Presentation on theme: "Geologic Time Correlation and Dating of the Rock Record."— Presentation transcript:
Geologic Time Correlation and Dating of the Rock Record
Time Relative –Order of deposition of a body of rock based on position Absolute –A number representing the time a body of rock was deposited
Relative Time Determining of sequence of events Which came first?
In what order did these events occur in American History? D
Relative Time Tools –Smith Fossil Succession –Steno Superposition Original horizontality –Lyell Cross-cutting relationships Intrusions Inclusions
Principle of fossil succession Fossils occur in a consistent vertical order in sedimentary rocks all over the world. (William"Strata Bill" Smith, late 1700's, England). This principle is valid and does not depend on any pre-existing ideas of evolution. (In fact, Charles Darwin's ideas on evolution did not appear until 50 years later ).
Principle of fossil succession Geologists interpret fossil succession to be the result of evolution - the natural appearance and disappearance of species through time. "Fossil species succeed one another in a definite and recognizable order" Fossils at the base of a thick sequence of sediments (so older, by previous principles) are less like present-day species than those near the top
Principle of fossil succession * Fossils unlike present-day species, but like each other, are found in widely separated sites * A fossil species which is observed to occur above (and so younger than, by previous principles) a second fossil species in one locality will always occur above that second species, wherever found.
Principle of fossil succession
Unconformities Unconformities are buried surfaces of erosion or non-deposition
Unconformities 1.Angular unconformities Implies tectonic deformation and erosion of underlying strata. 2.Nonconformity Sedimentary strata overlying igneous or metamorphic rocks (in an erosional - not intrusive- contact) 3.Disconformity An irregular surface of erosion between two units of parallel strata
Relative Dating Principle of Cross-cutting relationships Principle of Intrusions Principle of Inclusions
Principle of Cross-cutting relationships
Principle of Inclusions In which picture is the Granite older?
Principle of Intrusions
Intrusions vs Unconformities A xenolith is a fragment of country rocks which has been broken off during an intrusion, and has become surrounded by magma. The xenolith is older than the igneous rock which contains it. Through erosion and resedimentation, younger rocks will often have pieces of the older rock included (sedimentary).
Principle of Inclusions
History of Geologic Time Geologic Systems –Body of rock that contains fossils of diverse animal life –Corresponds to geologic period Sedgewick –Named Cambrian Murchison –Named Silurian
Stratigraphy Study of stratified rocks, especially their geometric relations, compositions, origins, and age relations Stratigraphic units –Strata Distinguished by some physical, chemical, or paleontological property Units of time based on ages of strata –Geologic Systems Correlation –Demonstrate correspondence between geographically separated parts of a stratigraphic unit Lithologic Temporal
Units of Time Time-rock unit –Chronostratigraphic unit –All the strata in the world deposited during a particular interval of time Erathem, System, Series, Stage Time unit –Geochronologic unit –Interval during which a time-rock unit is formed Eras, Period, Epoch, Age Boundary stratotype –Boundary between two systems, series or stages, formally defined at a single locality
Geologic Time Scale Chronologic units - Time/Age –Eons (largest): –Era – Periods –Epochs – Ages
Geologic Time Scale Geochronologic Units = Place –Eon (largest) = Eon –Era = Era –System = Period –Series = Epoch –Stage = Age
Biostratigraphy Biostratigraphic unit –Defined and characterized by their fossil content Stratigraphic range –Total vertical interval through which that species occurs in strata, from lowermost to uppermost occurrence
Biostratigraphy Index fossil –Abundant enough in the stratigraphic record to be found easily –Easily distinguished from other taxa –Geographically widespread and thus can be used to correlate rocks over a large area –Occurs in many kinds of sedimentary rocks and therefore can be found in many places –Has a narrow stratigraphic range, which allows for precise correlation if its mere presence is used to define a zone
Magnetic Stratigraphy Use of magnetic properties of a rock to characterize and correlate rock units Magnetic field –Reversals in polarity of field are recorded in rocks when they crystallize or settle from water
Magnetic Stratigraphy Chron –Polarity time-rock unit –Period of normal or reversed polarity Normal interval –Same as today –Black Reversed interval –Opposite to today –White
Lithostratigraphy Subdivision of the stratigraphic record on the basis of physical or chemical characteristics of rock Lithostratigraphic units –Formation Local three-dimensional bodies of rock –Group –Member Stratigraphic section –Local outcrop of a formation that displays a continuous vertical sequence Type section –Locality where the unit is well exposed, that defines the unit
Lithologic Correlation Cross-sections of strata –Establish geometric relationships –Interpret mode of origin
Lithologic Correlation Grand Canyon –McKee –Used Trilobite biostratigraphy to determine age relationships –Eastern portion of units is younger than western
Facies Transgression –Landward migration of shoreline –Grand Canyon Cambrian transgression Facies –Set of characteristics of a body of rock that presents a particular environment Facies changes –Later changes in the characteristics of ancient strata
Absolute Age 4.6 billion years old Early estimates –Salts in the ocean 90 million years old –Accumulation of sediment 100 m.y. or less Gaps in stratigraphic record Unconformities represent large breaks in accumulation Didnt include metamorphosed sedimentary rocks –Earths temperature Kelvin million years old
Absolute Ages How old is the Earth? 4.6 billion years (4,600,000,000 years) Radiometric dating (Uranium, Thorium). Mass spectrometer.
Early Attempts 1654 Archbishop Usher (Ireland), genealogy in Bible Earth was created October 22, 4004 BC, 9:00 am was added later Earth was 6000 years old. Led to the Doctrine of Catastrophism: Earth was shaped by series of giant disasters. Many processes fit into a short time scale.
Early Attempts 1770's, 1780's "Revolution" James Hutton, Father of Geology (Scotland) Published Theory of the Earth in 1785.
Hutton Hadrian's Wall built by Romans, after 1500 years no change. Suspected that Earth was much older. Slow processes shape earth. Mountains arise continuously as a balance against erosion and weathering
Hutton Doctrine of Uniformitarianism: "Present is key to the past". The physical and chemical laws that govern nature are uniform Unconformity at Siccar Point, Scotland "No vestige of a beginning, no prospect of an end"
Charles Lyell Charles Lyell 1800's compared amount of evolution shown by marine mollusks in the various series of the Tertiary System with the amount that had occurred since the beginning of the Pleistocene. Estimated 80 million years for the Cenozoic alone.
Various Geologists Thickness of total sedimentary record divided by average sedimentation rates (in mm/yr). In 1860, calculated to be about 3 million years old. In 1910, calculated to be about 1.6 billion years old.
Lord Kelvin In 1897, Lord Kelvin assumed that the Earth was originally molten and calculated a date based on cooling through conduction and radiation. Age of Earth was calculated to be about million years.
Lord Kelvin Problem: Earth has an internal heat source (radioactive decay) Discovery of radioactivity by Henri Becquerel in 1896.
John Joly In , John Joly (Irish) calculated the rate of delivery of salt to the ocean. River water has only a small concentration of salts. Rivers flow to the sea. Evaporative concentration of salts. Age of Ocean = Total salt in oceans (in grams) divided by rate of salt added (grams per year) Age of Earth = million years.
John Joly Problems: no way to account for recycled salt, salt incorporated into clay minerals, salt deposits.
von Helmholtz and Newcomb The German physicist Hermann von Helmholtz and the American astronomer Simon Newcomb joined in by independently calculating the amount of time it would take for the Sun to condense down to its current diameter and brightness from the nebula of gas and dust from which it was born. 100 million years, consistent with Thomson's calculations. However, they assumed that the Sun was only glowing from the heat of its gravitational contraction. They knew of no other ways for it to produce its energy.
Rutherford and Boltwood In 1905, they used radioactive decay to measure the age of rocks and minerals. Uranium decay produces He, leading to a date of 500 million years.
Rutherford and Boltwood In 1907, Boltwood suspected that lead was the stable end product of the decay of uranium. Published the age of a sample of urananite based on Uranium-Lead dating. Date was 1.64 billion years.
Age of Earth So far, oldest dated Earth rocks are 3.96 billion years. Canadian Shield. (NW Territories near Great Slave Lake, 3.96 byr). Detrital Zircons in sedimentary rocks are byr Older rocks include meteorites and moon rocks with dates on the order of 4.6 billion years.
Geologic Time Scale. The age for the base of each division is in accordance with recommendations of the International Commission on Stratigraphy for the year 2000.
Geologic Time Scale
The standard geologic time scale for the Paleozoic and other eras developed without benefit of a grand plan. Instead, it developed by the compilation of type sections for each of the systems.
Silurian - Murchison
Jurassic - Jura Mountains
Cretaceous Kreta = Chalk White Cliffs of Dover
Absolute Age Radioactive decay –Becquerel, 1895 Uranium undergoes spontaneous decay Atoms release subatomic particles and energy Change to another element –Parent isotope decays/daughter isotope produced
Principles of Radiometric Dating Naturally-occurring radioactive materials break down into other materials at known rates. This is known as radioactive decay. Radioactive parent elements decay to stable daughter elements.
What is an Isotope? Nuclide of an element with different masses
Absolute Age Three modes of decay –Loss of alpha particle Convert parent into element that has nucleus containing two fewer protons –Loss of beta particle Convert parent into element whose nucleus contains one more proton by losing an electron –Capture of beta particle Convert parent into element whose nucleus has one less proton
Absolute Age Radiometric dating –Radioactive isotopes decay at constant geometric rate After a certain amount of time, half of the parent present will survive and half will decay to daughter Half-life –Interval of time for half of parent to decay
Absolute Age Useful isotopes –Uranium 238 and thorium 232 Zircon grains –Uranium 238 and lead 206 Fission track dating –Rubidium-Strontium –Potassium-Argon, Argon-Argon –Radiocarbon dating Produced in upper atmosphere Half life = 5730 years Maximum age for dating: 70,000 years Bone, teeth, wood
Absolute Age Fission-Track Dating –Measure decay of uranium 238 by counting number of tracks –Tracks formed by subatomic particles that fly apart upon decay
Radioactive parent isotopes and their stable daughter products Each radioactive isotope has its own unique half-life. A half-life is the time it takes for half of the parent radioactive element to decay to a daughter product.
Radioactive parent isotopes and their stable daughter products Radioactive ParentStable DaughterHalf Life Potassium 40 Argon billion yrs Rubidium 87Strontium billion yrs Thorium 232Lead billion years Uranium 235Lead million years Uranium 238Lead billion years Carbon 14Nitrogen years
Radioactive decay occurs by releasing particles and energy. Alpha particles Beta particles Neutrons Gamma rays (high energy X-rays) are also produced.
Radioactive Decay Alpha particles (He) large, easily stopped by paper charge = +2 mass = 4
Radioactive Decay Beta particles penetrate hundreds of times farther than alpha particles, but easily stopped compared with neutrons and gamma rays. charge = -1 mass = negligible
Radioactive decay series of uranium- 238 ( 238 U) to lead- 206 ( 206 Pb).
Datable Minerals Most minerals which contain radioactive isotopes are in igneous rocks. The dates they give indicate the time the magma cooled. Potassium 40 is found in: potassium feldspar (orthoclase) muscovite amphibole glauconite (greensand; found in some sedimentary rocks; rare)
Datable Rocks Radioactive elements tend to become concentrated in the residual melt that forms during the crystallization of igneous rocks. More common in SIALIC rocks (granite, granite pegmatite) and continental crust.
Datable Rocks Radioactive isotopes don't tell much about the age of sedimentary rocks (or fossils). The radioactive minerals in sedimentary rocks are derived from the weathering of igneous rocks. If the sedimentary rock were dated, the age date would be the time of cooling of the magma that formed the igneous rock. The date would not tell anything about when the sedimentary rock formed.
How does 14 Carbon dating work? Cosmic rays from the sun strike Nitrogen 14 atoms in the atmosphere and cause them to turn into radioactive 14 C, which combines with oxygen to form radioactive CO 2.
How does 14 Carbon dating work? Living things are in equilibrium with the atmosphere, and the radioactive CO 2 is absorbed and used by plants. The radioactive CO 2 gets into the food chain and the carbon cycle.
How does 14 Carbon dating work? All living things contain a constant ratio of 14 C to 12 C (1 in a trillion). At death, 14 C exchange ceases and any 14 C in the tissues of the organism begins to decay to Nitrogen 14, and is not replenished by new 14 C.
How does 14 Carbon dating work? The change in the 14 C to 12 C ratio is the basis for dating. The half-life is so short (5730 years) that this method can only be used on materials less than 50,000 years old. Assumes that the rate of 14 C production (and hence the amount of cosmic rays striking the Earth) has been constant.
Deviation of carbon-14 ages to true ages from the present back to about 5000 B.C. Data are obtained from analysis of bristle cone pines from the western United States. Calculations of carbon- 14 are based on half-life of 5730 years. (Adapted from Ralph, E. K., Michael, H. N., and Han, M. C Radiocarbon dates and reality. MASCA Newsletter 9:1.)
Absolute Age Best candidates for most radiometric dating are igneous –Not necessarily useful for sediments Error in age estimate can be sizable
Absolute Age Absolute ages change –Error increases in older rocks –Techniques change Biostratigraphic correlations are usually more accurate –Radiometric dates used when fossils not present
How old is the Old Red Sandstone? a.Older than 425 myr b.Younger than 370 myr c.Between 425 and 370 myr d.Have no idea