PREFACE - GOADs Book

h-- Describe major events that occurred in the geological history of the Earth5前言.indd 52018/12/26 下午2:34

CHAPTER 1 Our Beautiful EarthFigure 1-1-1. An artist’s conception of the birth of a star and its planets. Image courtesy of NASA.1-1 The Formation of the EarthThe early Earth was a fiery ball of moltenmaterial, heated by gravitational forces,radioactive decay, and collisions with othermeteorites and celestial objects. One suchcollision is believed to have flung some of theEarth’s material into space, and this materialeventually condensed to form the Moon. Asthe Earth slowly stabilized and cooled, heavierelements settled inward to form a metallic core,while lighter material drifted outward to form alayer of molten rock termed the mantle, whichcompletely envelops the core. The exterior ofthe mantle eventually cooled to form the solidcrust, and this is the layer where all known livingorganisms exist. This formation process gave riseto the layered structure of the Earth (Figure 1-12), and further provided the foundation for thedevelopment of Earth’s magnetic field, withoutwhich life on Earth could not have emerged.Picture a giant cloud of dust and gas driftingaimlessly through space. A powerful externalforce, such as that caused by the explosion ofa star, disturbs this cloud and induces it to startspinning. This spinning effect creates heat andgravitational force, causing the cloud to collapseinto a giant spinning disk with an extremelydense and hot center (Figure 1-1-1). This diskis known as a protoplanetary disk, the centre ofwhich eventually coalesces into a star, while theouter leftover material aggregates to form planetsand other celestial objects. Our Solar System wasformed in this way about 4.6 billion years ago,based on dating of meteorites, asteroids, and theoldest Earth rocks, and similar dating suggeststhe Earth came into being about 4.54 billion yearsago.Chapter 1 Cover Image. Asia from space, with night lights in the background. Image courtesy of NASA.601.indd 62018/12/26 下午2:36

Our Beautiful Earth CHAPTER 1Despite this, we can still view the Earth as aplanet made up of spheres, due to the layeredstructure created by its formation process (Figure1-2-1). The interior of the Earth consists of thecore, which can be divided between an inner corewith a radius of about 1,250 km and an outer coreof about 2,200 km in thickness. The boundaryof the inner core and outer core is known asthe Lehmann-Bullen Discontinuity, as it wasindependently discovered in 1936 by the Danishseismologist Inge Lehmann (1888-1993) and theNew Zealand geologist Keith Edward Bullen(1906-1976). The mantle can also be dividedinto the lower mantle (about 2,550 km thick) andthe upper mantle (about 250 km thick), and theboundary in between is known as the GutenbergDiscontinuity, after the German-Americangeologist Beno Gutenberg (1889-1960). Thecrust ranges from 5-100 km in thickness, withcontinental crust being thicker and less dense thanoceanic crust. The boundary zone between thecrust and the mantle is known as the MohorovičićDiscontinuity, after the Croatian geologist AndrijaMohorovičić (1857-1936).CrustUpper mantleLower mantleOuter coreInner coreFigure 1-1-2. Schematic depicting the layered structureof the Earth, including the core (inner & outer core),mantle (lower & upper mantle), and the crust, drawn totheir actual proportions.1-2 The Spheres of the EarthThe Earth is not a perfect sphere, but is slightlyflattened at the poles and swollen at the Equator.The Earth has an average diameter of about12,742 kilometres (km), but this increases to12,756 km at the Equator and decreases to 12,713km at the poles, a difference of about 43 km.CrustContinental andOceanic CrustUpper mantleLower MantleMantleCoreOuter CoreInnerCore100 km2900 km6371 kmLehmann-BullenDiscontinuity5100 kmGutenbergDiscontinuity2900 km0 km100 kmMohorovičić 350 kmDiscontinuityFigure 1-2-1. Schematic depicting the spherical layers of the Earth, with their approximate thickness.Diameter of the Earth:12,742km701.indd 72018/12/26 下午2:36

PREFACE In the summer of 1977, the Voyager 1 and Voyager 2 space probes were launched, on a m