This is a brief description of the theory of Plate Tectonics, a new method of looking at Earth’s geological evolution developed in the 1960s. The observations that led to the concepts of plate tectonics, sea floor spreading and continental drift are reviewed, with emphasis on studies of the Earth’s magnetic field and its reversal properties. Data on seismology, topography of the ocean floor, the nature of the oceanic crust, and the loss of heat from the ocean floor are also discussed.

Mike Fuller was awarded the 2012 John Adam Fleming Medal at the AGU Fall Meeting Honors Ceremony, held on 5 December 2012 in San Francisco, Calif. The medal is for “original research and technical leadership in geomagnetism, atmospheric electricity, aeronomy, space physics, and related sciences”.

Fred Spilhaus was awarded the 2010 Waldo E. Smith Medal at the AGU Fall Meeting Honors Ceremony, held on 15 December 2010 in San Francisco, Calif. The medal is for “extraordinary service to geophysics.”

The number of tectonic plates on Earth described in the literature has expanded greatly since the start of the plate tectonic era, when only about a dozen plates were considered in global models of present-day plate motions. With new techniques of more accurate earthquake epicenter locations, modern ways of measuring ocean bathymetry using swath mapping, and the use of space based geodetic techniques, there has been a huge growth in the number of plates thought to exist. The study by Bird (2003) proposed 52 plates, many of which were delineated on the basis of earthquake locations. Because of the pattern of areas of these plates, he suggested that there should be more small plates than he could identify. In this paper, I gather together publications that have proposed a total of 107 new plates, giving 159 plates in all. The largest plate (Pacific) is about 20 % of the Earth's area or 104 Mm(2), and the smallest of which (Plate number 5 from Hammond et al. 2011) is only 273 km(2) in area. Sorting the plates by size allows us to investigate how size varies as a function of order. There are several changes of slope in the plots of plate number organized by size against plate size order which are discussed. The sizes of the largest seven plates is constrained by the area of the Earth. A middle set of 73 plates down to an area of 97,563 km2 (the Danakil plate at number 80, is the plate of median size) follows a fairly regular pattern of plate size as a function of plate number. For smaller plates, there is a break in the slope of the plate size/plate number plot and the next 32 plates follow a pattern of plate size proposed by the models of Koehn et al. (2008) down to an area of 11,638 km2 (West Mojave plate # 112). Smaller plates do not follow any regular pattern of area as a function of plate number, probably because we have not sampled enough of these very small plates to reveal any clear pattern.

R. Kerr's recent News of the Week story “Foreshadowing Haiti's catastrophe” (22 January, p. [398][1]) nicely summarized the geological and seismological knowledge behind the earthquake that struck Port-au-Prince on 12 January this year. However, he failed to draw an obvious conclusion. Why not

In order to investigate the latitudinal effect of the geomagnetic field variation, a new data set consisting of virtual geomagnetic poles (VGPs) from all latitudes has been produced. Since the updated data set was limited to data with VGPs within 45 degrees of the geographic poles, data from lava flows with low-latitude VGPs were added. More rigorous criteria were used to winnow the data. The data were divided into groups from different observation latitudes. In each group it was shown that the distribution of VGP latitudes could be described by a predominance of poles (average 82%) following a Fisher distribution with the rest following a distribution that would produce a uniform number of poles as a function of latitude. A distribution composed of two Fisher distributions also fit the data very well. For the case using a Fisher distribution plus a uniform distribution, the Fisher distribution changed such that the angular standard deviation (ASD) of VGPs from a set of observations taken at the equator is about 10 degrees and the ASD at 60 degrees observation latitude is about 19 degrees. These results are similar to some results seeking to determine the ASD of VGPs as a function of observation latitude using other methods, which have been recently published, but there are also discrepancies. The results allow us to model inclination distributions as a function of observation latitude for comparison with data in which only the inclination is known, such as data from drill holes. It is shown that in order not to have doubt about the polarity of the inclination data, a drill hole has to be located at an absolute latitude greater than 27 degrees for there to be less than a 5% error. This has major importance for the location of the "Mission to the MOHO" of the Integrated Ocean Drilling Program. The results also confirm that the sources of the nondipole field located in the outer core have to be more than twice as strong at high latitudes than at low latitudes so as to produce the observed increase in VGP scatter with observation latitude. The model of the secular variation proposed here is in no way a theory about how the secular variation happens, but it does allow those who wish to develop such a theory to have a model distribution with which to check their predictions.

Space geodetic estimates of the rate of Nazca-South America convergence and Nazca-Pacific spreading averaging over several years show that present day rates are significantly slower than the 3 million year average NUVEL-1A model. The implied rates of deceleration are consistent with longer term trends extending back to at least 20 Ma, about the time of initiation of Andes growth, and may reflect consequences of ongoing subduction and construction of the Andes, e.g., increased friction and viscous drag on the subducted slab as the leading edge of South America thickens.

The variation of power of the spherical harmonics of Earth's magnetic field as a function of degree is well known. At Earth's surface the power decreases as a function of degree until the crustal component dominates. At the core surface, the spectrum is flat apart from the dipole term, and each degree contributes about equally to the power. However, as a function of order the power tends to decrease for high harmonic orders. These represent sources at low geographic latitudes, and so a field in which high‐order power is low represents one in which low‐latitude sources are weaker than high‐latitude sources. Thus the explanation for the reduction in power for high‐order harmonics in Earth's core field is that in the core, low‐latitude sources of the nondipole field are less intense than high‐latitude sources. Earth's crustal field is represented by harmonic degrees greater than 15 and also shows the power falling off as a function of order, implying that there is an increase in the intensity of magnetic anomalies from equator to poles. This can be explained if the magnetizations have been produced by a dipole field aligned along the spin axis because such a field is twice as strong at the poles as at the equator. For the Martian field there is no central dipole source, but there are strong crustal sources that were created when Mars had a planetary dynamo and presumably an axial dipolar field. Martian harmonics show the same pattern of variation as a function of order as those on Earth. One explanation for this is that these crustal sources were magnetized with a dipolar field aligned along the present spin axis, implying that there has not been much Martian polar wander since the crust was magnetized.

This paper presents data to support the presence of (1) intra‐annual signals in the chemical composition (δ18O and Sr/Ca) of the skeletons of sclerosponges from the Bahamas and (2) variable rates of skeletal accretion. These conclusions are based on data obtained by using a microsampling method for the stable oxygen and carbon isotopes in which material was extracted at a resolution of one sample every 34 μm and a laser microprobe which obtained trace element data every 20 μm (Sr, Mg, and Pb). An age model was established using a combination of changes in the concentration of Pb, the change in the δ13C of the skeleton of the sclerosponges, and U/Th isotopic measurements. These methods yield a mean growth rate of 220 μm/yr but suggest that the growth rate in this particular sclerosponge was not constant. The calculated growth rate is within error identical to that determined by U/Th methods. The variable growth rate was confirmed through spectral analysis of the δ18O and Sr/Ca data that showed peaks corresponding to the annual cycle in these parameters as well as peaks corresponding to growth rates of approximately 128, 212, 270, and 400 μm/yr. The presence of these additional frequencies suggests a growth rate between approximately 100 and 300 μm/yr. These conclusions were supported by modeling of oxygen isotopic data measured on a scleractinian coral as well as model isotope data generated on synthetic time series. These findings have important implications for the use of sclerosponges as proxies of paleoclimate because they emphasize the need for a precise yearly chronology in order that proxy data can be compared with climatic variables.

Mechanical erosion rates are important factors in understanding how continents evolve. Mechanical erosion is much faster than chemical erosion, especially for highly elevated regions of the Earth's surface. It is a principal way in which mountain ranges are removed, exposing deep metamorphic roots, which comprise much of the older portions of the continental masses. In addition, there has to be a long-term balance between erosion and mountain building. A new data set allows us to explore in greater detail some of the many factors which control mechanical erosion rates. The most important factors are some expression of the average slope of a drainage basin, some measurement of the amount of water available for erosion, some environmental measurements, and also a measurement of basin length, for which we have no good explanation. The estimate of global mechanical erosion rate obtained here is considerably lower than those obtained by some other workers, some of whom have concentrated on the fact that smaller river basins tend to get eroded faster than larger basins, and it is mainly smaller basins which have not been measured and which are therefore not allowed for by simple arithmetic averaging of observed erosion rates. It is shown here that although smaller basins are eroded faster, this is mainly because they are steeper than larger basins. We also show that extrapolation of current data to smaller basins does not work because the observed continental area which is draining to the ocean cannot be attained by the simplest extrapolation scheme.

Water may be added to Earth's surface by the bombardment of large numbers of small comets, increasing free water on Earth's surface & deepening the oceans. Continental freeboard is tied to the ocean surface, because of the balance between erosion & mountain building. Addition of water will also affect continental thickness & area. I calculate the effect of increasing ocean volume on the balance between continental & oceanic area. One model also includes crustal volume growth: Significant oceanic depths are known to exist during the Archean, because sulphide deposits requiring deep oceans occur in greenstone belts. These observations prohibit the constant addition of large quantities of water (comparable to the present day ocean volume) throughout Earth history, but smaller quantities could be added.

Two sequences of 45 and 49 individual lava flows respectively have been sampled in eastern Iceland. The two sections range in age from 12.09 to 10.21 Ma as reported by Watkins and Walker [Watkins, N., Walker, G.P.L., 1977. Magnetostratigraphy of eastern Iceland. Am. J. Sci. 277, 513–584.] and are labelled as profiles C and D. Stepwise alternating field (AF) and thermal demagnetizations accompanied by investigations of rock magnetic properties indicate that the magnetization is primarily carried by titanomagnetite. Demagnetization experiments have identified eight and six transitional lavas, respectively, for these profiles. The transitional virtual geomagnetic poles (VGPs) of the (R-N) profile C reversal are located mainly between Patagonia and Antarctica with a tendency of the virtual poles to move towards west coast of South America, subsequently traveling to the northern hemisphere through several discrete steps located in the middle part of South America, then to west Africa and on to central Asia before the poles settle into normal polarity. The second and younger VGP path corresponding to profile D is a path characterized by a reverse-to-normal-to-reverse (R-N-R) motion of the virtual poles. This path is characterized by poles located in west Antarctica, Patagonia and the western and eastern part of South America. The passage from the southern to the northern hemisphere is also through a discrete sets of steps along the southwestern Pacific followed by a rapid motion to the northern Siberian region, continuing to the western equatorial part of South America before moving on to the central region of Asia, followed by a motion to the western Pacific prior to the final move to the eastern part of Antarctica. The data from eastern Iceland based on highly reliable transitional results, as indicated by the demagnetizations and rock magnetic experiments derived from relatively spaced sites and different ages, seem to indicate that the eastern Icelandic high latitude (66°N) profiles studied have several persistent transitional paleofield features that are uniquely observed at those sites particularly when compared to other volcanic records located at lower latitudes.