EAS 102 / BIO G 170 Lecture 5, Page 1 of 6 The “How To” of Reading the Geological Record: Tracking Rocks in Four Dimensions GEOCHRONOLOGY: TELLING TIME IN HISTORICAL PERIODS Where Are We? Whether the aim is to reconstruct the solar system’s formation (as it was in the last lecture), to chart continental drift in a movie-like series of world maps (as it will be in Lectures 9-12 and 16-18), or to trace the evolution of our own species (as it will be in Lectures 19-26), the technical problem generally comes down to reconstructing the four-dimensional historical trajectories of the rocks, minerals, and other debris that record the story. To accurately map the assembly and breakup Pangaea, for instance, one must be able to track the supercontinent’s constituent terranes by measuring (1) paleolatitude, (2) paleolongitude, and (3) paleoelevation as functions of (4) time. The more fully and accurately a terrane’s or a mineral grain’s four-dimensional trajectory can be reconstructed, the more precisely geological processes’ operation can be studied and the more exactly historical sequences of events can be reconstructed. Labs 3-8 illustrate with some of the original examples how geologists learned how to recognize plate-tectonic processes and reconstruct their operation. The basic concepts in this, the first of four lectures on reading the geological record, are discussed and illustrated in greater detail in Labs 3 and 4. The Discovery of Rocks as Historical Records It seems not to be obvious that rocks are products of everyday processes. Once the connection is made, however, it becomes clear that rocks are historical records. Herodotus (“The Father of History,” 484- c. 425 B.C.) and Leonardo da Vinci (1452-1519), among others, observed fossils in rocks and drew the obvious conclusions: the rocks are ancient sediments, and the Mediterranean Sea must have been higher in the past. More than once when leading Ithaca gradeschoolers on fossil-collecting trips, your professor has watched as the face of a latter-day Herodotus or Leonardo lit up with a corresponding realization: Eureka! Fossils show lake level must have been higher in the past! Among other things, this example shows that measuring geological time in years as well as in historical periods is important to avoid associating many-times-older fossils with a water-filled basin formed only in the last few ka of postglacial time, in the case of Cayuga Lake, or , in the case of the Mediterranean, one that had been as dry as Death Valley for much of the intervening few million years. Importance of the Uniformity Principle — As first discussed in Lecture 1, the discovery of the Earth’s historical record was intimately linked to recognition of the uniformity principle, and so was the realization that recorded events could be put in order on a geological time scale. The uniformity principle’s main implication in this regard is that rocks can be read back as historical records by applying a knowledge of time-invariant rock-forming processes — a knowledge that can be obtained from studying these processes and their products in the present. James Hutton’s phrase “The present is the key to the past” perhaps too concisely sums up the uniformity principle’s implications (or at least too concisely for literal-minded geologists who have mistakenly concluded the implication that processes always have proceeded at more or less the same rate). 1/20/03 EAS 102 / BIO G 170 Lecture 5, Page 2 of 6 Although the uniformity principle is implicit in Herodotus’ and Leonardo’s interpretations of rocks and fossils, many early scientists seemed to expect rocks to be Genesis written in stone. Isaac Newton’s Principia (Mathematical Principles of Natural Philosophy, in Latin; 1687) put the understanding of physical phenomena on a foundation firm enough to give definite, quantitative, testable predictions about the outcome of experiments; but it took another few generations for the understanding to become well and widely enough disseminated among natural philosophers to yield much in the way of applied science, much less technology. During the Enlightenment (c. 1687-1789), natural philosophers began to examine rocks with an eye to explaining them as products of newly understood processes. Ever before them was the prevailing belief that the Earth had been created around 4000 B.C., as many had deduced from chronological information in the Bible1. Sedimentology, marine ecology, and oceanography — the sciences most concerned with those aspects of the “present” most useful as keys to sedimentary rocks— did not develop until the mid- to late 19th Century, by which time “hard-rock” geology was already well advanced. What held up the birth of modern geological science until the end of the 18th Century was precisely what was holding up the Industrial Revolution: chemical knowledge good enough to be put into practice, whether as a key to the past in geology, or as a key to the future in chemical manufacturing. Pure chemistry advanced rapidly during the late 18th Century2. Its first industrial success was in dyestuffs manufacture in later 18th Century Edinburgh. Here was the very beginning of the Industrial Revolution. This success was due largely to a close-knit circle of natural philosophers who made it their business to understand the process behind their chemical products. The men applied the same common-sense approach to process-product relations in their shared hobby, geology. Among them was James Hutton, who made his money manufacturing ammonium chloride [NH4Cl] for the dyestuffs industry. Hutton’s Theory of the Earth (1788, 1795) founded modern geology squarely on the uniformity principle. Through him, the uniformity principle has come to be known in geology as the doctrine of uniformitarianism (see Lecture 1). What the Uniformity Principle’s Application Revealed — The first modern geological explorations of the 18th Century were in a sense an extension of geographic explorations. By Hutton’s time, Europeans’ systematic mapping of the globe by latitude and longitude had been under way for three centuries. Australia and Antarctica were still to be discovered when geologists began to take over from miners and sailors in systematically exploring the third spatial dimension, depth. Hutton’s interpretation of rocks in accord with the uniformity principle revealed time to be an unexpectedly, indeterminately vast fourth dimension. Uniformitarian reading of the geological record led Hutton to a further, socially more revolutionary conclusion: as consistent with the notion that nothing supernatural happens, nothing supernatural seems to have happened around 6 ka ago, when Earth was then generally supposed to have been created, or at any other time in a much longer history. 1 Creation is the starting point for the Hebrew calendar, which dates it to 7 September 3760 B.C. on the everyday Gregorian calendar. The Venerable Bede established the convention of numbering years relative to the birth of Christ in his De Temporum Ratione (725; by far the best early medieval computistic work, and the standard work on chronology and the calendar for centuries to come), thus setting the alarm for episode after episode of millennial hysteria. Bede estimated Creation to have taken place on or about 18 March 3952 B.C. For general review, see N. Dershowitz and E.M. Reingold, Calendrical Calculations (Cambridge, 1997). Apparently as unaware of the religious associations as they are of the associated scholarship, some geologists celebrate 23 October as “Bishop Ussher Day” in recognition of the 17th Century protestant archbishop who reckoned Creation to have taken place on or about that date in 4004 B.C. 2 See A. Clough and N.L. Clough, The Chemical Revolution (Batchworth, 1952). 1/20/03 EAS 102 / BIO G 170 Lecture 5, Page 3 of 6 The Geological Time Scale Geological Timekeeping Processes — Any process that generates a time-varying, geologically preserved product is potentially useful in telling geological time. The most useful processes have proved to be (1) sediment accumulation, the usual basis for establishing layers’ relative age within one rock outcrop; (2) evolutionary change in organisms as documented by fossils (change within lineages, including origination and extinction of species), the usual basis for establishing secdimentary rocks’ age relationships between outcrops; (3) radioactive decay, the usual basis for estimating absolute geological age (e.g. ages in years), as will be discussed in the next lecture; (4) sea level change; (5) change in the geomagnetic field’s strength and polarity. In geologically young organic material, (6) chemical decay is sometimes useful. Change in sea water’s 87Sr/86Sr ratio (7) is proving useful in geochronology, though the underlying geochemical processes involved are several and complicated. Deceleration of the Earth’s rotational rate (8) is potentially useful, but hardly ever used. Seasonal growth in organisms (9) can be useful in dating (e.g. tree-ring dating, which in some areas has been carried back a few ka’s), and so can seasonal sedimentation (10) (e.g. annual varves in lake sediments, which have been counted back a few ka’s in some areas, and annual layers in glacial ice, which have been counted back as much as 150 ka in some ice cores). Geological Time Scales — Today’s geological time scale is really several time scales in one. The relative geological time scale is concerned with geochronology, the measurement of time in historical periods (e.g. Enlightenment, Industrial Revolution, Victorian Period, Gilded Age, Cold War Era, Devonian Period). It includes the stratigraphic time scale (the one calibrated in eras, periods, epochs, and other units), the biochronological scale (a collection of many separate scales, each calibrated in the biochrons of variously related species), the geomagnetic time scale (the one calibrated in geomagnetic chrons, periods defined by reversals in the geomagnetic field’s polarity), the sea-level time scale (the one calibrated in cycles of sea-level rise and fall), and various isotopic time scales (e.g.