Geologic Time 1

Goals for understanding geologic time Relative dating vs. Absolute dating Relative dating principles – how geologists date layers of rock The dynamics of fossils Techniques of radiometric dating Why the earth is believed to be 4.6 b.y. The Geologic Time Scale what problems exist with the geologic time scale 2

Which geologic event took place first and when? Which rock layer is older, and how is earth history deciphered? How do we assign actual years to rock layers? Geology needs a Time Scale Consider: 1 2 ss Shale (sh) 3 3 Limestone (LS) 4 5 Sandstone (ss)

Relative Dating - placing the geologic occurrence in the proper sequence Which came first and WHY? To construct a “relative” time scale, rules were established (principles of relative dating). Nicholas Steno (1636-1686) Principle of Original Horizontality Law of Superposition Principle of Cross-Cutting Relations Principle of Inclusions 11

12 The Principle of Original Horizontality: Let’s unravel some geologic history from observations of various formations and their contacts. Nicholas Steno – 1669 proposed the following relative dating principles: The Principle of Original Horizontality: Sedimentary rock layers are deposited as horizontal strata. Any observed non-horizontal strata have been disturbed. Sediment input C B basin A 12

Original Horizontal Strata Limestone (ls) Shale (sh) Sandstone (ss) granitic rock 13

The Principle of Superposition In any undisturbed sequence of strata, the oldest stratum is at the bottom of the sequence, and the youngest stratum is on top. Unit 1 = old Unit 5 = young 5 4 3 2 1 14

Which strata is older? 5 4 3 2 1 youngest 5 4 oldest 3 2 1 15

The principle of Cross-Cutting Relationships Any geologic feature that cuts across another geologic feature is younger. 5 Unit 1 = older Unit 6 = youngest 4 3 2 6 1 Which came first: Unit 5 or Unit 6? 16

Which is older, the fault Which is younger, the dike or volcanic layer? Which is younger, the dike or country rock? fault dike Volcanic layer What type of fault is this? Normal country rock Determine the relative age of the two dikes. 1 2 17

18 The Principle of Inclusions A piece of rock (clast) that has become “included” in another rock body is older than the rock body it has become part of – why? Rock body A A A A Older (Rock A was there first.) Intrusion of pluton B 18

Which “granites” are older and younger? 19

Which rock body is older?: A ? ? C Can you identify the inclusions found in this Sierra Nevada Mountain batholitic material? 20

Superposition 21 Original Horizontality Principle of Inclusions Youngest Superposition Oldest Principle of Inclusions Cross-Cutting Relationship Which granite is older? Older A B C Asp Vn Younger 21

I this geology science class. Discuss with a friend: Explain the concept of relative dating. Draw a diagram, and explain each of the following dating principles: Original Horizontality Superposition Cross-Cutting Relations Inclusion Principle I will get an A on my exams and quizzes. 22

Ok – given the principles, what is wrong with this stack of rock (strata) youngest 7 6 5 3 2 1 oldest Missing time – or does time really stop? 23

24 The principle of Unconformities Igneous or metamorphic rock rock surface that represents a period of erosion or non- deposition referred to as “missing time” three major types of unconformities: disconformity angular unconformity non-conformity disconformity – unconformity in non-disturbed sedimentary layers angular unconformity – uncon. lies between angled strata and overlying horizontal strata non-conformity – sedimentary strata overlies crystalline rocks (ig and met) Unconformity Igneous or metamorphic rock 24

25 disconformity angular unconformity nonconformity Sedimentary rocks Xln rocks 25

I this geology science class. Explain what an unconformity is and what it represents 2. Diagram pictures that represent the three types of unconformities Discuss with a friend I will get an A on my exams and quizzes 26

How do we get a fossil? – preservation of past life Fossils – evidence of past life or “time pieces,” the remains or traces of prehistoric life Paleontology – study of fossils How do we get a fossil? – preservation of past life 2 conditions must exist for preservation rapid burial possession of hard parts Prehistoric bug Rapid burial of sediment covers the bug – fossil Bug dies Bug soft parts are eaten or dissolve 27

How do fossils help scientists relatively date layers Fossils – evidence of past life or “time pieces,” the remains or traces of prehistoric life Preservation of fossils Small percentage of fossils preserved throughout geologic time – WHY? Most organisms composed of soft parts. Organisms with hard parts and within a sedimentary environment are favored. Very rare to see vast array of other life forms How do fossils help scientists relatively date layers of rock (strata)? 28

William Smith – Principle of Fossil Succession Fossil organisms succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content. “Fossils are arranged according to their age by using the law of superposition.” Fossil succession: allows geologists to age date wide geographical areas documents the evolution of life Age of mammals Age of reptiles Age of fish Youngest Oldest 29

How do fossils help date rocks? 1200 miles 7 7 6 6 Disconformity 5 4 3 3 2 2 Which fossils are the youngest and oldest? 1 30

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I this geology science class. Give 2 reasons why many organisms are are not fossilized. Explain the law of fossil succession and how this law allows dating of strata. How has fossil succession helped geologists unravel earth history? Discuss with a friend: I will get an A on my exams and quizzes. 32

OK – We have relative dating and fossils – How do we get “absolute” ages on the rocks (numbers)? Radiometric dating – applying a number radioactive atoms (isotopes) decay at a constant rate over time 33

The time of decay can be measured. Radioactive decay of an unstable isotope atom + + + + Decay process + + + Pb206 (lead) + + U238 (Uranium) + + The time of decay can be measured. Isotope decay does not vary under various weathering conditions. Isotopes decay at a fixed rate. One isotope will decay into another isotope. 34

Stable Daughter Product Currently Accepted Half-Life Values How does radiometric dating work, and where does the age (number) come from? 35 Terms: Parent element: the “beginning” element that contains 100% of radioactive particles Daughter element: the element that the parent element decays into (or turns into over time) Half life: the time required for ½ of the parent to decay into the daughter element 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

U-235 Pb 207 704 m.y. 1.4 b.y. 2.1 b.y. 36 U-3 Daughter element 1/2 1 half life = 704 million years Daughter element 1/2 1/2 1/2 704 m.y. 1.4 b.y. 2.1 b.y. Parent element 36

I geology science class. Specifically define the differences between relative and absolute dating techniques. 2. Define the following absolute dating terms: parent/daughter elements, half-life 3. Explain how the half-life is used to calculate an absolute age. I will get an A on my exams and quizzes. 37

What is the importance of radiometric dating? produced thousands of dates for earth events rocks have been dated at more than 3 b.y. granite in South Africa dated at 3.2 b.y. granite contains inclusions of quartzite quartzite inclusions must be older Acasta gneiss in Northern Canada – 4.0 b.y. Earth believed to be 4.55 (4.6) b.y. old Radiometric dating: vindicated the ideas of Hutton, Darwin, and others consistent with relative dating techniques allowed “absolute” dating on the Geologic Time Scale 38

Relative dating + Absolute dating Lets make a Geologic Time Scale Relative dating + Absolute dating The Geologic Time Scale: It combines both relative and absolute dating Created during the nineteenth century in Western Europe and Great Britain Sub-divides the 4.6 billion-year-history of the earth Eons Eras Periods Epochs 39

Precambrian Building the Geologic Time Scale 40 Phanerozoic “visible life” fossil record becomes more detailed animals have hard shells and skeletons Building the Geologic Time Scale Proterozoic Multi-celled, soft body organisms “early life” Precambrian Archean Single cell life developed most “ancient” rocks found preserved rocks at the base of the Archean Hadean represents the earth’s time of formation no rocks are represented “hellish” conditions

41 Cenozoic Era Mesozoic Era Paleozoic Era birds and mammals flourished appearance of man Mesozoic Era marks the rise in dinosaurs dominant vertebrates first flowering plants first shrew-like mammals Paleozoic Era known as ancient life life progressed from marine invertebrates to fish, amphibians, and reptiles

42 Periods based on: Cretaceous, Jurassic, Triassic Cambrian period fossil types massive extinctions geographical locations characteristics of strata Cretaceous, Jurassic, Triassic age of reptiles dinosaurs dominant massive dinosaur extinction at 65 m.y. –Cretaceous “Jurassic Park” Cambrian period animals with hard shells diversification of life “the Cambrian explosion”

43 Epochs Age of Reptiles Age of fish Invertebrates not defined by extinction events, but % of fossils still living plants and animals found in the Pliocene epoch have living species today Eocene-few species surviving today Holocene human’s time Age of Reptiles Amphibians Age of fish Invertebrates How accurate is the Geologic Time Scale?

I the geologic Time Scale. 44 You should be able to draw the Geologic Time Scale and label it with the following: Eons, Eras, Periods, and Cenozoic/ Tertiary epochs. 2. List major characteristics of each period. 3. How did the strength of both absolute and relative dating techniques contribute the development of the geologic time scale?

Cenozoic, Mesozoic, Paleozoic The Geologic Time Scale – How much of Earth history is represented? Geologic Time Scale Cenozoic, Mesozoic, Paleozoic Eras 12% 88% Precambrian Eon 45

Difficulties in dating the Geologic Time Scale Not all rocks can be dated radiometrically. all minerals must contain 100% parent atoms. Sedimentary rocks can only rarely be dated. some parent atoms come from pre-existing rocks that have been weathered and transported. sedimentary rocks are dated in proximity of igneous bodies. Metamorphic rocks are challenging. some minerals do not necessarily represent the time when the rock was formed 46

How is the age of the Earth determined? Why is it difficult to determine the age of the earth? (think rock cycle) The external and internal forces constantly recycle earth material, obliterating rock clues to the earth’s past. What evidence suggests a 4.6 b.y. old earth? Precambrian rocks (Acasta gneiss, northern Canada) date at 4.0 billion years. Mineral grain found in sedimentary rock (Australia) dates at 4.4 billion years. What does the mineral grain in a sedimentary rock indicate about the 4.4 b.y. age relationship? 47

Acasta gneiss, northern Canada known as the Acasta gneiss complex dated at the Hadean Eon (4.0 billion years old) part of the Canadian Slave craton 48

Evidence from space to age date the earth-- moon dust and meteorites: moon dust from Apollo astronauts dated at 4.55 billion years the Allende Meteorite: a carbonaceous chondrite meteorite that was found in Chihuahua, Mexico, 1969 contains unaltered material from the formation of the solar system composed of tiny amounts of carbon that form the compounds of amino acids (essential for life) age dating of this and other meteorites is around based on earth rocks and interstellar space objects, earth is believed to be around 4.6 billion years old. 49

Allende Meteorite, Chihuahua, Mexico, Feb. 8, 1969 unaltered material from our solar system contains carbon (3 parts/1000) some carbon compounds in the form of amino acids dark areas – olivine with trace amounts of iron and carbon calcium and aluminum oxide compounds first matter to form during solar system formation older than earth carbonaceous chondrite 50

I what geology can do for me. Discuss with a friend: Identify at least 2 problems with the accuracy of the Geologic Time Scale. 2. Why does the Geologic Time Scale only represent about 12% of the earth’s geologic history (assuming the earth is 4.6 b.y. old)? I will get an A on my exams and quizzes. 51