Geological Time Geological Time As sediments pile up, layer after layer, and eventually form into sedimentary rocks, it is obvious that the newest, youngest later is on top, and the oldest layer is at the bottom of the pile. This is "The Principle of Superposition." However, it is basic common sense, and we can begin to assess relative ages of rock layers. A geologist may observe a sequence of rocks in one road cut: mudstones, sandstones, limestones, etc., and another sequence in the next road cut a mile up the road. If the sequences are identical, the geologist might begin to form the hypothesis that they are the same rock beds, and so of the same age, and this might allow him or her to infer that the rock beds were continuous between the two roadcuts, where they run under the ground/grass/poison oak and so cannot be seen. The whole science of studying rock layers is stratigraphy, and the process of matching identifying rocks laid down at the same time is correlation. Unfortunately, sequences of common kinds of sedimentary rock frequently look alike.

If there is a special, rather distinctive or rare sediment in the sequence, that would act as a "marker bed" to allow the geologist to be more confident in identifying two sequences as identical. Examples might be beds laid down by a tsunami (a wave kicked up in the ocean by an earthquake), or a volcanic ash bed. These marker beds are not only distinctive, but are formed practically instantaneously, so that they are precise time markers. We do not know how old they are in thousands or millions of years, but we do know that they formed on the same day! So simply matching rock types (lithostratigraphy) can be a powerful method of correlation. However, there is a more powerful way, based on the existence of specific time markers in rock sequences. If Klingons had visited the Earth every zillion years, and had sprinkled colored BBs over the entire surface of the planet in some sort of religious ceremony (different color for each visit), we could explore the planet, seeing always that certain colors were in older layers than others. We could then draw time lines, guaranteed accurate in different continents, using the BB layers.

Every time we found blue BBs, we would know, perhaps, that we were in older rocks than the layers that had red BBs. We might not know the actual dates to within 100 m.y., but we could work out the relative sequence of colors. That did not happen, but we have a perfectly acceptable substitute, in the form of fossils. Since life formed on this planet, it has evolved. The organisms alive at any time are different from their ancestors, and different from their descendants. What we have to do is to examine the sequence of fossils that occur in sedimentary rocks, and (laboriously) to collect up and down the layers, noting the changes, and getting to know which fossils are the best ones for telling us the age at which they were alive. This process is biostratigraphy. (Just as an aside, you don't *have* to believe in evolution to make this work. All you have to recognize is that fossils have changed through time. You can believe that fossils were planted by Klingons if you like, and you can still be an effective biostratigrapher.

In fact, biostratigraphy began, and was very successful, long before Darwin explained the mechanism for evolution.) Early geologists in Western Europe collected fossils in the extensive chalk deposits that extend from England and France all the way to Russia. The fossils (called Cretaceous after the Latin for "chalk") were generally alike, and they were different from those collected in the rocks sequences just below (named the Jurassic after the Jura Mountains of eastern France). So rather quickly, geologists began to set up a sequence of time periods, during which certain sedimentary sequences of rocks were laid down, each period with a recognizable set of fossils that reliably occurred in the rocks and defined the period. Geologists competed to discover and name these sets of rocks-with-their-own-fossils, and they were often named after the areas in which they were defined. So Devonian rocks were named after the county of Devon, in southern England; Permian rocks were named after the city of Perm, in Russia. Overall, the geological time scale has now been stabilized, codified, and refined to the point where a good fossil collection from any particular rock bed can usually be identified as belonging to a particular time.

Often we can do much better than "period": periods are typically divided into stages, and each stage is dividied into fossil zones. Some fossils are more useful than others for telling time. Animals that evolve slowly are obviously not as useful for telling time as those that evolve quickly, and organisms that are widespread in geography and in habitat are more useful than those that are restricted in their range. Those that are abundant are more useful than those that are rare. It helps if the evolutionary changes that occur are easily recognised. So certain fossils (called zone fossils) have come to be used more than others. I showed pictures of some fossil cephalopods that were big, beautiful, and had complex markings on the shell that allowed them to be identified not only as certain species, but also as belonging to specific time periods. Relative Ages in Geology I finished off this topic by telling a story about the uses of microfossils in telling relative time. Large fossils (ones you can see with the naked eye) are desirable, but an oil company drilling holes in rocks could not expect to hit any given large fossil.

Instead, oil companies rely on microfossils, which are often the preserved shells of single-celled organisms that live floating on the surface waters of the ocean. These creatures fulfil many of the criteria for good zone fossils: they are abundant, they float around so that their shells drop into all kinds of sediment, and they typically evolve fast. And if they are present by the thousands in sediment layers, a drill bit will certainly hit them, and a geologist will be able to gather them from the drill cores taken from the drill hole. Oil companies may spend $100,000 a day to drill an oil well, so it is important to know precisely what age of rock the drill bit has reached. If it has gone beyond (below) the level at which the company expected to find oil, the hole is a "dry" one and it is time to stop. A trained paleontologist will be able to tell the age of the rock at the base of the hole within a matter of hours, so may be able to save the company more than her annual salary, perhaps several times a year!

Earth History Curriculum - Chapter 1, Lesson 2: Geologic Time Scale Age millions of years The largest defined unit of time is the supereon, composed of eons. Eons are divided into eras, which are in turn divided into periods, epochs and ages. The terms eonothem, erathem, system, series, and stage are used to refer to the layers of rock that correspond to these periods of geologic time. Geologists qualify these units as Upper, Middle, and Lower. Examples are "Upper Jurassic" and "Middle Cambrian". The adjectives are capitalized when the subdivision is formally recognized, and lower case when not; thus "early Miocene" but "Early Jurassic." The names of subdivisions can vary when discussing either position in the geologic record or corresponding time period, by use of either upper/lower (for series) or late/early (for epochs). For example, the Lower Jurassic Series in geochronology corresponds to the Early Jurassic Epoch in chronostratigraphy . Geologic units from the same time but different parts of the world often look different and contain different fossils, so the same period was historically given different names in different locales.

For example, in North America the Lower Cambrian is called the Waucoban series that is then subdivided into zones based on succession of trilobites. In East Asia and Siberia, the same unit is split into Tommotian, Atdabanian, and Botomian stages. A key aspect of the work of the International Commission on Stratigraphy is to reconcile this conflicting terminology and define universal horizons that can be used around the world. History of the time scale In classical antiquity, Aristotle saw that fossil seashells from rocks were similar to those found on the beach and deduced that the fossils were once part of living animals. He reasoned that the positions of land and sea had changed over long periods of time. Leonardo da Vinci concurred with Aristotle's view that fossils were the remains of ancient life. The 11th-century Persian geologist Avicenna examined various fossils and deduced that they originated from the petrifaction of plants and animals. He also first proposed one of the principles underlying geologic time scales: the law of superposition of strata.

His contemporary, Abu Rayhan Biruni (973-1048), discovered the existence of shells and fossils in regions that once were seas and later became dry land, such as the Indian subcontinent. Based on this evidence, he realized that the Earth is constantly evolving and proposed that the Earth had an age, but that its origin was too distant to measure. Later in the 11th century, the Chinese naturalist, Shen Kuo (1031ï¿½1095), also recognized the concept of ' deep time '. The principles underlying geologic (geological) time scales were later laid down by Nicholas Steno in the late 17th century. Steno argued that rock layers (or strata) are laid down in succession, and that each represents a "slice" of time. He also formulated the law of superposition, which states that any given stratum is probably older than those above it and younger than those below it. While Steno's principles were simple, applying them to real rocks proved complex.

Over the course of the 18th century geologists realized that: Sequences of strata were often eroded, distorted, tilted, or even inverted after deposition; Strata laid down at the same time in different areas could have entirely different appearances; The strata of any given area represented only part of the Earth's long history. The first serious attempts to formulate a geological time scale that could be applied anywhere on Earth were made in the late 18th century. The most influential of those early attempts (championed by Abraham Werner , among others) divided the rocks of the Earth's crust into four types: Primary, Secondary, Tertiary, and Quaternary. Each type of rock, according to the theory, formed during a specific period in Earth history. It was thus possible to speak of a "Tertiary Period" as well as of "Tertiary Rocks." Indeed, "Tertiary" (now Paleocene-Pliocene) and "Quaternary" (now Pleistocene-Holocene) remained in use as names of geological periods well into the 20th century. The Neptunist theories popular at this time (expounded by Werner) proposed that all rocks had precipitated out of a single enormous flood.

A major shift in thinking came when James Hutton presented his Theory of the Earth; or, an Investigation of the Laws Observable in the Composition, Dissolution, and Restoration of Land Upon the Globe before the Royal Society of Edinburgh in March and April 1785. It has been said that "as things appear from the perspective of the twentieth century, James Hutton in those reading became the founder of modern geology" Hutton proposed that the interior of the Earth was hot, and that this heat was the engine which drove the creation of new rock: land was eroded by air and water and deposited as layers in the sea; heat then consolidated the sediment into stone, and uplifted it into new lands. This theory was dubbed "Plutonist" in contrast to the flood-oriented theory. The identification of strata by the fossils they contained, pioneered by William Smith , Georges Cuvier , Jean d'Omalius d'Halloy , and Alexandre Brogniart in the early 19th century, enabled geologists to divide Earth history more precisely. It also enabled them to correlate strata across national (or even continental) boundaries.

If two strata (however distant in space or different in composition) contained the same fossils, chances were good that they had been laid down at the same time. Detailed studies between 1820 and 1850 of the strata and fossils of Europe produced the sequence of geological periods still used today. The process was dominated by British geologists, and the names of the periods reflect that dominance. The "Cambrian," (the Roman name for Wales) and the "Ordovician," and "Silurian", named after ancient Welsh tribes, were periods defined using stratigraphic sequences from Wales. The "Devonian" was named for the English county of Devon, and the name "Carboniferous" was simply an adaptation of "the Coal Measures," the old British geologists' term for the same set of strata. The "Permian" was named after Perm, Russia, because it was defined using strata in that region by Scottish geologist Roderick Murchison . However, some periods were defined by geologists from other countries.

The "Triassic" was named in 1834 by a German geologist Friedrich Von Alberti from the three distinct layers (Latin trias meaning triad) ï¿½red beds, capped by chalk, followed by black shalesï¿½ that are found throughout Germany and Northwest Europe, called the 'Trias'. The "Jurassic" was named by a French geologist Alexandre Brogniart for the extensive marine limestone exposures of the Jura Mountains . The "Cretaceous" (from Latin creta meaning 'chalk') as a separate period was first defined by Belgian geologist Jean d'Omalius d'Halloy in 1822, using strata in the Paris basin and named for the extensive beds of chalk (calcium carbonate deposited by the shells of marine invertebrates). British geologists were also responsible for the grouping of periods into Eras and the subdivision of the Tertiary and Quaternary periods into epochs. When William Smith and Sir Charles Lyell first recognized that rock strata represented successive time periods, time scales could be estimated only very imprecisely since various kinds of rates of change used in estimation were highly variable.

While creationists had been proposing dates of around six or seven thousand years for the age of the Earth based on the Bible, early geologists were suggesting millions of years for geologic periods with some even suggesting a virtually infinite age for the Earth. Geologists and paleontologists constructed the geologic table based on the relative positions of different strata and fossils, and estimated the time scales based on studying rates of various kinds of weathering, erosion, sedimentation, and lithification . Until the discovery of radioactivity in 1896 and the development of its geological applications through radiometric dating during the first half of the 20th century (pioneered by such geologists as Arthur Holmes ) which allowed for more precise absolute dating of rocks, the ages of various rock strata and the age of the Earth were the subject of considerable debate. The first geologic time scale was eventually published in 1913 by the British geologist Arthur Holmes . He greatly furthered the newly created discipline of geochronology and published the world renowned book The Age of the Earth in 1913 in which he estimated the Earth's age to be at least 1.6 billion years.

In 1977, the Global Commission on Stratigraphy (now the International Commission on Stratigraphy ) started an effort to define global references ( Global Boundary Stratotype Sections and Points ) for geologic periods and faunal stages. The commission's most recent work is described in the 2004 geologic time scale of Gradstein et al.. A UML model for how the timescale is structured, relating it to the GSSP, is also available. End of Reading Time and the Rock record Geological Time and the Rock Record The rock record of interest to students of Geology 107 is dominantly the record of sedimentary rocks. Igneous rocks include volcanic rocks, which can and do cover large areas of Earth's surface. But they do not in general have many fossils in them. Occasionally a lava flow may overwhelm and preserve organisms as fossils, but that is rare. Volcanic ash beds are important in dating other rock layers, however, as we shall see. Metamorphic rocks are usually so altered by temperature and/or pressure that any potential fossils in them are destroyed. Sedimentary rocks are laid down and accumulated on the surface of the earth under normal temperatures and pressures.

The particles that make up the sediments - clays, silts, sand, pebbles, and boulders, are formed from pre-existing rocks by weathering and are then transported by various agents of erosion until they are finally deposited in a permanent resting place. Molded then by further sedimentary layers deposited on top of them, sediments are compressed, so that any water that lies between the grains is squeezed out in dewatering, often leaving behind any minerals it contained in solution to form cements that bind the sediments together. Eventually the squeezing and cementation allows one to say that a rock has formed from the original sediment. The process is called lithification. If in the process, organisms or traces of organisms have been buried along with the sedimentary particles, then those organisms may survive well enough to be exposed, recognized, and collected as fossils. We will return to the process of fossilization. We need to be able to place any fossils we find in the context of the rocks we find them in. The evidence allows us to do two different interpretations. The paleoenvironment of deposition. The time of deposition. Paleoenvironmental analysis. We will spend more time on this later.

At the moment, we need to recognize how the analysis works. It depends explicitly on the principle of uniformitarianism. Introduced formally by Charles Lyell in a famous book in 1832 (though it was used by Herodotus and Leonardo da Vinci, among others), it can be summarized loosely in the phrase, "The present is the key to the past." If we know what sand looks like on a modern beach, and we understand how it gets to be deposited in the way it is, then when we see the same structures in sandstones, we can recognize that the ancient sandstones were deposited as sands on some beach millions or billions of years ago. Example: Interpretation of a desert shoreline 3.5 billion years ago (Warrawoona rocks of Australia, where the oldest fossils on Earth were discovered - cyanobacteria). Stratigraphy As sediments pile up, layer after layer, and eventually form into sedimentary rocks, it is obvious that the newest, youngest later is on top, and the oldest layer is at the bottom of the pile. This is the principle of superposition. It dates back to the 1600s, but it is basic common sense.

Archeologists as well as geologists rely heavily on it. The concept gives us a way of relative dating of rock layers: youngest on top. Another concept is considerably weaker: the principle of original horizontality. It suggests that most surfaces of sediments are roughly horizontal, so any new layer of sediment must inevitably be roughly horizontal too. It easy to think of situations where this is not true. And what that means is that one cannot really trust rocks that look similar to be the same age at any major distance from the place one is studying. There may be exceptions, but they have to be demonstrated. So a geologist may observe a sequence of rocks in one road cut: mudstones, sandstones, limestones, etc., and another sequence in the next road cut a mile up the road. If the sequences are identical, the geologist might begin to form the hypothesis that they are the same rock beds, and so of the same age, and this might allow him or her to infer that the rock beds were continuous between the two roadcuts, even if they run under the ground/grass/poison oak and so cannot be seen.

The whole science of studying rock layers is stratigraphy, and the process of matching identifying rocks laid down at the same time is correlation. Unfortunately, sequences of common kinds of sedimentary rock frequently look alike. If there is a special, rather distinctive or rare sediment in the sequence, that would act as a "marker bed" to allow the geologist to be more confident in identifying two sequences as identical. Examples might be beds laid down by a tsunami (a wave kicked up in the ocean by an earthquake), or a volcanic ash bed. These marker beds are not only distinctive, but are formed practically instantaneously, so that they are precise time markers. We do not know how old they are in thousands or millions of years, but we do know that they formed on the same day! So simply matching rock types (lithostratigraphy) can be a powerful method of correlation. However, there is a more powerful way, based on the existence of specific time markers in rock sequences. The same problem arises in archaeology, of course. One must find markers that change with time. Archaeologists use pottery, or coins, or weapons, but they too can be diachronous (they may not exactly mark the same time level).

Even so, archaeologists can often use things that people have dropped: pottery fragments in archeology, and in ocean sedimentation and in landfills (Coke bottles and beer cans of various designs and metals). It would be nice for paleontologists if Klingon spaceships had dropped color-coded BBs on Earth every million years or so: each layer of BBs would mark a different time in the rock record. What paleontologists have instead is fossils that change with time. Since life first formed on this planet, it has evolved. The organisms alive at any time were different from their ancestors, and different from their descendants. What we have to do is to examine the sequence of fossils that occur in sedimentary rocks, and (laboriously) to collect up and down the layers, noting the changes, and getting to know which fossils are the best ones for telling us the age at which they were alive. This process is biostratigraphy (see below). (Just as an aside, you don't *have* to believe in evolution to make this work. All you have to recognize is that fossils have changed through time.

You can believe that fossils were planted by Klingons if you like, and you can still be an effective biostratigrapher. In fact, biostratigraphy began, and was very successful, long before Darwin explained the mechanism for evolution.) Change through time is a fact, and, of course, change through time is evolution. You might want to argue about the way it happened, but evolution itself is a fact, and oil companies bet millions of dollars on it every year. Here's how: Large fossils (ones you can see with the naked eye) are desirable, but an oil company drilling holes in rocks could not expect to hit any given large fossil. Instead, oil companies rely on microfossils, which are often the preserved shells of single-celled organisms that live floating on the surface waters of the ocean. These creatures fulfil many of the criteria for good zone fossils: they are abundant, they float around so that their shells drop into all kinds of sediment, and they typically evolve fast. And if they are present by the thousands in sediment layers, a drill bit will certainly hit them, and a geologist will be able to gather them from the drill cores taken from the drill hole.

Oil companies may spend $100,000 a day to drill an oil well, so it is important to know precisely what age of rock the drill bit has reached. If it has gone beyond (below) the level at which the company expected to find oil, the hole is a "dry" one and it is time to stop. A trained paleontologist will be able to tell the age of the rock at the base of the hole within a matter of hours, so may be able to save the company more than her annual salary, perhaps several times a year! Biostratigraphy denotes the use of fossils in stratigraphy. The process of evolution continually molds species of organisms, and new species evolve while old species become extinct. This gives a directionality to time, and since successive sets of species living in a region over time are never exactly alike, the flow of time is marked by the deposition of assemblages of fossils that can be differentiated by a skilled paleontologist if the preservation is good enough. By comparing sets of fossils over wide areas, one can gradually define a system of time-dependent guide fossils that will denote a particular time in relative terms.

Of course, not all organisms evolve rapidly, and not all organisms preserve well as fossils, and not all organisms are abundant enough in the fossil record to be used consistently as guide fossils. Some organisms are better guides to environment than to time. Thus, while biostratigraphy is at present the most widely used, most reliable, and cheapest way of telling the age of a rock formation, it is not easy and it is not free from ambiguity. There are other, powerful time markers: see section on absolute dating (below). Biostratigraphy is only one of a series of methods used in trying to refine the history of a set of rocks. In the end, we have been able to divide up the rock record based on the fossils found in it. There is a hierarchy of names for time periods in Earth history: we will concentrate on the larger ones, Eras (e.g. Phanerozoic), and Periods (e.g. Devonian, Jurassic). Stages and zones are finer subdivisions, down to periods that may be as small as 100,000 years (more precise that radiometric age dates). Here is the way these divisions came about.

Early geologists in Western Europe collected fossils in the extensive chalk deposits that extend from England and France all the way to Russia. The fossils (called Cretaceous after the Latin for "chalk") were generally alike, and they were different from those collected in the rocks sequences just below (named the Jurassic after the Jura Mountains of eastern France). So rather quickly, geologists began to set up a sequence of time periods, during which certain sedimentary sequences of rocks were laid down, each period with a recognizable set of fossils that reliably occurred in the rocks and defined the period. Geologists competed to discover and name these sets of rocks-with-their-own-fossils, and they were often named after the areas in which they were defined. So Devonian rocks were named after the county of Devon, in southern England; Permian rocks were named after the city of Perm, in Russia. Overall, the geological time scale has now been stabilized, codified, and refined to the point where a good fossil collection from any particular rock bed can usually be identified as belonging to a particular time. Often we can do much better than "period": periods are typically divided into stages, and each stage is dividied into fossil zones.

Some fossils are more useful than others for telling time. Animals that evolve slowly are obviously not as useful for telling time as those that evolve quickly, and organisms that are widespread in geography and in habitat are more useful than those that are restricted in their range. Those that are abundant are more useful than those that are rare. It helps if the evolutionary changes that occur are easily recognised. So certain fossils (called zone fossils) have come to be used more than others. A set or assemblage of fossils is better than a single guide fossil. Absolute Ages in Geology Absolute geological ages can be determined because newly formed mineral crystals sometimes contain unstable, radioactive, atoms. Radioactive isotopes break down according to a completely predictable timetable that no known physical or chemical agent can alter, and as they do so they may change into other elements. For example, potassium-40, 40K, breaks down to form 40Ar, argon-40. Potassium (K) is an element that is a vital ingredient of the common mineral potassium feldspar (K-feldspar), which occurs in many kinds of igneous rocks.

As a K-feldspar crystal forms, precipitating from a molten magma, it will contain a large proportion of "ordinary" K, and a small but fixed proportion of redioactive K-40. Over time, that K-40 breaks down to form atoms of argon-40, Ar-40, which is an inert gas that simply sits there within the crystal. After geological time, that crystal may be collected by a geologist who wishes to know its age. By measuring the amount of Ar-40 in the crystal, one can calculate the time since crystallization. The principle is simple, though the techniques are often laborious. K-40 breaks down to form Ar-40 at a rate such that half of it has gone in about 1.3 billion years. If we measure the Ar-40 in a K-feldspar crystal, and find that half the original amount of K-40 has gone, then the age of the crystal is 1300 m.y. One problem is that the rate of breakdown is so very slow that it is difficult to measure in a lab.

That rate is what gives time to the clock, so we may be as much as 1% off because we can't easily determine how fast the clock runs. However, other radioactive clocks exist (using Uranium, for example), and usually when a crystal is dated using two or three different methods, the resul;ts come out very close to one another. Therefore, though we know there are errors in the method, they are small if the geologist and the chemist did their work well. As a result, for example, we know that a huge extinction event at the end of the Permian period occurred at 250 � 2 million years ago (250 Ma), and the end of the dinosaurs occurred at 65 Ma � 1 m.y. Perhaps the most well-known is the radiocarbon method, based on the breakdown of C-14, carbon-14. However, the half-life of C-14 is only about 5370 years, so only a tiny fraction of the original C-14 is left after about 40,000 years. Therefore C-14 dating is valuable to archeologists and historians, but not to most geologists.

There are other problems with the C-14 method, too. Naturally, there are problems. Crystals may have been reheated, or even recrystallized, setting their radioactive clocks back to zero well after the time the rock originally formed. Chemical alteration of the rock may have removed some of the newly produced element, also giving a date younger than the true age. Worst of all, however, most elements used for radioactive age dating are not used by living organisms to build shells or bones. Usually, we cannot date fossils directly. Instead, we have to measure the age of a lava flow or volcanic ash layer as close to the fossil-bearing bed as possible. Sometimes reliable dates can be obtained this way, but, for example, many of the arguments about hominid evolution in East Africa have resulted from problems in assessing age dates from lavas and ashes close to hominid remains. Absolute methods are also expensive and time-consuming, and an oil company drilling a well at tens of thousands of dollars a day may not like to wait several days to find out the age of the rock it is drilling through, when a paleontologist can do it faster.

By convention, absolute ages in millions of years are given in megayears (Ma), while time periods or intervals are expressed in millions of years (m.y.). Geologic time scale Geologic time scale 2008/9 Schools Wikipedia Selection . Related subjects: Geology and geophysics Diagram of geological time scale. The geologic time scale is a chronologic schema (or idealized Model) relating stratigraphy to time that is used by geologists and other earth scientists to describe the timing and relationships between events that have occurred during the history of Earth . The table of geologic periods presented here agrees with the dates and nomenclature proposed by the International Commission on Stratigraphy, and uses the standard colour codes of the United States Geological Survey. Evidence from radiometric dating indicates that the Earth is about 4.570 billion years old. The geological or deep time of Earth's past has been organized into various units according to events which took place in each period. Different spans of time on the time scale are usually delimited by major geological or paleontological events, such as mass extinctions .

For example, the boundary between the Cretaceous period and the Paleogene period is defined by the extinction event, known as the Cretaceous–Tertiary extinction event , that marked the demise of the dinosaurs and of many marine species. Older periods which predate the reliable fossil record are defined by absolute age. Graphical timelines The second and third timelines are each subsections of their preceding timeline as indicated by asterisks. Millions of Years The Holocene (the latest epoch) is too small to be shown clearly on this timeline. Terminology The largest defined unit of time is the supereon composed of Eons . Eons are divided into Eras , which are in turn divided into Periods , Epochs and Stages. At the same time paleontologists define a system of faunal stages, of varying lengths, based on changes in the observed fossil assemblages. In many cases, such faunal stages have been adopted in building the geological nomenclature, though in general there are far more recognized faunal stages than defined geological time units. Geologists tend to talk in terms of Upper/Late, Lower/Early and Middle parts of periods and other units , such as "Upper Jurassic ", and "Middle Cambrian ".

Upper, Middle, and Lower are terms applied to the rocks themselves, as in "Upper Jurassic sandstone," while Late, Middle, and Early are applied to time, as in "Early Jurassic deposition" or " fossils of Early Jurassic age." The adjectives are capitalized when the subdivision is formally recognized, and lower case when not; thus "early Miocene" but "Early Jurassic." Because geologic units occurring at the same time but from different parts of the world can often look different and contain different fossils, there are many examples where the same period was historically given different names in different locales. For example, in North America the Lower Cambrian is referred to as the Waucoban series that is then subdivided into zones based on trilobites. The same timespan is split into Tommotian, Atdabanian and Botomian stages in East Asia and Siberia. A key aspect of the work of the International Commission on Stratigraphy is to reconcile this conflicting terminology and define universal horizons that can be used around the world.

History of the time scale Earth history mapped to 24 hours One of the principles underlying geologic time scales was the principle of superposition of strata, first proposed in the 11th century by the Persian geologist, Avicenna (Ibn Sina). While discussing the origins of mountains in The Book of Healing in 1027, he outlined the principle as follows: "It is also possible that the sea may have happened to flow little by little over the land consisting of both plain and mountain, and then have ebbed away from it. ... It is possible that each time the land was exposed by the ebbing of the sea a layer was left, since we see that some mountains appear to have been piled up layer by layer, and it is therefore likely that the clay from which they were formed was itself at one time arranged in layers. One layer was formed first, then at a different period, a further was formed and piled, upon the first, and so on.

Over each layer there spread a substance of differenti material, which formed a partition between it and the next layer; but when petrification took place something occurred to the partition whcih caused it to break up and disintegrate from between the layers (possibly referring to unconformity). ... As to the beginning of the sea, its clay is either sedimentary or primeval, the latter not being sedimentary. It is probable that the sedimantary clay was formed by the disintegration of the strata of mountains. Such is the formation of mountains." Later in the 11th century, the Chinese naturalist, Shen Kuo (1031-1095), also recognized the concept of ' deep time'. The principles underlying geologic (geological) time scales were later laid down by Nicholas Steno in the late 17th century. Steno argued that rock layers (or strata) are laid down in succession, and that each represents a "slice" of time. He also formulated the law of superposition, which states that any given stratum is probably older than those above it and younger than those below it. While Steno's principles were simple, applying them to real rocks proved complex.

Over the course of the 18th century geologists realized that: Sequences of strata were often eroded, distorted, tilted, or even inverted after deposition; Strata laid down at the same time in different areas could have entirely different appearances; The strata of any given area represented only part of the Earth's long history. A comparative geological timescale The first serious attempts to formulate a geological time scale that could be applied anywhere on Earth took place in the late 18th century. The most influential of those early attempts (championed by Abraham Werner, among others) divided the rocks of the Earth's crust into four types: Primary, Secondary, Tertiary, and Quaternary. Each type of rock, according to the theory, formed during a specific period in Earth history. It was thus possible to speak of a "Tertiary Period" as well as of "Tertiary Rocks." Indeed, "Tertiary" (now Paleocene-Pliocene) and "Quaternary" (now Pleistocene-Holocene) remained in use as names of geological periods well into the 20th century.

In opposition to the then-popular Neptunist theories expounded by Werner (that all rocks had precipitated out of a single enormous flood), a major shift in thinking came with the reading by James Hutton of his Theory of the Earth; or, an Investigation of the Laws Observable in the Composition, Dissolution, and Restoration of Land Upon the Globe before the Royal Society of Edinburgh in March and April 1785, events which "as things appear from the perspective of the twentieth century, James Hutton in those reading became the founder of modern geology" What Hutton proposed was that the interior of the Earth was hot, and that this heat was the engine which drove the creation of new rock: land was eroded by air and water and deposited as layers in the sea; heat then consolidated the sediment into stone, and uplifted it into new lands. This theory was dubbed "Plutonist" in contrast to the flood-oriented theory. The identification of strata by the fossils they contained, pioneered by William Smith, Georges Cuvier, Jean d'Omalius d'Halloy and Alexandre Brogniart in the early 19th century, enabled geologists to divide Earth history more precisely.

It also enabled them to correlate strata across national (or even continental) boundaries. If two strata (however distant in space or different in composition) contained the same fossils, chances were good that they had been laid down at the same time. Detailed studies between 1820 and 1850 of the strata and fossils of Europe produced the sequence of geological periods still used today. The process was dominated by British geologists, and the names of the periods reflect that dominance. The "Cambrian," (the Roman name for Wales ) and the "Ordovician," and "Silurian", named after ancient Welsh tribes, were periods defined using stratigraphic sequences from Wales. The "Devonian" was named for the English county of Devon, and the name "Carboniferous" was simply an adaptation of "the Coal Measures," the old British geologists' term for the same set of strata. The "Permian" was named after Perm, Russia , because it was defined using strata in that region by a Scottish geologist Roderick Murchison. However, some periods were defined by geologists from other countries.

The "Triassic" was named in 1834 by a German geologist Friedrich Von Alberti from the three distinct layers ( Latin trias meaning triad) — red beds, capped by chalk, followed by black shales— that are found throughout Germany and Northwest Europe, called the 'Trias'. The "Jurassic" was named by a French geologist Alexandre Brogniart for the extensive marine limestone exposures of the Jura Mountains. The "Cretaceous" (from Latin creta meaning ' chalk') as a separate period was first defined by a Belgian geologist Jean d'Omalius d'Halloy in 1822, using strata in the Paris basin and named for the extensive beds of chalk ( carbonate ">calcium carbonate deposited by the shells of marine invertebrates ). British geologists were also responsible for the grouping of periods into Eras and the subdivision of the Tertiary and Quaternary periods into epochs. When William Smith and Sir Charles Lyell first recognized that rock strata represented successive time periods, time scales could be estimated only very imprecisely since various kinds of rates of change used in estimation were highly variable.

While creationists had been proposing dates of around six or seven thousand years for the age of the Earth based on the Bible , early geologists were suggesting millions of years for geologic periods with some even suggesting a virtually infinite age for the Earth. Geologists and paleontologists constructed the geologic table based on the relative positions of different strata and fossils , and estimated the time scales based on studying rates of various kinds of weathering, erosion , sedimentation, and lithification. Until the discovery of radioactivity in 1896 and the development of its geological applications through radiometric dating during the first half of the 20th century (pioneered by such geologists as Arthur Holmes) which allowed for more precise absolute dating of rocks, the ages of various rock strata and the age of the Earth were the subject of considerable debate. In 1977, the Global Commission on Stratigraphy (now the International Commission on Stratigraphy) started an effort to define global references ( Global Boundary Stratotype Sections and Points) for geologic periods and faunal stages. The commission's most recent work is described in the 2004 geologic time scale of Gradstein et al..

A UML model for how the timescale is structured, relating it to the GSSP, is also available. Table of geologic time The following table summarizes the major events and characteristics of the periods of time making up the geologic time scale. As above, this time scale is based on the International Commission on Stratigraphy. (See lunar geologic timescale for a discussion of the geologic subdivisions of Earth's moon.) The height of each table entry does not correspond to the duration of each subdivision of time. Geologic time scale Geologic Time table Geologic Time Table The development of life on Earth By Dr J Floor Anthoni (2004) This extensive time table follows the development of Earth from the moment of the Big Bang until today. Where the geometry of Earth was known, also world maps are shown. To follow the history of the Earth and its life forms, start at the bottom of the last table. For successful printing, set the left and right margins in your page settings to minimal, e.g left 0.5 cm (0.2"). Information from various sources. Maps adapted from Stephen Jay Gould The Book of Life, 1993. .

Holocene 10kya till present AD 1350-1800 Little Ice Age. since then a warming trend. Major habitat changes and deforestations caused by humans. A major extinction wave due to introduced pests and habitat destruction. The last major ice age ends and the sea level rises by 80-110m worldwide, causing new continental margins, dunes and beaches. Climate still fluctuates in ten little ice ages. Humans spread across America and all islands. Major extinctions of large animals and birds due to humans. Pleistocene 1.6-0.01 Mya Climate fluctuating cold to mild . The era of ice ages. Numerous glacial advances and retreats; fiords formed. Uplift of the Sierra Nevada; deserts on large scale; Sahara formed. Ten major ice ages of 100ky each between 1Mya and 100kya. Last ice age from 100kya to 10kya. Planetary spread of Homo Sapiens over Eurasia; extinction of many species due to the ice ages; extinction of many large mammals and birds due to humans.

Cenozoic - Tertiary Pliocene 5-2 Mya Cooler climate; continued uplift and mountain building, with widespread glaciation in Northern Hemisphere. Uplift of Panama joins North and South America.Large carnivores. First known appearance of hominids (humanlike primates) . Miocene 25-5 Mya Moderate climate; extensive glaciation begins again in Southern Hemisphere. Moderate uplift of Rocky Mountains. Whales, apes, grazing mammals. Spread of grasslands as forests contract. . Oligocene 38-25 Mya Rise of Alps and Himalayas. Lands generally low. Volcanoes in Rocky Mountains. South America separates from Antarctica. Forests decline to make way for grasslands. Large browsing animals; monkey-like primates appear. Fish: eel, barracuda, seahorse, cod, trout. Reptiles: turtles, tortoises, lizards, snakes, crocodiles. Birds: flightless and flying. Origin of many modern families of flowering plants. . Eocene 55-38 Mya Mild to very tropical climate. Many lakes in western North America. Australia separates from Antarctica; India collides with Asia.

Primitive horses, tiny camels, marsupials; modern and giant types of bird. Most groups now well formed; barnacles, oysters, cuttlefish, crabs, sponges, freshwater snails. Formation of grasslands. . Paleocene 65-55 Mya Mild to cool climate. Wide, shallow continental seas largely disappear.First known primitive primates and mammal carnivores. . Cretaceous 144-66.4 Mya Tropical to subtropical climate. Elevation of Rocky Mountains at end of period. Africa and South America separate. In warm, shallow seas, vast layers of chalk laid down by marine organisms. Birds well developed. Marsupials, insectivores and flowering plants become abundant. At end of period extinction of dinosaurs, belemnites, ammonites and most cycads. Fish survived, mammals and some reptiles. Angiosperms dominate the land flora, colonise most land and diversify. . Jurassic 208-144 Mya Mild climate. Continents low, with large areas covered by seas. Mountains rise from Alaska to Mexico. Dinosaurs' zenith. Flying reptiles, small mammals and birds appear.

Many ammonites and other mollusks dominate the sea. Gymnosperms, especially cycads and ferns. First flowering plants (angiosperms). . Permian 286-245 Mya Extremely violent climate changes: deserts, swamps, ice. Extensive glaciation in Southern Hemisphere. Seas drain from land; worldwide aridity. Urals formed. Appalachians formed by end of Paleozoic. Large amphibians. Reptiles diversify. Bugs and beetles (metamorphosis). Age of the seed plants. Origin of conifers, cycads and ginkgos; possible origin of flowering plants; earlier forest types wane. At end of period extinctions of many groups: trilobites, eurypterids, many kinds of corals, bryozoa, sea lilies, brachiopods. Early fishes (placoderms) and many kinds of shark disappeared. . Carboniferous 360-286 Mya Slower earth movements. Seabeds began to rise. Climate warm; conditions like those in subtropical zones; little seasonal variation, water plentiful. Lands low, covered by shallow seas or great coal swamps.

Mountain building in eastern US, Texas, Colorado. Age of amphibians. First reptiles, cotylosaurs. Variety of insects. Sharks abundant. Great swamps; forests of ferns, gymnosperms (naked seed plants) and horsetails. . Devonian 408-360 Mya Violent change in the Earth's landscape by volcanic activity and crustal movements, folding and mountain forming. Europe mountainous with arid basins. Mountains and volcanoes in eastern US and Canada. Rest of north America low and flat. Sea covers most of land. Climate became drier. Age of fishes. Sharks, rays. Fishes move into the open seas. Lunged fishes (paddle-fins). Amphibians appear. Mollusks abundant. Extinction of primitive vascular plants. Origin of modern groups of vascular plants with true leaves, roots and stems (liverworts). The Earth started to look green. Some plants started to produce seeds, rather than spores. . Silurian 438-408 Mya Mild climate. Continents generally flat; again flooded. Mountain building in Europe.

Rise of fishes (placoderms) and reef building corals. Shell-forming sea animals abundant. Sea lilies (stalked crinoids), eurypterids, land scorpions. Invasion of land by arthropods. Earliest vascular plants (psilopsids, lycophytes). Modern groups of algae and fungi. . Ordovician 505-438 Mya Mild climate. Shallow seas; retreating from land and spreading back; teeming with life. continents low; sea covers US. Limestone deposits. All plants and animals still restricted to the water. Agnatha (no jaw fishes, first vertebrates). First primitive fishes (ostracoderms, vertebrates). Invertebrates dominant. Crustaceans, trilobites, graptolites, brachiopods, bryozoa, echinoderms, corals, mollusks, cephalopods. First fungi. Possible invasions of land by plants. . Cambrian 540-505 Mya Mild climate; extensive seas, spilling over continents. Shelled marine invertebrates.

Explosive diversification of eukaryotic organisms. swimming, floating, crawling, clinging, burrowing sea animals. Trilobites, brachiopods, radiolarians, sponges, echinoderms,starfish, seacucumbers, jellyfish, worms, eurypterids (water scorpions). Plants only as algae. . Proterozoic 2500-540 Mya Dry and cold climate to warm and moist. Eukaryotic cells and multicellularity by close of era. Earliest known fossils, including soft-bodied marine invertebrates. . Archean 3960-2500 Mya Extensive mountain building. Shallow seas. Accumulation of free oxygen. Origin of life. Prokaryotes, bacteria, blue-green algae. . Azoic 4500-3960 Mya Planet forms from accumulation of cosmic dust and from meteorite impacts. These impacts and the heat caused by compression, melt the interior of the planet. The heat melts the Earth's interior to enable heavy elements to migrate to its centre, while light elements migrate to its crust.

4400 Mya the liquid core with the mantle appeared. Planet cools. Formation of Earth's crust (4200 Mya). Extensive volcanic activity prevents life from forming. . sun and solar system 5000 Mya Universe is 2/3 its present size. From a left-over dust cloud originating from a supernova explosion, the sun forms, attracting most of the material. But rotational momentum has kept enough debris in space to form the planets. The sun's interior heats to 15 million degrees, hot enough for nuclear fusion: protons fuse into helium . Unknown period The exact age of the universe is not known (15 to 9 Gya). Below this line, all time is relative to the Big Bang; above it, time is relative to the present. first galaxies 1000 My Universe has expanded to 1/5 its present size. Nuclear reactions inside stars (supernovas) have made most of the heavy elements from which terrestrial planets are made and the elements necessary for life. The Milky Way, our glaxy, was formed 10,000 Mya. The universe cools further to 3ºK, its temperature today. .

Universe becomes transparent 300,000 y Matter clusters into ever larger units. Stars are formed. The universe becomes transparent. Its radiation, the backround radiation, is still observable today. . Formation of nucleotides 10-5 to 10-2 s All forms of particles are formed: neutrons, protons, electrons, etc. The universe expands to 1/1000 present size, temperature cools to 3000ºK. The light elements hydrogen and helium are formed. . Moment of inflation 10-12 - The universe has cooled to 1015 degrees, being a soup of matter (quarks) and radiation, a dense plasma gas under very high pressure. As the universe continues to expand, it cools further and matter wins over antimatter. The universe inflates rapidly to almost 1/1000 present size. . Moment of infinite temperature 0-10-12 s The very moment of the big bang is shrouded in mystery because scientists believe that conventional physics won't apply at the very high temperatures in excess of a million billon degrees 1015ºC.

Electromagnetic radiation and matter are indistinguishable. . Geologic time | Paleontology Wiki | Fandom powered by Wikia For other uses, see time scale . The geological time scale is used by geologists and other scientists to describe the timing and relationships between events that have occurred during the history of the Earth . The table of geologic periods presented here is in accordance with the dates and nomenclature proposed by the International Commission on Stratigraphy , and uses the standard color codes of the United States Geological Survey . Evidence from radiometric dating indicates that the Earth is about 4,570 million years old (expressed with m.y.a., i.e. million years ago, or " Ma " as in "it dates from 4570 Ma"). The geological or deep time of Earth's past has been organized into various units according to events which took place in each period. Different spans of time on the time scale are usually delimited by major geological or paleontological events, such as mass extinctions . For example, the boundary between the Cretaceous period and the Paleogene period is defined by the extinction event that marked the demise of the dinosaurs and of many marine species .

Older periods which predate the reliable fossil record are defined by absolute age. Graphical timelines The second and third timelines are each subsections of their preceding timeline as indicated by asterisks. Millions of Years </center> The Holocene (the latest epoch ) and the present (from 1950 to now) are too small to be shown clearly on this timeline. Terminology The largest defined unit of time is the Eon . Eons are divided into Eras, which are in turn divided into Periods, Epochs and Stages. At the same time paleontologists define a system of faunal stages , of varying lengths, based on changes in the observed fossil assemblages. In many cases, such faunal stages have been adopted in building the geological nomenclature , though in general there are far more recognized faunal stages than defined geological time units. Geologists tend to talk in terms of Upper/Late, Lower/Early and Middle parts of periods and other units , such as "Upper Jurassic ", and "Middle Cambrian ".

Upper, Middle, and Lower are terms applied to the rocks themselves, as in "Upper Jurassic sandstone," while Late, Middle, and Early are applied to time, as in "Early Jurassic deposition" or "fossils of Early Jurassic age." The adjectives are capitalized when the subdivision is formally recognized, and lower case when not; thus "early Miocene" but "Early Jurassic." Because geologic units occurring at the same time but from different parts of the world can often look different and contain different fossils, there are many examples where the same period was historically given different names in different locales. For example, in North America the Lower Cambrian is referred to as the Waucoban series that is then subdivided into zones based on trilobites . The same timespan is split into Tommotian , Atdabanian and Botomian stages in East Asia and Siberia . It is a key aspect of the work of the International Commission on Stratigraphy to reconcile this conflicting terminology and define universal horizons that can be used around the world. History of the time scale Earth history mapped to 24 hours The principles underlying geologic (geological) time scales were laid down by Nicholas Steno in the late 17th century.

Steno argued that rock layers (or strata) are laid down in succession, and that each represents a "slice" of time. He also formulated the principle of superposition , which states that any given stratum is probably older than those above it and younger than those below it. While Steno's principles were simple, applying them to real rocks proved complex. Over the course of the 18th century geologists came to realize that: 1) Sequences of strata were often eroded, distorted, tilted, or even inverted after deposition ; 2) Strata laid down at the same time in different areas could have entirely different appearances; 3) The strata of any given area represented only part of the Earth's long history. The first serious attempts to formulate a geological time scale that could be applied anywhere on Earth took place in the late 18th century. The most influential of those early attempts (championed by Abraham Werner , among others) divided the rocks of the Earth's crust into four types: Primary, Secondary, Tertiary, and Quaternary. Each type of rock, according to the theory, formed during a specific period in Earth history.

It was thus possible to speak of a "Tertiary Period" as well as of "Tertiary Rocks." Indeed, "Tertiary" and "Quaternary" remained in use as names of geological periods well into the 20th century. The identification of strata by the fossils they contained, pioneered by William Smith , Georges Cuvier , Jean d'Omalius d'Halloy and Alexandre Brogniart in the early 19th century, enabled geologists to divide Earth history more precisely. It also enabled them to correlate strata across national (or even continental) boundaries. If two strata (however distant in space or different in composition) contained the same fossils, chances were good that they had been laid down at the same time. Detailed studies of the strata and fossils of Europe produced, between 1820 and 1850, the sequence of geological periods still used today. British geologists dominated the process, and the names of the periods reflect that dominance. The "Cambrian," "Ordovician," and "Silurian" periods were named after ancient British tribes (and defined using stratigraphic sequences from Wales).

The "Devonian" was named for the English county of Devon , and the name "Carboniferous" was simply an adaptation of "the Coal Measures," the old British geologists' term for the same set of strata. The "Permian" was named after Perm , Russia , because it was defined using strata in that region by a British geologist Roderick Murchison . However, some periods were defined by geologists from other countries. The "Triassic" was named in 1834 by a German geologist Friedrich Von Alberti from the three distinct layers (Latin trias meaning triad) — red beds , capped by chalk , followed by black shales — that are found throughout Germany and Northwest Europe, called the 'Trias'. The "Jurassic" was named by a French geologist Alexandre Brogniart for the extensive marine limestone exposures of the Jura Mountains .

The "Cretaceous" (from Latin creta meaning ' chalk ') as a separate period was first defined by a Belgian geologist Jean d'Omalius d'Halloy in 1822, using strata in the Paris basin [1] and named for the extensive beds of chalk ( calcium carbonate deposited by the shells of marine invertebrates ). British geologists were also responsible for the grouping of periods into Eras and the subdivision of the Tertiary and Quaternary periods into epochs. When William Smith and Sir Charles Lyell first recognized that rock strata represented successive time periods, time scales could be estimated only very imprecisely since various kinds of rates of change used in estimation were highly variable. While creationists had been proposing dates of around six or seven thousand years for the age of the Earth based on their Christian heritage, early geologists were suggesting millions of years for geologic periods with some even suggesting a virtually infinite age for the Earth. Geologists and paleontologists constructed the geologic table based on the relative positions of different strata and fossils, and estimated the time scales based on studying rates of various kinds of weathering , erosion , sedimentation , and lithification .

Until the discovery of radioactivity in 1896 and the development of its geological applications through radiometric dating during the first half of the 20th century (pioneered by such geologists as Arthur Holmes ) which allowed for more precise absolute dating of rocks, the ages of various rock strata and the age of the Earth were the subject of considerable debate. In 1977, the Global Commission on Stratigraphy (now the International Commission) started an effort to define global references ( Global Boundary Stratotype Sections and Points ) for geologic periods and faunal stages. The commission's most recent work is described in the 2004 geologic time scale of Gradstein et al. ( ISBN 0-521-78673-8 ), and is used as the basis of this page. More information on the development of the geologic time scale can be found in the articles history of geology and history of paleontology . Table of geologic time

Question: Which geological period is named after the chalk laid down?
Answer: Cretaceous