1 Earth’s History

2 Planet Earth is approximately 4.5 X 10 9 years old –Rocks of the crust provide clues to Earth’s past By analyzing these clues we can infer events from the past

3 Earth’s History Principle of Uniformitarianism –Major assumption in geology –Events in the past occurred the same way that they are occurring today. Examples Include: Weathering/erosion Deposition Volcanism Plate tectonics

4 Geologic Time Plate motions and mountain building events can be placed on the geologic time scale

5 Geologists have divided Earth’s history into time units based on the the fossil record Geologic Time

6 A study of the fossil record shows –A great variety of plants, animals, and simpler life forms have lived on Earth in the past –That life forms have evolved through time –Most life forms of the geologic past have become extinct

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8 Geologic Dating

9 There are two types of Geologic Dating AbsoluteAbsolute –Determines how many years old something is RelativeRelative –Used to determine if one thing is younger or older than another

10 Tools used to determine Using radiometric dating Law of superposition Use of index fossils Correlation of rock layers Absolute AgeRelative Age

11 Relative Dating

12 Law of Superposition Sediments are laid down underwater in horizontal layers and form sedimentary rocks

13 Law of Superposition In a series of sedimentary rocks the bottom layer is the oldest and the top layer is the youngest –Lower layers must be in place before younger rocks can be deposited on top of them –Exception: when something occurs to overturn layers

14 Grand Canyon

GRAND CANYON- LAW OF SUPERPOSITION YOUNGEST ON TOP OLDEST ON BOTTOM

16 Law of Superposition Rock layers are older than folds found in them –Layers were there before they were folded

17 Folds/Tilted

18 Law of Superposition Rock layers are older than faults found in them This is logical: you can’t break a rock if it does not exist; so rock containing a fault must be older than the fault

19 Law of Superposition Fossils are generally the same age as the rock layers in which they are found –Animal remains are deposited along with the sediments that will turn into sedimentary rocks

20 Law of Superposition Igneous intrusions are younger than the rock that they cut through or flow out of

21 Igneous Intrusion - Cross Cutting

22 Unconformity When a new rock layer is formed atop an eroded surface

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24 Upper Silurian Carbonates Tilted Ordovician Shales and Sandstones Taconic Unconformity unconformity

25 Practice: what happened here?

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28 Correlation Matching similar rock layers in different locations to see if they formed at the same time

Which fossil might be found in Devonian rock layers?

33 Volcanic Ash Falls Can also be used to correlate rock layers over a large area Ash is a good indicator because: –The ash from one explosion has distinct characteristics –ash can be deposited around the globe –The event occurs at one, geologically brief, time

34 Fossils Naturally occurring preserved remains or impressions of living things Generally only hard parts get preserved –Bones, teeth, shells Hard parts are replaced by naturally occurring minerals

Eurypterus NY State Fossil Silurian index fossil

36 Fossils Can be a mold (impression) –Ex: footprint Or a cast –Ex: filled in footprint

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38 Fossils (Cont) Other types –Ice –Tar –Carbonaceous film –Amber –Imprints –Petrified

41 Fossils Generally found in sedimentary rock layers  Why don’t fossils exist in most igneous or metamorphic rock?

42 Fossils The living organism that made the fossil lived during the time the rock layer was forming –i.e. when the sediment was being deposited

43 Fossils Fossils can provide information about ancient environments –Marine fossils indicate a marine environment, wooly mammoths indicate a cold environment, etc…

44 Fossils A study of the fossil record shows: –A great variety of plants, animals and simpler life forms have lived on Earth in the past –That life forms have evolved through time –Most life forms of the geologic past have become extinct

45 Index Fossils Index Fossils--used to date rock layers (strata) –Fossils from creatures that existed for a geologically short period of time Ex. less than 2.0 x 10 7 –Fossils from creatures that had a wide geographic distribution

Location ALocation BLocation C Rock layer 1WWW Z Rock layer 2W Z Y Z Rock layer 3W X X X Z Which letter would make a good index fossil?

47 Absolute Dating

48 Radioactive Decay When an unstable radioactive element changes into a stable element

49 Unstable Radioactive IsotopeStable Decay Product Carbon 14 (C 14) Uranium 238 (U 238 ) Nitrogen 14 (N 14) Lead 206 (Pb 206 ) ParentDaughter Example

50 Radiometric Dating Half life Half life : The time required for 1/2 of a parent material to break down to daughter material

51 Another look

52 Radiometric Dating The age of objects can be determined by measuring the relative amounts of radioisotope and decay products

53 Radiometric Dating The half lives of radioisotopes vary depending upon the isotope –Examples: C 14 = 5,770 years U 238 = 4,500,000,000 years Rb 87 = 47,000,000,000 years

54 Carbon 14 is used to date biological remains –Carbon is incorporated into the cells of living organisms and begins to decay when the organism dies Uranium 238 is used to date rocks –Larger half life

55 Half Life Practice Problems

56 If half the K -40 of a rock has decayed into Ar-40, how old is the rock? How many years would it take for 75 % of a sample of K-40 to disintegrate?

57 How old are: A sample that has ½ c-14 and ½ n-14? A sample that has ¼ K-40 and ¾ Ar-40? A sample that has ½ U- 238 ½ Pb-206?

58 How old are: A sample that has 1/8 Rb-87 and 7/8 Sr-87? A sample that has ¼ C-14 and ¾ N-14? A sample that has ¾ C-14 and ¼ N-14?

59 What fraction remains of: A 50 billion year old sample has ____ Rb-87 and ____ Sr-87. A 2.6 billion year old sample has____ K-40 and ____Ar-40.

This is so boring Stranz should take a break ;) 60