08 Time and Geology

Uniformitarianism People two or three centuries ago did not realize the expanse of geologic

time. Christianity placed geologic events within a biblical chronology, and catastrophic events were blamed for features of the landscape. For example, Grand Canyon split open and remained that way ever since. Clams in the mountains was explained by a worldwide inundation drowning the earth’s mountains. No known physical laws could account for such events so they were attributed to divine intervention. Theories that explain scientific phenomena as the result of major catastrophes is considered catastrophism. The earth opened up and the ocean poured out of the crack, earth’s crust precipitated from the ocean basins.

In the 18th century, James Hutton concluded that the earth is a dynamic, ever-changing place in which new rocks, lands and mountains arise continuously as a balance against their destruction by erosion and weathering. Taking a cyclic view of the planet, he believed that the past history of our globe must be explained by what can be seen to be happening now, the present is the key to the past. We use present observations, physical and chemical laws (natural laws are invariant with time and are an accumulation of our observations) to infer the origin of particular features seen today. The principle of Uniformitarianism states that forces are operating today in the same fashion as they have for millions of years. The geologic processes operating at present are the same processes that have operated in the past, they may vary in their geographic location and geologic intensity.

What made the idea of uniformitarianism so difficult to accept was the vast amount of time implied. For example, layers of shale 2000 meters thick exist, but at observed rates silt and clay settle at rates of less than a mm a year onto the ocean floor. 2000 meter thick deposits would therefore require more than a million years to form, much longer than people in the eighteenth century believed the earth existed. Acceptance of the principle of uniformitarianism led to the realization that geology involves time periods much greater than a few thousand years.

Absolute age-age given in years or some other unit of time.Relative time-the sequence in which events took place.

Relative TimeRelative age refers to the sequence in which events took place (not the number of years involved). The sequence of geologic events can be determined by piecing together the history of the individual parts. Most of the individual parts of the larger problem are solved by applying several simple principles. In this way the sequence of events or the relative time involved can be determined.

(1) original horizontality(2) superposition(3) cross cutting relationships

Refer to figure 8.2 representing an area similar to the grand canyonThe principle of original horizontallity states that beds of sediment deposited in water form as horizontal layers. Thus, if we observe rock layers

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that are folded or inclined at a steep angle, they must have been moved into that position by crustal disturbance after their deposition. The principle of superposition states that within a sequence of undisturbed sedimentary rocks, the layers get younger going from bottom to top. If sedimentary rock is formed by sediment settling onto the ocean floor, then the first (or bottom) layer must be there before the next layer can be deposited on top of it. The principle also applies to layers formed by multiple lava flows and beds of ash from volcanic eruptions.Cross-cutting relationships states that a disrupted pattern is older than the cause of disruption. When igneous intrusions or faults cut through other rocks, they are assumed to be younger than the rocks they cut.

Inclusions are pieces of one rock unit that are contained within another. The rock mass adjacent to the one containing the inclusions must have been there first in order to provide the rock fragments. Therefore the rock mass containing inclusions is the younger of the two.

Layers of rock are said to be conformable when they are found to have been deposited. Although particular sites may exhibit conformable beds representing significant spans of geologic time, there is no place on earth that contains a full set of conformable strata. Throughout earth history, the deposition of sediment has been interrupted over and over again. All such breaks in the rock record are termed unconformities. An unconformity represents a long period of time during which deposition ceased, erosion removed previously formed rocks, and then deposition resumed.Angular unconformities consist of tilted or folded sedimentary rocks that are overlain by younger, more flat lying strata. Angular unconformities indicate that during the pause in deposition, a period of deformation (folding or tilting) as well as erosion occurred.Disconformities are a type of unconformity in which the strata on either side are essentially parallel. Many disconformities are difficult to identify because the rocks above and below are similar and there is little evidence of erosion.Nonconformities are a type of unconformity in which the break separates older metamorphic or intrusive igneous rocks from younger sedimentary strata. Intrusive igneous masses and metamorphic rocks originate far below the surface. Therefore, for a nonconformity to develop, there must be a period of uplift and the erosion of overlying rocks. Once exposed at the surface, the igneous or metamorphic rocks are subjected to weathering and erosion prior to sedimentation.

CorrelationIn order to develop a geologic time scale that is applicable to the whole earth, rocks of similar age in different regions must be matched up. Correlation means determining the age relationships between rock units or geologic events in separate areas. Correlation is necessary for understanding the geologic history of a region, a continent, or the whole

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earth. Substantiation of the plate tectonics theory depends on intercontinental correlation of rock units and geologic events, piecing together evidence that the continents were once one great body. Part of the evidence supporting the theory of plate tectonics (and continental drift) depended on correlating the age of rocks in South America and Africa.(1) Physical continuity - is tracing physically the course of a rock unit to

correlate rocks between two different places.(2) Similarity of rock types - is correlation of two regions by assuming that

similar rock types in two regions formed at the same time. This method must be used cautiously if the rocks being correlated are common ones. Correlation by similarity in rock types is more reliable when they are very unusual sequences of rock.

(3) Correlation by fossils - Fossils are common in sedimentary rock. Plants and animals that lived at the time the rock formed were buried by sediment, and their fossil remains are preserved in sedimentary rock. Fossils in one layer of sedimentary rock often differ markedly from fossils in layers above and below. The significance of fossils as geologic tools was made evident by an English engineer and canal builder, William Smith. Smith discovered that each rock formation in the canals contained fossils unlike those in the beds either above or below it. Further, he noted that sedimentary strata in widely separated areas could be identified by their distinctive fossil content. Based upon Smith’s observations, one of the most important and basic principles in historical geology was formulate: Fossil organisms succeed one another in a definite and recognizable order, and therefore any time period can be recognized by its fossil content. This is known as the principle of faunal succession. When fossils are arranged according to their age by using the law of superposition on the rocks in which they are found, they do not present a random or haphazard picture. To the contrary, fossils show progressive changes from simple to complex and reveal the advancement of life through time. For example, in the fossil record there is represented, in succession, an age of trilobites. an age of fishes, an age of reptiles, and an age of mammals. These ages pertain to groups that were especially plentiful and characteristic during particular time periods. Within each of the ages, there are many subdivisions based on certain species. This same succession of dominant organisms, never out of order, is found on every major landmass. No matter where on earth they are found, individual fossil species always occur in the same sequence relative to one another. By comparing fossils found in a layer of rock in one area with similar fossils in another area, we can correlate the two rock units.

Since fossils were found to be time indicators, they became the most useful means of correlating rocks of similar age in different regions. Most useful are index fossils - a fossil from a very short lived species known to exist during a specific period of geologic time. A single index fossil allows the geologist to correlate the rock in which it is found with all other rock layers in the world containing that fossil. Rock formations, however, do not always

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contain a specific index fossil. In such situations, groups of fossils are used to establish the age of the bed. Several different fossils in a rock is referred to as a fossil assemblage. Fossil assemblages are generally more useful for dating rocks than a single fossil because the sediment must have been deposited at a time when all the species represented existed.

Overlapping ranges of fossils help date rocks more exactly than using a single fossil.

^ Age of bed containing

Age of only fossil A and C

bed Age of bed containing

Time

Fossil A

Fossil B

Fossil C

containing

fossils A, B, and C

only Age of bed fossil A containing

only^ fossil A

and B

The Standard Geologic Time ScaleBased on fossil assemblages, the geologic time scale subdivides geologic time. The geologic time scale is a sort of calendar to which events and rock units can be referred. The geologic time scale is a relative time scale representing an extensive fossil record. It consists of three eras, which are subdivided into periods, which are in turn subdivided into epochs. Geologists can therefore use fossils in rock to refer the age of the rock to the geologic time scale.

EON ERA- The eras are bound by profound worldwide changes in life forms.

PERIOD-Each period is bound by less profound changes in life forms.

EPOCH

Phanerozoic EonMeans “visible life.”The rocks and deposits of the

Cenezoic Means ‘new life’

Quaternary

Tertiary

Recent (holocene)Pleistocene

PlioceneMioceneOligoceneEocenePaleocene

We live in the Holocene Epoch of the Quaternary Period.

The most

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Phanerozoic eon contain an abundance of fossils that document major evolutionary trends.

recent ice ages were part of the Pleistocene epoch.

Mesozoic Means ‘middle life’. Reptiles were the dominant animals.

CretaceousJurassicTriassic

The time of the dinosaurs

Paleozoic Means ‘old life’. Began with the appearance of abundant and complex life including creatures with shells and other hard parts.

PermianPennsylvanianMississippianDevonianSilurianOrdovicianCambrian

Proterozoic EonArchean Eoncollectively Precambrian Time

Precambrian time collectively is the vast amount of time that preceded the Paleozoic Era. Contains few fossils.

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Absolute AgeWidespread use of fossils for correlation led to the development of the standard geologic time scale. Originally based on relative age relationships, the subdivision of the standard geologic time scale have now been assigned absolute ages through radioactive dating.

Radioactive DatingAn atom is composed of electrons, protons, and neutrons. Protons and neutrons are found in the center nucleus of the atom. By adding together the number of protons and neutrons in the nucleus, the mass number of the atom is determined. The atomic number is equal to the number of protons. Every element has a different number of protons, and thus a different atomic number. Atoms of the same element may have different numbers of neutrons in the nucleus. Such atoms, called isotopes, have different mass numbers but the same atomic number.The forces that bind protons and neutrons together in the nucleus are strong. Some isotopes have unstable nuclei, and the forces that bind the protons and neutrons together are not sufficiently strong. As a result, the nuclei spontaneously break apart, or decay, a process called radioactivity. Three types of radioactive decay are common(1) Alpha particles may be emitted from the nucleus. An alpha particle is

composed of 2 protons and 2 neutrons. Therefore the emission of an alpha particle reduces the mass number by 4 and the atomic number by 2.

(2) When a beta particle, or electron, is given off from a nucleus, the mass number remains unchanged, because electrons have no mass. However, since the electron comes from a neutron (a neutron is a combination of a proton and electron), the proton is left in the nucleus, and therefore the nucleus contains one more proton than before. Therefore, the atomic number increases by 1.

(3) Sometimes an electron is captured by the nucleus. The electron combines with a proton and forms a neutron. The mass number remains unchanged. However, since the nucleus now contains one less proton, the atomic number decreases by 1.

The radioactive isotope is referred to as the parent, and the isotope resulting from the decay of the parent are termed the daughter products.

Radioactivity provides a reliable means of calculating the ages of rocks and minerals which contain particular radioactive isotopes, a procedure referred to as radiometric dating. Radiometric dating is reliable because the rates of decay for many isotopes have been precisely measured and do not vary. Therefore, each radioactive isotope used for dating has been decaying at a fixed rate since the formation of the rocks in which it occurs and the products of decay have been accumulating at a corresponding rate. For example, when uranium is incorporated into a mineral that crystallizes form magma, there is now lead (the stable daughter product) from previous decay. As the uranium in the newly formed mineral disintegrates, atoms of

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the daughter product are trapped and measurable amounts of lead eventually accumulate. The time required for one-half of the nuclei in a sample to decay, called half-life, is a common way of expressing the rate of radioactive disintegration. When the quantities of parent and daughter are equal, we know that one half life has transpired. When one-quarter of the original parent atoms remain and three-quarters have decayed to the daughter product, the parent daughter ratio is 1:3 and we know that two half lives have passed. Therefore, if the half-life of a radioactive isotope is known and the parent/daughter ratio can be determined, the age of the sample can be calculated.

Notice that the percentage of radioactive atoms that decay during one half-life is always the same. However the actual number of atoms that decay with the passing of each half-life continually decreases. Thus as the percentage of radioactive parent atoms declines, the proportion of stable daughter atoms rises, with the increase in daughter atoms just matching the drop in parent atoms.

It is important to realize that an accurate radiometric date can be obtained only if the mineral remained a closed system during the entire period since its formation. That is, a correct date is not possible unless there was neither the addition nor loss of parent or daughter isotopes. Most successfully dated rocks are igneous, which, since they solidified from a melt, are unlikely to be contaminated by previously formed daughter products. Metamorphic rock can yield inaccurate ages because heat during metamorphism can drive out some of either the daughter product or the radioactive isotope. Sedimentary rock cannot be radioactively dated.

RadioCarbon: Dating “young” events.To date very recent events, carbon-14 (also called radiocarbon), the radioactive isotope of carbon, is used. Because of its half life of 5730 years, it can be used for dating events of about 50,000 years to those from recent geologic history. Carbon-14 is continuously produced in the upper atmosphere as a consequence of cosmic ray bombardment, in which cosmic rays shatter the nuclei of gasses, releasing neutrons. Some of the neutrons are absorbed by nitrogen (atomic number 7, mass number 14), causing its nucleus to emit a proton. As a result, the atomic number decreases by 1 (to 6), and a different element, carbon-14, is created. This isotope of carbon is incorporated into carbon dioxide, circulates in the atmosphere, and is absorbed by living matter. As a result, all organisms contain a small amount of carbon-14. As long as an organism is alive, the decaying radiocarbon is continually replaced, and the proportion of C-14 and C-12 (the stable and most common isotope of carbon) remain constant. However, when the plant or animal dies, the amount of carbon-14 gradually decreases as it decays to nitrogen-14 by beta emission. By comparing the proportions of C-14 and C-12 in a sample, radiocarbon dates can be determined.

Combining Relative and Absolute Ages.

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Radioactive dating can provide absolute time brackets for events whose relative ages are known. By combining many radioactive dates from sites carefully selected for well-known relative age relationships, absolute ages have been assigned to the geologic time scale.