Heart of Avalonia |
Radiometric Dating, Paleomagnetism and Geochronology
The existence of the microcontinent of Avalonia, along with its time of formation, its place of formation and its motions relative to other ancient landmasses are all inferred from scientific theories, models and maps (paleomaps) derived through application of radiometric dating, paleomagnetism and methods of geochronology. These methods are simple in principle, but often extraordinarily laborious, painstaking and expensive in practice. It is only through decades of global research by geoscientists in many countries working with rock samples, mass spectrometers, magnetometers, ocean bottom surveys and innumerable other complex methods and equipment that Avalonia has been given a geologic history. That history remains uncertain and very much a work in progress.
Radiometric dating is the only broadly applicable method of estimating the absolute age of rocks. Radioactive elements are sometimes incorporated into the stable crystalline structure of mineral grains as molten rock solidifies. These elements break down at known rates to produce other elements that remain locked within the crystal structure of the host mineral grains. By measuring the concentrations of radioactive parent and resultant daughter elements in mineral grains, the time elapsed since solidification of the grains can be approximately determined. Some radioactive elements decay so slowly that a significant percentage of the original element will survive over the entire span (4.54 billion years) of earth's history. Such elements, particularly uranium which decays eventually to lead, are useful for dating the fairly old rocks found in Avalonia. Uranium is often found in zircon (zirconium silicate) crystal grains, but the decay product, lead (chemically excluded from the grains when they are formed), is only found to the extent that it was produced by radioactive breakdown. The ratio of lead to uranium in zircon can thus reveal its age. This frequently makes a hunt for tiny zircon crystals a key part of the procedure for establishing the absolute age of rocks found in Avalonia. Zircon grains found in igneous rocks can reveal the age of solidification of those rocks. Zircon grains found in sedimentary rocks are older than the rocks containing them and, accordingly, provide only an upper bound on the age. Radiometric ages of ancient rocks can have uncertainties of millions of years. Several of the rock formations in the Avalon region of Newfoundland have been radiometrically dated. Sometimes, the motivation has been academic, to better understand fossils or plate tectonics, and sometimes commercial, to better assess economic mineral potential. In cases where neither motivation has been sufficient, rocks remain undated.
Paleomagnetism involves the analysis of magnetism (remnant magnetism) occurring in rocks containing magnetic minerals (usually magnetite or hematite) that were magnetized at their time of formation by the earth's magnetic field. Igneous rocks containing magnetite or hematite can retain a record of their orientation relative to earth's magnetic field at their time of solidification and cooling. Sedimentary rocks may possess magnetic grains which become aligned with earth's field at the time of settling. Paleomagnetic samples must be extracted carefully, preserving knowledge of their orientation in bedrock, dated radiometrically or by other means and then analyzed with a magnetometer to determine an inferred orientation of earth's north-south magnetic axis when the rock was formed. Averaged over time, it is assumed that the magnetic axis of the earth coincides with the planet's axis of rotation (although the field alternates in direction from time to time). Paleomagnetic analysis of rocks of differing ages appears to show wandering of the magnetic axis of the earth. This apparent polar wandering can be corrected by assuming that the latitude of the sample at time of formation differed from its present latitude. In practice, curve-fitting methods are used to statistically combine data from many samples to determine a best-fitting trajectory for a tectonic plate that minimizes apparent polar wandering. Latitude trajectories and rotational motions of plates can be determined in this manner. Determination of the paleolongitude (relative to an arbitrary meridian or to other plates) of ancient tectonic plates requires introduction of other geological data in addition to paleomagnetic data since the earth's magnetic field is assumed symmetric about the north-south axis. In practice, the magnetic axis differs from the geographic (rotation) axis over short geological time intervals. Therefore some longitude information can, in principle, be extracted from paleomagnetic data. Paleomagnetic methods have provided one of the key foundations in the development of the theory of plate tectonics.
Geochronology describes the process of placing of a rock formation in the sequence of events comprising the geologic history of the earth. If precise radiometric dates were readily available for all rocks, then rock-forming events could be sequenced based solely on comparisons of radiometric dates. Unfortunately, most rocks cannot be dated with radiometric methods, and the cost and complexity of the procedure precludes its use in a “broad-brush” manner. Many other dating methods are available, albeit with limitations. For example, once the trajectory of rotation and changing latitude of a tectonic plate has been well established, paleomagnetic data can be used to date rocks on that plate. Sedimentary rocks formed in the more recent periods of geological time can be dated using fossils which resemble other fossils found in rocks of known age. Most fossil species exist for only short spans of geological time before becoming extinct.
There are several straightforward methods for determining the relative age of bodies of rock which are in contact with each other. For instance, beds of sedimentary rock incorporate layers of increasing age with increasing depth (assuming, of course, that the beds have not been severely tilted or flipped over). An unconformity occurs when rock layers are built up, then eroded back over long time intervals, following which new layers are added. Such unconformities represent a gap in the geological time-record preserved in the rock layers. The relative ages of rocks exposed to igneous and metamorphic activity are indicated through observations of the results of that activity. A rock metamorphosed by heat and pressure from surrounding rocks is older than those surrounding rocks. A vein of igneous rock cutting across a body of rock is younger than than the body of rock that it cuts. All of these methods, and others, must be combined to establish the geochronology of rocks and landforms, leading to a coherent understanding of the earth's development in a particular region. As a general (and obvious) rule, the older the rocks, the less certain is their history. The rocks of the Avalon region of Newfoundland are young enough not to have been heavily reworked (metamorphism can erase much of the geological record) by tectonic processes, but old enough to challenge state-of-the-art geochronological methods for years to come.
The existence of the microcontinent of Avalonia, along with its time of formation, its place of formation and its motions relative to other ancient landmasses are all inferred from scientific theories, models and maps (paleomaps) derived through application of radiometric dating, paleomagnetism and methods of geochronology. These methods are simple in principle, but often extraordinarily laborious, painstaking and expensive in practice. It is only through decades of global research by geoscientists in many countries working with rock samples, mass spectrometers, magnetometers, ocean bottom surveys and innumerable other complex methods and equipment that Avalonia has been given a geologic history. That history remains uncertain and very much a work in progress.
Radiometric dating is the only broadly applicable method of estimating the absolute age of rocks. Radioactive elements are sometimes incorporated into the stable crystalline structure of mineral grains as molten rock solidifies. These elements break down at known rates to produce other elements that remain locked within the crystal structure of the host mineral grains. By measuring the concentrations of radioactive parent and resultant daughter elements in mineral grains, the time elapsed since solidification of the grains can be approximately determined. Some radioactive elements decay so slowly that a significant percentage of the original element will survive over the entire span (4.54 billion years) of earth's history. Such elements, particularly uranium which decays eventually to lead, are useful for dating the fairly old rocks found in Avalonia. Uranium is often found in zircon (zirconium silicate) crystal grains, but the decay product, lead (chemically excluded from the grains when they are formed), is only found to the extent that it was produced by radioactive breakdown. The ratio of lead to uranium in zircon can thus reveal its age. This frequently makes a hunt for tiny zircon crystals a key part of the procedure for establishing the absolute age of rocks found in Avalonia. Zircon grains found in igneous rocks can reveal the age of solidification of those rocks. Zircon grains found in sedimentary rocks are older than the rocks containing them and, accordingly, provide only an upper bound on the age. Radiometric ages of ancient rocks can have uncertainties of millions of years. Several of the rock formations in the Avalon region of Newfoundland have been radiometrically dated. Sometimes, the motivation has been academic, to better understand fossils or plate tectonics, and sometimes commercial, to better assess economic mineral potential. In cases where neither motivation has been sufficient, rocks remain undated.
Paleomagnetism involves the analysis of magnetism (remnant magnetism) occurring in rocks containing magnetic minerals (usually magnetite or hematite) that were magnetized at their time of formation by the earth's magnetic field. Igneous rocks containing magnetite or hematite can retain a record of their orientation relative to earth's magnetic field at their time of solidification and cooling. Sedimentary rocks may possess magnetic grains which become aligned with earth's field at the time of settling. Paleomagnetic samples must be extracted carefully, preserving knowledge of their orientation in bedrock, dated radiometrically or by other means and then analyzed with a magnetometer to determine an inferred orientation of earth's north-south magnetic axis when the rock was formed. Averaged over time, it is assumed that the magnetic axis of the earth coincides with the planet's axis of rotation (although the field alternates in direction from time to time). Paleomagnetic analysis of rocks of differing ages appears to show wandering of the magnetic axis of the earth. This apparent polar wandering can be corrected by assuming that the latitude of the sample at time of formation differed from its present latitude. In practice, curve-fitting methods are used to statistically combine data from many samples to determine a best-fitting trajectory for a tectonic plate that minimizes apparent polar wandering. Latitude trajectories and rotational motions of plates can be determined in this manner. Determination of the paleolongitude (relative to an arbitrary meridian or to other plates) of ancient tectonic plates requires introduction of other geological data in addition to paleomagnetic data since the earth's magnetic field is assumed symmetric about the north-south axis. In practice, the magnetic axis differs from the geographic (rotation) axis over short geological time intervals. Therefore some longitude information can, in principle, be extracted from paleomagnetic data. Paleomagnetic methods have provided one of the key foundations in the development of the theory of plate tectonics.
Geochronology describes the process of placing of a rock formation in the sequence of events comprising the geologic history of the earth. If precise radiometric dates were readily available for all rocks, then rock-forming events could be sequenced based solely on comparisons of radiometric dates. Unfortunately, most rocks cannot be dated with radiometric methods, and the cost and complexity of the procedure precludes its use in a “broad-brush” manner. Many other dating methods are available, albeit with limitations. For example, once the trajectory of rotation and changing latitude of a tectonic plate has been well established, paleomagnetic data can be used to date rocks on that plate. Sedimentary rocks formed in the more recent periods of geological time can be dated using fossils which resemble other fossils found in rocks of known age. Most fossil species exist for only short spans of geological time before becoming extinct.
There are several straightforward methods for determining the relative age of bodies of rock which are in contact with each other. For instance, beds of sedimentary rock incorporate layers of increasing age with increasing depth (assuming, of course, that the beds have not been severely tilted or flipped over). An unconformity occurs when rock layers are built up, then eroded back over long time intervals, following which new layers are added. Such unconformities represent a gap in the geological time-record preserved in the rock layers. The relative ages of rocks exposed to igneous and metamorphic activity are indicated through observations of the results of that activity. A rock metamorphosed by heat and pressure from surrounding rocks is older than those surrounding rocks. A vein of igneous rock cutting across a body of rock is younger than than the body of rock that it cuts. All of these methods, and others, must be combined to establish the geochronology of rocks and landforms, leading to a coherent understanding of the earth's development in a particular region. As a general (and obvious) rule, the older the rocks, the less certain is their history. The rocks of the Avalon region of Newfoundland are young enough not to have been heavily reworked (metamorphism can erase much of the geological record) by tectonic processes, but old enough to challenge state-of-the-art geochronological methods for years to come.
heartofavalonia.org Exploring Geologic History