This chapter reviews the present-day composition of the continental crust, the methods employed to derive these estimates, and the implications of the continental crust composition for the formation of the continents, Earth differentiation, and its geochemical inventories. We review the composition of the upper, middle, and lower continental crust. We then examine the bulk crust composition and the implications of this composition for crust generation and modification processes. Finally, we compare the Earth's crust with those of the other terrestrial planets in our solar system and speculate about what unique processes on Earth have given rise to this unusual crustal distribution.

/pdf/composition-of-the-continental-crust-2hxz49vsod.pdf

Composition of the Continental Crust

Recent revisions to the geomagnetic time scale indicate that global plate motion model NUVEL-1 should be modified for comparison with other rates of motion including those estimated from space geodetic measurements. The optimal recalibration, which is a compromise among slightly different calibrations appropriate for slow, medium, and fast rates of seafloor spreading, is to multiply NUVEL-1 angular velocities by a constant, α, of 0.9562. We refer to this simply recalibrated plate motion model as NUVEL-1A, and give correspondingly revised tables of angular velocities and uncertainties. Published work indicates that space geodetic rates are slower on average than those calculated from NUVEL-1 by 6±1%. This average discrepancy is reduced to less than 2% when space geodetic rates are instead compared with NUVEL-1A.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/94GL02118%4010.1002/%28ISSN%291944-8007.GRL40

Effect of recent revisions to the geomagnetic reversal time scale on estimates of current plate motions

3.01 – Composition of the Continental Crust

We present and interpret Global Positioning System (GPS) measurements of crustal motions for the period 1988–1997 at 189 sites extending east-west from the Caucasus mountains to the Adriatic Sea and north-south from the southern edge of the Eurasian plate to the northern edge of the African plate. Sites on the northern Arabian platform move 18±2 mm/yr at N25°±5°W relative to Eurasia, less than the NUVEL-1A circuit closure rate (25±1 mm/yr at N21°±7°W). Preliminary motion estimates (1994–1997) for stations located in Egypt on the northeastern part of Africa show northward motion at 5–6±2 mm/yr, also slower than NUVEL-IA estimates (10±1 mm/yr at N2°±4°E). Eastern Turkey is characterized by distributed deformation, while central Turkey is characterized by coherent plate motion (internal deformation of <2 mm/yr) involving westward displacement and counterclockwise rotation of the Anatolian plate. The Anatolian plate is de-coupled from Eurasia along the right-lateral, strike-slip North Anatolian fault (NAF). We derive a best fitting Euler vector for Anatolia-Eurasia motion of 30.7°± 0.8°N, 32.6°± 0.4°E, 1.2°±0.1°/Myr. The Euler vector gives an upper bound for NAF slip rate of 24±1 mm/yr. We determine a preliminary GPS Arabia-Anatolia Euler vector of 32.9°±1.2°N, 40.3°±1.1°E, 0.8°±0.2°/Myr and an upper bound on left-lateral slip on the East Anatolian fault (EAF) of 9±1 mm/yr. The central and southern Aegean is characterized by coherent motion (internal deformation of <2 mm/yr) toward the SW at 30±1 mm/yr relative to Eurasia. Stations in the SE Aegean deviate significantly from the overall motion of the southern Aegean, showing increasing velocities toward the trench and reaching 10±1 mm/yr relative to the southern Aegean as a whole.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999JB900351

Global Positioning System constraints on plate kinematics and dynamics in the eastern Mediterranean and Caucasus

The Ames Salmonella/microsome mutagenicity assay (Salmonella test; Ames test) is a short-term bacterial reverse mutation assay specifically designed to detect a wide range of chemical substances that can produce genetic damage that leads to gene mutations. The test employs several histidine dependent Salmonella strains each carrying different mutations in various genes in the histidine operon. These mutations act as hot spots for mutagens that cause DNA damage via different mechanisms. When the Salmonella tester strains are grown on a minimal media agar plate containing a trace of histidine, only those bacteria that revert to histidine independence (his(+)) are able to form colonies. The number of spontaneously induced revertant colonies per plate is relatively constant. However, when a mutagen is added to the plate, the number of revertant colonies per plate is increased, usually in a dose-related manner. The Ames test is used world-wide as an initial screen to determine the mutagenic potential of new chemicals and drugs. The test is also used for submission of data to regulatory agencies for registration or acceptance of many chemicals, including drugs and biocides. International guidelines have been developed for use by corporations and testing laboratories to ensure uniformity of testing procedures. This review provides historical aspects of how the Ames was developed and detailed procedures for performing the test, including the design and interpretation of results.

http://people.uleth.ca/%7Eselibl/Biol3200/AmesTest/AmesTestreview.pdf

The Ames Salmonella/microsome mutagenicity assay

The dynamic role of volatiles on the surface of Mars has been a subject of longstanding interest. In the pre-Viking era, much of the debate was necessarily addressed by theoretical considerations. A particularly influential treatment by Leighton and Murray put forth a simple model relying on solar energy balance, and led to the conclusion that the most prominent volatile exchanging with the atmosphere over seasonal cycles is carbon dioxide. Their model suggested that due to this exchange, atmospheric CO2 partial pressure is regulated by polar ice. While current thinking attributes a larger role to H2O ice than did the occasional thin polar coating this model predicted, the CO2 cycle appears to be essentially correct. There are a number of observational constraints on the seasonal exchange of surface volatiles with the atmosphere. The growth and retreat of polar CO2 frost is visible from Earth-based telescopes and from spacecraft in Mars orbit, both at visible wavelengths and in thermal IR properties of the surface. Recently, variations in Gamma ray and neutron fluxes have also been used to infer integrated changes in CO2 mass on the surface. Measurements made by Viking's Mars Atmospheric Water Detector experiment were sensitive to atmospheric H2O vapor abundance. Surface condensates and their transient nature were detected by the Viking landers. The study here is motivated by recent data collected by the Mars Global Surveyor, affording the opportunity to not only detect the lateral distribution of volatiles, but also to constrain the variable volumes of the reservoirs. We elaborate on a technique first employed by Smith et al. By examining averages of a large number of topographic measurements collected by the Mars Orbiter Laser Altimeter (MOLA), that study showed that the zonal pattern of deposition and sublimation of CO2 can be determined. In their first approach, reference surfaces were fit to all measurements in narrow latitude annuli, and the time dependent variations about those mean surfaces were examined. In their second approach, height measurements from pairs of tracks that cross on the surface were interpolated and differenced, forming a set of crossover residuals. These residuals were then examined as a function of time and latitude. The initial studies averaged over longitude to maximize signal and minimize noise in order to isolate the expected small signal. In this follow-up study we now attempt to extract the elevation change pattern also as a function of longitude, and we focus on the crossover approach.

Depth and Distribution of CO2 Snow on Mars

A spherical harmonic model complete to degree and order seventy has been developed from Doppler tracking of the Lunar Orbiters, the Apollo 15 and 16 subsatellites, and the Clementine spacecraft. The model combines 361,000 observations from Clementine with 347,000 historical observations. The model was developed using the method of Lerch [1991] in order to derive a more realistic realization of the uncertainties associated with this model. An a priori power law of the form 15 × 10−5/12, where l is the spherical harmonic degree, was applied in this model. The Clementine data provide a powerful constraint on the low degree harmonics, and the sectoral terms through degree twenty, by virtue of the spacecraft orbit geometry, and the excellent quality of the tracking data. The gravity anomalies in our model, GLGM-2 (Goddard Lunar Gravity Model-2), have a dynamic range of −294 to +358 mGals when evaluated on the lunar surface at a reference radius of 1738 km. The error predicted from the GLGM-2 covariance ranges from 14 mGals on the equatorial near side to 44 mGals over the high latitude regions of the lunar far side. The model resolves the mascon basins first deduced from Lunar Orbiter tracking [Muller and Sjogren,1968]. Mare Orientale is resolved as a horseshoe-shaped low with an amplitude of −225 mGals centered on the inner and outer Rook rings. Although direct tracking is not available over substantial regions of the lunar farside, the model still resolves far side basins such as South-Pole Aitken, Hertzsprung, Korolev, Moscoviens, and Tsiolkovsky.

High Degree and Order Spherical Harmonic Models for the Moon From Clementine and Historic S-Band Data

LAGEOS laser ranging observations collected by the global tracking network between May 1976 and December 1988 have been analysed to yield a comprehensive geodetic parameter solution. Some of the stations in the participating network have a continuous tracking record over the full LAGEOS mission lifetime and can be used to monitor positions in a limited network for over 12 years. However, the introduction of several new stations of improved precision has allowed determination of relative positions to centimeter accuracy for each quarter of as year solution since the beginning of 1980. A nine year history of these three-dimensional positions yields horizontal inter-station baseline rates to an accuracy of a few mm/yr as well as vertical displacement rates to similar accuracy. Comparisons of the measurements of plate motion between sites centrally located within the plates with those predicted by current geological models agree to better than 9 mm/yr RMS. The phenomena of post-glacial rebound is considered as an explanation for the vertical motion.

Plate Motions and Deformation from Lageos

We will discuss the fractal nature of surface structures on asteroid 433 Eros. Additional information is contained in the original extended abstract.

Fractal Analyses of Small Scale Features on Eros

Major new computations of terrestrial gravitational field models were performed by the Geodynamics Branch of Goddard Space Flight Center (GSFC). This development has incorporated the present state of the art results in satellite geodesy and have relied upon a more consistent set of reference constants than was heretofore utilized in GSFC's GEM models. The solutions are complete in spherical harmonic coefficients out to degree 50 for the gravity field parameters. These models include adjustment for a subset of 66 ocean tidal coefficients for the long wavelength components of 12 major ocean tides. This tidal adjustment was made in the presence of 550 other fixed ocean tidal terms representing 32 major and minor ocean tides and the Wahr frequency dependent solid earth tidal model. In addition 5-day averaged values for Earth rotation and polar motion were derived for the time period of 1980 onward. Two types of models were computed. These are satellite only models relying exclusively on tracking data and combination models which have incorporated satellite altimetry and surface gravity data. The satellite observational data base consists of over 1100 orbital arcs of data on 31 satellites. A large percentage of these observations were provided by third generation laser stations (less than 5 cm). A calibration of the model accuracy of the GEM-T2 satellite only solution indicated that it was a significant improvement over previous models based solely upon tracking data. The rms geoid error for this field is 110 cm to degree and order 36. This is a major advancement over GEM-T1 whose errors were estimated to be 160 cm. An error propagation using the covariances of the GEM-T2 model for the TOPEX radial orbit component indicates that the rms radial errors are expected to be 12 cm. The combination solution, PGS-3337, is a preliminary effort leading to the development of GEM-T3. PGS-3337 has incorporated global sets of surface gravity data and the Seasat altimetry to produce a model complete to (50,50). A solution for the dynamic ocean topography to degree and order 10 was included as part of this adjustment.

/pdf/gravitational-model-improvement-at-the-goddard-space-flight-7z7wr2t6s5.pdf

David E. Smith

Papers

Depth and Distribution of CO2 Snow on Mars

High Degree and Order Spherical Harmonic Models for the Moon From Clementine and Historic S-Band Data

Plate Motions and Deformation from Lageos

Fractal Analyses of Small Scale Features on Eros

Gravitational model improvement at the Goddard Space Flight Center