Geological History of the Earth PDF

GENERAL GEOLOGY I. GEOLOGICAL HISTORY OF THE EARTH Geology is an earth science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which they change. Geology is the study of the Earth - how it wo...

GENERAL GEOLOGY I. GEOLOGICAL HISTORY OF THE EARTH Geology is an earth science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which they change. Geology is the study of the Earth - how it works and its 4.5-billion-year history. Geologists study some of society's most important problems, such as energy, water, and mineral resources; the environment; climate change; and natural hazards like landslides, volcanoes, earthquakes, and floods. The geological history of Earth follows the major events in Earth's past based on the geologic time scale, a system of chronological measurement based on the study of the planet's rock layers (stratigraphy). Geology for Civil Engineers Page 1 Cambrian Period (570-510 Million Years Ago) An explosion of life populated the seas, but land areas remained barren. Animal life was wholly invertebrate, and the most common animals were arthropods called trilobites (now extinct), with species numbering in the thousands. Multiple collisions between the Earth's crustal plates gave rise to the first supercontinent, known as Gondwanaland. When Pangaea broke up, the northern continents of North America and Eurasia became separated from the southern continents of Antarctica, India, South America, Australia and Africa. Ordovician Period (510-439 million years ago) The predecessor of today's Atlantic Ocean began to shrink as the continents of that time drifted closer together. Trilobites were still abundant; important groups making their first appearance included the corals, crinoids, bryozoans, and pelecypods. Armored, jawless fishes—the oldest known vertebrates—made their appearance as well; their fossils are found in ancient estuary beds in North America. Silurian Period (439-408.5 million years ago) Life ventured on to land in the form of simple plants called psilophytes, with a vascular system for circulating water, and scorpion-like animals akin to now extinct marine arthropods called eurypterids. Trilobites decreased in number and variety, but the seas teemed with reef corals, cephalopods, and jawed fishes. Devonian Period (408.5-362.5 million years ago) This period is also known as the age of fishes, because of their abundant fossils in Devonian rocks. Fishes had also become adapted to fresh water as well as to salt water. They included a diversity of both jawless and jawed armored fishes, Geology for Civil Engineers Page 2 early sharks, and bony fishes, from the last of which amphibians evolved. (One subdivision of the sharks of that time is still extant.) On land areas, giant ferns were widespread. Permian Period (290-245 million years ago) The Earth's land areas became welded into a single land mass that geologists call Pangaea, and in the North American region the Appalachians were formed. Cycad-like plants and true conifers appeared in the northern hemisphere, replacing the coal forests. Environmental changes resulting from the redistribution of land and sea triggered the greatest mass extinction of all time. Trilobites and many fishes and corals died out as the Palaeozoic era came to an end. Triassic Period (245-208 million years ago) The beginning of the Mesozoic era was marked by the reappearance of Gondwanaland, as Pangaea split apart into northern (Laurasia) and southern (Gondwanaland) supercontinents. Forms of life changed considerably in the Mesozoic, known as the age of reptiles. New pteridosperm families appeared, and Geology for Civil Engineers Page 3 conifers and cycads became major floral groups, along with ginkgoes and other genera. Such reptiles as dinosaurs and turtles appeared, as did mammals. Jurassic Period (208-145.6 million years ago) As Gondwanaland rifted apart, the North Atlantic Ocean widened and the South Atlantic was born. Giant dinosaurs ruled on land, while marine reptiles such as ichthyosaurs and plesiosaurs increased in number. Primitive birds appeared, and modern reef-building corals grew in coastal shallows. Crab-like and lobster-like animals evolved among the arthropods. Cretaceous Period (145.6-65 million years ago) Dinosaurs flourished and evolved into highly specialized forms, but they abruptly disappeared at the end of the period, along with many other kinds of life. (Theories to account for these mass extinctions are currently of great scientific interest.) The floral changes that took place in the Cretaceous were the most marked of all alterations in the organic world known to have occurred in the history of the Earth. Gymnosperms were widespread, but in the later part of the period angiosperms (flowering plants) appeared. Geology for Civil Engineers Page 4 Tertiary Period (65-1.64 million years ago) In the Tertiary, North America's land link to Europe was broken, but its ties to South America were forged towards the end of the period. During Cenozoic times, life forms both on land and in the sea became more like those of today. Grasses became more prominent, leading to marked changes in the dentition of plant-eating animals. With most of the dominant reptile forms having vanished at the end of the Cretaceous, the Cenozoic became the age of mammals. Thus, in the Eocene epoch, new mammal groups developed such as small, horse-like animals; rhinoceroses; tapirs; ruminants; whales; and the ancestors of elephants. Members of the cat and dog families appeared in the Oligocene epoch, as did species of monkeys. In Miocene times, marsupials were numerous, and anthropoid (human-like) apes first appeared. Placental mammals reached their zenith, in numbers and variety of species, in the Pliocene, extending into the Quaternary period Quaternary Period (1.64 million years ago to present) Intermittent continental ice sheets covered much of the northern hemisphere. Fossil remains show that many primitive pre-human types existed in south-central Africa, China, and Java by Lower and middle Pleistocene times; but modern humans (Homo sapiens) did not appear until the later Pleistocene. Late in the period, humans crossed over into the New World by means of the Bering land bridge. The ice sheets finally retreated, and the modern age began. IMPORTANT PRINCIPLES OF GEOLOGY There are a number of important principles in geology. Many of these involve the ability to provide the relative ages of strata or the manner in which they were formed. PRINCIPLE OF UNIFORMITARIANISM - states that the geologic processes observed in operation that modify the Earth's crust at present have worked in much the same way over geologic time. A fundamental principle of geology advanced by the 18th century Scottish physician and geologist James Hutton, is that "the present is the key to the past." In Hutton's words: "the past history of our globe must be explained by what can be seen to be happening now." PRINCIPLE OF INTRUSIVE RELATIONSHIPS - concerns crosscutting intrusions. In geology, when an igneous intrusion cuts across a formation of sedimentary rock, it Geology for Civil Engineers Page 5 can be determined that the igneous intrusion is younger than the sedimentary rock. There are a number of different types of intrusions, including stocks, laccoliths, batholiths, sills and dikes. PRINCIPLE OF INCLUSION AND COMPONENTS - states that, with sedimentary rocks, if inclusions (or clasts) are found in a formation, then the inclusions must be older than the formation that contains them. For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer. A similar situation with igneous rocks occurs when xenoliths are found. These foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix. As a result, xenoliths are older than the rock which contains them. PRINCIPLE OF ORIGINAL HORIZONTALITY - states that the deposition of sediments occurs as essentially horizontal beds. Observation of modern marine and non-marine sediments in a wide variety of environments supports this generalization (although cross-bedding is inclined, the overall orientation of cross-bedded units is horizontal). PRINCIPLE OF SUPERPOSITION - states that a sedimentary rock layer in a tectonically undisturbed sequence is younger than the one beneath it and older than the one above it. Logically a younger layer cannot slip beneath a layer previously deposited. This principle allows sedimentary layers to be viewed as a form of vertical time line, a partial or complete record of the time elapsed from deposition of the lowest layer to deposition of the highest bed. PRINCIPLE OF FAUNAL SUCCESSION - is based on the appearance of fossils in sedimentary rocks. As organisms exist at the same time period throughout the world, their presence or (sometimes) absence may be used to provide a relative age of the formations in which they are found. Based on principles laid out by William Smith almost a hundred years before the publication of Charles Darwin's theory of evolution, the principles of succession were developed independently of evolutionary thought. The principle becomes quite complex, however, given the uncertainties of fossilization, the localization of fossil types due to lateral changes in habitat (facies change in sedimentary strata), and that not all fossils may be found globally at the same time. Geology for Civil Engineers Page 6