Portion Of Mantle Research Articles

Discontinuities in the earth's upper mantle as indicated by reflected seismic energy

Thirty-three seismograms from nine large quarry blasts ranging in size from 50,000 to 2,138,000 lb of explosives were analyzed for possible reflections from inhomogeneities in the earth's upper mantle. Of the 33 seismograms, four were obtained at temporary seismograph stations positioned between 90 and 243 km from the explosions and an array of three to four seismometers was used at each of the stations. The remaining twenty-nine seismograms were obtained from ten permanent seismograph stations located between 76 and 1,009 km from the explosions. Seven of these latter seismograms were obtained from the seismograph station at Salt Lake City, Utah, and six were obtained from the seismograph station at Eureka, Nevada. Each arrival on these 13 seismograms was noted and then correlated to determine which arrivals were common to all seismograms having nearly constant epicentral distances. Of the nine quarry blasts recorded, seven were detonated at Promontory, Utah, and two were detonated at Lakeside, Utah, which lies about 33 km west of Promontory. This multiplicity of blasts resulted in two groups of seismograms for both the Salt Lake City and Eureka stations with one group at each station having a different epicentral distance from the other group at the same station. A comparison was made between the seismograms of each station based on the apparent velocity of the arrivals across this difference in epicentral distance. Seismic arrivals having apparent velocities that would be representative of deep reflections were selected from the aforementioned arrivals common to most records. The remaining 16 seismograms, which were from eight permanent seismograph stations located at epicentral distances in excess of 500 km, were used to check the results from the analysis of the Salt Lake City, Eureka, and temporary stations. Times of possible reflected events are presented which could result from energy reflected at discontinuities in the upper mantle at depths of about 190, 520, and 910 km. The depths were computed using average velocities based on velocity-depth curves given by Jeffreys and Gutenberg (Jacobs 1953, p. 187) for the deeper portions of the upper mantle and assuming that linear ray paths pertained.

Studies on Isaria cretacea. Nutritional and morphological characteristics of two strains and morphogenesis of the synnema.

A strain of Isaria cretacea van Beyma was isolated from soil and from powdery leaves of a tomato plant. The white phototropic synnema is referred to as an indeterminate synnema because it bears spores on the lower portion only and the tip is capable of indeterminate growth.This isolate (strain (A)) can utilize ammonia, amino and nitrate nitrogen, and a number of carbohydrates and related compounds as carbon sources for growth. Germination requires an energy source but little or no exogenous vitamins. Vegetative growth requires exogenous biotin and the pyrimidine moiety of thiamin. Biotin can be replaced with biotin-L-sulphoxide and biocytin and partially with desthiobiotin, but not with homobiotin, norbiotin, aspartic acid, pimelic acid, oleic acid, Tween 80, or various combinations of them.Strain (A) continuously gave rise de novo to a variant (variant strain (B)) which did not develop synnemata and which differed from the parent in possessing a higher growth rate. The two could enter into a heterokaryotic state which suppressed sporulation and synnemata formation.The synnema consists of three structures: a terminal growing point, a central column of closely packed, sterile hyphae, and a mantle of hyphae. Spores are borne on the portion of mantle enveloping the lower half of the column.Morphogenesis of the synnema can be divided into four phases: differentiation of certain hyphae of the assimilative mat into those capable of developing into synnema, orientation of the hyphae away from their origin, continued growth, and branching and sporulation of the hyphae. The entire synnema remains metabolically active and may continue growing, since 2 hours after injecting C14-labelled glucose into the agar beneath a mature synnema, all hyphae of the structure become labelled.Neither negative geotropism nor phototropism is essential for the initiation of synnemata formation. Synnemata formation is influenced by the carbon source but not by the nitrogen source. The requirement for biotin and thiamin appears related to the requirement for general hyphal growth.

The nature of the Mohorovicic discontinuity

New evidence for the composition of the silicate mantle of the primary meteorite body suggests that a considerable portion of the Earth's upper mantle may well be composed of silicate material of basaltic composition. Consequently the Mohorovicic discontinuity may not separate basaltic crustal material from ultrabasic upper‐mantle material but rather mark a zone in which basaltic material is transforming into a high‐pressure modification possibly represented by the rock‐type eclogite. Estimates of the temperatures and pressures at the average levels of the Mohorovicic discontinuity under oceans and continents give two points on the ‘equilibrium line’ for the transformation basalt → eclogite which is roughly given by the relationship p = 21.80 — 488 (p in bars, 9 in °C). The geophysical consequences of this hypothesis are far reaching. It removes difficulties arising from the observed equivalence of oceanic and continental heat flows, provides satisfactory ‘parent magma’ from which the granitic and basaltic zones in the crust have differentiated, and suggests a mechanism for elevating or depressing regions of the Earth's surface simply by increasing or decreasing the temperature at the level of the Mohorovicic discontinuity beneath that region.

Effects of low-velocity layers

Effects of low-velocity channels in the atmosphere, the ocean and the solid earth are discussed. There are two major low-velocity channels in the atmosphere, one with its axis at the tropopause, and another at a height of about 80 km. They produce “zones of silence” and permit the transmissions of waves involving the whole atmosphere. Low-velocity layers in the ocean result from the combined effects of temperature, pressure and salinity. In the earth, the sudden decrease of velocity at the boundary of the core produces a low-velocity at the boundary of the core produces a low-velocity channel for elastic waves. In the earth's crust there are two major low velocity shannels, one below theMohorovicic discontinuity, the other at a depth of about 15 Km. Misinterpretation of their effects has caused incorrect conclusions concerning the structure of the outer portion of the earth's mantle.

Wave velocities at depths between 50 and 600 kilometers†

abstract Wave velocities in the interior of the earth are usually calculated by means of the Wiechert-Herglotz method. Incorrect interpretation of the form of the travel time curve for a certain distance interval then leads to errors not only in results concerning the corresponding depth interval but (in decreasing amounts) also for all greater depths. A new method is described and applied to that portion of the earth's mantle in which earthquake foci exist. Values obtained with this method for velocities at various depths are independent of each other.

The problem of the earth's density variation*

Summary The present paper examines the problem of the earth's density variation and includes a quantitative discussion of the errors likely to be involved. Figures are given for the density distribution of the earth's outer mantle, and it is shown that all these are probably accurate within about 0.05 gm/cm.3 On the figures presented, the density in the earth's mantle ranges from 3.32 gm/cm.3 at the base of the crustal layers to 5.68 gm/cm.3 at the base of the mantle. The mean density of the central core is shown to be 10.7 gm/cm.3 within an error of order 0.1 gm/cm.3 Application of equations derived by Birch from Murnaghan's theory of finite strain indicates agreement with the density figures found, but it is to be noted that the rigidity of the lower portion of the earth's mantle appears to increase more slowly with increase of depth than would be consistent with the equations of Birch. Values which are expected to need very little future amendment are also given for the pressure and gravitational attraction within the earth's mantle.

Scientific Serials

Wiedemann's Annalen der Physik tind Chemie, No. 7—Polarised fluorescence, by L. Sohncke. The polarisation of fluorescent light is capable of giving hints concerning the manner in which the molecules of a solid substance vibrate, and its study may form the basis of the kinetic theory of solids. Theoretically, all doubly-refracting crystals should emit polarised fluorescence. This is found to be the case. Crystals of the regular system are the only crystals which do not. The author has investigated the fluorescence of a large number of substances in confirmation of this view.—Uniformities in the spectra of solid bodies, by F. Paschen. The author investigates the distribution of energy in the spectrum of glowing iron oxide at various temperatures. Of the formula hitherto proposed for its expression, that of Weber most closely approaches the reality. It gives a nearly parabolic curve in which the energy declines on both sides from a maximum which decreases in wave-length as the temperature rises. But the want of symmetry in Weber's curve is greater than in reality. The author finds a new formula, for which he claims that it covers all the observations.—The electrical behaviour of vapours from electrified liquids, by G. Schwalbe. The author finds that the vapours rising from electrified liquids are not capable of bearing away with them any portion of the electric charge, and that Exner's theory of atmospheric electricity must therefore be abandoned.—The damping action of magnetic fields upon rotating insulators, by William Duane. Cylinders and discs of glass, sulphur, paraffin, ebonite, or quartz, oscillating between the poles of a magnet with their axes vertical and at right angles to the lines of force, experience a damping action proportional to the field intensity and to the speed of rotation. This is not due to an action on the suspending threads, nor on the viscosity of the air, nor an electrostratic effect from the current in the coils, nor to induction currents in the substance, as was proved by test experiments and calculations. It must therefore be regarded as a hitherto unobserved magnetic effect upon the insulators in question.—Effect of magnetism upon electromotive force, by A. H. Bucherer. The author finds that in solutions of neutral ferrous salts no E. M. F. exceeding 0˙00001 volt can be produced by the magnetisation of one of the two iron electrodes. The E.M. F.s observed by Gross and others must be attributed to changes of concentration produced by the magnetised electrode during its solution.—On the measurement of flame temperatures by thermo-elements, especially the temperature of the Bunsen burner, by W. J. Waggener. The temperatures were determined by various thermo-couples in different parts of the flame. The highest temperature, 1700°C., was indicated in the lower portion of the external mantle. But an infinitely thin thermo-element free from conduction would probably indicate over 17700°. A wire 0˙05 mm. thick still suffers from conduction, and it is actually fused in the hottest portion. A more refractory metal is required for these measurements.