic polarity reversals is global in nature
- The correlation of sedimentary sequences using magnetic stratigraphy helps in understanding the timing and extent of geological events (mass extinctions, climate changes, sea-level fluctuations)
- Magnetic stratigraphy is particularly useful for dating sedimentary sequences that lack other datable materials (fossils, volcanic ash layers) and can provide age constraints for these sequences
- Examples of geological events studied using magnetic stratigraphy include the Cretaceous-Paleogene boundary, the Eocene-Oligocene transition, and the Messinian salinity crisis
Key Terms to Review (22)
Apparent Polar Wander: Apparent polar wander refers to the perceived movement of the Earth's magnetic poles over geological time, as indicated by the orientation of magnetic minerals in ancient rocks. This phenomenon occurs because the continents have moved while the magnetic poles have remained relatively stable, resulting in a record that shows how the magnetic poles appear to have shifted. Understanding apparent polar wander is crucial for interpreting the tectonic movements of continents and reconstructing the geological history of the Earth.
Brunhes-Matuyama Reversal: The Brunhes-Matuyama reversal is a significant geomagnetic event that occurred approximately 780,000 years ago, marking the transition from the normal magnetic field (Brunhes Chron) to a reversed magnetic field (Matuyama Chron). This event is crucial in paleomagnetism as it provides insight into Earth's magnetic field history and serves as a time marker in geological records for understanding climate changes and biostratigraphy.
Chemical Remanent Magnetization: Chemical remanent magnetization (CRM) refers to the permanent magnetic signature acquired by rocks as they form through chemical processes, particularly during the crystallization of minerals that contain magnetic properties. This type of magnetization occurs when iron-bearing minerals align themselves with the Earth's magnetic field during or shortly after their formation, providing crucial information about past geomagnetic conditions. Understanding CRM is key to studying the history of the Earth's magnetic field and plate tectonics.
Continental drift: Continental drift is the geological theory that continents have moved over geological time relative to each other, resulting in the current configuration of Earth's landmasses. This idea, originally proposed by Alfred Wegener in the early 20th century, links the movement of continents to the processes of plate tectonics and the dynamics of the Earth's lithosphere, reshaping our understanding of geological history and mechanisms.
Detrital Remanent Magnetization: Detrital remanent magnetization (DRM) is the magnetic signature preserved in sediments, primarily caused by the alignment of magnetic minerals during deposition. This phenomenon occurs when detrital particles, containing ferromagnetic minerals, settle in a magnetic field and lock in their orientation, recording the Earth's magnetic field at that time. Understanding DRM is crucial for paleomagnetic studies as it provides insight into ancient geomagnetic conditions and tectonic movements.
Drummond Matthews: Drummond Matthews was a British geophysicist known for his significant contributions to the study of paleomagnetism, particularly in understanding the magnetic properties of ocean floor rocks. His research provided critical evidence for the theory of seafloor spreading and plate tectonics, linking paleomagnetism to geological processes. This work has had lasting implications in both geology and geophysics, influencing our understanding of Earth's magnetic history and its effects on continental drift.
Frederick Vine: Frederick Vine is a prominent geophysicist known for his significant contributions to the understanding of plate tectonics and paleomagnetism, particularly through the development of the Vine-Matthews hypothesis. This hypothesis provides a mechanism for understanding how magnetic patterns in oceanic crust are formed and how they relate to seafloor spreading, which is essential for interpreting geological history and plate movements.
Geodynamo: The geodynamo is the process by which the Earth's magnetic field is generated through the movement of molten iron and other conductive materials in its outer core. This process involves the convection currents driven by heat from the inner core and the rotation of the Earth, resulting in a self-sustaining magnetic field that extends far into space. Understanding this phenomenon helps explain various applications in geophysics, such as plate tectonics and the behavior of geomagnetic fields over time.
Geomagnetic polarity timescale: The geomagnetic polarity timescale is a chronology that describes the history of Earth’s magnetic field reversals, indicating periods of normal and reversed magnetic polarity. These reversals are recorded in the rock record and are essential for understanding plate tectonics and the age of geological formations. The timescale provides key insights into the Earth's geodynamic processes and is used extensively in paleomagnetism to date rocks and sediments.
Jaramillo Subchron: The Jaramillo Subchron is a significant geological interval within the Matuyama Chron, marking a brief period of geomagnetic reversal that occurred approximately 0.9 million years ago. This subchron is characterized by a temporary reversal of the Earth's magnetic field, where the magnetic north pole shifted to the south, providing crucial data for understanding paleomagnetism and its implications for tectonic activity and climate changes throughout Earth’s history.
Magnetic anomalies: Magnetic anomalies are variations in the Earth's magnetic field caused by the distribution of magnetic materials in the crust. These anomalies can provide crucial insights into geological structures, tectonic activities, and mineral deposits by revealing differences between expected and observed magnetic field strengths.
Magnetic field reversal: Magnetic field reversal refers to the phenomenon where the Earth's magnetic north and south poles switch places over geological time scales. This process has been recorded in various geological formations, providing important data on the history of Earth's magnetic field and its influence on paleomagnetism, which helps in understanding tectonic plate movements and the age of rocks.
Magnetic Stratigraphy: Magnetic stratigraphy is a geological technique that uses the magnetic properties of rocks and sediments to establish a chronological framework and correlate geological layers. This method relies on the record of Earth's magnetic field reversals, known as magnetostratigraphy, which allows scientists to date sedimentary sequences and understand their depositional environments. By linking these magnetic signatures to a global magnetic time scale, researchers can gain insights into the geological history and the evolution of the Earth's magnetic field.
Natural remanent magnetization: Natural remanent magnetization (NRM) refers to the magnetization that a rock or sediment acquires during its formation or alteration, which retains the Earth's magnetic field direction at the time of its solidification. This property is crucial in studying paleomagnetism, as it helps reconstruct past magnetic fields and understand tectonic movements over geological timescales.
Normal Polarity: Normal polarity refers to the orientation of Earth's magnetic field when the magnetic north pole is near the geographic North Pole. This condition is significant in paleomagnetism, as it allows geoscientists to understand past magnetic field reversals and their timing, which helps in reconstructing the history of Earth's tectonic plates and the formation of oceanic crust.
Paleomagnetism: Paleomagnetism is the study of the magnetic properties of rocks and sediments, particularly the record of Earth's magnetic field preserved in them over geological time. This concept connects various geophysical aspects, such as understanding the historical movement of tectonic plates, the behavior of Earth’s magnetic field, and how these factors can be utilized in applications like magnetic surveying and interpreting data related to the Earth’s core dynamics.
Paleopoles: Paleopoles are the theoretical locations of the Earth's magnetic poles at various points in geological history, reconstructed from the magnetization of rocks. These positions provide vital information about the historical movement of tectonic plates and the geomagnetic field over time, linking paleomagnetism to the understanding of continental drift and plate tectonics.
Pangaea: Pangaea was a supercontinent that existed during the late Paleozoic and early Mesozoic eras, bringing together nearly all of the Earth's landmasses into one massive landform. Its existence is crucial for understanding continental drift, plate tectonics, and the historical arrangement of continents over time, all of which are essential in paleomagnetism as it relates to ancient magnetic field directions recorded in rocks.
Plate Tectonics: Plate tectonics is the scientific theory that explains the movement of the Earth's lithosphere, which is divided into several large and small plates that float on the semi-fluid asthenosphere beneath. This theory helps to understand various geological phenomena such as earthquakes, volcanic activity, and mountain-building processes, as well as the historical arrangement of continents over geological time.
Reversed Polarity: Reversed polarity refers to a phenomenon where the Earth's magnetic field has flipped, resulting in the magnetic north and south poles being switched. This process is significant in understanding the historical changes in Earth's magnetic field, which is crucial for paleomagnetism, as it helps scientists interpret the age of rocks and understand tectonic plate movements.
Rodinia: Rodinia is a supercontinent that existed during the late Proterozoic era, approximately 1.3 billion to 750 million years ago. This ancient landmass is significant in understanding the geological history of Earth, particularly through paleomagnetism, which studies the magnetic properties of rocks to determine the historical positions of continents.
Thermal remanent magnetization: Thermal remanent magnetization (TRM) is the magnetization acquired by rocks and sediments when they cool below their Curie temperature, allowing them to retain a magnetic field aligned with the Earth's magnetic field at that time. This process is crucial in paleomagnetism as it provides insights into the historical geomagnetic field and the past movements of tectonic plates, helping to reconstruct the Earth's magnetic history.