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Geologic Time and Principles of Stratigraphy•Recall Archbishop James Usher (1581-1665) who made a first attempt to calculate the age of the earthusing the Bible.•Date: October 4004 B.C.•Very young earth! WRONG!ITS AS OLD AS DIRT!•James Hutton and Charles Lyell (1797-1875)•Estimated soil formation at thousands of years/cm. –Physical and chemical laws are uniform and have been operating on earth, causing changes for a LONG time. –The present is the key to the past!•Charles Lyell published Principles of Geology 1830.•Discussed uniformitarianism. Small changes in present day processes could, over long periods of time,produce significant changes.Geologic Time•Two methods to assess geologic time: RELATIVE dating and ABSOLUTE dating.•What’s the difference?Geologic time involves events ranging in time from a few seconds (earthquakes, meteorite impacts) to millionsand even billions of years (the movement of tectonic plates, the formation of the earth itself).Geologists use two different methods to determine geologic time and the ages of geologic features andevents: 1. Relative age dating involves establishing the order of features and events, without necessarily determining their actual age in years. 2. Absolute age dating involves determining the actual age (in thousands, millions, or billions of years) of a rock or other geologic feature.Relative Dating • Age refers to the sequence in which events took place, not an actual age (in years). • Method used before radioactivity was discovered. • Using original horizontality, superposition, cross-cutting relationships and fossil assemblages one can determine which rock is older than the other.Relative Dating TechniquesNicolaus Steno (1638-1686) • Principle of Superposition • Principle of Original Horizontality • Principle of Lateral ContinuityThese principles led to the study of stratigraphy (study of layered rocks).Steno’s Law of Superposition In any sequence of undisturbed strata (layers of rock), the oldest is at the bottom and successively higher layers are successively younger.
Steno’s Principle of Original Horizontality • Sediments that compose rock are under the influence of gravity. Therefore sediments are deposited horizontally and parallel to the surface they are accumulating onto. • If the sedimentary rock layers are tilted on an angle, what does this mean?Steno’s Principle of Lateral ContinuitySediments are deposited layers on top of each other. If you find the layers of rock are the same type of rock and sequence of layers here and over there, we can assume they were once continuous.•A fault can disrupt once continuous layers of rock.•Streams can erode a valley making rock layers discontinuous.More StratigraphyCharles LyellIn addition to Steno’s Principles, Lyell came up with another couple of rules to stratigraphy. • Principle of Cross Cutting Relationships • Principle of InclusionsPrinciple of Cross-Cutting Relationships•Can determine the relative ages of rocks compared to one another.•Applies to rocks, faults, unconformities, intrusions, etc.•For example, If one rock layer cuts across another rock layer then the rock layer doing the ‘cutting into’ isyounger.Principle of Inclusions•Fragments of one rock type can be ‘included’ into a totally different rock type (when the two rocks are incontact with one another).•Whenever two rock types are in contact, the rocks containing the pieces of the other will be younger of thetwo.•In order to be ‘included’ into another rock you must exist first.Breaks in the Stratigraphic Record•Layers of rock are said to be conformable when they are found to have been deposited layer after layerwithout interruption. Although particular sites may exhibit conformable beds representing significant spansof 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 suchbreaks in the rock record are termed unconformities. An unconformity represents a long period of timeduring which deposition ceased and/or erosion removed previously formed rocks.•There are three kinds of unconformities…..Disconformity•A Disconformity is an irregular surface of erosion between parallel strata. A disconformity implies acessation of sedimentation, as well as erosion, but no tilting.•Many disconformities are difficult to identify because the rocks above and below are similar and there islittle evidence of erosion.Angular Unconformity
•Angular unconformities consist of tilted or folded sedimentary rocks that are overlain by younger, moreflat lying strata.•Angular unconformities indicate that during the pause in deposition, a period of deformation (folding ortilting) as well as erosion occurred.Nonconformity•Nonconformities are a type of unconformity in which the break separates older metamorphic or intrusiveigneous rocks from younger sedimentary strata.•Intrusive igneous masses and metamorphic rocks originate far below the surface. Therefore, for anonconformity to develop, there must be a period of uplift and the erosion of overlying rocks. Once exposedat the surface, the igneous or metamorphic rocks are subjected to weathering/erosion prior tosedimentation.The Fossil RecordWhat about life represented in the rock record?William Smith’s Principle of Faunal Succession • Examined fossils in rocks. Found that you could match them up all over the world based on the assemblages (group of fossils) in rocks. • Rocks around the world can be correlated based on their fossil content (formed at the same TIME under the same CONDITIONS just in different locations).Principle of Faunal SuccessionPrinciple of Faunal Succession suggests that throughout geologic time, there has been a regularsuccession of organisms. Fossils succeed one another in a definite and recognizable order, and thereforeany time period can be recognized by its fossil content.•When fossils are arranged according to their age by using the law of superposition on the rocks in whichthey are found, they do not present a random or haphazard picture. To the contrary, fossils showprogressive 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 andcharacteristic during particular time periods. Within each of the ages, there are many subdivisions based oncertain species. This same succession of dominant organisms, never out of order, is found on every majorlandmass.Fossils and StratigraphyGeologic RangeBecause of faunal succession, each fossil species can be given a geologic rangethe first appearance of the fossil in the rock record to the last appearance of the fossil in the rock record. It is the range that an organism existed and left behind fossil evidence.Geologic ranges vary among groups of organisms.Correlation and Faunal SuccessionIndex FossilsFossils that are good for correlating and dating rocks all over the world. • Easy to identify. • Abundant. • Short life span for the species (small geologic range).
• Cosmopolitan Lifestyle • Geographically WidespreadCorrelation by fossils.Certain index fossils are keys to matching and dating sedimentary strata in widely separated outcropsPrinciple of Faunal Succession•Fossil assemblages are generally more useful for dating rocks than a single fossil because the sedimentmust have been deposited at a time when all the species represented existed.•The fossil remains of living things are present in the rock layers at definite intervals, and exist within adiscrete period of time.Absolute DatingThe geologic time scale was based on relative dating. Absolute ages have now been assigned by way of radiometric dating.Radiometric dating is a reliable means of calculating the ages of rocks and minerals that contain radioactive elements. How? • Determining the actual age of a rock, in years, requires some sort of natural “clock”. The clock that geologists use is the radioactive decay of certain elements within rocks. • Radioactive decay is when an unstable element changes (decays) into another element, or a new variation of itself. This change occurs at a precise rate that can be determined by experimentation.Absolute dating involves atoms. • An atom is made of electrons, protons and neutrons. • The atomic number is the number of protons. • The atomic mass is the number of protons and the number of neutrons in the nucleus. • Atoms can be considered STABLE, where they will not want to bond with another atom (if the outermost shell is full of electrons).Radioactive elements are not ‘stable’ like atomsthey vary in the number of neutrons in the nucleus forming different isotopes. Atoms of different isotopes are the same element but they have different atomic weights (due to varying number of neutrons). In order to become stable radioactive elements must lose particles (radioactive decay).By convention, the unstable element is called the “parent”, while the element resulting from the decay ofthe parent is called the “daughter”.The speed (rate) at which a particular parent element changes into a daughter element is expressed as thehalf-life: the time required for half of the parent atoms to change into daughter atoms.Two types of radioactive decay are common. • Alpha decay is when an unstable parent atom emits an alpha particle (a small atom consisting of 2 protons and 2 neutrons). • Beta decay is when an unstable parent emits a beta particle (an electron).Both types of decay produce a new, stable daughter atom.Bottom Line: To determine the age when a rock formed, you need to know two things:
•- the percentage of parent and daughter atoms in the rock•- the half-life (rate of decay)The percentage of parent atoms decreases, and the percentage of daughter atoms increases, as time goesby. By measuring the rate at which this occurs (the half-life), and measuring the percentage of parent anddaughter atoms in the rock, we can determine the age of the rock!The half-life is the time it takes for half of the parent to decay into daughter. • We start with 100% parent. • If one half life passes, 50% parent and 50% daughter isotope. How much time passed to accomplish this. • If another half life were to pass, OF THE 50% percent parent, half of THAT will decay. After two half lives, we should record 75% daughter and 25% parent.Different radioactive elements decay at different rates. You will choose your radioactive element wisely depending on what you are dating. C-14 = 5730 yrs; U-238 = 4.5byrs.A form of radioactive decay that is particularly useful for determining the ages of the remains of plants andanimals is the decay of carbon-14 to Nitrogen-14. • C-14 is made continuously in the upper atmosphere, so all living things absorb a fraction of C-14 in their tissues as they grow. After death, C-14 decays gradually to N-14. By measuring the percentage of C-14 versus N-14 in plant and animal remains, we can determine their age. • Carbon-14 radiometric dating can be used to determine ages of organic remains up about 50,000 years old.FORMATION and AGE of the EARTHOur solar system consists of 8 planets (not including Pluto) orbiting the Sun in a single plane, all traveling inthe same direction. Most scientists think this reflects the formation of the solar system from a nebula - aspinning cloud of gas and dust that slowly contracted under its own gravity, flattened into a disc, andeventually coalesced into the Sun and the planets, including Earth.When did the Earth and the rest of the solar system form?Radiometric dating (based on the decay of uranium into lead) shows us that the oldest rocks yet found onearth are nearly 4.0 billion years old. Furthermore, certain isolated mineral grains (eroded out of rocks thatare no longer present) have been dated as old as 4.3 billion years.However, scientists presently accept 4.6 billion years as the age of formation of the earth. This is the age(based again on the decay of uranium into lead) of the oldest moon rocks and meteorites. Since we assumethat all objects in the solar system formed at about the same time (see previous slide), we assume that theearth is about as old as the moon and meteorites.Our earth today is a result of geologic processes that have acted over an immense span of geologic time!