Geologic Time Scale & Fossil Dating

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75 fill-in-the-blank flashcards covering the geologic time scale, characteristics of each eon, Earth’s formation through accretion, fossil evidence, and carbon-14 dating.

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75 Terms

1
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Scientists use the __ to refer to large spans of Earth’s history.

geologic time scale

2
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Earth’s history is divided first into four major __.

eons

3
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Within the geologic hierarchy, eons are divided into __.

eras

4
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Eras are further subdivided into __.

periods

5
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Periods are finally broken down into __.

epochs

6
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The earliest eon of Earth is called the __ Eon.

Hadean

7
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The Hadean Eon lasted from 4,540 million years ago to __ million years ago.

4,000

8
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During the Hadean Eon, Earth and its __ were formed.

Moon

9
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The second eon is known as the __ Eon.

Archean

10
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The Archean Eon ended about __ million years ago.

2,500

11
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__ life forms first appeared during the Archean Eon.

Single-celled

12
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Some early __ formed during the Archean Eon.

continents

13
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The third eon is the __ Eon.

Proterozoic

14
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The Proterozoic Eon ended around __ million years ago.

541

15
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__ life forms emerged during the Proterozoic Eon.

Multicellular

16
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The first plants and animals appeared later in the __ Eon.

Proterozoic

17
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The current eon is the __ Eon.

Phanerozoic

18
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The Phanerozoic Eon began approximately __ million years ago.

541

19
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The __ Eon is the shortest eon so far.

Phanerozoic

20
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Complex life has __ during the Phanerozoic Eon.

flourished

21
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Around 300 million years ago, the supercontinent __ formed.

Pangaea

22
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Before the Hadean, Earth’s layers formed through __.

accretion

23
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__ is the process in which small particles collide and combine to form larger bodies.

Accretion

24
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As Earth grew, it drew more __ into its gravitational field.

cosmic dust

25
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The attraction of cosmic dust to Earth still continues __.

today

26
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Scientists believe all life on Earth shares a __ ancestor.

common

27
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Petrified remains of past organisms are called __.

fossils

28
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Studying a rock layer’s composition and fossils helps scientists estimate its __.

age

29
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Plant fossils occurring below animal fossils indicate that __ formed first.

plants

30
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For relatively young specimens, scientists use __ dating.

carbon

31
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Carbon dating is reliable only for organic matter younger than about __ years.

50,000

32
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Carbon-14 is an __ isotope of carbon.

unstable

33
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Carbon-14 has a half-life of about __ years.

5,000

34
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To determine a fossil’s age, scientists compare its Carbon-14 content to that of __ tissue.

living

35
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Older fossils generally represent more __ organisms.

basic

36
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Similar body structures in fossils and living species suggest a recent __ ancestor.

common

37
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Greater similarity in __ indicates organisms share a more recent common ancestor.

DNA

38
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Because older material is deeper underground, scientists can infer the __ sequence of rock layers.

order

39
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Earth formed from debris orbiting the __ during the Hadean Eon.

Sun

40
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The geologic time scale spans from Earth’s formation to the __ day.

present

41
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Understanding Earth’s formation has involved contributions from all branches of __ science.

natural

42
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As Earth attracted more matter, it began forming __ layers.

rock

43
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Deeper sediment layers were deposited __ than the layers above them.

earlier

44
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Earth is estimated to have formed about __ million years ago.

4,540

45
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The Moon was formed during the __ Eon.

Hadean

46
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Some continental crust stabilized during the __ Eon.

Archean

47
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During accretion, smaller particles collide and __ to make larger ones.

combine

48
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As an object grows during accretion, its __ field becomes stronger.

gravitational

49
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The four eons together represent the entire __ history of Earth.

geologic

50
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The order Hadean, Archean, Proterozoic, Phanerozoic is a __ sequence.

chronological

51
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During the Phanerozoic Eon, Pangaea eventually __ into separate continents.

broke apart

52
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Evidence that plants evolved first is provided when plant fossils are found __ animal fossils.

below

53
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Carbon dating is sometimes called __ dating.

radiocarbon

54
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Because Carbon-14 is unstable, it gradually __ over time.

decays

55
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A half-life measures the time required for half of a radioactive isotope to __.

decay

56
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Using rock strata, scientists establish the __ order of historical events.

relative

57
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Higher rock layers typically contain more __ fossils than lower layers.

complex

58
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Earth’s internal layers began forming __ the Hadean Eon.

before

59
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The constant pull of cosmic dust shows Earth is still gradually __ in mass.

growing

60
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Fossil dating helps scientists assemble a general __ of life on Earth.

timeline

61
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Comparing __ structures helps scientists trace evolutionary relationships.

body

62
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The abbreviation “Ma” stands for “million __ ago.”

years

63
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In the geologic hierarchy, __ come directly after eons.

eras

64
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Periods are __ than eras but larger than epochs.

shorter

65
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Epochs are the __ divisions mentioned in the geologic time scale.

smallest

66
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The Archean–Proterozoic boundary is set at __ million years ago.

2,500

67
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The Proterozoic–Phanerozoic boundary is set at __ million years ago.

541

68
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Branches of __ science all contribute to understanding Earth’s formation.

natural

69
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Fossils found in higher strata are generally __ than those below them.

younger

70
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The term “petrified” means organic material has been replaced by __.

minerals

71
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Because of its short half-life, Carbon-14 is useful only for dating relatively __ fossils.

recent

72
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Carbon dating cannot be used on samples older than about __ thousand years.

fifty

73
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Carbon-14 dating estimates the time since an organism __.

died

74
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Accretion not only increased Earth’s size but also led to the formation of distinct __ such as core and mantle.

layers

75
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The four major eons are Hadean, Archean, Proterozoic, and __.

Phanerozoic