Dating earths history

Content
  • Relative and absolute ages in the histories of Earth and the Moon: The Geologic Time Scale
  • Changing Views of the History of the Earth
  • AGE OF THE EARTH
  • Age of the Earth
  • Dating Rocks and Fossils Using Geologic Methods

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Relative and absolute ages in the histories of Earth and the Moon: The Geologic Time Scale

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 environments. To date past events, processes, formations, and fossil organisms, geologists employ a variety of techniques. These include some that establish a relative chronology in which occurrences can be placed in the correct sequence relative to one another or to some known succession of events.

Radiometric dating and certain other approaches are used to provide absolute chronologies in terms of years before the present. The two approaches are often complementary, as when a sequence of occurrences in one context can be correlated with an absolute chronlogy elsewhere. Local relationships on a single outcrop or archaeological site can often be interpreted to deduce the sequence in which the materials were assembled.

This then can be used to deduce the sequence of events and processes that took place or the history of that brief period of time as recorded in the rocks or soil. For example, the presence of recycled bricks at an archaeological site indicates the sequence in which the structures were built. Similarly, in geology, if distinctive granitic pebbles can be found in the sediment beside a similar granitic body, it can be inferred that the granite, after cooling, had been uplifted and eroded and therefore was not injected into the adjacent rock sequence.

Although with clever detective work many complex time sequences or relative ages can be deduced, the ability to show that objects at two separated sites were formed at the same time requires additional information. A coin, vessel, or other common artifact could link two archaeological sites, but the possibility of recycling would have to be considered.

It should be emphasized that linking sites together is essential if the nature of an ancient society is to be understood, as the information at a single location may be relatively insignificant by itself. Similarly, in geologic studies, vast quantities of information from widely spaced outcrops have to be integrated. Some method of correlating rock units must be found. In the ideal case, the geologist will discover a single rock unit with a unique collection of easily observed attributes called a marker horizon that can be found at widely spaced localities.

Any feature, including colour variations, textures, fossil content, mineralogy , or any unusual combinations of these can be used. It is only by correlations that the conditions on different parts of Earth at any particular stage in its history can be deduced. In addition, because sediment deposition is not continuous and much rock material has been removed by erosion , the fossil record from many localities has to be integrated before a complete picture of the evolution of life on Earth can be assembled.

Using this established record, geologists have been able to piece together events over the past million years, or about one-eighth of Earth history, during which time useful fossils have been abundant. The need to correlate over the rest of geologic time, to correlate nonfossiliferous units, and to calibrate the fossil time scale has led to the development of a specialized field that makes use of natural radioactive isotopes in order to calculate absolute ages.

The precise measure of geologic time has proven to be the essential tool for correlating the global tectonic processes that have taken place in the past. Precise isotopic ages are called absolute ages, since they date the timing of events not relative to each other but as the time elapsed between a rock-forming event and the present.

The same margin of error applies for younger fossiliferous rocks, making absolute dating comparable in precision to that attained using fossils. To achieve this precision, geochronologists have had to develop the ability to isolate certain high-quality minerals that can be shown to have remained closed to migration of the radioactive parent atoms they contain and the daughter atoms formed by radioactive decay over billions of years of geologic time.

In addition, they have had to develop special techniques with which to dissolve these highly refractory minerals without contaminating the small amount about one-billionth of a gram of contained lead and uranium on which the age must be calculated. Since parent uranium atoms change into daughter atoms with time at a known rate, their relative abundance leads directly to the absolute age of the host mineral.

In fact, even in younger rocks, absolute dating is the only way that the fossil record can be calibrated. Without absolute ages, investigators could only determine which fossil organisms lived at the same time and the relative order of their appearance in the correlated sedimentary rock record. Unlike ages derived from fossils, which occur only in sedimentary rocks, absolute ages are obtained from minerals that grow as liquid rock bodies cool at or below the surface.

When rocks are subjected to high temperatures and pressures in mountain roots formed where continents collide, certain datable minerals grow and even regrow to record the timing of such geologic events. When these regions are later exposed in uptilted portions of ancient continents, a history of terrestrial rock-forming events can be deduced. Episodes of global volcanic activity , rifting of continents, folding, and metamorphism are defined by absolute ages.

The results suggest that the present-day global tectonic scheme was operative in the distant past as well. Continents move, carried on huge slabs, or plates, of dense rock about km 62 miles thick over a low-friction, partially melted zone the asthenosphere below. In the oceans , new seafloor, created at the globe-circling oceanic ridges , moves away, cools, and sinks back into the mantle in what are known as subduction zones i.

Where this occurs at the edge of a continent, as along the west coast of North and South America, large mountain chains develop with abundant volcanoes and their subvolcanic equivalents. These units, called igneous rock , or magma in their molten form, constitute major crustal additions. By contrast, crustal destruction occurs at the margins of two colliding continents, as, for example, where the subcontinent of India is moving north over Asia.

Great uplift, accompanied by rapid erosion, is taking place and large sediment fans are being deposited in the Indian Ocean to the south. Rocks of this kind in the ancient record may very well have resulted from rapid uplift and continent collision. When continental plates collide, the edge of one plate is thrust onto that of the other. The rocks in the lower slab undergo changes in their mineral content in response to heat and pressure and will probably become exposed at the surface again some time later.

Rocks converted to new mineral assemblages because of changing temperatures and pressures are called metamorphic. Virtually any rock now seen forming at the surface can be found in exposed deep crustal sections in a form that reveals through its mineral content the temperature and pressure of burial. Such regions of the crust may even undergo melting and subsequent extrusion of melt magma, which may appear at the surface as volcanic rocks or may solidify as it rises to form granites at high crustal levels.

Magmas produced in this way are regarded as recycled crust, whereas others extracted by partial melting of the mantle below are considered primary. Even the oceans and atmosphere are involved in this great cycle because minerals formed at high temperatures are unstable at surface conditions and eventually break down or weather, in many cases taking up water and carbon dioxide to make new minerals. If such minerals were deposited on a downgoing i.

These components would then rise and be fixed in the upper crust or perhaps reemerge at the surface. Such hot circulating fluids can dissolve metals and eventually deposit them as economic mineral deposits on their way to the surface. Geochronological studies have provided documentary evidence that these rock-forming and rock-re-forming processes were active in the past. Seafloor spreading has been traced, by dating minerals found in a unique grouping of rock units thought to have been formed at the oceanic ridges, to million years ago, with rare occurrences as early as 2 billion years ago.

Other ancient volcanic units document various cycles of mountain building. The source of ancient sediment packages like those presently forming off India can be identified by dating single detrital grains of zircon found in sandstone. Magmas produced by the melting of older crust can be identified because their zircons commonly contain inherited older cores.

Episodes of continental collision can be dated by isolating new zircons formed as the buried rocks underwent local melting. Periods of deformation associated with major collisions cannot be directly dated if no new minerals have formed. The time of deformation can be bracketed, however, if datable units, which both predate and postdate it, can be identified.

The timing of cycles involving the expulsion of fluids from deep within the crust can be ascertained by dating new minerals formed at high pressures in exposed deep crustal sections. In some cases, it is possible to prove that gold deposits may have come from specific fluids if the deposition time of the deposits can be determined and the time of fluid expulsion is known.

Where the crust is under tension, as in Iceland, great fissures develop. These fissures serve as conduits that allow black lava , called basalt , to reach the surface. The portion that remains in a fissure below the surface usually forms a vertical black tubular body known as a dike or dyke. Precise dating of such dikes can reveal times of crustal rifting in the past. Dikes and lava, now exposed on either side of Baffin Bay , have been dated to determine the time when Greenland separated from North America—namely, about 60 million years ago.

Combining knowledge of Earth processes observed today with absolute ages of ancient geologic analogues seems to indicate that the oceans and atmosphere were present by at least 4 billion years ago and that they were probably released by early heating of the planet. The continents were produced over time; the oldest preserved portions were formed approximately 4 billion years ago, but this process had begun about by 4. Absolute dating allows rock units formed at the same time to be identified and reassembled into ancient mountain belts, which in many cases have been disassociated by subsequent tectonic processes.

The most obvious of these is the Appalachian chain that occupies the east coast of North America and extends to parts of Newfoundland as well as parts of Ireland, England, and Norway. Relic oceanic crust , formed between million and million years ago, was identified on both sides of the Atlantic in this chain, as were numerous correlative volcanic and sedimentary units. Evidence based on geologic description, fossil content, and absolute and relative ages leave no doubt that these rocks were all part of a single mountain belt before the Atlantic Ocean opened in stages from about million years ago.

Relative geologic ages can be deduced in rock sequences consisting of sedimentary, metamorphic, or igneous rock units. In fact, they constitute an essential part in any precise isotopic, or absolute, dating program. Such is the case because most rocks simply cannot be isotopically dated. Therefore, a geologist must first determine relative ages and then locate the most favourable units for absolute dating.

It is also important to note that relative ages are inherently more precise, since two or more units deposited minutes or years apart would have identical absolute ages but precisely defined relative ages. While absolute ages require expensive, complex analytical equipment, relative ages can be deduced from simple visual observations. Most methods for determining relative geologic ages are well illustrated in sedimentary rocks. These rocks cover roughly 75 percent of the surface area of the continents, and unconsolidated sediments blanket most of the ocean floor.

They provide evidence of former surface conditions and the life-forms that existed under those conditions. The sequence of a layered sedimentary series is easily defined because deposition always proceeds from the bottom to the top. This principle would seem self-evident, but its first enunciation more than years ago by Nicolaus Steno represented an enormous advance in understanding.

Known as the principle of superposition , it holds that in a series of sedimentary layers or superposed lava flows the oldest layer is at the bottom, and layers from there upward become progressively younger. On occasion, however, deformation may have caused the rocks of the crust to tilt, perhaps to the point of overturning them. Moreover, if erosion has blurred the record by removing substantial portions of the deformed sedimentary rock, it may not be at all clear which edge of a given layer is the original top and which is the original bottom.

Identifying top and bottom is clearly important in sequence determination, so important in fact that a considerable literature has been devoted to this question alone. Many of the criteria of top—bottom determination are based on asymmetry in depositional features. Oscillation ripple marks, for example, are produced in sediments by water sloshing back and forth. When such marks are preserved in sedimentary rocks, they define the original top and bottom by their asymmetric pattern.

Certain fossils also accumulate in a distinctive pattern or position that serves to define the top side. In wind-blown or water-lain sandstone , a form of erosion during deposition of shifting sand removes the tops of mounds to produce what are called cross-beds. The truncated layers provide an easily determined depositional top direction. The direction of the opening of mud cracks or rain prints can indicate the uppermost surface of mudstones formed in tidal areas.

When a section of rock is uplifted and eroded, as during mountain-building episodes, great volumes of rock are removed, exposing a variety of differently folded and deformed rock units. The new erosion surface must postdate all units, dikes, veins, and deformation features that it crosses. Even the shapes formed on the erosional or depositional surfaces of the ancient seafloor can be used to tell which way was up. A fragment broken from one bed can only be located in a younger unit, and a pebble or animal track can only deform a preexisting unit—i.

In fact, the number of ways in which one can determine the tops of well-preserved sediments is limited only by the imagination, and visual criteria can be deduced by amateurs and professionals alike. One factor that can upset the law of superposition in major sediment packages in mountain belts is the presence of thrust faults.

Boltwood gave up work on radiometric dating and went accurate timelines of the history of Earth. 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 environments. To date past events, processes.

Primer Public Event: I Came From Where? Imagining the Human Future video Public Event:

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 environments. To date past events, processes, formations, and fossil organisms, geologists employ a variety of techniques.

The age of the Earth is 4. Following the development of radiometric age-dating in the early 20th century, measurements of lead in uranium-rich minerals showed that some were in excess of a billion years old. It is hypothesised that the accretion of Earth began soon after the formation of the calcium-aluminium-rich inclusions and the meteorites.

AGE OF THE EARTH

Despite seeming like a relatively stable place, the Earth’s surface has changed dramatically over the past 4. Mountains have been built and eroded, continents and oceans have moved great distances, and the Earth has fluctuated from being extremely cold and almost completely covered with ice to being very warm and ice-free. These changes typically occur so slowly that they are barely detectable over the span of a human life, yet even at this instant, the Earth’s surface is moving and changing. As these changes have occurred, organisms have evolved, and remnants of some have been preserved as fossils. A fossil can be studied to determine what kind of organism it represents, how the organism lived, and how it was preserved.

Age of the Earth

A few days ago, I wrote a post about the basins of the Moon — a result of a trip down a rabbit hole of book research. Here’s the next step in that journey: In the science of geology, there are two main ways we use to describe how old a thing is or how long ago an event took place. There are absolute ages and there are relative ages. People love absolute ages. An absolute age is a number. When you say that I am 38 years old or that the dinosaurs died out 65 million years ago, or that the solar system formed 4. We use a variety of laboratory techniques to figure out absolute ages of rocks, often having to do with the known rates of decay of radioactive elements into detectable daughter products.

If, in the year AD , you had asked an educated European how old the planet Earth was and to recount its history he would have said that it was about years old and that its ancient history was given by the biblical account in Genesis. If you asked the same question of an educated European in AD you would have received a quite different answer.

Танкадо использовал ТРАНСТЕКСТ, чтобы запустить вирус в главный банк данных. Стратмор вяло махнул рукой в сторону монитора. Сьюзан посмотрела на экран и перевела взгляд на диалоговое окно. В самом низу она увидела слова: РАССКАЖИТЕ МИРУ О ТРАНСТЕКСТЕ СЕЙЧАС ВАС МОЖЕТ СПАСТИ ТОЛЬКО ПРАВДА Сьюзан похолодела.

Dating Rocks and Fossils Using Geologic Methods

Прямо перед ним над деревьями возвышалось Аюнтамьенто – старинное здание ратуши, которое окружали три акра бело-голубой мозаики азульехо. Его арабские шпили и резной фасад создавали впечатление скорее дворца – как и было задумано, – чем общественного учреждения. За свою долгую историю оно стало свидетелем переворотов, пожаров и публичных казней, однако большинство туристов приходили сюда по совершенно иной причине: туристические проспекты рекламировали его как английский военный штаб в фильме Лоуренс Аравийский.

Коламбия пикчерз было гораздо дешевле снять эту картину в Испании, нежели в Египте, а мавританское влияние на севильскую архитектуру с легкостью убедило кинозрителей в том, что перед их глазами Каир. Беккер перевел свои Сейко на местное время – 9. 10 вечера, по местным понятиям еще день: порядочный испанец никогда не обедает до заката, а ленивое андалузское солнце редко покидает небо раньше десяти.

Несмотря на то что вечер только начинался, было очень жарко, однако Беккер поймал себя на том, что идет через парк стремительным шагом. Голос Стратмора в телефонной трубке звучал еще настойчивее, чем утром. Новые инструкции не оставляли места сомнениям: необходимо во что бы то ни стало найти канадца. Ни перед чем не останавливаться, только бы заполучить кольцо.

Беккера очень удивило, что это кольцо с какой-то невразумительной надписью представляет собой такую важность.

Расслабьтесь, мистер Беккер. Если будет ошибка, мы попробуем снова, пока не добьемся успеха. – Плохой совет, мистер Беккер, – огрызнулся Джабба.  – Нужно сразу быть точным. У шифров-убийц обычно есть функция злопамятства – чтобы не допустить использования метода проб и ошибок. Некорректный ввод только ускорит процесс разрушения. Два некорректных ввода – и шифр навсегда захлопнется от нас на замок.

– И замолчал. – Подумайте, мистер Клушар, – тихо, но настойчиво сказал Беккер.  – Это очень важно. Клушар заморгал. – Я не знаю… эта женщина… он называл ее… – Он прикрыл глаза и застонал. – Как.

Что. Этого не может. Он заперт внизу. – Нет. Он вырвался оттуда. Нужно немедленно вызвать службу безопасности. Я выключаю ТРАНСТЕКСТ! – Она потянулась к клавиатуре.

Он показал на прилавок, где лежала одежда и другие личные вещи покойного. – Es todo. Это. – Si. Беккер попросил дать ему картонную коробку, и лейтенант отправился за .

Защитник Джорджтауна перехватил опасную передачу, и по трибунам пронесся одобрительный гул. Сьюзан наклонилась к Дэвиду и шепнула ему на ухо: – Доктор. Он смотрел на нее с недоумением. – Доктор, – повторила.  – Скажи первое, что придет в голову. – Ассоциативный ряд? – по-прежнему недоумевал Дэвид. – Стандартная для АНБ процедура.

Earth Science: Crash Course History of Science #20