Chapter 4: Geologic Time

Transcription

1 Chapter 4: Geologic Time The history of the Earth is written in its rocks Rocks record events and life-forms that are long gone. The rock record is far from complete the history of the Earth becomes more difficult to decipher as we look further back in time. One of the goals of geology is to learn the history of the Earth. Much of the Earth s history is contained in sedimentary rocks, as they are deposited sequentially and contain fossils. How Old is the Earth? Bishop Ussher (Archbishop of Armagh, Primate of All Ireland and Vice-Chancellor of Trinity College in Dublin, ) interpreted the Bible literally and calculated the creation of the Earth to be on Sunday, October 23, 4004 BCE. In addition, here determined that the ark touched down on Mt. Ararat on Wednesday May 5, 1491 BCE. He based these dates on a chronology of the Bible and added up the generations of men and women in the Bible. His calculations resulted in an Earth that is ~6000 years old and only about 116 generations have passed since creation! More recently, creationists have switched to an age of the Earth that is in excess of 10,000 years; this is because archeological evidence is so clear about the existence of human civilizations predating 4004 BCE. 1

2 James Hutton (Scottish naturalist) published a paper in 1785 entitled Theory of the Earth. He originated the idea known as uniformitarianism, the fundamental principle on which geology is now based. The previous idea was known as catastrophism and postulated that geologic formations were caused by sudden catastrophic upheavals. Catastrophism states that geologic features and landscapes are the result of catastrophic events (Noah s flood). Uniformitarianism states that the physical, chemical and biological processes that operate today have operated in the geologic past. These small changes over long periods of time are responsible for the geologic features such as mountains. Hutton believed that rocks are formed and modified by a continuous and uniform series of natural phenomena such as rain, tides, winds and gradual shifts in the Earth's crust. "Soils form by the weathering of rocks; tides and the pounding of waves erode the coast; layers of sediment accumulate; and the general cycles of sedimentation, uplift of hills and mountains, and erosion can be seen everywhere." Bottom line: past phenomena and rock formations can be explained by current ordinary processes (e.g. weathering and erosion) that we can observe plus time. 2

3 This unconformity on the Scottish coast led Hutton to utter on the enormity of geologic time, that we find no vestige of a beginning, no prospect of an end. James Hutton is recognized as the father of a new science known as Geology. He was violently opposed and branded a heretic. 3

4 In 1830, Charles Lyell published his Principles of Geology in which he adopted Hutton's views of uniformitarianism - adding more evidence to this new theory. He convincingly destroyed the theory of catastrophism. Faced with the overwhelming evidence, people finally began to accept the concept that ordinary, slow geologic processes can have a profound effect on the Earth. It also became accepted that the Earth was older than could be accounted for by biblical chronologies. Nevertheless, even though it became accepted that the Earth was older than originally thought and that our planet "evolved" with time, it was still unacceptable (scientifically and religiously) to suggest that life evolved. Recognition of the age of the Earth was an important first step to Darwin's theory of evolution and natural selection. 4

5 Geology Needs a Time Scale Geological events by themselves have little meaning until they are put into the perspective of time. One of geology s major contributions to human knowledge is the geologic time scale and the revelation that the Earth's history is immense. Geological Dating Absolute Dating: we may put a numerical date on geologic events such as the formation of the Earth 4.6 billion years ago or that dinosaurs became extinct 65 million years ago. These dating methods became available since the advent of modern radiometric dating techniques using nuclear clocks to pinpoint events in geologic time. Relative Dating: rocks and events are placed in their proper sequence of formation. We may have no idea how long ago (number of years) an event took place, but can place it before and/or after other events. There are several principles involved in developing a relative time scale - they seem simple or intuitive but were revolutionary when they were first formulated. 1. Law of Superposition 2. Principle of Original Horizontality 3. Principle of Cross-Cutting Relationships 4. Principle of Lateral Continuity 5

6 Law of Superposition in an undeformed sequence of sedimentary rocks, each bed is older than the one above and younger than the one below. This law was first clearly stated by Nicolaus Steno (1669). Fig. 8.2 Applying the law of superposition to these layers in the upper portion of the Grand Canyon - rocks of the Supai Group (red) are oldest, followed by the Hermit Shale (green), Coconino Sandstone (yellow), Toroweap Fm. (gray) and the Kaibab Limestone (brown). Principle of Original Horizontality layers of sediment are generally deposited in a horizontal position. If we find rocks that have been folded or tilted, they must have been moved into that position after their deposition. 6

7 Principle of Cross-Cutting Relationships We can determine the relative order of the formation of rocks and other feature (ex. faults) from their geometric arrangement. Fault A occurred after the deposition of the sandstone but before the conglomerate. Dike A is younger than Dike B and its associated sill. The batholith was emplaced after Fault B but before Dike B. Fig. 8.4 Unconformities breaks in the rock record; represents a long period during which deposition ceased, erosion occurred and then deposition resumed. Angular Unconformity - tilted or folded strata overlain by younger, typically flatter-lying strata. Nonconformity - younger strata deposited on the erosion surface of igneous or metamorphic rocks. Disconformity - erosion surface between two relatively flatlying formations. This is much more common and less conspicuous. 7

8 Angular Unconformity Nonconformity Fig. 8.7 Fig

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10 Using Relative Dating Principles What happened next? Exercises in Relative Dating 10

11 Exercises in Relative Dating 11

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13 Fossil Succession Age of Trilobites Age of Geeks? Age of Reptiles Age of Fishes Age of Mammals 13

14 Absolute Dating We can use some elements as atomic clocks Recall that atoms have a nucleus, composed of protons (+) and neutrons (neutral), which is orbited by electrons (-). Neutrons may be thought of as a proton combined with an electron - yielding a neutral charge. Recall also that an atom s mass number is the sum of the number of protons and neutrons - variation in the number of neutrons yields different mass numbers - these variants are called isotopes. Example: Uranium has an atomic number of 92, which is its number of protons all uranium atoms have 92 protons. BUT its number of neutrons can vary yielding different isotopes: 234 U 235 U 238 U They each look and behave (chemically) exactly the same 14

15 Radioactivity Radioactivity is the spontaneous decay or breaking apart of the nucleus of an isotope. There are several decay processes that result in the emission of sub-atomic particles, changing the number of protons in the nucleus and thus the identity of the element. Unstable radioactive isotopes are called parents and the products of radioactive decay are called daughter products. Alpha (α) Decay Alpha particles (composed of 2 protons & 2 neutrons) are released from the nucleus which reduces the mass number of the isotope by 4 and the atomic number by 2. Beta (β) Decay Beta particles (electrons) are released from the nucleus, causing a neutron to become a proton, and increasing the atomic number of the element by 1. 15

16 Electron Capture This decay mechanism involves combining an electron with a proton to create a neutron - one less proton in the nucleus causes a decrease in the atomic number. As an example, 238 U decays to 206 Pb by a series of alpha and beta decays: 238 U => 8α + 6β Pb Recap 16

17 Half-Life The half-life is the amount of time required for one-half of the nuclei in a sample to decay. The half-life for an isotope is essentially a constant. Half-lives range from less than a second to billions of years. The figure illustrates the decay of a radioactive isotope to its daughter isotope as a function of time. When equal amounts of the parent and daughter exist then 1 half-life has take place. This is the foundation for isotopic age dating techniques such as the 14 C method. 17

18 The systematic and predictable change in the isotopic composition of a mineral can be used as a very effective clock. Certain minerals, such as zircon (in the photos) may contain radioactive U and Th. These radioisotopes will decay by various mechanisms until they reach a stable isotope - in this case, various isotopes of Pb. By measuring the amount of the parent (U & Th) and daughter product (Pb isotopes), the age of the mineral may be determined. Alkali feldspar contains radioactive 40 K which decays to 40 Ar. By measuring the relative amounts of 40 K and 40 Ar in a sample, the age of the mineral may be determined. Geologic Time Geologic time is immense. Initially, geologists attempted to organize or systematize geologic time based upon relative age dating. However, as the technology for absolute dating developed, the geologic time scale became quantitative. 18

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