摘要:

The ratio of natural remanence (NRM) to saturation remanence (IRMs) is found to be exceedingly high for a number of basaltic to ultramafic samples from the 3.4 billion year old Barberton Mountain Land greenstone belt, South Africa. Although conventional paleointensity methods indicate paleofields of several oersteds, demagnetization curves plotted on logarithmic scale clearly show the NRM in these samples to be unlike thermal remanence (TRM). It seems most probable that the observed NRM is a chemical remanence (CRM) related to the greenschist metamorphic event, and that the intense magnetizations are perhaps the result of internally generated, rather than external fields. The probability that the intense magnetization observed in these ancient terrestrial samples is unrelated to the strength of the external field suggests caution in the interpretation of paleointensity results from extraterrestrial materials.

ISSN:0094-8276    

DOI:10.1029/GL008i002p00129

年代:1981

数据来源: WILEY

摘要:

A paleomagnetic reconnaissance (43 samples) of the Raggedy Mountain Gabbro Group, which is comprised of a layered complex of anorthosites, anorthositic gabbros, and massive gabbro, shows: (1) stable remanent magnetization and (2) consistent directions which differ significantly from those of the nearby Cambrian Wichita Granite Group, with exception of one site near the granite contact. These results suggest a Precambiran age for the gabbro group. The difference in pole positions between the granitic and mafic groups demonstrates that the plutonic episodes responsible for the mafic and granitic magmas are significantly separated in time. Models of formation of the Wichita aulacogen should be altered to account for the difference in time between the mafic and silicic plutonism.

ISSN:0094-8276    

DOI:10.1029/GL008i002p00133

年代:1981

数据来源: WILEY

摘要:

Viscous decay of magnetic polarity messages in rocks is slowed 3‐to‐6 fold by the reversing nature of the geomagnetic field, compared with the effect expected in a field that is constantly polarized oppositely to the original remanence. The process is studied analytically and numerically for the known geomagnetic polarity sequence, as well as for simulated sequences having constant‐length and Poisson‐distributed lengths of polarity intervals. For equal average‐interval lengths, the random Poisson reversing process causes more rapid decay than does the periodic reversin

ISSN:0094-8276    

DOI:10.1029/GL008i002p00137

年代:1981

数据来源: WILEY

摘要:

Experimental static high‐pressure data on the volume equation of state of the B1 and B2 phases of CaO in the range 1 bar‐650 kbar agree with existing low‐pressure data and with an equation of state calculated from shock‐wave experiments. The density of the B2 phase is close to that of the lower mantle, so the properties of CaO could be consistent with theories of the earth's formation by inhomogeneous ac

ISSN:0094-8276    

DOI:10.1029/GL008i002p00140

年代:1981

数据来源: WILEY

摘要:

Laboratory deformation studies of metals and some oxides indicate that these solids are easily deformed while undergoing a phase transformation. This superplastic effect, if it occurs for mantle silicates, may result in an instability of mantle flow. The instability is illustrated by considering a sheared layer of viscous fluid containing a phase transition. With a simple idealization of the phase transition weakening effect, an oscillatory fluid motion which cycles material through the phase transition is shown to reduce the rate of viscous dissipation associated with shearing the fluid layer. Some possible consequences for mantle flow include the partial decoupling of mantle layers at phase transitions and short wavelength free air gravity anomalies due to induced normal stress variations at the base of the lithosphere.

ISSN:0094-8276    

DOI:10.1029/GL008i002p00143

年代:1981

数据来源: WILEY

摘要:

Preliminary results of an analysis of global precipitation data from WMO (World Meteorological Organization) stations suggest that even remote maritime baseline stations, far removed from major continents, could become predisposed to acid rain if there is a deficiency of non‐marine calcium relative to non‐marine sulfate. The regional stations show greater complexity than the baseline stations in their precipitation chemistry. The overall results of this analysis suggest that not all non‐marine sulfate and nitrate in precipitation could be present as

ISSN:0094-8276    

DOI:10.1029/GL008i002p00147

年代:1981

数据来源: WILEY

摘要:

A principal component image enhancement has been effective in applying Landsat data to geologic mapping in a heavily forested area of eastern Virginia. A chestnut‐oak forest unit, which occurs on metavolcanic rocks and some metaclastic rocks in the western Piedmont and on highly weathered upland gravel deposits in the eastern Piedmont, can be discerned on a digitally enhanced Landsat winter image. The image enhancement procedure consists of a principal component transformation, a histogram normalization, and the inverse principal component transformation. The enhancement preserves the independence of the principal components, yet produces a more readily interpretable image than does a single principal component transformation.To determine how the chestnut‐oak forest unit was being enhanced, average Landsat multispectral scanner (MSS) values were extracted for four sample forest types and were calculated through the inverted principal component transformation. Slope and intercept values for the linear histogram normalization were chosen to keep the scale between the raw MSS bands and the inverted principal component (PC) bands constant. Plots of the inverted principal component data show that the most separation between forest types is in inverted PC band 5. The chestnut‐oak forest unit is characterized by a high value for inverted PC band 5 as opposed to a low value for inverted PC band 4. In contrast, raw MSS band 4 is greater than MSS band 5 for the four forest types sampled in the winter image. Such observations cannot be readily deduced from analysis of only the principal component rotation matrix. They may ultimately provide a means to map the distribution of chestnut‐oak forest from

ISSN:0094-8276    

DOI:10.1029/GL008i002p00151

年代:1981

数据来源: WILEY

摘要:

A coherent pulse Doppler sonar with a few centimeters range resolution provides effective monitoring of slow water velocity with excellent resolution. The experiments presented in the paper were conducted with a sonar transducer resting on the floor of a waterway influenced by tidal effects with the transducer producing a beam tilted from the horizontal. All the results are shown in the form of Doppler Spectra composed of radial velocity estimates separated by a few mm/s, which are presented at adjacent range gates spaced by 4.5 or 9 cm, so that a vertical profile of horizontal velocity extending from the waterway bottom to the water surface is derived from the data. Continuous monitoring of these profiles shows considerable detail in the evolution of the ebb/flood characteristics of the flow during tidal cycles.

ISSN:0094-8276    

DOI:10.1029/GL008i002p00155

年代:1981

数据来源: WILEY

摘要:

Seasat radar observations of the Gulf of Alaska by the Synthetic Aperture Radar (SAR) and the Wind Speed Scatterometer (SASS) indicates that L‐band backscatter is modulated by wind speed and possibly by wind direction. In particular, L‐band radar cross‐section of the ocean at 23°±3° angle of incidence and wind speeds between 5m/s and 20m/s is related to wind speed and direction by the following:σoαUa[1+bcos(2ϕ)](1)where σo= L‐band radar cross‐sectionU = wind speedϕ = wind‐radar anglea = 0.5 ± 0.1b = 0.05 ± 0.05This relationship suggests that a combination of Seasat SAR and Scatterometer data may yield high resolution

ISSN:0094-8276    

DOI:10.1029/GL008i002p00159

年代:1981

数据来源: WILEY

摘要:

The stratospheric aerosol layer between 15 and 25 km altitude is an important factor in the balance of the global radiation budget. The eruption of the Mount St. Helens in Washington, USA, on 18 May 1980 violated the quasi stationary situation in the stratosphere which was established after the decay of the post‐Fuego aerosol during the years 1974 to 1976. At tropopause level the cloud was observed over the east coast of the US and Canada on 21 May 1980. The further transport over the Atlantic was estimated by first guess trajectory constructions. The arrival of the eruption cloud over Europe was observed by the authors employing in‐situ techniques and remote sensing. The in‐situ measurements of atmospheric SO2were performed during an aircraft ascent up to 13.7 km on 26 May 1980 over southern Scandinavia. In contrast to an aircraft ascent over southern Germany on 24 May 1980 a tenfold increase in SO2mixing ratio at tropopause level was observed.Remote sensing by ground based ruby lidar at Garmisch‐Partenkirchen (47.5°N, 11°E) from 25 May to 29 May 80 indicates an aerosol peak at a height of 11 to 12 km, which was initially coupled to the tropopause, but later clearly separated from t

ISSN:0094-8276    

DOI:10.1029/GL008i002p00163

年代:1981

数据来源: WILEY