Chapter 11, Geologic Time - Subduction.net

Chapter Outlines

NOTE: This is intended to help students ‘organize’ their understanding of each topic. It is not a comprehensive study guide for quizzes or midterms, i.e. study your text! From Lyell’s book ‘Principles of Geology’

Chapter 11 – Geologic Time

Geologic time is deep. Earth’s age of approximately 4.5 billion years is just a start. But, in geology, relative time is often more useful than absolute time (numerical time). Our knowledge of Earth’s history comes from a number of sources. The geologic time scale is constructed through scientific methods and calculations as well as from the interrelationships of geological features as observed in the field. The principle of uniformitarianism (geologic processes in action today, were also in action in the past and the distant past) is helpful in that we can accurately measure the rates of geologic processes we see today and apply them to the geologic past. Famous outcrop in Scotland

interpreted by Hutton 1) Historical notes a) Catastrophism – the early belief that landscape had developed by ancient catastrophes b) Prior to the 19th century, accepted age of Earth was based on religious beliefs i) Chinese/Hindu: Old beyond comprehension ii) Biblical: James Ussher (mid-1600s) concluded that Earth was only a few thousand years old c) James Hutton, MD: ‘Father of Geology’ (1726 – 1797) i) realized geologic processes require vast amounts of time ii) The fundamental principle of geology "The present is the key to the past" iii) ‘Principle of uniformitarianism’ grew from his ideas d) Charles Lyell (1797 – 1875) Popularized Hutton’s concepts in the Charles Lyell book ‘Principles of Geology’. 2) Relative time dating, the placing of rocks and events in sequence Principles and rules of relative dating: i) Law of superposition—oldest rocks are on the bottom This illustration has examples of: ii) Principle of original horizontality—sediment is deposited horizontally  Original horizontally Superposition iii) Principle of cross-cutting relationships —younger feature cuts through an older feature  Lateral continuity iv) Inclusions (one rock contained within another)—rock containing the inclusions is younger  Cross-cutting relationship  inclusions v) Unconformities - an unconformity is a break in the rock record. Types: (1) Angular unconformity—tilted rock beds are overlain by flat-lying rock beds (2) Disconformity—strata above and below the unconformity are parallel (3) Nonconformity - metamorphic or igneous rocks below & younger sedimentary rocks above

3) Correlation of rock layers – the matching of rocks of similar age in but different regionsRelies on fossils or other distinctive properties of a rock unit 4) Fossils - Remains or traces of prehistoric life a) Types of fossils: i) Petrified—cavities and pores are filled with precipitated mineral matter ii) Formed by replacement—cell material is removed and replaced with mineral matter iii) Mold—shell or other structure is buried and then dissolved by underground water A disconformity (dashed lines represent iv) Cast—hollow space of a mold is filled with mineral matter strata correlation) v) Carbonization—organic matter becomes a thin residue of carbon vi) Impression—replica of the fossil's surface is preserved in fine-grained sediment vii) Preservation in amber— hardened resin of ancient trees surrounds an organism viii) Indirect evidence: Tracks, burrows, coprolites —fossil dung and stomach contents, gastroliths—stomach Fossils of fish stones used to grind food by some extinct reptiles b) Conditions favoring preservation: Rapid burial and possession of hard parts c) Fossils and correlation Page 1 of 2

Compiled by Arthur Reed

Print Date: June 24, 2011

i)

Principle of fossil succession: (1) Fossils succeed one another in a definite and determinable order (2) Proposed by William Smith—late 1700s and early 1800s ii) Index fossils (1) Widespread geographically (2) Existed for a short range of geologic time 5) Absolute time (numerical time) – age in years, commonly established by radioactive dating Using fossil Earth is estimated to be 4.5 billion years old. The oldest rock found on Earth so far is 4.03 billion correlation years old (Canada). Oldest mineral is a zircon at 4.4 billion years (Australia). 6) Radioactivity and radiometric dating (radioactivity is naturally occurring) a) Atomic structure reviewed i) Nucleus - Protons (positively charged), neutrons (a proton & electron combined=neutral charge) ii) ‘Orbiting’ the nucleus are electrons—negative electrical charges iii) Atomic number – an element's identifying number - number of protons in the atom's nucleus defines the element iv) Mass number - number of protons plus the number of neutrons in an atom's nucleus Isotope is a variant of the same parent atom, with different number of neutrons and therefore a different mass number from that of the parent Decay by half-lives b) Radioactivity – the spontaneous breaking apart (decay) of atomic nuclei i) Parent—an unstable isotope ii) Daughter products—isotopes formed from the decay of a parent iii) Types of radioactive decay: Alpha emission, Beta emission, Electron capture c) Radiometric dating – yields numerical dates i) Half-life—the time required for half of the radioactive element to decay to the daughter Types of ii) Uranium 238 converts (decays) spontaneously to lead 206 at the rate of ½ of its atoms every radioactive decay 4.5 billion years (its half-life) d) Carbon-14 dating – used to date recent events (approximately the last 40,000 years) The percentage of the isotope carbon-14 in a living organism is a known constant. Once that organism dies, the carbon-14 begins to decay to a ‘daughter’ element at the rate of ½ of its atoms every 5,730 years. Measuring how much carbon-14 still remains in the residue of that organism will tell you exactly when it died e) Importance of radiometric dating i) Radiometric dating is a complex procedure that requires precise measurement ii) Rocks from several localities have been dated at more than 3bya iii) Confirms the idea that geologic time is immense 7) Geologic time scale - divides geologic history into units a) Originally created using relative dating b) Subdivisions i) Eon – the greatest expanse of time Four eons: Phanerozoic, Proterozoic, Archean, Hadean—the oldest eon ii) Era – subdivision of an eon. Eras of the Phanerozoic eon (important for this class): Cenozoic (‘recent life’), Mesozoic (‘middle life’), Paleozoic (‘ancient life’) One version of the geologic iii) Pre-Cambrian - general term for time prior to the Paleozoic time scale iv) Eras are subdivided into periods, periods are subdivided into epochs c) Difficulties in dating the time scale i) Not all rocks are datable (sedimentary ages are rarely reliable) ii) Nearby materials are often used to ‘bracket’ events and arrive at ages 8) Human civilization (recorded history) has only been on Earth for 0.000001 of the life-span of this planet. Is this a significant/relevant fact? Try not to let the huge time span trouble you, just think of it as simply numbers on a time scale. Illustration from text showing the brevity of human existence on Earth!

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Compiled by Arthur Reed

Print Date: June 24, 2011