The work of geologists is to tell the true story of Earth's history-more precisely, a story of Earth's history that is ever truer. A hundred years ago, we had little idea of the story's length-we had no good yardstick for time. Today, with the help of isotopic dating methods, we can determine the ages of rocks nearly as well as we map the rocks themselves. For that, we can thank radioactivity, discovered at the turn of the last century. A hundred years ago, our ideas about the ages of rocks and the age of the Earth were vague. But obviously, rocks are very old things.
During the Paleoproterozoic era the earliest part of the Proterozoic eon an event known as the oxygen catastrophe occurred: a relatively sudden increase in the amount of available oxygen which was the result of a complex chain of events. Since the Archean eon, early bacteria had been excreting oxygen as a waste product. Initially, most of the oxygen was consumed in the oxidation of minerals and metals such as iron. As the amount of unoxidized iron began to decrease, the amount of oxygen in the atmosphere increased.
This poisoned some types of anaerobic Archean bacteria, but spurred others to use oxygen in their metabolism, a much more efficient way of processing energy. Aerobic organisms became dominant in the Proterozoic eon. The Mesoproterozoic era the middle part of the Proterozoic eon saw the development of eukaryotes, single-celled organisms with a nucleus.
During the end of the Neoproterozoic era the most recent part of the eonin a division known as the Ediacaran period, the earliest complex multicellular organisms appeared. These soft-bodied creatures appear to have lived on the bottom of shallow seas, not unlike modern corals or sponges.
They were diverse in size, structural complexity, shape, and symmetry. The Ediacaran period is the most recently recognized of all the eons, eras, and periods, named for the Ediacara area in Australia, where many of the fossils have been found.
Alongside the rapidly changing life forms of the Proterozoic eon, significant geological processes were occurring. The supercontinent called Rodinia formed at the end of the Stenian period in the Mesoproterozoic. The first ice ages occurred during the Proterozoic era. The end of the Proterozoic is marked by a dramatic event in the fossil record known as the Cambrian explosion. At this time, a remarkable increase in the numbers and types of species is seen, as well as the first hard-bodied animals, i.
During this time, life evolved from the simplest sponges, jellyfish, and worms to include almost everything we can think of that is alive today. Geological periods during the Phanerozoic are divided into smaller epochs based on changes in the kinds of life that appear in the fossil record. The larger number of fossilized species present and the relatively short period of time since their deposit allow this more precise dating.
The largest divisions of the Phanerozoic eon are the Paleozoic, Mesozoic, and Cenozoic eras. Each lasted for millions of years and each is broadly characterized by the degree of development that the life within it has undergone.
The Paleozoic is divided into the Cambrian, Ordovician, Silurian, Devonian, Carboniferous which is sometimes divided into the Mississippian and Pennsylvanian eras and Permian periods. Each of these is further divided into several epochs, some named for places where their major characteristics were discovered, others simply divided into early, middle, and late epochs.
During the Paleozoic erainsects, plants, the first vertebrate animals, amphibians, reptiles, fish, sharks, and corals all appeared. Often, it is the changes in the kinds of animals and plants that are used to decide boundaries between the different periods.
Despite the emphasis on life in describing the various ages of the Paleozoic, geologic processes were still. Supercontinents formed and broke apart, several ice ages advanced and retreated, temperatures fluctuated, and sea levels rose and fell.
These diverse processes influenced the many changes in life that are recorded in the fossils of the era-coal deposits in Europe laid down during the Carboniferous period are one of its more famous features. At the end of the Paleozoic eraa disastrous event known as the Permian-Triassic extinction led to the destruction of almost all Paleozoic species.
Though there have been efforts to link this extinction to a meteorite impact, no convincing evidence of a large enough collision during this time period has been found. Dinosaurs appeared during the Mesozoic era. The names of the periods in the Mesozoic era may sound familiar: Triassic, Jurassic, and Cretaceous.
During this million-year era, all the familiar dinosaurs such as triceratops, tyrannosaurus, stegosaurus, diplodocus, and apatosaurus flourished at different times. Some modern animals have ancestors that first appeared during the Mesozoic era, including birds, crocodiles, and mammals.
Plants continued to develop, and the first flowering plants appeared.
Earth Science: Geologic Ages and Dating TechniquesIntroductionEarth is about billion years old. Geologists divide this age into major and minor units of time that describe the kinds of geological processes and life forms that existed in them. Earth's geologic record was formed by constant change, just like those that occur routinely today. Sep 30, Geologic age dating is an entire discipline of its own. In a way, this field, called geochronology, is some of the purest detective work earth scientists do. There are two basic approaches: relative geologic age dating, and absolute geologic age dating. rows The geologic time scale (GTS) is a system of chronological dating that relates geological .
The end of the Mesozoic era can be seen clearly in some rock layers. Known as the K-T Cretaceous-Tertiary boundary, this dark line of sediment is rich in the element iridium.
Another massive extinction of species occurred at this time, possibly because of one or more meteorite impacts along with a period of intense volcanic activity. This would have decreased the amount of sunlight reaching Earth's surface, killing plants and, eventually, animals. Not all geologists and paleontologists are convinced that the K-T extinction was a catastrophic event; some argue that it occurred over a few million years after slower climate changes.
The Cenozoic erathe current era of geologic time, is divided into the Paleogene and Neogene periods, and further into the Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene, and Holocene epochs. During the Cenozoic, the supercontinent of Gondwana broke apart, and the continents reached their current positions. Several ice ages occurred, and the poles became ice-covered. The first mammals began to flourish in the Paleocene; the first apes appeared in the Miocene; and the first human ancestors in the Pliocene.
Modern humans, along with large animals such as mammoths and wooly rhinoceroses, appeared in the Pleistocene. The Holocene epochcurrently ongoing, began with the end of the last ice age, less than 10, years ago. Though this vast span of time was largely understood by the end of the nineteenth century, geologists, paleontologists, and scientists of other disciplines were still curious about Earth's absolute ageusing different approaches to tackle the problem.
In the s William Thomson -more commonly known as Lord Kelvin, applied his theories of thermodynamics to determine Earth's age. He surmised that Earth was between 20 and 40 million years old by calculating the time it should take for it to cool from a liquid to a solid.
Though his calculations and some of his assumptions were correct, he failed to account for heat added by radioactivity. Around the turn of the twentieth century, Irish geologist John Joly - estimated Earth's age by analyzing the salt content of the seas. He then assumed that the oceans had started off as freshwater, and that all the salt had washed into them from the land. This relied on the assumption that the rate of salt coming into the oceans was constant and that no salt had ever been removed from the seas.
By this calculation he arrived at an age of about million years. Scientists needed a method that relied on something measurable over Earth's entire lifespan. In rocks older than about million years, it becomes impossible to use fossils to calculate their age because very few, if any, exist in these rocks. There are, however, a number of naturally radioactive elements that have been decaying since the formation of Earth.
With the discovery of radiation and the calculation of half-lives in the twentieth century it finally became possible to determine the age of Earth's oldest rocks. Radioactive decay is the spontaneous change in the nucleus of an element by the escape of a proton or neutron. Once a particle escapes the nucleus of an atom, it becomes a different isotope of the same element, or sometimes a different element altogether.
The ratio of the original parent element to the daughter element produced by decay determines how long the element has been decaying. The half-life of an isotope is the amount of time it takes for half of the sample to decay. InNew Zealand -born British physicist Ernest Rutherford - discovered that uranium and thorium decayed into isotopes of lead.
By Bertram Boltwood -an American chemist studying radioactive materials, had calculated the age of certain rocks based on analysis of their radioactivity.
Radiometric dating, a well-regarded way to establish the age of rocks, is still based on the same principles laid out by Rutherford and Boltwood.
It assumes that the half-lives of elements do not change over time, and that the sample has not been contaminated by the addition or removal of radioactive material. Zirconium crystals are usually analyzed because they trap uranium in their structure. Analyzing the decay of uranium to lead is useful because the half-life of uranium is million years. Even longer dates can be measured with potassium-to-argon decay, with a half-life of 1.
Carbon dating is useful for measuring very short ages on the geologic time scale. With a half-life of 5, years, carbon decay is useful for measuring dates up to about 70, years. This makes the method particularly useful for dating samples from the Holocene and late Pleistocene epochs.
Radiometric dating is the key to developing and understanding an absolute time scale of Earth and its geologic ages. When geological events, rock formations, and individual species can be placed accurately in time, it becomes possible to understand their relationships to each other and to events and circumstances present today.
Many scientific disciplines rely on an understanding of the geological past to make accurate observations and predictions. Some of these sciences, like meteorology, hydrology, and oceanography have important roles to play in understanding and possibly mitigating the effects of global climate change and population growth.
By studying climate changes in the past or uncovering the reasons for mass extinctions, it might be possible to foresee disasters and figure out how to avert them. Gould, Stephen J. New York : W. Schopf, J. Shimer, John A. Jensen, Soren. Knoll, Andrew H. Pope, Kevin O.
D'hondt, and Charles R. American Museum of Natural History. BBC News. University of Maryland, Department of Geology. University of Texas at Dallas, Department of Geosciences. Cite this article Pick a style below, and copy the text for your bibliography. May 11, Retrieved May 11, from Encyclopedia. Then, copy and paste the text into your bibliography or works cited list. Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.
Historical Background and Scientific Foundations In the mid-seventeenth century, James Ussher -the Archbishop of Ireland, compiled a chronology of Earth by adding up the generations named in the Bible. Shortly thereafter, Earth was pelted with meteorites during the late heavy bombardment, increasing the environment's hostility to life. Despite the emphasis on life in describing the various ages of the Paleozoic, geologic processes were still under way. Modern Cultural Connections Radiometric dating is the key to developing and understanding an absolute time scale of Earth and its geologic ages.
Weiner, Jonathan. Planet Earth.
New York: Bantam Books, Periodicals Jensen, Soren. David King Jr. Learn more about citation styles Citation styles Encyclopedia. Brookfiel Vermont: Ashgate Publishing Variorum series. III, Transactions of the Royal Society of Edinburgh published Retrieved 6 September Basin and Range.
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The oldest rock on Earth is the Acasta Gneissand it dates to 4. Solid Earth. Bibcode : SolE In Felix M. Gradstein; James G. Ogg; Mark D. Schmitz; abi M. Ogg eds. The geologic time scale 1st ed. Amsterdam: Elsevier. July Bibcode : PLoSO Geological history of Earth.
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Spread of Homo erectus. Start of Quaternary glaciations. Rise of the Pleistocene megafauna and Homo habilis. Greenland ice sheet develops. Zanclean flooding of the Mediterranean Basin.
Cooling climate. Ardipithecus in Africa. Messinian Event with hypersaline lakes in empty Mediterranean Basin. Moderate Icehouse climatepunctuated by ice ages and re-establishment of East Antarctic Ice Sheet ; Gradual separation of human and chimpanzee ancestors.
Sahelanthropus tchadensis in Africa. Warmer during middle Miocene climate optimum. Orogeny in Northern Hemisphere. Widespread forests slowly draw in massive amounts of CO 2gradually lowering the level of atmospheric CO 2 from ppmv down to around ppmv during the Miocene. Horses and mastodons diverse. Grasses become ubiquitous. Ancestor of apesincluding humans. Grande Coupure extinction. Start of widespread Antarctic glaciation.
Major evolution and dispersal of modern types of flowering plants. Moderate, cooling climate.
Thermal ionization mass spectrometer used in radiometric dating. Radiometric dating calculates an age in years for geologic materials by measuring the presence of a short-life radioactive element, e.g., carbon, or a long-life radioactive element plus its decay product, e.g., potassium/argon Relative dating cannot tell us the actual age of a rock; it can only tell us whether one rock is older or younger than another. The most common form of relative dating is called stratigraphic.
Archaic mammals e. Creodonts" Condylarths ", Uintatheresetc. Appearance of several "modern" mammal families. Primitive whales diversify. Orogeny of the Alps in Europe begins. Hellenic Orogeny begins in Greece and Aegean Sea. The Azolla event decreased CO 2 levels from ppm to ppm, setting the stage for a long period of cooling. Starts with Chicxulub impact and the K-Pg extinction event. Climate tropical. Modern plants appear; Mammals diversify into a number of lineages following the extinction of the non-avian dinosaurs.
First large mammals up to bear or small hippo size. Alpine orogeny in Europe and Asia begins. Flowering plants proliferate, along with new types of insects. More modern teleost fish begin to appear. Ammonoideabelemnitesrudist bivalvesechinoids and sponges all common. Many new types of dinosaurs e.
TyrannosaursTitanosaursHadrosaursand Ceratopsids evolve on land, as do Eusuchia modern crocodilians ; and mosasaurs and modern sharks appear in the sea.
Birds toothed and toothless coexist with pterosaurs. Monotremesmarsupials and placental mammals appear. Break up of Gondwana. Gymnosperms especially conifersBennettitales and cycads and ferns common. Many types of dinosaurssuch as sauropodscarnosaursand stegosaurs. Mammals common but small. First birds and lizards. Ichthyosaurs and plesiosaurs diverse.
BivalvesAmmonites and belemnites abundant. Sea urchins very common, along with crinoidsstarfish, spongesand terebratulid and rhynchonellid brachiopods. Breakup of Pangaea into Gondwana and Laurasia. Nevadan orogeny in North America. Rangitata and Cimmerian orogenies taper off.
Atmospheric CO 2 levels times the present day levels - ppmv, compared to today's ppmv  [f].
Archosaurs dominant on land as dinosaurs and in the air as pterosaurs. Ichthyosaurs and nothosaurs dominate large marine fauna.
Mar 13, A hundred years ago, our ideas about the ages of rocks and the age of the Earth were vague. But obviously, rocks are very old things. Judging from the number of rocks there are, plus the imperceptible rates of the processes forming them-erosion, burial, fossilization, uplift-the geologic record must represent untold millions of years of jankossencontemporary.com is that insight, first expressed in Author: Andrew Alden. Planetary scientists can estimate the relative age of the craters by comparing their appearance to craters of known age. However, the best they can do with these techniques is an estimate. Numerical or Absolute Geologic Dating. The numerical or absolute geologic age gives a narrow age range for the formation of a structure, such as a lava flow. Dating, in geology, determining a chronology or calendar of events in the history of Earth, using to a large degree the evidence of organic evolution in the sedimentary rocks accumulated through geologic time in marine and continental jankossencontemporary.com date past events, processes, formations, and fossil organisms, geologists employ a variety of techniques.
Cynodonts become smaller and more mammal-like, while first mammals and crocodilia appear. Dicroidium flora common on land. Many large aquatic temnospondyl amphibians.
Ceratitic ammonoids extremely common. Modern corals and teleost fish appear, as do many modern insect clades.
Radiometric Age Dating
Andean Orogeny in South America. Cimmerian Orogeny in Asia. Rangitata Orogeny begins in New Zealand. Landmasses unite into supercontinent Pangaeacreating the Appalachians.
End of Permo-Carboniferous glaciation. Synapsids including pelycosaurs and therapsids become plentiful, while parareptiles and temnospondyl amphibians remain common.
In the mid-Permian, coal -age flora are replaced by cone -bearing gymnosperms the first true seed plants and by the first true mosses. Beetles and flies evolve. Marine life flourishes in warm shallow reefs; productid and spiriferid brachiopods, bivalves, foramsand ammonoids all abundant.
Ouachita and Innuitian orogenies in North America. Altaid orogeny in Asia. Hunter-Bowen Orogeny on Australian continent begins c. Carbon- iferous [g]. Winged insects radiate suddenly; some esp. Protodonata and Palaeodictyoptera are quite large.
Amphibians common and diverse. First reptiles and coal forests scale treesferns, club treesgiant horsetailsCordaitesetc.
How Does Radiometric Dating Work? - Ars Technica
Highest-ever atmospheric oxygen levels. Goniatitesbrachiopods, bryozoa, bivalves, and corals plentiful in the seas and oceans. Testate forams proliferate. Uralian orogeny in Europe and Asia.
Variscan orogeny occurs towards middle and late Mississippian Periods. Large primitive treesfirst land vertebratesand amphibious sea-scorpions live amid coal -forming coastal swamps. Lobe-finned rhizodonts are dominant big fresh-water predators.
In the oceans, early sharks are common and quite diverse; echinoderms especially crinoids and blastoids abundant. Coralsbryozoagoniatites and brachiopods ProductidaSpiriferidaetc. Glaciation in East Gondwana.
Tuhua Orogeny in New Zealand tapers off. First clubmosseshorsetails and ferns appear, as do the first seed -bearing plants progymnospermsfirst trees the progymnosperm Archaeopterisand first wingless insects. Strophomenid and atrypid brachiopodsrugose and tabulate corals, and crinoids are all abundant in the oceans.
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Goniatite ammonoids are plentiful, while squid-like coleoids arise. Trilobites and armoured agnaths decline, while jawed fishes placodermslobe-finned and ray-finned fish, and early sharks rule the seas.
First tetrapods still aquatic. First vascular plants the rhyniophytes and their relativesfirst millipedes and arthropleurids on land.
First jawed fishesas well as many armoured jawless fishpopulate the seas. Sea-scorpions reach large size. Tabulate and rugose corals, brachiopods PentameridaRhynchonellidaetc. Trilobites and mollusks diverse; graptolites not as varied. Also continued into Devonian period as the Acadian Orogenyabove.
Taconic Orogeny tapers off. Lachlan Orogeny on Australian continent tapers off. Invertebrates diversify into many new types e. Early coralsarticulate brachiopods OrthidaStrophomenidaetc. Conodonts early planktonic vertebrates appear. First green plants and fungi on land. Ice age at end of period. Floian formerly Arenig. Stage Major diversification of life in the Cambrian Explosion. Numerous fossils; most modern animal phyla appear.