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Paper No. : 03 Archaeological Anthropology Module : 24 Geological Time Scale Development Team Principal Investigator Prof. Anup Kumar Kapoor Department of Anthropology, University of Delhi Paper Coordinator Dr. Manoj Kumar Singh Department of Anthropology, University of Delhi Prof. Mani Babu, Content Writer Department of Anthropology, Manipur University Content Reviewer Prof. Anup Kumar Kapoor Department of Anthropology, University of Delhi 1 Archaeological Anthropology Anthropology Geological Time Scale Description of Module Subject Name Anthropology Paper Name 03 Archaeological Anthropology Module Name/Title Geological Time Scale Module Id 24 2 Archaeological Anthropology Anthropology Geological Time Scale Table of contents: 1. Introduction 2. Early Principles Behind Geologic Time 3. Construction Of Geologic Time Scale (GTS) 4. The Time Scale Creator 5. Recognizing Geologic Stages 6. Divisions Of Geologic Time 6.1 Proterozoic Or Precambrian Eon 6.2 The Palaozoic Era 6.3 The Mesozoic Era 6.4 The Cenozoic Era Learning outcomes To know about the geological time scale To understand the early Principles Behind Geologic Time To know about the construction Of Geologic Time Scale (GTS) To know and understand its divisions 3 Archaeological Anthropology Anthropology Geological Time Scale 1. Introduction The geography and landscape of a region are always changing. Geological research works reveal that the mountains and valleys that surround us or the position of the coastline today have not always been as we know them now. The land that we walk on, in the majority of cases, has risen up from the depths of an ancient sea, and the distribution of land and sea will change through time. These changes result from complex geological processes: sediments that are transformed into new rocks and erosion of rocks that already exist into sediments; uplift or emergence of land areas, with the consequent retreat of the sea, and flooding of other areas, that are invaded by seas and oceans, where accumulation of sediment starts again that will later be transformed into other rocks, followed by renewed emergence and further destruction, etc. By studying the internal structure and composition of rocks, their age (that measured in millions of years) and the way that they are distributed in a region, geologists can reconstruct the way in which the landscape and geography of the region has changed, when the mountains were uplifted that are emerged now, and so on. All of these geological processes are extraordinarily slow from a human perspective, the duration of which is counted in terms of millions of years. Geologists now able to reconstruct the sequence of events that has shaped the Earth‘s surface from the study of petrology, stratigraphy and palaeontology. Many events have occurred since the formation of the Earth about 4.5 billion years ago (or 4500 million years ago). Some of these events have been recorded in the rocks that make up the crust. A chronological organization of these events is recorded on a geologic time scale as a framework for deciphering the history of our planet, Earth. This system of chronological measurement that relates stratigraphy to time, to describe the timing and relationships between events that have occurred throughout Earth's history is termed as the Geologic Time Scale (GTS). The geological time has started with the deposition of sedimentary rocks; the oldest stratum was created about some four and half billion years ago. 2. Early Principles behind Geologic Time The history of systematic principle of Geologic time begins with a number of works by Geologists and the like since early centuries. Mention may be made of the work of one Danish physician Nicholas Steno (1638-1687) in 1669 who described how the position of a rock layer could be used to show the relative age of the layer. He devised three main principles that underlie the interpretation of geologic time and these principles have formed the framework for the geologic area of stratigraphy, which is the study of layered rock. The first is the principle of superposition which points out that in an undisturbed pile sediments, layer on the bottom was deposited first and so is the oldest, followed in succession by the layers above them, and the topmost one being the youngest in formation. 4 Archaeological Anthropology Anthropology Geological Time Scale The second one is the principle of horizontality, which states that all rock layers were originally deposited horizontally. The third, that is, the principle of original lateral continuity refers that originally deposited layers of rock extend laterally in all directions until either thinning out or being cut off by a different rock layer. Contribution of a Scottish physician and geologist James Hutton (1726-1797) was the theory of ‗uniformitarianism‘ - which states that the surface of the earth was an ever-changing environment and ―the past history of our globe must be explained by what can be seen to be happening now‖. The theory of uniformitarianism states that the continuing uniformity of existing processes/physical laws is responsible for present and past conditions on earth. This theory was later well-known with a catch- phrased ―the present is the key to the past‖. Another significant work was by William Smith, a surveyor by profession who was in charge of mapping a large part of England. He was the first to understand that certain rock units could be identified by the particular assemblages of fossils they contained. Using this information, he was able to correlate strata with the same fossils for many miles, giving rise to the principle of biologic succession. This principle states that: each age in the earth‘s history is unique such that fossil remains will be unique. This permits vertical and horizontal correlation of the rock layers based on fossil species. The English Geologist, Charles Lyell‘s work (―Principles of Geology‖) during the early 1800s had significant contributions in framing the foundation of Geologic time scale – where he proposed two important principles – i) the principle of cross-cutting relationships and ii) Inclusion principle. His first principle states that a rock feature that cuts across another feature must be younger than the rock that it cuts. The second principle, the Inclusion principle, proposes that small fragments of one type of rock but embedded in a second type of rock must have formed first, and were included when the second rock was forming. 3. Construction of Geologic Time Scale (GTS) In fact the geologic time scale is the framework for deciphering the history of the Earth and has three important components (Gradstein, et. al. 2004) - (1)The international chronostratigraphic divisions and their correlation in the global rock record, (2) The means of measuring absolute (linear) time or elapsed durations from the rock record, and 5 Archaeological Anthropology Anthropology Geological Time Scale (3) The methods of effectively joining the two scales. In general the rock record of Earth‘s history is subdivided in a ―chronostratigraphic‖ scale of standardized global stratigraphic units, and is based on relative time units, in which global reference points at boundary stratotypes define the limits of the main formalized units, such as ―Devonian‖. The chronostratigraphic scale is an agreed convention, whereas its calibration to absolute (linear) time is a matter for discovery or estimation. No geologic time scale can be final on the fact that there occur continual improvements in data coverage, methodology, and standardization of chronostratigraphic units (Gradstein, et. al. 2004). There have been major developments in Geological Time Scale research has since 1989 by various international forums. Mention may be made of the works of the International Commission on Stratigraphy (ICS) mainly on refining the international chronostratigraphic scale, such as in the Ordovician or Permian periods, traditional European- or Asian-based geological stages have been replaced with new subdivisions that allow global correlation. Moreover, numerous high-resolution radiometric dates have been generated that has led to improved age assignments of key geologic stage boundaries. The use of global geochemical variations, Milankovitch climate cycles, and magnetic reversals has become important calibration tools (Gradstein, et. al. 2004). 4. The Time Scale Creator One goal of ICS is to provide detailed global and regional ―reference‖ scales of Earth history. Such scales summarize our current consensus on the inter-calibration of events, their relationships to international divisions of geologic time and their estimated numerical ages. On-screen display and production of user-tailored time-scale charts is provided by the Time-Scale Creator, a public JAVA package available from the ICS website (www.stratigraphy.org) and www.tscreator.com, (Gradstein and Ogg, 2006). 5. Recognizing Geologic Stages Geologic stages are recognized through their fossil content and not by their boundaries. Since the morphology of fossil taxa and their unique range in the rock record form the most unambiguous way to assign a relative age, these are used as the main method to distinguish and correlate strata among different regions. Obviously the evolutionary successions and assemblages of each fossil group are generally grouped into zones. And, the T S Creator program (www.tscreator.com) includes a majority of zonations and/or event datums (first or last appearances) for widely used groups of fossils 6 Archaeological Anthropology Anthropology Geological Time Scale through time. Trends and excursions in stable-isotope ratios, especially of carbon 12/13 and strontium 86/87, have become an increasingly reliable method to correlate among regions. Geologists recognize two major segments in the geologic time scale called ‗eons‘. Eons is divided into three smaller time units called ‗eras‘. Eons refer to the longest subdivision based on the abundance of certain fossils recorded. Eras are the next subdivision to eons, marked by major changes in the fossil record. Eras are again divided into ‗periods‘ based on types of life existing at the time.