Geophysics seminar 15:00-17:00 on Thursday March 10, 2016
Room D114, Physicum, Kumpula campus, Helsinki
We welcome everyone to our guest seminar by Prof. David Evans (Yale University, USA) and Dr. Phil McCausland (University of Western Ontario, Canada).
Elusive Keystone Laurentia – Ancestral North America’s Journey Through Two Supercontinents Before Pangea
Dr. Phil J.A. McCausland, Western University of Canada
For over four decades geologists, geophysicists and other scientists have wrestled with the implications of the plate tectonic revolution for the Earth's geological history before Alfred Wegener's supercontinent Pangea. Continental (and plate) mobility clearly happened, but opinions diverge on the past geographic arrangements of the continents and even on fundamental geodynamic mechanisms going back into Precambrian time. Laurentia -the ancestral North American continent including Greenland and Scotland- has contributed crucial paleomagnetic and other information to debates concerning the assembly and dispersal of supercontinents. Laurentia seems to have played a "keystone" role in two major supercontinents prior to Pangea, and will likely provide further key information for ultimately solving these paleogeographic and geodynamic problems. At stake is our understanding of the large-scale driving forces for Earth's secular history.
Biography of Earth’s magnetic field: Birth, adolescence, adulthood, mid-life crisis, maturity
Prof. David Evans, Yale University
It is easy to take the stability of Earth’s magnetic field for granted, but its character may have differed greatly in the deep past. This talk summarizes a long history of paleomagnetic research into the behavior of the field through Earth history, as seen in new context by recent computer simulations of core evolution during the same interval. It is proposed that the field was born at least 3.5 billion years ago (Ga), went through an “adolescent” period of hyperactive-multipolar(!) behavior at 2.0-1.87 Ga, stabilized into an “adulthood” of axial dipolar conditions between 1.8 and 0.8 Ga, reverted to a “mid-life crisis” of behavior similar to the adolescent period between 0.8 and 0.5 Ga, and finally settled into the current “mature” mode with which we are all familiar: a stable, occasionally reversing axial dipole. Alternative mechanisms such as true polar wander or unusual styles of plate tectonics may also account for the two periods of anomalous paleomagnetic signals (~1.9 and 0.6 Ga); each alternative can be tested by geodynamic considerations and predicted geological consequences.