Geochronology and Thermochronology
Geochronology and Thermochronology is designed for graduate and upper-level undergraduate students with a solid background in mathematics, geochemistry, and geology, Read an interview with the editors to find out more: University of Arizona, USA Richard W, Carlson, Carnegie Institution for Science, USA Paul R, Renne, Berkeley Geochronology Center and University of California, USA Kari M, Cooper, University of California, USA Darryl E, Granger, Purdue University, USA Noah M, McLean, University of Kansas, USA Blair Schoene, Princeton University, USA
Geochronology and Thermochronology
Occasionally debates arise and hands are wrung about what parts of a scientific discipline really distinguish it from others. Geoscientists often find themselves trying to define the unique perspectives or essential skills at the heart of their field as if failure to properly indoctrinate students in them might put the entire profession at risk. Without commenting on the wisdom of such disciplinary exceptionalism, a reasonable person asked to engage in it could, after some thought, suggest that if there is something distinctive about Earth science, it might have something to do with time. Naturalistic thinking about the evolution and workings of the Earth have been around for centuries if not millennia, and considerations of time at scales far surpassing human experience are a central and obligatory part of any serious endeavor in this area. The facility to deal easily with enormous timescales is such an ingrained part of Earth and planetary science that occasional meditative realizations of even the most hardened scientists are sometimes required to remind them that our ability to envision geologic time accurately and precisely has been in some ways hard won. Before quantitative measurements were available of the durations of time separating events of the past from the present, and of the rates of geologic processes, practically all attempts to understand Earth were, to paraphrase a key historical figure in geochronology (Lord Kelvin), meagre and of a most unsatisfactory kind. Quantitative geochronology as a concept, and especially radioisotopic geochronology as a field in and of itself, revolutionized our understanding of the Earth and planets. More importantly, geochronology continues to be one of, if not the most, important foundation and means of exploration in modern geoscience.
The tools and applications of geochronology find use in a variety of fields besides Earth and planetary science, including archeology, evolutionary ecology, and environmental studies. But the impact of geochronology on Earth science was fundamentally transformative. For one thing, it laid out the boundary conditions for reconstructing the history of the planet and quantitative understanding of the significance of ongoing physical processes like erosion, sedimentation, magmatism, and deformation. It also established, for the first time, a realistic temporal context of existence-not just of life as we know it, but for the recognizable planetary environment that hosts life. This is because the timescales of Earth history and Earth processes (including biotic evolution at that scale) require a fundamentally different temporal perspective than human experience (much less historical records) can offer. While some important geologic and evolutionary processes happen over very short timescales and require chronometers with commensurate sensitivity, many of the most challenging and important observations we make about the Earth reflect processes that occur either very slowly or very rarely, relative to the perspective of humans as individuals, civilizations, or even species. Modern radioisotopic techniques span vast timescales from seconds to billions of years, finding application in problems ranging from the age and pace of individual volcanic eruptions to condensation of the solar nebula and ongoing planetary accretion. The transformative power of geochronology comes from its capacity to expand our understanding beyond the reach of the pathetically short timescales of intuitive human or social perspectives.
1.1 GEO AND CHRONOLOGIES
Extending the timescale of our understanding does not mean just establishing a chronology of events that occurred earlier than historical records or generational folklore allow. It goes without saying that establishing pre-historical records of changes on and in Earth and other planets is practically useful: knowing when a vo