STRENGTH STRUCTURE PRENTICE-HALL GEOLOGY SERIES Edited by N orman E. A. H inds Strength and Structure of the Earth, by Reginald Aldworth Daly Geophysical E xploration, by C. A. Heiland Sedimentation, by Gustavus E. Anderson STRENGTH AND STRUCTURE OF THE EARTH By REGINALD ALDWORTH DALY STURCIS HOOPER PROFESSOR OF GEOLOGY HARVARD UNIVERSITY N ew Y o r k : 1940 PRENTICE-HALL, INC. Copyright, 1940, by PRENTICE-HALL, INC. 70 Fifth Avenue, N ew York ALL RIGHTS RESERVED. NO PART OF THIS BOOK MAY BE REPRODUCED IN ANY FORM, BY MIMEOGRAPH OR ANY OTHER MEANS, WITHOUT PERMISSION IN WRITING FROM THE PUBLISHERS, PRINTED IN THE UNITED STATES OF AMERICA CONTENTS CHAPTER PAGE 1. Introductory................................................................. 1 . The Problem.............................. 1 Vocabulary..................................................................... 5 Discontinuities in the Earth...................................... 16 Calculated Internal Densities.................................... 21 Figure of the Earth; Geoid, Ellipsoid, Spheroid. 24 2. D evelopment of the Idea oe Isostasy.......... 36 Bouguer to Petit.......................................................... 36 Pratt................................................................................ 38 Airy................................................................................. 42 Hall................................................................................. 50 Jamieson......................................................................... 50 Dutton............................................................................ 51 Gilbert............................................................................. 53 Helmert; Schweydar.................................................... 54 Suggestions Regarding Isostatic Compensation.. 56 3. Testing Isostasy with the Plumb-Line......... 65 Deflections of the Vertical.......................................... 65 Hayford’s First Investigation.................................... 67 Topographic Deflections......................................... 70 Construction of the Geoid...................................... 78 Recognition of Isostasy.......................................... 81 Effect of Isostatic Compensation on the Direc­ tion of the Vertical.............................................. 87 Statistical Tests of Conclusions............................ 94 Degree of Isostatic Compensation........................ 99 VI Contents CHAPTER PAGE 3. Testing Isostasy with the Plumb-Line (Cont.) Amended Ellipsoid of Reference........................... 102 Hayford’s Second Investigation................................ 102 Conclusions Regarding the Dimensions of Uncom­ pensated Loads......................................................... 104 4. Measurement oe Gravity; Comparison oe Intensities................................................................. 106 Gravitation.................................................................... 106 Relation of Gravity to Latitude............................... 109 Measurement of Gravity............................................ I ll Reduction of Values of Gravity................................ 114 Free-air Reduction................................................... 114 Bouguer Reduction.................................................. 116 Isostatic Reductions................................................ 120 Indirect Reduction.................................................. 126 Clearing-house for Systematic Record of Anom­ alies............................................................................. 129 5. Gravimetric Tests oe Isostasy in the United States, by Hayford and B owie........................ 131 Introduction.................................................................. 131 Assumed Conditions for Isostatic Compensation.. 132 Theoretical Gravity at a Station.............................. 139 Correction for Elevation........................................ 139 Correction for the Effects of Topography and Compensation...................................................... 140 Specimen Isostatic Anomalies................................... 149 Comparison with the Corresponding Free-air and Bouguer Anomalies.................................................. 149 Local Versus Regional Compensation..................... 152 Relation of Anomaly to Varying Depth of Com­ pensation.................................................................... 154 Anomalies in Terms of Masses................................. 154 General Conclusions.................................................... 156 6. Later Gravimetric Tests oe Isostasy in N orth America........................................................ 159 Bowie Report of 1912.................................................. 159 Bowie Report of 1917.................................................. 161 Contents vii CHAPTER PAGE 6. Later Gravimetric Tests oe Isostasy in N orth America (Cont.) Bowie Report of 1924.................................................. 174 United States Values of Gravity Reported Since 1924............................................................................. 182 Putnam’s Discussion of United States Data.......... 184 Gilbert’s Estimate of Uncompensated Loads........ 187 Heiskancn on Gravity in the United States.......... 187 Gravity in Canada....................................................... 190 Concluding Remarks................................ 191 7. Testing Isostasy in Europe................................. 196 Introduction.................................................................. 196 Gravity in Alps and Apennines.............................. 197 Heiskanen’s Studies..................................................... 205 Caucasus.................................................................... 205 Norway....................................................................... 208 Gravity in Finland....................................................... 210 Gravity Measurements in Cyprus............................ 211 Additional European Data........................................ 213 Review and Forecast................................................... 215 8. Testing Isostasy in Africa and Asia............. 218 Earlier Work in East Africa...................................... 218 Bullard on Gravity in East Africa........................... 219 Gravity in Siberia........................................................ 224 Conditions in India...................................................... 224 The Geoid in India...................... 225 Gravity Anomalies in India................................... 236 Observations in Turkestan......................................... 247 Gravity Anomalies in Japan...................................... 248 Some Leading Results................................................. 250 9. Testing Isostasy at Se a ........................................ 252 Introduction........................................... 252 Observations of Hansen, Schiotz, and Hecker.. 253 Measurement of Gravity at Sea, by Vening Meinesz...................................................................... 256 Mean Gravity Anomalies Over the Open Ocean 259 Gravity Anomalies Over Mediterranean Seas. 264 Strip of Negative Anomalies, East Indies.......... 269 viii Contents CHAPTER PAGE 9. Testing Isostasy at Sea (Cont.) Negative Strip of the West Indies........................... 285 Anomalies Over Ocean Deeps Facing Mountain- arcs.............................................................................. 290 Gravity Profiles off Shores......................................... 294 Gravity Anomalies in the Vicinity of Volcanic Islands............................................................... 298 Gravity Anomalies at Stations Near Great Deltas 303 Conclusions.................................................................... 305 10. N ature’s Experiments with Icecaps............... 309 Introduction.................................................................. 309 Jamieson Hypothesis................................................... 310 Warping of Fennoscandia, Past and Present......... 312 Objections to Jamieson’s Isostatic Hypothesis.... 316 Recoil of the Glaciated Tract of Northeastern North America.......................................................... 326 Nansen’s Evidence from the Strandflat.................. 330 Summary........................................................................ 334 11. Retrospect...................................................................... 337 Introduction.................................................................. 337 Meaning of the Word “Isostasy”............................. 338 Proofs of Close Approach of the Earth to Ideal Isostasy...................................................................... 342 Relation of Gravity Anomaly to Spheroid of Ref­ erence .......................................................................... 344 Need of Great Extension of the Gravitational Survey......................................................................... 345 Effect of Errors of Mensuration and Computation 348 Effect of Changing the Assumed Type of Isostatic Compensation........................................................... 349 Effect of Changing the Assumed Depth of Com­ pensation.................................................................... 350 Effect of Local Abnormalities of Density of Rock 351 How Permanent are Areas of One-sign Anomaly?