2Determining geological ages Relative ages – placing rocks and geologic events in their proper sequence, oldest to youngest.Absolute dates – define the actual numerical age of a particular geologic event. For example, large dinosaurs died out 65 mya. The Lavas along Rt 22 and Rt 78 were deposited about 205 mya.
3Relative Age Dating assigns a non-specific age to a rock, rock layer or fossil based on its position in the Strata relative to other rocks, rock layers or fossils.
4Relative Age Dating is based on a list of principles or rules.
5First principle of relative dating Law of superpositionDeveloped by Nicolaus Steno in 1669In an undeformed sequence of sedimentary or volcanic rocks the oldest rocks are at the base; the youngest are at the top
35Another method of examining the Geologic Record involved examining instances where rock layers are missing (Unconformities).
36The processes that would bring about the removal of these missing layers require large amounts of time.
37Unconformities (loss of rock record) An unconformity is a break in the rock record produced by erosion and/or nondepositionTypes of unconformitiesNonconformity – sedimentary rocks deposited above metamorphic or igneous rocks (basement) with time lostAngular unconformity – tilted rocks overlain by flat-lying rocksDisconformity – strata on either side of the unconformity are parallel (but time is lost)
38Disconformity Angular unconformity Layered (a) sedimentary rocks 8_9NonconformityIgneousintrusive rockMetamorphicrock(b)YoungersedimentaryrocksAngularunconformityOlder, foldedsedimentaryrocks(c)DisconformityTrilobite(490 million years old)Brachiopod(290 million years old)
47Sea level rises, new sediment is deposited Development of a NonconformityAn intrusion occursThe overburden is eroded awayPennsylvanian sandstone over Precambrian granite is a nonconformitySea level rises, new sediment is deposited
48Nonconformity- Sedimentary Rock layers over older Igneous or Metamorphic
49Nonconformity in the Grand Canyon - Sediments deposited over Schist
50Cross Cutting Relationships in strata Zoroaster Granite across Vishnu Schist
51Rock Layer Correlation Correlation is the matching of rock layers from one area to another.Matching rocks in different locations due to their similar characteristicsKey BedsStratigraphic MatchingUsing Index Fossils (fossils that lived and died in one particular geologic time) to match rock layers
52Correlating Rock age using Index Fossils and Stratigraphic Matching
53Correlation of rock layers Matching strata of similar ages in different regions is called correlation
54Correlation of strata in southwestern United States Sections are incompleteMatch with fossils and lithology
55Matching Rock Layers in Africa and South America
57Because of sea-level changes Fossils are more reliable than sequences of sediment facies However, falling sea level is useful for worldwide correlation. Why?Sauk SequenceWESTEASTTransgressionMiddleCambrianLowerCambrianNote how western BAS is older than eastern BAS
58Index Fossil Requirements Index Fossils mustbe easy to identifyhave been very abundanthave lived in a wide geographic areahave existed for a short geologic time (ie: someone’s picture in a yearbook)
62Radiometric Dating- Proportion of Parent to Daughter Isotopes To get amount of parent material for each half-life, know that after one half-life, you have ½ of parent isotope left, then double your denominator for each half-life thereafter.
64Half Life The original isotope is called the parent The new isotope is known as the daughter isotopeProduced by radioactive decayAll parent isotopes decay to their daughter isotope at a specific and unique rateBased on this decay rate, it takes a certain period of time for one half of the parent isotope to decay to its daughter productHalf life – the time it takes for half of the atoms in the isotope to decay
66Comparison with known tree ring sequences Can go back 10,000+ years Based on living and fossil wood Paleoclimate information Paleohydrology Archeology.
67EX: The half life of C-14 is 5,730 years So it will take 5,730 years for half of the C-14 atoms in an object to change into N-14 atomsSo in another years, how many atoms will be turned into N-14?HALF LIFEIn another 5,730 years, another half of the remaining atoms will degrade to N-14, and so on.So after 2 half lives, one forth of the original C-14 atoms remainAfter 3 half lives, one eighth of the original c-14 atoms still remainKeeping cutting in half
68Radiocarbon DatingC-14 is useful for dating bones, wood and charcoal up to 75,000 yoLiving things take in C from the environment to make their bodiesMost is C-12 but some is C-14The ratio of these two types in the enviro is always the sameBy studying the ratio in an organism it can be compared to the ratio in the environment presently