Chapter 9 Geologic Time I. Some Historical Notes About Geology People have studied the earth (materials and processes) for centuries. Before the birth of modern geology in the late 1700s most explanations were based in religion. A. Catastrophism - James Ussher (Anglican Bishop in Ireland) counted generations in the Bible - Stated that the earth was formed in 4004 B.C. - Catastrophism is the belief that most of the earth's landscapes are the result of catastrophic events. - This idea was popular in the 1600 and 1700s. B. The Birth of Modern Geology - The birth of modern geology began with the work of James Hutton. - J.Hutton put forth the principle of uniformitarianism. - Uniformitarianism states that the physical processes that operate today are the same as the processes that were at work in the past. - Charles Lyell further advanced the idea. C. Geology Today - Today we know that the Earth is about 4.5 billion years old. - There have been many cycles of mountain building and erosion. II. Relative Dating vs. Absolute Dating - The primary idea behind relative dating is to place geologic events in order. - There is no attempt made at assigning numerical dates to events. - Absolute dating is the way to use the isotope analysis of rocks in order to assign the numerical dates to events. III. Relative Dating basic laws and principles A. Principle of Superposition - Nicolaus Steno proposed the Principle of Superposition (1669). - This Principle states - in a stack of undeformed sedimentary rocks, the layer on the bottom is older than the layer on top. B. Principle of Original Horizontality - Sedimentary layers are deposited horizontally. - If sedimentary rocks are tilted or folded, they have been changed after deposition. - This enables one to place deformation events in sequence. C. Principle of Cross-Cutting Relationships (fig 9.4) - When a fault or igneous body cuts across a rock unit, it is younger than that unit. - The sedimentary rock unit had to exist in order to be cut. - Fig.9.4. Cross-cutting relationships D. Inclusions - Inclusions are pieces of one rock unit included in another. - The rocks that have been caught up in sediments are older than the new rock created with them. Fig. 9.5 These diagrams illustrate two ways that inclusions can form. E. Unconformities Unconformities are breaks in the rock record. Unconformities represent periods of non-deposition or periods of erosion. Parts of the rock record are missing. Angular Unconformity - Angular unconformities form when younger flat lying strata overlie older, tilted strata. 2. Disconformity - Disconformities form when a period of erosion or non-deposition occurs, but there is no tilting of strata. - Strata are at the same orientation above and below the unconformity. 1. 3. Nonconformity - Nonconformities are form when sedimentary rocks overlie older igneous rocks. Fig. 9.6. This cross-section through the Grand Canyon illustrates the three basic types of unconformities F. Using Relative Dating Principles - Relative dating uses these principles to put geologic events in order. IV. Correlation of Rock Layers - Correlation is the process of matching up rock units of similar age on a local level and over a long distance. - On the local level, this is done mainly by visual inspection of the rocks. - Correlation over a long distance requires the examination of a fossils. V. Fossils: Evidence of Past Life - Fossils are evidence of past life. - They provide evidence of environmental conditions. - They also can be used to determine the age of rocks. A. Conditions Favoring Preservation - The two conditions that favor preservation are the presence of hard parts and rapid burial. - When an organism dies, the soft tissue decays. - Hard parts like bone, teeth and shell are more easily preserved. - If an organism is buried soon after death, it is less likely to be scattered and destroyed by scavengers. B. Fossils and Correlation - Principle of Fossil Succession - fossil organisms succeed each other in a definite and determinable order. - Fossils can be used to determine the age of rocks they are in. - Index Fossils are fossil organisms that only lived for a short time, but were numerous and widespread. - If that fossil appears within the rock, the age of the rock is known. - It is also possible to determine environmental conditions that were present. Fig. 9.12. Overlapping ranges of fossils help date rocks more exactly than using a single fossil. VI. Dating with Radioactivity - The other means of dating rocks is done with radioactivity. - <a href="/keyword/radiometric-dating/" >radiometric dating</a> provides numerical dates where relative dating only places events in order. A. Reviewing Basic Atomic Structure - There are three kinds of atomic particles. - Protons and neutrons are contained within the nucleus. - Protons are positively charged. - Neutrons are neutral. - The third type of particle is the electron. - Electrons have a negative charge and orbit the nucleus. - Atomic number is the number of protons and defines the element. - Mass number is the number of protons and neutrons. - Isotopes are atoms of an element but have different numbers of neutrons. B. Radioactivity - Radioactivity is the process by which unstable nuclei break apart. - In the process, they emit a particle. 1. Alpha particles consist of two protons and two electrons, charge =+2, mass = 4 2. Beta particles electrons ejected from the nucleus when a neutron decays, charge = -1, mass = negligible. 3. Gamma particles protons (high energy X-rays, light) highly penetrating electromagnetic radiation. No charge or mass. - In this process, the parent material converts into a new element the daughter product. - The ratio of parent to daughter is a function of time. - Radioactive decay provides a very accurate clock that marks the passage of geologic time. - This clock can be read by examining the ratio of parent isotope to daughter isotope. - The process of radioactive decay is unaffected by heat, pressure or any other geologic processes. - <a href="/keyword/radiometric-dating/" >radiometric dating</a> is the process of using radioactive decay to determine the age of a rock. C. Half-life - Half-life is the time required for one-half of the parent material to decay into the daughter element. - If the ratio of parent material to daughter is 1:1 then one half-life has transpired. - As time passes, the amount of parent material decreases and the amount of daughter material increases in a predictable pattern. Fig. 9.15 The radioactive decay curve shows change that is expotential. Half of the radioactive parent remains after one half-life. After a second half life one-quarter of the parent remains and so forth. D. <a href="/keyword/radiometric-dating/" >radiometric dating</a> - There are five isotopes commonly used in <a href="/keyword/radiometric-dating/" >radiometric dating</a> of ancient rocks. - These are Uranium-238, Uranium-235, Thorium-232, Rubidium-87, Potassium-40 - These have been useful because of the length of their half-lives and the fact that these elements are easily found in igneous rocks. Fig. 8.16 (from Lab Manual) Some isotopes useful for <a href="/keyword/radiometric-dating/" >radiometric dating</a> and their decay parameters. Examples The length of an isotope's half-life determines what sort of dating it is useful for. C14 has a half-life of 5730 years, + or - 30 years. Thus, this best-known dating method is actually only useful for the past 70,000 years or so (otherwise all the C14 atoms would be gone). It is crucial for archaeology, but less useful for geology. C14 is a component of the atmosphere. It's ratio is well-known, and is absorbed into living things at that constant ratio with C13 and C12. When those living organisms die, the C14 begins to decay into C12, starting the radioactivity clock. 1. Sources of Error - <a href="/keyword/radiometric-dating/" >radiometric dating</a> assumes that the system is closed. - This means that none of the parent or daughter has been removed from the system. - It also assumes that none of the parent or daughter has been added. - Changes in the amount of the elements in study negatively impact the date. 2. Importance of <a href="/keyword/radiometric-dating/" >radiometric dating</a> - <a href="/keyword/radiometric-dating/" >radiometric dating</a> has produced thousands of dates. - Granite from South Africa has been dated at 3.2 billion years. - <a href="/keyword/radiometric-dating/" >radiometric dating</a> has demonstrated that the Earth is very old. - This gives credibility to the idea that the small incremental changes can produce great change. - <a href="/keyword/radiometric-dating/" >radiometric dating</a> supports a uniformitarian view of earth history. VII. The Geologic Time Scale - Geologists divide geologic times into units. - These units are arranged based on knowledge of index fossils. - The arrangement of these units into a chronology of earth history has produced the geologic time scale. A. Structure of the Time Scale - The geologic time scale divides 4.5 billion years of earth history into events - The largest division is the eon. - The Phanerozoic Eon (meaning &quot;visible life&quot;) began 540 million years ago. Fig. 9.17. The geologic time scale. The numerical dates were added long after the time-scale had been established using relative dating techniques. The Phanerozoic Eon is <a href="/keyword/further-subdivided/" >further subdivided</a> into eras. These are the Paleozoic (means paleo &quot;Ancient&quot;, zoic &quot; life&quot;) Era, the Mesozoic (means meso &quot;Middle&quot; , zoic - life) Era, and the Cenozoic (means ceno &quot;Recent&quot; life) Era. - Each era is <a href="/keyword/further-subdivided/" >further subdivided</a> into periods. B. Precambrian Time - 4 billion years of time exist before the Cambrian Period of the Phanerozoic Eon. - This interval has long been known as the &quot;Precambrian&quot;. - It represents about 88% of earth history. - It is now divided into the Hadean Eon, The Archean Eon, and the Proterozoic Eon. - At this time, not enough data exists to divide these into eras and periods. -