Correlating beds using index fossils Stratigraphic Classification: Piles of stratified rocks need to be classified. A formation is an easily identifiable rock unit that differs from layers above and below Classification of Stratigraphic Rock Units Classification Hierarchy Supergroup Group Formation Member Bed Local example (Strata under UP) Transvaal Supergroup Pretoria Group Hekpoort Formation n/a n/a Formation is a unit of similar strata, easily recognizable from adjacent strata The principle of fossil succession allows strata in different parts of the world to be correlated, and worldwide relative ages to be worked out Fig. 12.14a W. W. Norton 1
Correlation across continents has allowed geologists to construct a chart, showing the entire history of Earth. This is known as a geological column or a stratigraphic chart Eon Names of the subdivisions of the geological column Phanerozoic (visible life) Proterozoic (early life) Archaean (ancient) Eras Cenozoic (recent life) Mammals and flowering plants Mesozoic (middle life) Reptiles Palaeozoic (old life) Invertebrates, fish, amphibians Precambrian Hadean Within the Phanerozoic, each Era is divided into Periods Young Cenozoic: Palaeogene, Neogene Mesozoic: Triassic, Jurassic, Cretaceous Palaeozoic: Old Cambrian Ordovician, Silurian, Devonian, Carboniferous, Permian Neogene Palaeogene The Geological Column and Time Scale Correlation and stratigraphy only tells us about relative ages of stratigraphic units, not the absolute ages. Absolute ages could not be determined until the early 20 th century Absolute ages allow geologists to: 1. See how old the Earth and the solar system really are 2. To place absolute ages on the geological column 2
Early scientific attempts to measure geological time were based on: 1. Sedimentation rates-lyell (3-1500 Ma) 2. Salinity of seawater Halley and Joly (90 Ma) 3. Rates of heat loss-kelvin (100 Ma) Calculations on salinity of the sea were inaccurate because: 1. Salts are constantly removed from seawater (evaporites) 2. Salts are constantly added to seawater by submarine volcanism (e.g. at mid-oceanic ridges). Lord Kelvin s calculations did not take into account that heat could be added by radioactive decay. Ironically, radioactive decay is the way in which the age of rocks can be measured. Ernest Rutherford: discovery of radioactivity in rocks 1904 In an atom of an element, the number of protons and electrons is fixed, but the number of neutrons is variable. Atoms of the same element, with different numbers of neutrons are called isotopes. Naturally Occurring Isotopes of Carbon 3
If the ratio of neutrons to protons is too high or too low, the nucleus becomes unstable. It spontaneously changes to a more stable nucleus, even if a nucleus of a different element is produced Beta Decay Electron Capture Alpha Decay (2n+2p) The initial (unstable) radioactive element is called the parent. The stable element produced is called the daughter. e.g. 14 C 14 N 238 U 206 Pb Radioactive decay can happen in one of five ways: 1. Emission of electron (β - particle) n p = β - 2. Emission of positron (β + ) p n = β + 3. Electron capture p+electronn 4. Emission of α particle (2p+2n) 5. Emission of γ rays (loss of energy) Rates of Decay: The transformation (decay) from parent to daughter varies in speed between elements. Chemical and physical environment does not affect rates of decay. The proportion of parent to daughter atoms is a function of time Fig. 12.17bc W. W. Norton t t t t t= half-life 4
Radioactivity and Absolute Time Fig. 12.17a decay of parent atoms decay of daughter atoms W. W. Norton The decay begins when a radioactive isotope of an element (e.g. 40K) is incorporated from an igneous melt into a mineral. Therefore we can measure the time at which an igneous rock crystallises "To seek the first principles of things." (Lucretius) Useful dating techniques: Uranium (238U- 4.5Ga, 235U- 710 Ma) Thorium (232Th- 14 Ga) Potassium (40K- 1.3 Ga) Rubidium (87Rb- 47 Ga) [Carbon (14C- 5730 a)] 5
Radiometric dates provide absolute ages to the Geologic Column The Geological Column and Time Scale The Earth (and the rest of the solar system) is about 4.6 Billion years old (4,600,000,000 years: 4.6 Ga). This is based on the ages of Moon rock (which is never recycled into the Moon s mantle, as plate tectonics does not operate on the moon), and from the age of meteorites that land on Earth Another useful method of getting relative ages: Magnetic reversals. Every 700000 years or so (though it is highly variable) the polarity of the Earth s magnetic field changes between North and South. Lavas/tuffs which cool within each period inherit the same polarity Earth s Magnetic Field Magnetization of Magnetite 6
Lavas record magnetic reversals Magnetic reversals over the past 20 million years 7