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1 Copyright (c) 2005 Pearson Education Canada, Inc. Chapter 8: Geologic Time PowerPoint Presentation PowerPoint Presentation Stan Hatfield. SW Illinois.

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Presentation on theme: "1 Copyright (c) 2005 Pearson Education Canada, Inc. Chapter 8: Geologic Time PowerPoint Presentation PowerPoint Presentation Stan Hatfield. SW Illinois."— Presentation transcript:

1 1 Copyright (c) 2005 Pearson Education Canada, Inc. Chapter 8: Geologic Time PowerPoint Presentation PowerPoint Presentation Stan Hatfield. SW Illinois College Stan Hatfield. SW Illinois College Ken Pinzke. SW Illinois College Ken Pinzke. SW Illinois College Charles Henderson. University of Calgary Charles Henderson. University of Calgary Tark Hamilton. Camosun College Tark Hamilton. Camosun College

2 Geological Time is of Vast Duration Sediments were buried under a Mountain Range, metamorphosed, half the crust was uplifted & eroded, glaciers carried it, lake storms removed all but the biggest rocks.

3 Geologic Time Geologic Time Relative age dates – placing rocks and events in their proper sequence of formation. e.g. Tertiary is younger than Cretaceous (rocks, fossils, climate) Relative age dates – placing rocks and events in their proper sequence of formation. e.g. Tertiary is younger than Cretaceous (rocks, fossils, climate) Numerical dates – specifying the actual number of years that have passed since an event occurred (known as absolute age dating using isotope clocks) Numerical dates – specifying the actual number of years that have passed since an event occurred (known as absolute age dating using isotope clocks) Geologic time scale – Earths history is ~4.5 Ga long and written disproportionately by the rocks formed and processes which operated at different times and places. Geologic time scale – Earths history is ~4.5 Ga long and written disproportionately by the rocks formed and processes which operated at different times and places.

4 Ordovician Strata: Chute Vernal, PQ

5 Relative Dating – Key Principles Relative Dating – Key Principles Law of Superposition Law of Superposition Developed by the Danish physician Nicolaus Steno working in Italy in 1669 Developed by the Danish physician Nicolaus Steno working in Italy in 1669 In an undeformed sequence of sedimentary rocks (or layered igneous rocks), the oldest rocks are on the bottom In an undeformed sequence of sedimentary rocks (or layered igneous rocks), the oldest rocks are on the bottom Most sedimentary & volcanic rocks are deposited in sequences of essentially flat lying beds Most sedimentary & volcanic rocks are deposited in sequences of essentially flat lying beds

6 Principle of Original Horizontality Principle of Original Horizontality Layers of sediment are generally deposited in a horizontal position Layers of sediment are generally deposited in a horizontal position Rock layers that are flat have not been disturbed Rock layers that are flat have not been disturbed Principle of Cross-Cutting Relationships Principle of Cross-Cutting Relationships Younger features cut across older features Younger features cut across older features Principle of Inclusions Principle of Inclusions Younger rocks/features include older ones Younger rocks/features include older ones All 3 types of rocks can include older rocks and minerals All 3 types of rocks can include older rocks and minerals Relative Dating – Key Principles Relative Dating – Key Principles

7 Relative Dating – Key Principles: Horizontality & Law of Superposition Relative Dating – Key Principles: Horizontality & Law of Superposition Permian Strata, South Rim Correlation

8 Lower Paleozoic Strata: Devon, UK Deposition, Deformation At the Closing of Iapetus Fold Hinge

9 Formation of Sedimentary Inclusions

10 Unconformities Unconformities Types of Unconformities Types of Unconformities Disconformity – strata on either side of the unconformity are parallel (least missing time) Disconformity – strata on either side of the unconformity are parallel (least missing time) Angular unconformity – tilted rocks are overlain by flat-lying rocks (10s of Ma missing) Angular unconformity – tilted rocks are overlain by flat-lying rocks (10s of Ma missing) Nonconformity – metamorphic or igneous rocks in contact with sedimentary strata (most missing time, eroded down to crystalline basement rocks) Nonconformity – metamorphic or igneous rocks in contact with sedimentary strata (most missing time, eroded down to crystalline basement rocks) Relative Dating – Key Principles Relative Dating – Key Principles

11 Siccar Point, Scotland & The development of an angular unconformity.

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14 c d e f g h i k l m Figure out the sequence of geological events, Oldest on the bottom, youngest on the top. j What kind of unconformity is i? What kind of Contact is m?

15 c d e f g h i k l m Figure out the sequence of geological events, Oldest on the bottom, youngest on the top. j i is a disconformity m is intrusive, & baked margins. Dyke A Dyke B Intr-l Fault B k j I Fault A h g f e d c

16 Which type of unconformity here at the South Rim represents the greatest amount of missing time and why?

17 Correlation of Rock Layers Matching of rocks of similar ages in different regions is known as correlation Matching of rocks of similar ages in different regions is known as correlation Correlation often relies upon fossils Correlation often relies upon fossils William Smith (late 1700s and early 1800s) noted that sedimentary strata in widely separated areas could be identified and correlated by their distinctive fossil content William Smith (late 1700s and early 1800s) noted that sedimentary strata in widely separated areas could be identified and correlated by their distinctive fossil content Other than fossils we often rely on widespread instantaneous events like volcanic ash falls Other than fossils we often rely on widespread instantaneous events like volcanic ash falls Unconformities bound packages of stratigraphy Unconformities bound packages of stratigraphy

18 Correlation across slightly overlapping stratigraphy Arizona to Utah.

19 Correlation often relies upon fossils Correlation often relies upon fossils Principle of Fossil (Faunal) Succession – fossil organisms succeed one another in a definite and determinable order that documents the evolution of life; therefore any time period can be recognized by its fossil content Principle of Fossil (Faunal) Succession – fossil organisms succeed one another in a definite and determinable order that documents the evolution of life; therefore any time period can be recognized by its fossil content Index fossils – represent best fossils for correlation; they are widespread ecologically & geographically and are limited to a short time span (i.e., they evolved rapidly) Index fossils – represent best fossils for correlation; they are widespread ecologically & geographically and are limited to a short time span (i.e., they evolved rapidly) Correlation of Rock Layers

20 Time from Concurrent Range Zones

21 Reviewing Basic Atomic Structure Reviewing Basic Atomic Structure Isotope (only a few of these are radioactive) Isotope (only a few of these are radioactive) Variant of the same parent atom, same atomic number Variant of the same parent atom, same atomic number Differs in the number of neutrons (weight) Differs in the number of neutrons (weight) Results in a different mass number than the common type of atom for this element Results in a different mass number than the common type of atom for this element Too many additional neutrons makes a nucleus unstable and prone to radioactive decay Too many additional neutrons makes a nucleus unstable and prone to radioactive decay Dating with Radioactivity

22 3 kinds of radioactive decay

23 Parent – an unstable radioactive isotope Parent – an unstable radioactive isotope Daughter product – the isotopes that result from the decay of a parent Daughter product – the isotopes that result from the decay of a parent Half-Life – the time required for one-half of the radioactive nuclei in a sample to decay Half-Life – the time required for one-half of the radioactive nuclei in a sample to decay Half-life differs for every Parent-Daughter pair of isotopes Half-life differs for every Parent-Daughter pair of isotopes Dating with Radioactivity

24 Exponential Decay of Parent Isotopes (& exponential growth of daughters) The Decay Equation: A = A 0 e –λt A 0 is the initial amount of the parent isotope

25 Radiometric Dating Radiometric Dating Principle of Radioactive Dating Principle of Radioactive Dating The percentage of radioactive atoms that decay during one half-life is always the same (50 percent) The percentage of radioactive atoms that decay during one half-life is always the same (50 percent) However, the actual number of atoms that decay continually decreases However, the actual number of atoms that decay continually decreases Comparing the ratio of parent to daughter yields the age of the sample Comparing the ratio of parent to daughter yields the age of the sample Dating with Radioactivity

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27 Radiometric dating Radiometric dating Useful radioactive isotopes for providing radiometric ages Useful radioactive isotopes for providing radiometric ages Rubidium-87 Strontium-87 ~Precambrian Rubidium-87 Strontium-87 ~Precambrian Feldspar, micas, amphiboles Feldspar, micas, amphiboles Thorium-232 Lead-207 ~Pc Thorium-232 Lead-207 ~Pc Zircon Zircon Two isotopes of uranium (235 and 238) to Lead (208 & 206), 4.5 Ga & 713 Ma respectively, ~Pc to Mesozoic Two isotopes of uranium (235 and 238) to Lead (208 & 206), 4.5 Ga & 713 Ma respectively, ~Pc to Mesozoic Zircon Zircon Potassium-40 Argon-40 or relative to Argon-39 >2 Ma Potassium-40 Argon-40 or relative to Argon-39 >2 Ma Feldspar, micas, volcanic glass Feldspar, micas, volcanic glass Dating with Radioactivity

28 Slow U 238 U 210 Pb (½ life = 4.5 Ga) Used for Ancient Zircons & <200a 210 Pb

29 Rules for Radiometric Dating Rules for Radiometric Dating Mineral contains both parent & daughter as for Zircon with 238 U and 206 Pb or feldspar with 87 Rb and 87 Sr Mineral contains both parent & daughter as for Zircon with 238 U and 206 Pb or feldspar with 87 Rb and 87 Sr Formed at time of event to be dated as for a lava flow, dyke or contact metamorphism Formed at time of event to be dated as for a lava flow, dyke or contact metamorphism The mineral is a closed system and has neither gained nor lost parent & daughter The mineral is a closed system and has neither gained nor lost parent & daughter Dating with Radioactivity

30 Radiometric dating Radiometric dating Sources of error Sources of error A closed system is required A closed system is required If temperature is too high, daughter products may be lost If temperature is too high, daughter products may be lost To avoid potential problems, only fresh, unweathered rock samples should be used To avoid potential problems, only fresh, unweathered rock samples should be used Dating with Radioactivity

31 Production of 14 C in the upper atmosphere (& decay once organisms die) Cosmic rays Expel neutrons from Atmospheric gases (N,O). This expels a proton From nitrogen Forming radioactive 14 C. The extra neutron in radioactive 14 C decays back to 14N With a ½ life of 5730 years.

32 Dating with carbon-14 (radiocarbon dating) Dating with carbon-14 (radiocarbon dating) Half-life of only 5730 years Half-life of only 5730 years Used to date very recent events Used to date very recent events Carbon-14 is produced in the upper atmosphere Carbon-14 is produced in the upper atmosphere Useful tool for anthropologists, archaeologists, historians, and geologists who study very recent Earth history Useful tool for anthropologists, archaeologists, historians, and geologists who study very recent Earth history Only works for plant or animal tissue, not rocks Only works for plant or animal tissue, not rocks Dating with Radioactivity

33 The Geological Time Scale

34 Structure of the geologic time scale Structure of the geologic time scale Names of the eons Names of the eons Phanerozoic (visible life) – the most recent eon, began just over 540 million years ago Phanerozoic (visible life) – the most recent eon, began just over 540 million years ago Proterozoic Proterozoic Archean Archean Hadean – the oldest eon Hadean – the oldest eon Geologic Time Scale

35 The Geological Time Scale

36 Structure of the geologic time scale Structure of the geologic time scale Era – subdivision of an eon Era – subdivision of an eon Eras of the Phanerozoic eon Eras of the Phanerozoic eon Cenozoic (recent life) Cenozoic (recent life) Mesozoic (middle life) Mesozoic (middle life) Paleozoic (ancient life) Paleozoic (ancient life) Eras are subdivided into periods Eras are subdivided into periods Periods are subdivided into Epochs Periods are subdivided into Epochs Geologic Time Scale

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38 Precambrian time Precambrian time Nearly 4 billion years prior to the Cambrian period Nearly 4 billion years prior to the Cambrian period Not divided into smaller time units because the events of Precambrian history are not known in great enough detail Not divided into smaller time units because the events of Precambrian history are not known in great enough detail Also, first abundant fossil evidence does not appear until the beginning of the Cambrian Also, first abundant fossil evidence does not appear until the beginning of the Cambrian

39 Difficulties in dating the geologic time scale Difficulties in dating the geologic time scale Not all rocks can be dated by radiometric methods Not all rocks can be dated by radiometric methods Grains comprising detrital sedimentary rocks are older than the rock in which they formed Grains comprising detrital sedimentary rocks are older than the rock in which they formed The age of a particular mineral in a metamorphic rock may not necessarily represent the time when the rock formed as porphyroblasts may grow for many Ma The age of a particular mineral in a metamorphic rock may not necessarily represent the time when the rock formed as porphyroblasts may grow for many Ma The rock needs minerals with the right parent and daughter isotope pairs and in the right age span to be measureable. The rock needs minerals with the right parent and daughter isotope pairs and in the right age span to be measureable.

40 Difficulties in dating the geologic time scale Difficulties in dating the geologic time scale Datable materials (such as volcanic ash beds and igneous intrusions) are often used to bracket various episodes in Earth history and arrive at ages Datable materials (such as volcanic ash beds and igneous intrusions) are often used to bracket various episodes in Earth history and arrive at ages Dates change as brackets become narrower and methods refined; e.g., base of Triassic is now 252 Ma, base of Permian is now 299 Ma, base of Cambrian is 543 Ma… Dates change as brackets become narrower and methods refined; e.g., base of Triassic is now 252 Ma, base of Permian is now 299 Ma, base of Cambrian is 543 Ma…

41 Igneous events permit Absolute dating Of sedimentary rocks.

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43 Dating Terrains On other Worlds

44 Crater counts & cross cutting relations mostly Precambrian

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46 Dendrochronology (wiggle matching) For wood in 14 C realm <70Ka

47 Extinctions: Dinosaur Graveyards

48 Bolide impact & Deccan Trap Volcanism both occur at 65Ma. Biodiversity dwindled for 10 Ma prior to this. The K/T massive extinction is debated.

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