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1 Geologic Time Earth 9th edition, Chapter 9 Geology 100 2 Key Concepts Geologic time and the “rock record.” Putting events in order – Relative dating laws and principles. Using principles of relative dating – Interpretation and correlation of rock units. Evidence of past life and environments – Fossils. Determining ages of events – Radioactive dating. The geologic time scale. 3 Determining geological ages Relative age dates Placing rocks and events in their proper sequence of formation Numerical dates Specifying the actual number of years that have passed since an event occurred Also known as absolute age dating 4 Principles of relative dating Law of superposition Developed Nicolaus Steno in 1669 In an undeformed sequence of sedimentary rocks (or layered extrusive igneous rocks), the oldest rocks are on the bottom 5 6 Superposition illustrated in the Grand Canyon Principles of relative dating Principle of original horizontality Layers of sediment are generally deposited in a horizontal position Rock layers that are flat have not been disturbed Principle of cross-cutting relationships Younger features cut across older features 7 8 9 10 11 12 13 14 15 Cross-cutting Relative Geologic Dating Cross-cutting Cross-cutting Inclusions Inclusions Inclusions Unconformity Principles of relative dating Types of unconformities Angular unconformity Tilted rocks are overlain by flat-lying rocks Disconformity Strata on either side of unconformity are parallel Nonconformity Metamorphic or igneous rocks are in contact with sedimentary rocks 16 17 18 19 20 21 Formation of an angular unconformity Unconformity Types Any unconformities in this picture? Unconformities present in the Grand Canyon Starting at the bottom: nonconformity Starting at the bottom: angular unconformity 1 22 23 24 Toward the top: disconformities From the top: Redwall limestone Correlation of rock layers Matching of rocks of similar ages in different regions is known as correlation Correlation often relies upon fossils William Smith (late 1700’s) noted that sedimentary strata in widely separated areas could be identified and correlated by their distinctive fossil content 25 Correlation of rock layers Principle of fossil succession Fossil organisms succeed one another in a definite and determinable order Any time period can be recognized by its fossil content Index fossils Widespread geographically Limited to short span of geologic time 26 27 28 Determining the ages of rocks using fossils Relative dating Using radioactivity in dating Reviewing basic atomic structure Nucleus Protons zPositively-charged particles with mass of 1 a.m.u. Neutrons zNeutral particles with mass of 1 a.m.u. Electrons Negatively-charged particles that orbit the nucleus 29 Using radioactivity in dating Reviewing basic atomic structure Atomic number An element’s identifying number Equal to the number of protons in the atom’s nucleus Mass number Sum of the number of protons and neutrons in the atom’s nucleus 30 Using radioactivity in dating Reviewing basic atomic structure Isotope Variant of the same parent atom Differs in the number of neutrons Results in a different mass number than the parent atom 31 Using radioactivity in dating Radioactivity Spontaneous changes (decay) in the structure of atomic nuclei Types of radioactive decay Alpha emission Emission of 2 protons and 2 neutrons (an alpha particle) Mass number is reduced by 4 and the atomic number is lowered by 2 32 Using radioactivity in dating Types of radioactive decay Beta emission An electron (beta particle) is ejected from the nucleus Mass number remains unchanged and the atomic number increases by 1 34 35 Types of radioactive decay* Using radioactivity in dating 2 Parent An unstable radioactive isotope Daughter product The isotopes resulting from the decay of a parent Half-life The time required for one-half of the radioactive nuclei in a sample to decay 36 37 38 39 A radioactive decay curve Radioactive Decay Uranium – 238 decay series Using radioactivity in dating Principle of radioactive dating The percentage of radioactive atoms that decay during one half-life is always the same (50%) However, the actual number of atoms that decay continually decreases Comparing the ratio of parent to daughter yields the age of the sample 40 41 Using radioactivity in dating Sources of error A closed system is required To avoid potential problems, only fresh, unweathered rock samples should be used 42 Using radioactivity in dating Dating with carbon-14 (radiocarbon dating) Half-life of only 5730 years Used to date very recent events Carbon-14 is produced in the upper atmosphere Useful tool for anthropologists, archeologists and Quaternary geologists 43 Using radioactivity in dating Importance of radiometric dating Radiometric dating is a complex procedure that requires precise measurement Rocks from several localities have been dated at more than 3 billion years Confirms the idea that geologic time is immense 44 Geologic time scale A “calendar” of Earth history Subdivides geologic history into units Originally created using relative dates Structure of the geologic time scale Eon The greatest expanse of time 45 Geologic time scale Names of the eons Phanerozoic “Visible life” Most recent eon (we’re still there…) Began about 540 million years ago Proterozoic 2.5 Ga Æ 540 Ma Archean Oldest rocks (up to ~4 Ga) Hadean Oldest eon (no rock record) 46 Geologic time scale Era – subdivision of an eon 3 Eras of the Phanerozoic eon Cenozoic (“recent life”) Mesozoic (“middle life”) Paleozoic (“ancient life”) Eras are subdivided into periods Periods are subdivided into epochs 47 48 49 50 Major divisions of geologic time Simplified divisions of geologic time Complex divisions of geologic time Geologic time scale Precambrian time Nearly 4 billion years up to the start of the Cambrian period Not divided into smaller time units Events of Precambrian history are not known in great enough detail zFirst abundant fossil evidence does not appear until the beginning of the Cambrian 51 Geologic time scale Difficulties in dating the geologic time scale Not all rocks can be dated by radiometric methods Grains comprising detrital sedimentary rocks are not the same age as the rock in which they formed The age of a particular mineral in a metamorphic rock may not necessarily represent the time when the rock formed 52 Geologic time scale Difficulties in dating the geologic time scale Datable materials (such as volcanic ash beds and igneous intrusions) are often used to bracket various episodes in Earth history and arrive at ages 53 55 Dating sedimentary strata using radiometric dating End of Chapter 9 4