Presentation on theme: "General Geology: Geologic time"— Presentation transcript:
1 General Geology: Geologic time Instructor: Prof. Dr. Boris Natalin
2 Interpreting Earth history is the primary goal of geology Rocks contain information about their origin.Rocks exist as individual material bodies (e.g. layer or intrusion) occupying some space in the Earth.These bodies have contacts with each other which can be interpreted in terms of time – e.g. magmatic rock (batholith or dyke) intrudes sedimentary rocks.Geological event must be put into time perspective
3 Early estimate of geologic time Herodotus (450 B.C) observed steady growth of the Nile delta and conclude that the age of the Earth should be more that 20,000 yearsDark ages and the Book of the Genesis – “Begat” method - Archbishop James Ussher of Ireland ( ) declared that the Earth was created in the evening of October 22, 4004 BC.Comte de Buffon (cooling of iron bolls; age of the earth is 75,000 years)Salinity of the oceans (John Joly)Total age 90 Ma
4 James Hutton“The results, therefore, of our enquiry is, that we find no vestige of a beginning – no prospects of an end”
5 Geologic time Absolute (numerical) date Relative dating This date pinpoint the time in history when something took placeRelative datingRocks are placed in their proper sequence of formation
6 is the founder of relative dating Nicolaus Steno ( )is the founder of relative dating
7 Nicolaus Steno"The prodromus of Nicolaus Steno's dissertation concerning a solid body enclosed by a process of nature within a solid"Steno introduced three principals of spatial and temporal relationships of rocks1. Original horizontality2. Original continuity3. Superposition
14 Relationships of sedimentary rocks ConformityThe relationships between adjacent sedimentary strata that have been deposited in orderly sequence with little or no evidence of time lapse; true stratigraphic continuityUnconformity-A break or gap in geologic record-The structural relationships between rocks that are not in normal succession
19 Formation of an angular unconformity - Accumulation - Deformation - Subsidence- New accumulationAn angular unconformity represents an extended period during which deformation and erosion occurred
20 Angular unconformity Disconformity Nonconformity Younger sediments rest upon the eroded surface of tilted or folded rocks (An episode of deformation separates the rocks)DisconformityAn unconformity between beds that are parallel(A time gap exist between two rock groups)NonconformityAn unconformity between sedimentary rocks above and igneous or metamorphic rocks below (A magmatic or metamorphic episode separates two groups of rocks)
21 Record of relative time as determined by structural relation of rocks
28 Relative dating and correlation Relative ages of rocks determined in individual outcrops must be correlated with each other.Correlation by physical criteria (type of rocks, succession of layers, thickness of beds, metamorphism, structures, etc.).Correlation by fossils (rocks containing similar fossils are synchronous).
29 Correlation by physical criteria MethodsWalking along outcropComparing the position of bedsComparing distinctive minerals or rocksResultsSuccession of deposited bedsStratigraphic column
32 Stratigraphic succession and stratigraphic columns
33 Fossils and correlation William Smith ( )Principle of faunal successionRocks containing similar fossils are synchronous
34 Index fossils Graptolite These fossils are wide spread geographically and are limited to a short span of geologic timeGraptoliteAmmonite
35 Relative age from assemblage of fossil - Time intervals of fossils A, B, and C allows to divide geological history into 3 intervals
36 Fossils and correlation Age of TrilobiteAge of FishesAge of Coal SwampsAge of ReptilesAge of Mammals
37 Radiometric dating (absolute date) Earth is about 4.6 billon years oldDinosaurs became extinct 66 million years ago
38 Atoms Atom is composed of electrons, protons, and neutrons Atomic number is the number of protons in nucleusAtomic mass number is the number of protons and neutronsIn the same element, a number of neutrons can vary, and these variations or isotopes define the mass of element.
39 Radioactivity Some isotopes are unstable The breaking apart, or decay, of a nucleus is called radioactivityThere are tree types of radioactive decayAlpha emission (α) → two protons and two neutrons Beta emission (β) → (an electron or a positron) is emitted from an atomElectron capture → a proton-rich nuclide absorbs an inner atomic electronalpha particle is the same as a helium-4 nucleus, which has mass number 4
42 Radioactivity and radiometric dating Rate of decay for many isotopes have been precisely measured and it do vary under the physical conditions that exist in Earth’s outer layers.Radioactive isotopes can be used for dating of rocks because content of parent and daughter elements can be measured.A radioactive mineral is captured during magma formation. If system is closed after the cooling the amount of appeared daughter element gives us a time elapsed.
43 Half-life as a rate measure Half of the radioactive parent element remains after one half-lifeOne quarter of the radioactive parent element remains after the second half-lifeChange is exponential
44 Radiometric dating Choice of the method Expected age and the half-life Content of parent/daughter elements in rocks
45 Potassium-Argon dating The half life is 1.3 billion yearsIsotopes are common in micas and feldspars
46 Potassium-Argon dating K39 (93% of total K), K40 ( of total K), and K41(7.9% of total K)K40 is radioactiveK40 decay by:- electron capture (11% to argon-40- beta emission 89% to calcium-40Ca40 is not useful
47 Potassium-Argon dating: errors System must be closedSamples must be freshCross check by other method must be applied
48 Radiocarbon dating Carbon-14 → Nitrogen-14 The half life is 5730 years Isotopes are common organic materialThe method dates events as far back as 75,000 years
49 Radiocarbon datingIsotope of carbon is incorporated into carbon dioxide in atmosphere and then is absorbed by leaving material
50 Radiocarbon dating Carbon-14 is incorporated to carbon dioxide Carbon dioxide is absorbed by living materAs long as an organism is alive the content of carbon-14 is stableAfter the death of an organism the radioactive decay of carbon-14 causes decrease of its content in organic tissue
51 The geologic time scale Relative dating of rocks have been used since Steno time but isotopic dating (absolute age) appeared only in 20thcentuary.The scale is mainly based on evolution of fossilsEon → Era → Period → Epoch → Stage
52 Precambrian → Paleozoic → Mesozoic → Cenozoic 542 Ma
54 Why relative dating is still important? Radiometric (isotopic) dating is mainly used for magmatic rocks.Sedimentary rocks can only rarely be dated by radiometric meansMetamorphic rocks are affected by several deformational and metamorphic eventsRadiometric dating is possible!
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