Geophysical Abstracts 152 January-March 1953

54 GEOPHYSICAL ABSTRACTS 152, JANUARY-MARCH 1953
14368. Munk, Walter [H.], and Revelle, Roger. On the geophysical interpreta­
tion of irregularities in the rotation of the earth: Royal Astron. Soc.
Monthly Notices, Geophys. supp., v. 6, no. 6, p. 331-347, 1952.
Astronomical measurements for the past 300 years have revealed fluctuations
of the annual average of the angular velocity of the earth. Variations of several
milliseconds in the length of the day have occurred over periods of a few decades.
It was formerly thought that these changes occurred abruptly, and accordingly
past investigations have searched for catastrophic causes. This limitation has
been removed by the recent work of Brouwer, who has shown that the observed
changes in the length of day can be fitted to cumulative random series.
Munk and Revelle investigate possible causes involving changes in the moment
of inertia due to redistributions of matter and variable motion relative to the
earth's crust. Variations in the distribution of air mass are completely inade­
quate to account for the observed fluctuation of the length of day and variations
in oceanic circulation have a negligible effect, but random fluctuations in the
elevation of continental blocks of the crust, if sufficiently large to cause observed
changes in the length of day, would bring about displacements in the pole of rota­
tion far greater than those observed from 1890 to 1950. A symmetrical oscilla­
tion of the crust, with movements in opposite directions in the high and low
latitudes, and its implications seem difficult to explain as regards a source
mechanism. Tide-gage data are inadequate to preclude this possibility. Electro­
magnetic coupling of the mantle to a turbulent core could account for the observed
fluctuations, provided the conductivity of the mantle is about 10~~9 emu. Vestine's
calculation of the westward drift of the magnetic field between 1890 and 1950
shows the proper relationship, qualitatively and quantitatively, to Brouwer's
calculations of the length of day over the same period.
An organized vertical crustal movement, even if a priori unlikely, cannot be
altogether ruled out as a possible cause for the fluctuations in the length of day,
especially if most of the motion is ascribed to the Antarctic, where tide-gage
data are lacking. There is positive evidence for ascribing the fluctuations to
variable motion in the earth's core, which also has "the advantage of being even
less accessible than the Antarctic."
Tabulated values for certain mean sea-level fluctuations and some pertinent
areal integrals are included. P. E. B.
14369. Munk, Walter [H.], and Revelle, Roger. Sea level and the rotation of
the earth: Am. Jour. Sci., v. 250, no. 11, p. 829-883, 1952.
From ancient beach lines, Daly has proposed an eustatic lowering of sea level
by about five meters during the last few thousand years. This would decrease
the length of day by 30 milliseconds if the lowering is due to "rapid" growth
of ice caps, and by 90 milliseconds if, as suggested by Daly, the lowering is due
to isostatic after flow in the earth's mantle following a rapid meeting of ice caps.
The corresponding value derived from ancient observations of eclipses, 27±18
milliseconds, indicates rapid growth of ice caps. For the last hundred years,
tide-gauge records and the retreat of glaciers indicate a rise in the sea .level
by 10 cm. This rise would displace the north pole of rotation by something
like 10 feet towards the North Atlantic, the precise value and direction depending
on the source of melted water. Such a displacement is not inconsistent with
astronomic evidence. Authors' Abstract.