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Rock Record and Geologic Time

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1 Rock Record and Geologic Time
4/1/2017 Rock Record and Geologic Time Grand Canyon in Arizona (stratification, bedding) stratification, bedding, stratum (strata) Geologic Time - October 4, 2007

2 Timescale (Relative)

3 Timescale (Absolute)

4 How do we tell geologic time?
4/1/2017 How do we tell geologic time? Relative time Principles of stratigraphy - 17th – 18th century Fossils - 18th century Paleomagnetism – 20th century Absolute time Radiometric dating – late 19th-20th century The Earth is very old billion years or more according to recent estimates. Most of the evidence for an ancient Earth is contained in the rocks that form the Earth's crust. The rock layers themselves -- like pages in a long and complicated history -- record the surface-shaping events of the past, and buried within them are traces of life --the plants and animals that evolved from organic structures that existed perhaps 3 billion years ago. Also contained in rocks once molten are radioactive elements whose isotopes provide Earth with an atomic clock. Within these rocks, "parent" isotopes decay at a predictable rate to form "daughter" isotopes. By determining the relative amounts of parent and daughter isotopes, the age of these rocks can be calculated. Thus, the results of studies of rock layers (stratigraphy), and of fossils (paleontology), coupled with the ages of certain rocks as measured by atomic clocks (geochronology), attest to a very old Earth! Geologic Time - October 4, 2007

5 Relative dating Law of superposition – oldest rocks are on the bottom
4/1/2017 Relative dating Placing rocks and events in sequence Law of superposition – oldest rocks are on the bottom Principle of original horizontality – sediments are deposited in flat, horizontal layers In October 1666 two fishermen caught a huge shark near the town of Livorno, and Ferdinando II de' Medici, Grand Duke of Tuscany, ordered its head to be sent to Steno. Steno dissected the head and published his findings in He noted that the shark's teeth bore a striking resemblance to certain stony objects, found embedded within rock formations, that his learned contemporaries were calling glossopetrae or "tongue stones". Ancient authorities, such as the Roman author Pliny the Elder, in his Naturalis Historiae, had suggested that these stones fell from the sky or from the moon. Others were of the opinion, also following ancient authors, that fossils naturally grew in the rocks. Steno's contemporary Athanasius Kircher, for example, attributed fossils to a "lapidifying virtue diffused through the whole body of the geocosm", consided an inherent characteristic of the earth — an Aristotelian approach. Steno, however, argued that glossopetrae looked like shark teeth because they were shark teeth, derived from the mouths of ancient sharks, and had been buried in mud or sand of the sea floor that now formed rock on dry land. There were differences in composition between glossopetrae and living sharks' teeth, but Steno argued, using the contemporary corpuscular theory of matter, that the chemical composition of fossils could be altered without changing their form. Steno's work on shark teeth led him to the question of how any solid object could come to be found inside another solid object, such as a rock or a layer of rock. The "solid bodies within solids" that attracted Steno's interest included not only fossils, as we would define them today, but minerals, crystals, encrustations, veins, and even entire rock layers or strata. He published his geologic studies in De solido intra solidum naturaliter contento dissertationis prodromus, or Preliminary discourse to a dissertation on a solid body naturally contained within a solid in Steno was not the first to identify fossils as being from living organisms; his contemporaries Robert Hooke and John Ray also argued that fossils were the remains of once-living organisms. Steno, in his Dissertationis prodromus of 1669 is credited with three of the defining principles of the science of stratigraphy: the law of superposition: " the time when any given stratum was being formed, all the matter resting upon it was fluid, and, therefore, at the time when the lower stratum was being formed, none of the upper strata existed"; the principle of original horizontality: "Strata either perpendicular to the horizon or inclined to the horizon were at one time parallel to the horizon"; the principle of lateral continuity: "Material forming any stratum were continuous over the surface of the Earth unless some other solid bodies stood in the way"; and the principle of cross-cutting discontinuities: "If a body or discontinuity cuts across a stratum, it must have formed after that stratum."[1] These principles were applied and extended in 1772 by Jean-Baptiste L. Romé de l'Isle. Another principle, known simply as Steno's law, or Steno's law of constant angles, states that the angles between corresponding faces on crystals are the same for all specimens of the same mineral, a fundamental breakthrough that formed the basis of all subsequent inquiries into crystal structure.[2] Nicolas Steno (17th Century) was one of the first Western naturalists to appreciate the connection between fossil remains and strata. His observations led him to formulate important stratigraphic concepts (i.e., the "law of superposition" and the "principle of original horizontality"). In the 1790s, the British naturalist William Smith hypothesized that if two layers of rock at widely differing locations contained similar fossils, then it was very plausible that the layers were the same age. William Smith's nephew and student, John Phillips, later calculated by such means that the Earth was about 96 million years old. Geologic Time - October 4, 2007

6 Rio Colorado – ‘Red River’
4/1/2017 Glen Canyon Dam, Lake Powell Rio Colorado – ‘Red River’ Red sediment now trapped in Lake Powell so the river is no longer red. Grand Canyon in Arizona Geologic Time - October 4, 2007

7 Superposition is well illustrated by the strata in the Grand Canyon

8 Original Horizontality
4/1/2017 Original Horizontality A stratigraphic section of Ordovician rock exposed in central Kentucky, USA. The sediments composing these rocks were formed in an ocean and deposited in horizontal layers. Geologic Time - October 4, 2007

9 Relative dating Other geologic principles Cross-cutting relationships - fractures, faults and intrusions must be YOUNGER than rocks they cut Inclusions – one rock contained within another (rock containing the inclusions is younger)

10 Cross-cutting Relationships
fractures, faults and intrusions must be YOUNGER than rocks they cut

11 Inclusions 4/1/2017 Pearson Prentice Hall, Inc.
Geologic Time - October 4, 2007

12 4/1/2017 Geologic Time - October 4, 2007

13 (8) Erosion of folded unit and of intrusion B
4/1/2017 (8) Erosion of folded unit and of intrusion B (7) Intrusion of B (or before 6) (6) Folding of all previously deposited layers (5) Deposition of layer E (4) Deposition of layer I (3) Deposition of layer F (2) Deposition of layer H (1) Oldest Event: Deposition of layer D (12) Youngest Event: Deposition of unit C (11) Erosion of unit G and intrusion A (10) Intrusion of A (9) Deposition of unit G Geologic Time - October 4, 2007

14 Telling Time with Fossils
use first occurrence and last occurrence rapidly evolving (short-lived) organisms divide time into the finest divisions best index fossils have a wide geographic range (planktonic ocean organisms)

15 Index (Zone) Fossils trilobites – Cambrian
ammonoids – Devonian to Cretaceous bivalves – Devonian to Cretaceous foraminifera - Cenozoic

16 Radioactivity and radiometric dating
Spontaneous breaking apart (decay) of atomic nuclei Radioactive decay Parent – an unstable isotope Daughter products – isotopes formed from the decay of a parent

17 Radioactivity and radiometric dating
Half-life – the time for one-half of the radioactive nuclei to decay Requires a closed system Cross-checks are used for accuracy Complex procedure Yields numerical dates

18 The radioactive decay curve
4/1/2017 The radioactive decay curve Pearson Prentice Hall, Inc., 2006 Geologic Time - October 4, 2007

19 Proportion of Parent Atoms Remaining as a Function of Time
Half-lives and remaining parent isotope

20 Radiometric/Isotopic dating
4/1/2017 Radiometric/Isotopic dating Radioactive elements (parents) decay to nonradioactive (stable) elements (daughters). The rate at which this decay occurs is constant and knowable (measurable). Therefore, if we know the rate of decay and the amount present of parent and daughter, we can calculate how long this reaction has been proceeding. Short-range dating techniques There are a number of other dating techniques that have short ranges and are so used for historical or archaeological studies. One of the best-known is the carbon-14 (C14) radiometric technique. Carbon-14 is a radioactive isotope of carbon, with a half-life of 5,730 years (very short compared with the above). In other radiometric dating methods, the heavy parent isotopes were synthesized in the explosions of massive stars that scattered materials through the Galaxy, to be formed into planets and other stars. The parent isotopes have been decaying since that time, and so any parent isotope with a short half-life should be extinct by now. Carbon-14 is an exception. It is continuously created through collisions of neutrons generated by cosmic rays with nitrogen in the upper atmosphere. The carbon-14 ends up as a trace component in atmospheric carbon dioxide (CO2). An organism acquires carbon from carbon dioxide during its lifetime. Plants acquire it through photosynthesis, and animals acquire it from consumption of plants and other animals. When an organism dies, it ceases to intake new carbon-14 and the existing isotope decays with a characteristic half-life (5730 years). The proportion of carbon-14 left when the remains of the organism are examined provides an indication of the time lapsed since its death. The carbon-14 dating limit lies around 58,000 to 62,000 years [5]. Geologic Time - October 4, 2007

21 Radioactivity and radiometric dating
Carbon-14 dating Half-life of only 5730 years Used to date very recent events Carbon-14 produced in upper atmosphere Incorporated into carbon dioxide Absorbed by living matter Useful tool for anthropologists, archeologists, historians, and geologists who study very recent Earth history

22 Stable Daughter Product Currently Accepted Half-Life Values
4/1/2017 Parent Isotope Stable Daughter Product Currently Accepted Half-Life Values Uranium-238 Lead-206 4.5 billion years Uranium-235 Lead-207 704 million years Thorium-232 Lead-208 14.0 billion years Rubidium-87 Strontium-87 48.8 billion years Potassium-40 Argon-40 1.25 billion years Samarium-147 Neodymium-143 106 billion years Table from: Geologic Time - October 4, 2007

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