The Pulse of the Earth

Problems of current interest relating to the earth's physical history will be discussed in this volume. Each chapter constitutes a subject in itself, but the sequence I have chosen will, I hope, show and explain the deeper correlation of several terrestrial pro­ cesses which, at first sight, appear to be heterogeneous. The geologist follows the changing face of the earth, the oscillations of the sea-level, the pulsation of folding and mountain-building, the periodicity of the ice-ages, the rhythmical cadence of Life. Just as the physician will draw his conclttsions from outward symptoms when examining his patient, so the geologist tries to discover the deeper significance of the sequence of observed phenomena by ieeling the pulse of the earth. The many additions and revisions which have had to be made in this second. edition include three new chapters, several new sections in other chapters, I09 new textfigures, I2 tables and 2 plates. A leu) fundamental geological terms have been explained in Chapter I.

Contenu

I. Space and Time.- The universe, the solar-system and the earth.- Cosmic dimensions. Spiral galaxies. The expanding universe. The origin of the solar-system..- The birth of the moon.- The earth and the moon. Lunar volcanism..- The earth's interior.- Interpretation of seismic data. Two different theories on the internal constitution of the earth..- The age of the earth and the universe.- Geological figures. Astronomical methods and the age of meteorites..- The history of the earth's crust.- Precambrian and later history. Alterations of the earth's aspect. A geological cycle. Erosion. Sedimentation. Geosynclines. Folding. Facies. Unconformity. Magma. Periodicity. Contents of the following chapters..- References.- II. Mountain-Chains.- Analysis of the earth's structural history. Mountain-chains of different ages. Epochs of compression. Classification of mountain-belts..- Epochs of compression.- The enormous extent of Pre-Cambrian areas. The foundering of older zones and a presumed accretion of the continents. Centrifugal migration. Dynamic centra of tectonic activity. Alternating increase and decrease of compression. A deep-seated cause and world-wide movements. Exceptions. The influence of the basement. Intersecting structures. Absence of centrifugal displacement..- Mountain-building.- Epochs of compression and elevation. Rejuvenation. The extent of mountains-belts of various ages..- Subsided blocks.- Appalachia and Nova Scotica, Llanoria, Cascadia, Scandia, the submerged area west of the Congo-Basin, Melanesia. Periods of foundering..- References.- III. Basins and Troughs.- Characteristics of basins and troughs. Nomenclature and systematic division..- Continental basins and troughs.- Marginal deeps. Intramontane troughs and idiogeosynclines. Nuclear basins. Discordant basins..- Deep-sea basins and troughs.- Examples in the East-Indies and a comparison with continental types. The abnormal character of the present situation. Periods of rejuvenation of the continental and submarine relief. An open question..- Chronological relations to other phenomena.- Relations to periods of mountain-building and decreasing compression. A deep internal terrestrial process of world-wide activity..- References.- IV. Crust and Substratum.- The composition of the earth's crust and its substratum. Sial and sima. Continental and oceanic sectors. Volcanic and petrographic consequences..- Magmatic clans.- Theory of the origin of igneous rocks belonging to the pacific and mediterranean clans of differentiation from a parental magma-layer of olivine basalt. Other theories..- Tectonic and magmatic cycles.- Crustal folding and the root of a mountain-chain. Belts of isostatic anomalies in the East and West Indies. The magmatic phenomena associated with a subsiding and subsequently buckling crust..- A peridotite-Layer in the substratum.- The relation of peridotite intrusions to certain epochs of down-buckling of a sialic mountain-root..- Zonal migration of geosynclines; continents and ocean-floors.- The pattern of continental shields. The origin of continents and ocean-floors. A primordial, world-wide sialic layer. The origin of an intermediate layer in the earth's crust..- Volcanism in basins.- The occurrence of basic rocks and the absence of acid rocks..- Fragmentation and growth of continents.- Continental rejuvenation. Periodical supply of sialic melts from the substratum..- Summary.- References.- V. Oscillations of the sea-level.- Regional transgressions and regressions.- The alternation of fresh and salt-water bodies in the Baltic Region. Other examples of local rhythms..- World-wide transgressions and regressions.- The synchronism and huge extent of the major trans- and regressions. Possible causes and coincidences with other phenomena. The amount of positive and negative shifts of the sea-level. An important problematical question..- References.- VI. The continental margin.- Problems of the continental shelves and the oceanic coasts of the continents. Classification of continental shelves..- The surface of the shelf.- Hypsometric relation between sea-level and shelf-surface. Older theories on the origin of the shelf. The problem of the shelf-edge at the isobath of 200 meters..- The influence of changing sea-level.- Amount of eustjatic changes. Their possible influence on the grading of the surface of the shelf..- Subsidence of the shelf-area.- Formations on the outer shelf-surface originally deposited in the littoral zone. Deeper surfaces of the shelf..- The marginal flexure of the continents.- Phenomena showing a widespread rejuvenation of the oceanic borders of the continents..- The theory of the marginal flexure..- Marine terraces.- The most recent movements along the marginal flexure. Possible dating of the different shelf-surfaces..- The geophysical side of the problem.- Gravity at the continental margin. The hypothesis of periodical convection currents in the marginal part of the continents. Correlation with other periodical phenomena..- Submarine valleys.- General statement. Classification. Cases to be excluded. The notched shelf-edge. Hypotheses involving a subaerial origin. Hypotheses involving submarine agencies. Gorges extending over the surface of the shelf. Submarine canyons showing a dendritic river-like pattern..- Summary and conclusions.- References.- VII. Island-Arcs.- Geographical distribution. Phenomena associated with island-arcs. Their counterpart on the continents..- The problem.- Restrictions. Short outline of problematic questions to be examined..- Physiographic features.- Genetic classifications. Angle of arc-curvature. The notion of deep-reaching thrustplanes. A simple working hypothesis based on physiographic evidence..- Earthquake zones.- Classification..- Normal-, intermediate-, and deep foci..- The Mechanism..- Distribution of dilatations and compressions. Similarity of the mechanism giving origin to normal tectonic earth shocks and deep-focus earthquakes..- Geographic distribution of deep-focus earthquakes..- Exceptional position of the western and southeastern Pacific..- Deep-reaching shear-zones..- The action of deep-reaching shear-zones and the theory of convection currents..- Terrestrial magnetism.- Agreement with the theory of thermal convection currents. Possible relation between districts of deep-focus earthquakes and anomalies of the vertical component of terrestrial magnetism..- Volcanism and plutonism.- The andesite line..- The boundary between sialic and basaltic sea-floor in the Pacific..- Petrographic provinces..- Distribution of igneous rocks belonging to the pacific and mediterranean provinces. Their mode of origin..- Igneous rocks in geanticlinal belts,.- Plutonism and volcanism in the inner arc. Their mutual relations to epochs of compression. Longitudinal faults and rift-valleys. Volcano-tectonic depressions..- Igneous rocks in geosynclinal troughs..- Their relation to epochs of compression..- Problems…