The Evolution of the Earth The formation of this planet and its atmosphere gave rise to life, which shaped the earth’s subsequent development. Our future lies in interpreting this geologic past by Claude J. All•gre and Stephen H. Schneider EARTH SEEN FROM SPACE has changed dramatically. One hundred million years after it had formedÑsome 4.35 billion years agoÑthe planet was probably under- going meteor bombardment (left ). At this time, it may have been studded with vol- canic islands and shrouded by an atmosphere laden with carbon dioxide and heavy with clouds. Three billion years ago its face may have been obscured by an orange haze of methane, the product of early organisms (center). Today clouds, oceans and continents are clearly discernible (right ). This illustration was pre- pared with the help of James F. Kasting of Pennsylvania State University. 66 SCIENTIFIC AMERICAN October 1994 Copyright 1994 Scientific American, Inc. ike the lapis lazuli gem it resembles, standing the origin of life and, perhaps, the blue, cloud-enveloped planet its future. CLAUDE J. ALLéGRE and STEPHEN H. that we recognize immediately Scientists used to believe the rocky SCHNEIDER study various aspects of the L earthÕs geologic history and its climate. from satellite pictures seems remark- planets, including the earth, Mercury, Ve- All•gre is a professor at the University ably stable. Continents and oceans, en- nus and Mars, were created by the rapid of Paris and directs the department of circled by an oxygen-rich atmosphere, gravitational collapse of a dust cloud, a geochemistry at the Institut de Physique support familiar life-forms. Yet this con- deßation giving rise to a dense orb. In du Globe de Paris. He is a foreign mem- stancy is an illusion produced by the the 1960s the Apollo space program ber of the National Academy of Sciences. human experience of time. The earth changed this view. Studies of moon Schneider is a professor in the depart- and its atmosphere are continuously al- craters revealed that these gouges were ment of biological sciences at Stanford tered. Plate tectonics shift the conti- caused by the impact of objects that University and is a senior fellow at the universityÕs Institute of International nents, raise mountains and move the were in great abundance about 4.5 bil- Studies. He is also a senior scientist at ocean ßoor while processes that no one lion years ago. Thereafter, the number the National Center for Atmospheric Re- fully comprehends alter the climate. of impacts appeared to have quickly de- search in Boulder, Colo. Such constant change has character- creased. This observation rejuvenated ized the earth since its beginning some the theory of accretion postulated by 4.5 billion years ago. From the outset, Otto Schmidt. The Russian geophysi- heat and gravity shaped the evolution cist had suggested in 1944 that plan- meant that it took a long time to build of the planet. These forces were gradu- ets grew in size gradually, step by step. up a large planet. A calculation made ally joined by the global eÝects of the According to Schmidt, cosmic dust by George W. Wetherill of the Carnegie emergence of life. Exploring this past lumped together to form particulates, Institution of Washington suggests that oÝers us the only possibility of under- particulates became gravel, gravel be- about 100 million years could pass be- came small balls, then big balls, then tween the formation of an object mea- tiny planets, or planetesimals, and, Þ- suring 10 kilometers in diameter and nally, dust became the size of the moon. an object the size of the earth. As the planetesimals became larger, The process of accretion had signif- their numbers decreased. Consequent- icant thermal consequences for the ly, the number of collisions between earth, consequences that have forceful- planetesimals, or meteorites, decreased. ly directed its evolution. Large bodies Fewer items available for accretion slamming into the planet produced im- Copyright 1992 Scientific American, Inc. mense heat in the interior, melting the stroyed by its very activity. But the de- Isotope geology has permitted geolo- cosmic dust found there. The resulting velopment of isotope geology in the gists to determine that the accretion of furnaceÑsituated some 200 to 400 kilo- 1960s had rendered this view obsolete. the earth culminated in the diÝerentia- meters underground and called a mag- Their imaginations Þred by Apollo and tion of the planet: the creation of the ma oceanÑwas active for millions of the moon Þndings, geochemists began coreÑthe source of the earthÕs magnet- years, giving rise to volcanic eruptions. to apply this technique to understand ic ÞeldÑand the beginning of the at- When the earth was young, heat at the the evolution of the earth. mosphere. In 1953 the classic work of surface caused by volcanism and lava Claire C. Patterson of the California In- ßows from the interior was supplement- ating rocks using so-called ra- stitute of Technology used the uranium- ed by the constant bombardment of dioactive clocks allows geolo- lead clock to establish an age of 4.55 huge planetesimals, some of them per- D gists to work on old terrains that billion years for the earth and many of haps the size of the moon or even Mars. do not contain fossils. The hands of a the meteorites that formed it. Recent No life was possible during this period. radioactive clock are isotopesÑatoms work by one of us (All•gre) on lead iso- Beyond clarifying that the earth had of the same element that have diÝerent topes, however, led to a somewhat new formed through accretion, the Apollo atomic weightsÑand geologic time is interpretation. As Patterson argued, program compelled scientists to try to measured by the rate of decay of one some meteorites were indeed formed reconstruct the subsequent temporal isotope into another [see ÒThe Earliest about 4.56 billion years ago, and their and physical development of the early History of the Earth,Ó by Derek York; debris constituted the earth. But the earth. This undertaking had been con- SCIENTIFIC AMERICAN, January 1993]. earth continued to grow through the sidered impossible by founders of geol- Among the many clocks, those based bombardment of planetesimals until ogy, including Charles Lyell, to whom on the decay of uranium 238 into lead some 120 to 150 million years later. At the following phrase is attributed: No 206 and of uranium 235 into lead 207 that timeÑ4.44 to 4.41 billion years vestige of a beginning, no prospect for are special. Geochronologists can deter- agoÑthe earth began to retain its at- an end. This statement conveys the idea mine the age of samples by analyzing mosphere and create its core. This pos- that the young earth could not be re- only the daughter productÑin this case, sibility had already been suggested by created, because its remnants were de- leadÑof the radioactive parent, uranium. Bruce R. Doe and Robert E. Zartman of the U.S. Geological Survey in Denver a decade ago and is in agreement with WetherillÕs estimates. How the Earth Got Its Core The emergence of the continents came somewhat later. According to the he differentiation of the planet took place quite quickly after the earth theory of plate tectonics, these land- Twas formed by the accretion of cosmic dust and meteorites. About 4.4 masses are the only part of the earthÕs billion years ago the core—which, with the mantle, drives the geothermal crust that is not recycled and, conse- cycle, including volcanism—appeared; gases emerging from the interior of quently, destroyed during the geother- the planet also gave rise to a nascent atmosphere. Somewhat later, although mal cycle driven by the convection in the issue has not been entirely resolved, it seems that continental crust the mantle. Continents thus provide a formed as the various elements segregated into different depths. form of memory because the record of early life can be read in their rocks. The testimony, however, is not extensive. Geologic activity, including plate tecton- ics, erosion and metamorphism, has R M A N destroyed almost all the ancient rocks. P P E T L E U Very few fragments have survived this LITHOSPHERE geologic machine. ASTHENOSPHERE Nevertheless, in recent years, several important Þnds have been made, again OLIVINE-RICH W E R M A N T L O L E MATERIAL using isotope geochemistry. One group, led by Stephen Moorbath of the Univer- sity of Oxford, discovered terrain in PEROVSKITE-RICH West Greenland that is between 3.7 and ER MAT IAL 3.8 billion years old. In addition, Sam- uel A. Bowring of the Massachusetts In- stitute of Technology explored a small area in North AmericaÑthe Acasta gneissÑthat is 3.96 billion years old. C O R E Ultimately, a quest for the mineral zircon led other researchers to even more ancient terrain. Typically found in continental rocks, zircon is not dis- 4.4 BILLION solved during the process of erosion YEARS AGO but is deposited as particles in sedi- ment. A few pieces of zircon can there- fore survive for billions of years and 4.2 BILLION YEARS AGO can serve as a witness to the earthÕs TO PRESENT more ancient crust. The search for old zircons started in Paris with the work of Annie Vitrac and Jo‘l R. Lancelot, now 68 SCIENTIFIC AMERICAN October 1994 Copyright 1994 Scientific American, Inc. at the University of Marseilles and the CONTINENTAL SHIFT has altered the 700 MILLION University of Montpellier, respectively, face of the planet for nearly a billion YEARS AGO as well as with the eÝorts of Moorbath years, as can be seen in the diÝerences and All•gre.