Allan Cox

ROCK GLACIERS IN THE ALASKA RANGE

Field studies and examination of aerial photographs of approximately 200 rock glaciers in the Healy (1:250,000) quadrangle in the central Alaska Range showed that there are three types of rock glacier in plan: lobate, in which the length is less than the width (200–3500 feet long and 300–10,000 feet wide); tongue-shaped, in which the length is greater than the width (500–5000 feet long and 200–2500 feet wide); and spatulate, tongue-shaped but with an enlargement at the front. Lobate rock glaciers line cliffs and cirque walls and probably represent an initial stage; the other two move down valley axes and represent more mature stages. The rock glaciers are composed of coarse, blocky debris that is cemented by ice a few feet below the surface. The top quarter of the thickness is coarse rubble, below which is coarse rubble mixed with silt, sand, and fine gravel. Fronts of active (moving) rock glaciers are bare of vegetation, are generally at the angle of repose, and make a sharp angle with the upper surface. Fronts of inactive (stationary) rock glaciers are covered with lichens or other vegetation, have gentle slopes, and are rounded at the top. Active rock glaciers average 150 feet in thickness, inactive rock glaciers, 70 feet. The upper surface of most rock glaciers is clothed with turf or lichens. Sets of parallel rounded ridges and V-shaped furrows—longitudinal near the heads of some rock glaciers and transverse, bowed downstream, on the lower parts of others—and conical pits, crevasses, and lobes mark the upper surfaces of many rock glaciers. The upper surface of a rock glacier at the head of Clear Creek moved 2.4 feet per year between 1949 and 1957, and the front advanced 1.6 feet per year. Heights of the upper edges of the talus aprons along the fronts of rock glaciers average 45 per cent of the heights of the fronts. Each of these observations implies that motion is not confined to thin surface layers but is distributed throughout the interiors of the rock glaciers, which in this permafrost region are probably frozen. “Viscosity” has been calculated for rock glaciers at between 1014 and 1015 poises; for glacial ice it has been estimated at between 1012 and 1014 poises. Maximum average shear stresses within active rock glaciers range from 1 to 2 bars; these values are much larger than those calculated for solifluction and creep features. Rock glaciers occur on blocky fracturing rocks which form talus that has large interconnected voids in which ice can accumulate. They are rare on platy or schistose rocks whose talus moves rapidly by solifluction. The rock glaciers lie in an altitudinal zone about 2000 feet thick, centered on the lower limit of existing glaciers[1][1]. Although the firn lines on glaciers rise 1200 feet in a distance of 25 miles northward across the Alaska Range, the lower limit of active rock glaciers rises only 800 feet. The firn line on southward-facing glaciers is 2000 feet higher than that on northward-facing glaciers, yet in any given area southward-facing rock glaciers average only 200 feet higher than northward-facing rock glaciers. Insulation by the debris cover is believed responsible for the difference in altitudinal ranges between rock glaciers and glaciers. It is concluded that rock glaciers move as a result of the flow of interstitial ice and that they require for their formation steep cliffs, a near-glacial climate cold enough for the ground to be perennially frozen, and bedrock that is broken by frost action into coarse blocky debris with large interconnected voids. The longitudinal furrows are thought to result from the accumulation of ice-rich bands in the swales between talus cones at the head of the rock glaciers and the subsequent melting of this ice as the rock glacier moves down-valley. The transverse ridges are thought to result from shearing within the rock glacier that would occur where the thickness increases or the velocity decreases downstream. An average of 30 feet of bedrock was removed from source areas to form the present rock glaciers, indicating an average rate of erosion of 1–3 feet per year when they are active. [1]: #fn-1

REVIEW OF PALEOMAGNETISM

This review is an attempt to bring together and discuss relevant information concerning the magnetization of rocks, especially that having paleomagnetic significance. All paleomagnetic measurements available to the authors are here compiled and evaluated, with a key to the summary table and illustrations in English and Russian. The principles upon which the evaluation of paleomagnetic measurements is based are summarized, with special emphasis on statistical methods and on the evidence and tests for magnetic stability and paleomagnetic applicability. Evaluation of the data summarized leads to the following general conclusions: (1) The earth9s average magnetic field, throughout Oligocene to Recent time, has very closely approximated that due to a dipole at the center of the earth oriented parallel to the present axis of rotation. (2) Paleomagnetic results for the Mesozoic and early Tertiary might be explained more plausibly by a relatively rapidly changing magnetic field, with or without wandering of the rotational pole, than by large-scale continental drift. (3) The Carboniferous and especially the Permian magnetic fields were relatively very “steady” and were vastly different from the present configuration of the field. (4) The Precambrian magnetic field was different from the present field configuration and, considering the time spanned, was remarkably consistent for all continents.

Evidence that the Laschamp polarity event did not occur 13 300–30 400 years ago

Abstract No evidence for a geomagnetic reversal between 30 400 and 13 300 yr ago has been found in a paleomagnetic study of sediments from Mono Lake, California, with ages controlled by 14 C dating. If the Laschamp reversed event occurred during this time, its duration can have been no longer than 1700 yr. The sediments record a well-defined excursion in the direction of the field 24 000 yr ago with a peak-to-peak amplitude of 25 deg and a period of 600 yr. This is attributed to geomagnetic secular variation due to local eastward drift of the nondipole field.

Magnetism of Pillow Basalts and Their Petrology

The average intensity of magnetization of a layer of pillow basalt depends on (l) variations in the intensity of the earths field as the lava is formed, (2) variations in the direction of magnetization within the layer, (3) the percent of nonmagnetic pore space between pillows, and (4) variations in the thermoremanence of the basalt due to variations in composition and in the degree of crystallization. To evaluate (4) we made a centimeter-by-centimeter examination of submarine pillow fragments and found that the remanence increases from almost zero in the glassy crust to high values of .04 emu/cc in the interior. The main control on remanence is the degree of crystallization. In very large pillows the remanence reaches a peak and decreases toward the center, the decrease apparently being due to an increase in grain size. The magnetization of the basalts resides in titanomagnetite grains possessing natural remanences in the range 1 to 2 emu/cc. The variation of natural remanence with grain size in the range 3 to 6 microns suggests pseudo-single domain behavior. Our best estimate of the average magnetization of a layer of submarine pillow basalt is .014 emu/cc. A layer less than 1 km thick with a value of remanence this high is adequate to account for most marine magnetic anomalies.

Brunhes-Matuyama polarity transition

Abstract A paleomagnetic record of the geomagnetic field during its change of polarity from the reversed Matuyama epoch to the normal Brunhes epoch has been obtained from sediments of ancient Lake Tecopa in southeastern California. The polarity switch occurs in siltstone of uniform composition, and anhysteretic magnetization experiments indicate that the magnetic mineralogy does not change markedly across the transition. Within the transition interval, intensity of the magnetization drops to a minimum of 10% of the intensity after the transition. The interval of low field intensity preceded and lasted longer than the interval during which the field direction reversed, the latter being shorter than the interval of low intensity by a factor of at least 2.5. The VGPs make a smooth transit from reversed to normal polarity, with the path lying in the sector of longitude between 30°E and 60°W. Pole paths for the Brunhes-Matuyama transition recorded in California and Japan are completely different, indicating that the dipole field decayed. The transition field appears to be nondipolar, and there is no evidence for an equatorial component. Since there is little dispersion of the VGPs about a great circle path, it is possible that large-scale drift of the nondipole field ceased during this polarity transition.

Pacific geomagnetic secular variation

We have considered several different types of records of long-period geomagnetic secular variation: direct measurements made in geomagnetic observatories; paleomagnetic measurements on Hawaiian lava flows with accurately known ages in the interval 0 to 200 years; paleomagentic measurements on Hawaiian lava flows with loosely determined ages within the interval 200 to 10,000 years ago; and worldwide paleomagnetic measurements of the average geomagnetic angular dispersion recorded in lava flows that formed during the past 0.7 million years. All these magnetic records indicate that, during this time, the nondipole component of the earths field was lower in the central Pacific than elsewhere, as it is today. This, in turn, indicates that there is some type of inhomogeneity in the lower mantle which is coupled to the earths core in such a way as to suppress the generation of the nondipole field beneath the central Pacific. With the present incomplete state of knowledge about the processes that give rise to the earths field, it is uncertain whether undulations in the core-mantle interface or lateral variations in the composition and physical state of the lower mantle are ultimately responsible for the pattern of secular variation seen at the earths surface.

Potassium-Argon Age and Paleomagnetism of the Bishop Tuff, California

Duplicate potassium-argon age determinations on each of three samples from widely separated localities indicate that the age of the Bishop Tuff, California, is about 0.7 million years. Two of the samples are from the basal ash fall that preceded the ash flow eruptions; one of these two samples was collected within 1 m of the contact of the Bishop Tuff with the underlying Sherwin Till. The third sample is from near the present exposed surface of the Bishop Tuff. The minimum age of the Sherwin Till (Kansan?) is thus 0.7 million years. The samples used for previously published age determinations of about 1 million years were probably contaminated with older material. Paleomagnetic results from five widely separated localities indicate that the welded part of the Bishop Tuff became magnetized when the geomagnetic field was normal and that it may have cooled in several centuries or less. The Brunhes-Matuyama polarity epoch boundary is now uncertain in the range of 0.7 to 1.0 million years.

Late Permian paleomagnetic pole from dikes of the Tarim craton, China

We have obtained a Late Permian paleomagnetic pole from dikes in the Tarim craton of China. Directions of magnetization from 21 mafic and ultramafic dikes at three localities pass a fold test at the 95% confidence level. The new paleomagnetic pole lies at lat 66°N, long 181° E; N = 21, K = 61, and α95 = 3.9°. On the basis of a comparison with paleomagnetic data from adjacent tectonic units, we conclude that (1) the Late Permian paleolatitude of the Tarim craton differs from that of the Sino-Korean craton, suggesting that the two cratons were distinct tectonic units in Late Permian time; (2) there was relative motion between the Tarim craton and the Siberian craton after Late Permian time; and (3) negligible relative motions have occured between different thrust sheets in the Keping-Bachu area of the north-western Tarim craton.

Plate Tectonics and Geomagnetic Reversals

The impetus for publishing this set of articles was the realization that, year after year, I and my students at Stanford were making copies (with varying degrees of legibility and legality) of the same classic articles on plate tectonics and geomagnetic reversals. At first we did this because there was so little material on the subject available, but even after the appearance of several excellent textbooks on plate tectonics and paleomagnetism, we still found ourselves using many of the original articles as supplementary reading. Students somehow seem to sense more acutely the excitement of discovery when they are given a sense of participation by having the scientist himself describe it to them. As scientists, most of us, when we write about our research, almost completely depersonalize it. Although there may be good reasons for this, it is a rather odd thing to do because we almost always feel a deep sense of personal involvement in our research. As some anomalous data fall into place or as a new idea begins to stir, the individual scientist, working perhaps late at night, experiences a feeling that surely is similar to the elation of the composer as he first hears new music in his mind, or to the exhilaration of the athlete as he breaks a record. Yet our science, as we write about it, tends to be science with the scientists left out. The effect is that we inadvertently may leave students with the impression that science is a routine, matter-of-fact business, which it surely isnt t. In the present book I have tried to put the scientists back into the scientific story. Wherever possible I have done this using the scientists own words, quoting from statements given to me in the spring of 1972 by many of the authors whose articles are included in this book. No pretense is made of presenting a complete history of research in plate tectonics and geomagnetic reversals. My intent, rather, is to convey something of the feeling of intense involvement and excitement experienced by earth scientists as they contributed to important new ideas.

Geomagnetic Polarity Epochs: Sierra Nevada II

Ten new determinations on volcanic extrusions in the Sierra Nevada with potassium-argon ages of 3.1 million years or less indicate that the remanent magnetizations fall into two groups, a normal group in which the remanent magnetization is directed downward and to the north, and a reversed group magnetized up and to the south. Thermomagnetic experiments and mineralogic studies fail to provide an explanation of the opposing polarities in terms of mineralogic control, but rather suggest that the remanent magnetization reflects reversals of the main dipole field of the earth. All available radiometric ages are consistent with this field-reversal hypothesis and indicate that the present normal polarity epoch (N1) as well as the previous reversed epoch (R1) are 0.9 to 1.0 million years long, whereas the previous normal epoch (N2) was at least 25 percent longer.