Proc. Natl. Acad. Sci. USA Vol. 94, pp. 12742–12743, November 1997 From the Academy
This paper is a summary of a session presented at the third annual German–American Frontiers of Science symposium, held June 20–22, 1997 at the Kardinal Wendel Haus in Munich, Germany.
BRADFORD M. CLEMENT*, ASTRID HOLZHEID†, AND ANDREAS TILGNER‡
*Department of Geology, Florida International University, Miami, FL 33199; †Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139; and ‡Theoretische Physik IV, Universista¨tBayreuth, D-95440, Bayreuth, Germany
The deep interior of Earth is inaccessible directly, yet deter- magnetic field that are shorter than the diffusive time scale of mining our planet’s structure and composition is critical for the inner core. understanding the origin and evolution of our planet. In recent The first three-dimensional dynamo model using earth-like years, important advances have been made using indirect conditions was solved recently (1). This model included an methods to infer the nature of the Earth’s deep interior. These inner core with a finite electrical conductivity. This dynamo advances have been made by applying an interdisciplinary model successfully predicted that the inner core should rotate approach, using seismic, geochemical, and geomagnetic meth- faster than the earth’s mantle. Seismic studies showed that the ods. inner core is rotating about 1.1°͞year faster than the mantle On the basis of seismic wave propagation through the earth (2). This dynamo model also caught the magnetic field in the we divide the planet into the crust (10–70 km thick), the mantle act of reversing its polarity—one of the most dramatic prop- (which extends down to 2,900 km), the liquid outer core (from erties of geomagnetic field behavior. 2,900 km to 5,080 km), and a solid inner core (5,080 km to 6,371 Polarity reversals remain one of the most enigmatic prop- km). The composition of the Earth is estimated by assuming erties of the field. Under certain conditions magnetic minerals that the planets and the Sun formed from a parent nebula of lock in the direction and intensity of Earth’s magnetic field at gas and dust. Therefore the elemental abundances in the Sun the time the rock forms. The resulting magnetizations can be together with those in chondritic meteorites (which are con- treated as fossil compasses and used to map out the changes sidered remnants of the early material that accreted to form in the geomagnetic field. By obtaining high-resolution paleo- the planets) can be used to infer the bulk composition of the magnetic records of sediments or lava flows that formed as the earth. The result is that the outer core must be composed of field was reversing, it is possible to document how the reversal an iron and nickel alloy (80 wt% Fe, 5 wt% Ni) along with a occurs. smaller percentage of a less dense element (up to 15%) A ubiquitous feature of polarity transition records is a necessary to meet the density required by seismic wave veloc- decrease in the intensity of the field to roughly 10% of its full ities through the core. Likely candidates for the lighter element polarity strength. While the intensity is low, the directions are oxygen, silicon, carbon, and to a lesser extent hydrogen or change by 180° and then the field grows in the new direction. nitrogen. The directional change takes 1,000 to 4,000 years to occur, and The core began to grow after the formation of the Earth as the intensity change takes longer. Changes in the field may the temperature increased to the point where dense, liquid iron occur much more rapidly, though. In what is arguably the best began to sink to the center of the planet. The time of the core documented record, the Steen’s Mountain record, the field formation can be estimated by using isotopic signatures of changes by as fast as 6° per day (3). The drop in the field radiogenic element pairs as natural chronometers. For exam- intensity and fast changes in the local field direction have ple, hafnium-182͞tungsten-182 ratios of various terrestrial and implications for the environment at Earth’s surface, but there extraterrestrial rocks (Hf has the tendency to concentrate in exist no known correlations between polarity reversals and the Earth’s mantle, whereas W will partition into the Earth’s biological extinctions. core) suggest that the last core formation events occurred at A number of recent polarity transition records exhibit least 4.7 ϫ 107 years after the formation of the Earth (4.5 ϫ recurring field behavior observed at widely separated sites. 109 years ago). This suggests that something provides a memory for the rapidly The geomagnetic field is a fundamental feature of our changing dynamo. The two possible candidates are the solid planet, and yet the origin of the field remains as one of the inner core and the core mantle boundary. major outstanding problems in the earth sciences. The field is Seismic tomography shows that the core–mantle boundary thought to be generated by some sort of dynamo process acting is laterally heterogeneous. Observed as differences in velocity in the outer core. Liquid iron and nickel flowing in a convect- of seismic waves, these variations are interpreted as lateral ing core provide a moving conductor capable of generating variations in temperature or composition. Either of these magnetic fields. But just how this happens is unknown. Solving interpretations could influence the geodynamo by entraining the nonlinear equations for a dynamo with earth-like condi- thermal convection in the outer core, or lateral variations in tions has proven extremely difficult. the electrical conductivity could focus the magnetic field lines Dramatic progress has been made recently by including the emerging from the core. effects of the inner core in dynamo models. The inner core is The primary energy source that maintains the dynamo solid iron and has a finite electrical conductivity. As a result of action is convection driven by the latent heat of crystalliza- its electrical conductivity, it is difficult for the magnetic field tion of the solid inner core. If the inner core provides a generated in the outer core to diffuse through the inner core. stabilizing influence on the magnetic field, then the growth The inner core therefore dampens out fluctuations in the The Frontiers of Science symposia is the latest in the series “From the Academy,” which is presented occasionally to highlight work of the © 1997 by The National Academy of Sciences 0027-8424͞97͞9412742-2$2.00͞0 Academy, including the science underlying reports of the National PNAS is available online at http:͞͞www.pnas.org. Research Council.
12742 Downloaded by guest on October 2, 2021 From the Academy: Clement et al. Proc. Natl. Acad. Sci. USA 94 (1997) 12743
of the inner core likely affected the frequency of polarity 2. Song, X. D. & Richards, P. G. (1996) Nature (London) 382, reversals. By extending the polarity reversal chronology back 221–224. into the Precambrian, paleomagnetism may provide con- 3. Coe, R. S., Prevot, M. & Camps, P. (1995) Nature (London) 374, 687–692. straints on the timing of the origin and growth of Earth’s inner core. Suggested Reading
1. Glatzmaier, G. A. & Roberts, P. H. (1995) Nature (London) 377, 1. Fuller, M., Laj, C. & Herrera-Bervera, E. (1996) Am. Sci. 84, 203–209. 552–561. Downloaded by guest on October 2, 2021