Harry Williams, Historical Geology1 Introduction: The standard geologic time scale was not developed in an organized and systematic manner. Instead, the scale took shape over centuries as a number of geologists discovered and named the various intervals, on the basis of time-rock units identified by superposition, fossils and correlation (absolute ages from radiometric dating have only recently been added). Often the units were named after the local area or the type of rocks present. HISTORICAL GEOLOGY LECTURE 4. THE GEOLOGIC TIME SCALE.

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3 Divisions: Eons:Archean – ( million years ago). No rocks remain from ~ million years ago. Proterozoic ( million years ago) modern tectonics and sedimentation started in this eon. (Archean & Proterozoic together = Precambrian) Phanerozoic (evident life) fossils become abundant. Eras: Based on major changes in fossils. Paleozoic ( m.y.); Mesozoic ( m.y.); Cenozoic (65m.y. - present day). Periods: Again, based on fossil changes. Named after local features.

Harry Williams, Historical Geology4 Early Paleozoic period names are based on outcrops in the west of Great Britain: A. Cambrian B. Ordovician C. Silurian D. Devonian

Harry Williams, Historical Geology5 Mesozoic and Cenozoic period names are from locations in Europe and Russia. Perm Basin in Russia

Harry Williams, Historical Geology6 1. Cambrian; after cambria (latin for Wales) mybp. 2. Ordovician; after ordovices, early celtic tribe, mybp. 3. Silurian; after silures, English/Welsh tribe, mybp. 4. Devonian; after Devonshire, mybp. 5. Mississippian; from the Mississippi River basin, mybp. 6. Pennsylvanian; rock outcrops in Pennsylvania, mybp. (last two only used in N. America, elsewhere = Carboniferous, based on coal beds in northern England). 7. Permian; from Perm, a province of Russia, mybp. 8. Triassic; rocks in Germany, 3 divisions, mybp. 9. Jurassic; the Jura mountains in Switzerland, mybp. 10. Cretaceous; from 'creta' for chalk, mybp. 11. Paleogene (65 – 23 mybp); beginning of Age of Mammals. 12. Neogene ( mybp); Neo = newer part of Cenozoic, pre-ice ages. 13. Quaternary; suggested by a French geologist, mybp. Time of ice ages. (Absolute ages were not available when these divisions were first made).

Harry Williams, Historical Geology7 Epochs: Pleistocene – time distinguished by many glaciations or ice ages. 1.8 million – 10,000 years ago. Holocene – after the last ice age. 10,000 years ago to present day.

Harry Williams, Historical Geology8 Radiometric Dating Methods. Based on radioactive decay i.e. the nucleus of certain elements (PARENT ELEMENT) spontaneously lose or gain atomic particles and in so doing change to a different element (DAUGHTER ELEMENT). Atomic number = number of protons – defines element. Atomic mass = number of protons+neutrons Mass of proton=1, neutron=1, electron=0

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10 Example: Uranium 238 (U 238 ) decays to Lead 206 (Pb 206 ) via a large number of intermediate steps.

Harry Williams, Historical Geology11 When dealing with a large number of atoms, studies have shown how long it will take FOR HALF OF THEM TO DECAY (change from parent to daughter material) - this is the HALF LIFE.

Harry Williams, Historical Geology12 The ratio of PARENT ELEMENT to DAUGHTER ELEMENT thus follows a predictable pattern. Therefore, radiometric dating methods are based on finding the radioactive element in a mineral from which none of the daughter element has escaped and calculating the ratio between them to find the age of the mineral. Lead 206

Harry Williams, Historical Geology13 These radiometric dating methods (except C 14 ) are best suited to IGNEOUS rocks, since the radioactive elements and their daughter elements are "locked" in the mineral crystals when the rock solidifies from magma. These methods are not well-suited to metamorphic rocks since heating of the original rock may release the daughter elements, "resetting" the radiometric clock to the time of the last episode of heating. Using these methods for sedimentary rocks will give the age of the rock and mineral particles that make up the rock, not the time of deposition of the sediment.

Harry Williams, Historical Geology14 Instead, sedimentary rocks are often given minimum and maximum ages by radiometrically dating enclosing or intruding igneous rocks (including volcanic ash falls).

Harry Williams, Historical Geology15 Or, a rock of unknown age can be correlated (e.g. by fossils) to a rock of known age. As a result of these methods, thousands of sedimentary rocks around the world have been dated and absolute ages have been added to the geologic time scale.

Harry Williams, Historical Geology16 Radiocarbon Dating Radiocarbon (C 14 ) has a half-life of 5730 years and is usually used for dating ORGANIC MATERIAL up to about 100,000 years old - useful for archaeology and very recent geologic events, such as the last ice age (ended 10,000 years ago). The organic material can be any kind e.g. bones buried in a swamp; pieces of tree buried in a river floodplain; shells buried in ocean floor muds.....etc.

Harry Williams, Historical Geology17 Production of C 14 in the atmosphere by cosmic rays (high energy particles). The C 14 spontaneously decays to N 14 by Beta emission.

Harry Williams, Historical Geology18 C 14 is naturally produced in the atmosphere by cosmic rays at a fairly steady rate (2 atoms/second/square cm). Since C 14 is absorbed by plants (the bottom of the food chain), it makes its way into all living things.

Harry Williams, Historical Geology19 REGULAR CARBON (non-radioactive carbon 12 - the most common carbon) is also absorbed by all living things, such that: RATIO OF C 14 /C 12 CONSTANT IN ALL LIVING ORGANISMS Once the organism dies the C 14 /C 12 ratio decreases, since the organism is no longer absorbing new C 14. If C 14 /C 12 ratio = 50% of modern ratio, material is 1 half-life, or 5730 years, old; if ratio = 25%, material is 2 half-lifes, or 11,460 years old and so on.

Harry Williams, Historical Geology20 How can you possibly remember the period names in the correct order? Answer = a mnemonic