2Determining geological ages Relative dating – placing rocks and events in their proper sequence of formation, without actual dates.Numerical dating – specifying the actual number of years that have passed since an event occurred (also known as absolute dating)
3Principles of Relative Dating: Law of Superposition In an undeformed sequence of surface-deposited rocks, the oldest rocks are on the bottom.Includes sedimentary rocks, lava flows, ash deposits and pyroclastic strata.Does not include intrusive rocks, which intrude from below.
5Principles of Relative Dating Principle of original horizontalityLayers of sediment are generally deposited in a horizontal positionRock layers that are flat have not been disturbedPrinciple of cross-cutting relationshipsYounger features cut across older features (faults, intrusions etc)
7Figure 18.4, #4 Is Fault A o/y than the ss layer? Is Dike A o/y than the ss?Was the conglom. deposited b/a fault A?Was the cong. deposited b/a fault B?Which fault is older, A or B?Is dike A o/y than the batholith?
8Figure 18.4 - a Is Fault A o/y than the ss layer? -Y Is Dike A o/y than the ss?Was the conglom. deposited b/a fault A?Was the cong. deposited b/a fault B?Which fault is older, A or B?Is dike A o/y than the batholith?
9Figure 18.4 - b Is Fault A o/y than the ss layer? -Y Is Dike A o/y than the ss? - YWas the conglom. deposited b/a fault A?Was the cong. deposited b/a fault B?Which fault is older, A or B?Is dike A o/y than the batholith?
10Figure 18.4 - c Is Fault A o/y than the ss layer? -Y Is Dike A o/y than the ss? - YWas the conglom. deposited b/a fault A? - AfterWas the cong. deposited b/a fault B?Which fault is older, A or B?Is dike A o/y than the batholith?
11Figure 18.4 -d Is Fault A o/y than the ss layer? -Y Is Dike A o/y than the ss? - YWas the conglom. deposited b/a fault A? - AfterWas the cong. deposited b/a fault B? - BeforeWhich fault is older, A or B?Is dike A o/y than the batholith?
12Figure 18.4 - e Is Fault A o/y than the ss layer? -Y Is Dike A o/y than the ss? - YWas the conglom. deposited b/a fault A? - AfterWas the cong. deposited b/a fault B? - BeforeWhich fault is older, A or B? - AIs dike A o/y than the batholith?
13Figure 18.4 - f Is Fault A o/y than the ss layer? -Y Is Dike A o/y than the ss? - YWas the conglom. deposited b/a fault A? - AfterWas the cong. deposited b/a fault B? - BeforeWhich fault is older, A or B? - AIs dike A o/y than the batholith? - Y
14Figure AnswersIs fault A o/y than the ss? – Y – fault cuts the ssIs dike A o/y than the ss? – Y – dike cuts ssWas the conglom. deposited b/a fault A? – after – conglom not cutWas the conglom deposited b/a fault B? –before – fault cuts itWhich fault is older?-A – conglom older than B but younger than AIs dike A o/y than the batholith? – Y – Dike A cuts Dike B, which in turn cuts the batholith.
15InclusionsAn inclusion is a piece of rock that is enclosed within another rock.Principle of cross-cutting relationships tells us rock containing the inclusion is younger than the inclusion itself.The presence of inclusions allow us to determine whether a intrusive igneous rock is older or younger than the rock above it.Let’s see how
16InclusionsMagma intrudes into an existing rock formation, surrounding small pieces of it.The magma becomes an intrusive igneous rock (e.g. granite).Even though it is underneath the pink rock, it is youngerThe contact between the two layers is not an unconformity, because it was never exposed at the surface.
17InclusionsFirst the “country rock” (the pink stuff) weathers away, exposing the granite (gray) at the surface.The granite also weathers away, leaving an erosional surface.
18InclusionsConditions change and the erosional surface becomes a depositional environment.The lower layers of the sedimentary formation contain inclusions of granite.This shows the granite is older than the sedimentary layers.The contact between the older igneous and younger sedimentary rocks is a type of unconformity, because it was at one time exposed at the surface.
19Unconformitya break in the rock record produced by erosion of rock units and/or nondeposition of sedimentsBetween sedimentary rocks and crystalline (non-layered) bedrockBetween two sets of layered sedimentary rocks deposited at two different timesAngular unconformity – tilted rocks are overlain by flat-lying rocks
20Formation of an angular unconformity Figure 18.7
21Unconformity TypesTo view this animation, click “View” and then “Slide Show” on the top navigation bar.
22Unconformities in the Grand Canyon Unconformities, especially between sedimentary strata, are hard to distinguish.
24Fossils: the remains or traces of living organisms Conditions favoring preservationRapid burialPossession of hard parts (shells or bonesCorrelation: Matching of rocks of similar ages in different regionsCorrelation often relies upon fossils
25Principle of Fossil Succession: Fossil organisms succeed one another in a definite and determinable order, so any time period can be recognized by its fossil content.
26Principle of Fossil Succession: Although developed over 50 years before Darwin’s work, it is now known that the reason this principle is valid is due to evolution.Fossil organisms become more similar to modern organisms with geologic timeExtinct fossils organisms never reappear in the fossil record
27Index Fossils Widespread geographically Limited to short span of geologic timeValuable for correlation: use of index fossils can often provide numerical dates for rock units and eventsSimilar accuracy to radiometric dating techniques.
28Using fossil groups to determine the age of rock strata
29Geologic time scale: a “calendar” of Earth history Subdivides geologic history into units based on appearance and disappearance of fossils from the geologic recordStructure of the geologic time scaleEon – the greatest expanse of timeEra – subdivision of an eonEras are subdivided into periodsPeriods are subdivided into epochs
30The “Precambrian”Used to refer to all geologic time before the Phanerozoic (Visible Life) EonRepresents almost 88% of geologic timeOriginally it was thought that no life existed before the Phanerozoic EonNow we know that the lack of fossil evidence in the Precambrian rocks is partially due to the lack of organisms with exoskeletons
31Eras of the Phanerozoic eon Cenozoic (“recent life”)Mesozoic (“middle life”)Paleozoic (“ancient life”)
32Notable divisions between the Eras Paleozoic-Mesozoic – 248 myaMass extinction of trilobites and many other marine organismsPossibly due to climate change that occurred with the formation of PangaeaMesozoic-Cenozoic – 65 myaMass extinction of dinosaurs and many other speciesProbably caused by meteor impactMade way for the domination of mammalsCenozoic- ????
42Basic atomic structure Proton – positively charged particle found in nucleus.Neutron – neutral particle, which is a combination of a proton and an electron, found in nucleus.Electrons – very small, negatively charged particle that orbits the nucleus. Also, an elementary charged particle that can be be absorbed by a proton or emitted by a neutron to change one into the other.
43Basic atomic structure Atomic numberAn element’s identifying numberEqual to the number of protons in the atom’s nucleusCarbon’s atomic number is always 6.Mass number (formerly “atomic weight”)Sum of the number of protons and neutrons in an atom’s nucleusIndicates the isotope of the element (e.g. C-12, C-14).
45Isotopes and Radioactivity Isotope: Variety of an atom with a different number of neutrons and mass numberSome isotopes (not all!) are inherently unstable, which means the forces binding nuclear particles together are not enough to hold the nucleus together. These are called radioactive isotopes.Examples of isotopes include O-16, O-18, C-12, C-13, and C-14. Only the last is radioactive.
47RadioactivityMany common radioactive isotopes are naturally occurring.Most radioactive processes release energy; formation of C-14 by neutron capture is an exception. It requires cosmic (solar) radiation.They also often release energy and sometimes eject atomic particles as they “decay” or change into a more stable substance.
48From Parent to Daughter In many cases atomic particle are ejected during radioactive decayProtons and/or neutrons ejected from nucleusProtons become neutrons or vice verseAtomic number changes so a new daughter element results.How long does a radioactive parent take to turn into a stable daughter?
50Half-lifethe time required for one-half of the radioactive nuclei in a sample to change from parent isotope to daughter isotope.Decay occurs at random. Can’t predict when an individual atom will decay.However, decay is statistically predictable.Comparison with coin toss
51Half Life (cont’d)After one half-life, 50% of the parent isotope will have become daughter isotope, regardless of the sample size.After 2 half-lives, 50% of the remaining parent isotope will have become daughter isotope. This means 75% of the original parent isotope will have changed.This is an exponential relationship.
52Figure 4.19 As a radioactive parent isotope decays to a daughter, the proportion of parent decreases (blue line) and the amount of daughter increases (red line). The half-life is the amount of time required for half of the parent to decay to daughter. At time zero, when the radiometric calendar starts, a sample is 100 percent parent. At the end of one half-life, 50 percent of the parent has converted to daughter. At the end of two half-lives, 25 percent of the sample is parent and 75 percent is daughter. Thus, by measuring the proportions of parent and daughter in a rock, the rock’s age in half-lives can be obtained. Because the half-lives of all radioactive isotopes are well known, it is simple to convert age in half-lives to age in years.Fig. 4-19, p.86
53Using half-lives of radioactive isotopes in an object to determine the numerical age Zircon (zirconium silicate) is a common accessory mineral in igneous, sedimentary and metamorphic rocks which contains traces of uranium and thoriumPotassium-40 is a radioactive isotope which occurs in K-spars and other minerals containing potassium.Zircon crystal
54Using half-lives of radioactive isotopes in an object to determine the numerical age Every radioactive isotope has a unique half-life, which can be determined by experimentFor radioactive isotopes other than C-14, the ratio of parent to daughter product in a sample determines the age of the sampleC-14 is compared to atmospheric concentration to determine age of organic material.After approximately 10 half-lives, the method is no longer effective as the amount of parent material is too small to measure.
57Importance of Radiometric Dating Radiometric dating is a complex procedure that requires precise measurementRocks from several localities have been dated at just under 4 billion yearsConfirms the idea that geologic time is immense.
58Formation and radioactive decay of Carbon-14 C-14 is created in the upper atmosphere when bombardment of (N-14)2 gas with high energy cosmic rays results in neutron capture.C-14 is unstable and eventually will turn back into N-14, by ejecting a negative ß (beta) particle. The half-life is 5730 years.
59Radiometric dating with Carbon-14 The % C-14 is equal to atmospheric C-14 in a living object, but decreases after deathTo determine the age of the sample, compare % C-14 in sample with % atmospheric C-14.Due to relatively small half-life, C-14 is used to date recent events only (10 half-lives is less than 60, 000 years)Most useful in the fields of archeology and anthropology, also for climate change studies
60Corrections for C-14 dating The % C-14 in our atmosphere has changed over timesolar flare activity determine cosmic ray activity, which causes C-14 formationNuclear testing (see 1963 graph)Use of dendochronology to create calibration curves
61Conversions of Dates and Ages into years BP If the age of the object is given as a date:AD (“year of the lord”): This is the same as a calendar date. The years BP value is how old the sample was in Ex: If an object is dated at 5 AD, the BP age is or 1945 years BPBC/BCE: This is also a date, indicating how many years before “the birth of Christ”. Ex: If an object is dated at 5 BC, it was already 5 when the AD numbering system began. In 1950, it was or 1955 years BPIf an age is given (ex: the object is 2000 years old, that’s 2000 years older than today. Assuming today is in 1999 (!) simply subtract 49 years from the age.