Geologic Time Terms Hadean Archean Proterozoic Phanerozoic Paleozoic Mesozoic Cenozoic(Tertiary) Cambrian Unconformity Angular unconformity Half-life Alpha particle Beta particle Gamma ray Neutron UT How do we determine if layers separated by large distances formed at the same time? WY How would we recognize a gap in the rock record if part of the rock record is missing and how much time the gap represents? CO Laws / Principles of Stratigraphy Timing of Geologic Events 1) relative-age dating (fossils, stratigraphy, structure) 2) absolute-age dating (isotopes, tree rings, etc.) Nicolaus Steno (1669) Law of Superposition Principle of Original Horizontality Principle of Lateral Continuity William Smith (1793) (1638-1686) Principle of Fossil Succession (1769-1839) 1
Law of Superposition Principle of Original Horizontality Youngest rocks Oldest rocks In a sequence of undisturbed layered rocks, the oldest rocks are on the bottom. Layered strata are deposited horizontally or nearly horizontally Use of Fossils to Correlate Rock Formations Principle of Fossil Succession William Strata Smith (1793) Recognized that different strata contained different fossils Recognized an order or succession of fossils and strata Used fossils to correlate formations from different outcrops Unconformity A surface that represents a break in the rock record due to erosion or nondeposition. Types of Unconformities Angular Unconformity Nonconformity Disconformity Angular Unconformity 2
Siccar Point, Scotland Devonian Old Red Sandstone Nonconformity Older tilted strata (shales and slates) Several unconformities are present in the Grand Canyon Disconformity 3 4 2 1 Several unconformities are present in the Grand Canyon Cambrian Tapeats Sandstone 4 3 2 1 Precambrian Wapatai Shale 3
South rim of the Grand Canyon 250 million years old South rim of the Grand Canyon 250 million years old Paleozoic Strata 550 million years old 1.7 billion years old Precambrian 550 million years old Nonconformity 1.7 billion years old Vishnu Schist Principle of Cross-cutting Relationships Host rocks (red) are older than the intruding rocks (black). Your turn Use the geologic principles to place the events in order What is this surface? Lava Flow (bed H) 4
How old is the Earth? By the mid 19th century a relative time scale had been worked out for the sedimentary rocks of Europe (Phanerozoic). They lacked an absolute time scale. Kelvin and classical physicists advocated 40 million max. Darwin and evolutionary biologists advocated billions of years. Discovery of radioactivity at about 1900 confirmed billions. Phanerozoic Geologic Time Scale Geologic Time Scale Time divisions (units) of Earth s history as recorded by rock formations based originally on relative-dating methods: Fossil groups or assemblages Fossil succession (order of fossils) Stratigraphic relationships Cross-cutting relationships and later Absolute (isotopic) ages The Age of the Earth Bishop Ussher - 17th Cent. (biblical): 4004BC Buffon - 18th Cent. (Cooling of spheres): ~50000 Y Hutton - late 18th Cent. (Geological cycles): Infinite Darwin - late 19th Cent. (Biological changes): Billions Kelvin - late 19th C (Sun s energy): 40 Million Max Modern - (Radiometric): 4.55 Billion 5
Absolute-Age Dating Absolute ages of geologic events and rock formations are based on radioactive elements and the rates at which they decay. Many isotopes of each element occur naturally Isotope: a variety of an element with the normal number of protons, but different number of neutrons http://ie.lbl.gov/education/isotopes.htm Rocks are composed of minerals, Minerals are composed of atoms of different elements 1. Proton: positive charge 2. Neutron: no charge 3. Electron: negative charge Isotope The number of protons determines the element the atomic number The neutrons of a given element may vary ISOTOPE: variations of the same element, with different # of neutrons, and so different atomic mass number 6
Isotopes of Carbon Radioactive decay: Spontaneous changes in structure of atomic nuclei Types of radioactive decay 1. Alpha emission: Emission of 2 protons and 2 neutrons Which of the following accurately describes alpha emission A) Atomic number lower by 2; atomic mass unchanged B) Atomic number lower by 2; atomic mass lower by 2 C) Atomic number lower by 2; atomic mass lower by 4 D) Atomic number lower by 4; atomic mass lower by 4 Example of alpha emission U 238 Th 234 2. Beta emission A neutron loses an electron and turns into a proton; the electron is ejected from the nucleus What is change in 1) Atomic number? 2) Atomic mass? -2-4 Which of the following accurately describes beta emission A) Atomic number unchanged; atomic mass unchanged B) Atomic number increases by 1; atomic mass unchanged C) Atomic number decreases by 1; atomic mass unchanged D) Atomic number increases by 1; atomic mass dec by 1 Th 234 Pa 234 3. Electron capture: An electron is captured, combines with a proton to form a neutron Parents and Daughters Parent: an unstable radioactive isotope Daughter product: isotopes resulting from decay of parent Keep track of the ratio # of daughter (D) to # of parents (N): D/N What is change in 1) Atomic number? 2) Atomic mass? -1 0 K 40 Ar 40 7
Radiometric Dating: Establishing an absolute time scale Minerals contain naturally radioactive elements K, U, Th, Rb, Sm These radioactive parent isotopes decay to stable daughter isotopes When minerals crystallize from melt, they contain parent only. If we measure the concentration of daughter isotope in a mineral and we know the decay rate, we can calculate when the mineral crystallized. Types of Radioactive Decay Particle composed of: Mass# Atomic # Example alpha 2 neutrons+ 4 2 U, Th, 2 protons beta- electron 0-1 40 K beta+ positron 0 +1 40 K gamma photon 0 0 all nuclear reactions neutron neutron 1 0 235 U Common types of radioactive decay An Example: U 238 to Pb 206 Radioactive decay curve The half-life of a radioactive isotope is the time required for half of the original number of radioactive parent atoms to decay to stable daughter products. Fraction of elements present 8
Half-lives: If the amount of radioactive isotope (the parent) is ¼ the amount originally present, how many half-lives have gone by? A. 1 B. 2 C. 3 D. 4 Naturally Radioactive Isotopes Parent Daughter Half life 40K 40Ar 1.3 x 10 9 y β + 87 Rb 87Sr 4.9 x 10 10 y β - Decay 238 U 206Pb 4.5 x 10 9 y 8α, γ 235 U 207Pb 7.1 x 10 8 y 7α, γ 232 Th 208Pb 1.4 x 10 10 y 6α, γ 14 C 14 N 5.7 x 10 3 y β - Radiometric Dating Example: 40 K - 40 Ar A K-feldspar (KAlSi 3 O 8 ) crystallizes in a granite and initially contains no Ar. Natural K is 0.012% 40 K 40 K decays to 40 Ar with a half-life of 1.31 x 10 9 years (1.3 billion years). If we measure the 40 Ar content of the feldspar, we can get a crystallization date of the mineral. Isotope measurements are made with a mass spectrometer. Some Major Events Latest warming 7000y Ice age ~1.8 MY Dinosaur extinction 66 MY Dinosaurs ~245 MY Vertebrates ~400 MY Multi-cell life forms ~550 Cambrian Explosion Snowball earth 600 MY Free O 2 ~ 2.5 GY (CH 4 and NH 3 decline) Single cell life forms ~3.7 GY Oceans: at least by 4.3 GY Accretion: 4.55 GY 9
The era of dinosaurs is subdivided into Triassic, Jurasssic, and Cretaceous. Together these are known as the: Dike @ 66 million years old A. Archean B. Proterozoic C. Paleozoic D. Mesozoic E. Cenozoic Ash bed @ 160 million years old This surface represents: A. A fault B. A fold C. An unconformity D. The Phanerozoic Geologic Time Terms Hadean Archean Proterozoic Phanerozoic Paleozoic Mesozoic Cenozoic(Tertiary) Cambrian Unconformity Angular unconformity Half-life Alpha particle Beta particle Gamma ray Neutron Why can t 14 C be used to date limestones? A. No carbon in limestone B. 14 C half-life too short C. 14 C half-life too long D. Daughter 14 N not retained by limestone 10