GEOLOGIC HISTORY

1
GEOLOGIC HISTORY
2
(No Transcript)
3
(No Transcript)
4
RELATIVE AGE DATING

• places events in a sequence, but does not
  identify their actual date of occurrence.
• 1st event
• 2nd event
• Last event

5
PRINCIPLES FOR RELATIVE AGE DATING
6
1 ORIGINAL HORIZONTALITY

• sedimentary rocks are originally deposited in
  horizontal layers.

7
(No Transcript)
8
2 SUPERPOSITION

• in undisturbed sedimentary rocks, the oldest
  layer is usually on the bottom.

youngest
oldest
9
3 CROSS-CUTTING RELATIONSHIPS

• an igneous rock is younger than the rocks it has
  intruded.
• A fault is younger than the rocks it cuts across.

10
RELATIVE AGE DATING(Practice)

• Determining Which is Older

11
(No Transcript)
12
Dating Intrusions, Extrusions, and Inclusions.

• intrusion
• igneous rock that formed when magma squeezed into
  preexisting rocks.
• an intrusion is younger than the rock it cuts
  through.

13

• extrusion
• forms when molten rock (lava) flows onto Earths
  surface and solidifies.
• an extrusion is younger than the rocks beneath
  it.
• it is older than the rocks deposited on top of
  it.

14

• inclusion
• a piece of rock that falls into an intrusion of
  magma.
• the inclusion is older than the magma intrusion.

15

• contact metamorphism
• occurs when magma or lava comes in contact with
  older bedrock and alters the minerals

16

• In sedimentary rocks.
• the sediments are older than the rock itself.
• Veins
• forms when a watery mineral solution fills a
  crack in the rock.
• a vein is younger than the rock around it.

17
Example 1

• youngest event (what happened last)
• deposition of Limestone
• deposition of Sandstone
• deposition of Shale
• oldest event (what happened first)

18
Example 8(trial one)

• youngest event (what happened last)
• Deposition of Limestone
• Tilting
• Deposition of Shale
• Deposition of Limestone
• Deposition of Sandstone
• oldest event (what happened first)

19
Unconformity

• a buried erosional surface.
• a surface of erosion between rock layers of
  different ages indicating that deposition was not
  continuous.
• part of the rock record is missing.
• symbol

20
In Most Cases..

• Erosion occurs.
• on land.
• Deposition occurs.
• in the water.

21
Geologic History of an Unconformity.

• Deposition in a marine environment.
• Uplift exposed the land to Erosion.
• The land Subsides below sea level.
• Deposition begins again.

22
When you are describing an unconformity you must
include.

• Uplift and Erosion
• Subsidence

23
(No Transcript)
24
Example 8. (trial two)

• youngest event (what happened last)
• Deposition of Limestone
• Subsidence
• Uplift and Erosion
• Tilting
• Deposition of Shale
• Deposition of Limestone
• Deposition of Sandstone
• oldest event (what happened first)

UNCONFORMITY
25

• Begin Geologic History Lab

classzone.com
26
Correlation Techniques.
27
correlation

• the matching of rock layers from one area to
  another.

28
1) Walking the outcrop.

• directly following the individual layers (or rock
  formations) at the Earths surface.
• Other methods must be used where rock formations
  are separated from one another.

29

• formation
• a layer or group of layers of rock that have
  similar characteristics.
• outcrop
• bedrock that is exposed at the Earths surface.

30
2) Similarity of Rock.

• rocks can be matched on the basis of similarity
  in appearance, color, and mineral composition.
• this method is useful only across small areas,
  and may be incorrect.

31
3) Fossil Evidence.

• one of the best methods for correlation.
• fossils
• any evidence of former life.

32
index fossil

• the remains of animals that lived and died within
  a particular time segment of Earths history.

33
Index fossils are useful in correlation because
they

• are easy to identify.
• are abundant.
• are widespread in occurrence.
• existed for only a brief period of time.

34

• Fossils are found almost exclusively in
  sedimentary rocks.
• Fossils provide clues to the environments in
  which the organism lived.

35
4) Volcanic Time Markers.

• volcanic ash
• small pieces of igneous rock that are shot into
  the air during eruptions.
• volcanic ash is deposited rapidly over a large
  area.
• Therefore, it can serve as an age marker in rocks
  that are 1000s of km apart.

36
ABSOLUTE AGE DATING
37
Absolute (or actual) Age Dating

• absolute age
• actual age in years.
• Age can be determined by.
• counting lake varves (annual lake sediment).
• counting tree rings.
• most common method is using radioactive dating.
• (measuring the amount of a particular
  radioactive element in a rock).

CLASSZONE.COM
38
(No Transcript)
39
Radioactive Decay is

• the natural and spontaneous breakdown,
• of the nucleus,
• of unstable atoms into more stable atoms.
• Radioactive decay
• releases energy and subatomic particles.
• occurs at a constant rate, which can not be
  changed.

40

• The radioactive isotope (Parent material) will
  break down naturally into another element called
  the decay product (Daughter material).
• The rate of radioactive decay is measured in
  terms of half-lives.

41
half-life

• the amount of time it takes for half of the
  radioactive element to decay.

42
The age of a rock can be determined by
comparing.

• the relative amount of the Undecayed substance
  (radioactive parent material)
• to the relative amount of Decay product (stable
  daughter material).

43
COMPLETE EXAMPLE

• Color in the bars.
• determine the relative amount of daughter and
  parent using fractions.
• use the number of half-lives to determine the age
  of a sample.

44
(No Transcript)
45
Graph Example
100
0

• 0 half-life parent daughter
• 1 half-life parent daughter
• 2 half-lives parent daughter
• Draw Graph
• The more daughter material in the sample, the
  older the sample.

50
50
75
25
46
The amount of time for a half-life is different
for each radioactive substance.

• See ESRT.

47
(No Transcript)
48
(No Transcript)
49

• Carbon -14 ..decays to
• half-life
• Uranium-238.decays to
• half-life

Nitrogen-14
5700 years.
Lead-206
4,500,000,000 years.
50

• Radioactive substances with short half-lives,
  such as C-14, are good for dating recent organic
  remains.
• Those with longer half-lives, such as U-238, are
  useful for dating older rocks.

51
PRACTICE QUESTIONS

• ABSOLUTE AGE DATING

52
GEOLOGIC HISTORY
53
Geologic History from the Rock Record

• some life forms exist only during specific
  intervals of time.
• therefore, fossils in rock can be used to
  determine relative age.
• ex) dinosaurs existed only during the Mesozoic
  Era.

54
Geologic Time Scale

• geologists have used fossil evidence to divide
  4.6 billion years of Earths history into smaller
  units.
• it is these divisions that make up the Geologic
  Time Scale.
• see ESRT p.8 9, Geologic History of New York
  State.

55
(No Transcript)
56
Precambrian

• represents 88 of Earths history.
• fossils in these rocks are rare and difficult to
  identify
• earliest life was small and lacked hard parts.
• these rocks may have been eroded away.

57
Uniformity

• the present is the key to the past.
• assumes that the geologic processes occurring
  today also occurred in the past.
• therefore, we can interpret past events by
  examining what is happening today.

58
WORKSHEET

• THE GEOLOGIC TIME SCALE

59
(No Transcript)
60
(No Transcript)
61
(No Transcript)
62
(No Transcript)
63
EVOLUTION OF EARTH AND LIFE

• Fossils
• a wide variety of life forms have lived over
  time.

Anomalocaris canadensis
Recent stromatolites at Hamelin Pool, Aus
Burgess Shale
Hallucigenia sparsa
64

• most of these life forms are now extinct.
• Since, the chances of fossilization are low.
• most forms of past life probably have not been
  identified.
• we compare fossils to similar life forms that
  exist today.

65
Environmental change.

• fossil evidence
• corals shallow, warm ocean water.

66

• coal deposits swamps.

67

• salt and gypsum evaporating oceans.

Dead Sea
68

• environments have changed as the Earths
  lithospheric plates have moved.
• as a landmass moves closer to the equator, its
  climate gets warmer.
• plate collisions form mountains.

69
Evolution and Extinction

• organic evolution
• theory that life forms change through time.
• offspring with favorable variations will survive.
• offspring with unfavorable variations will become
  extinct.

70
Mass Extinctions

• periods of time when large numbers of species
  went extinct.
• believed to have been caused by catastrophic
  events.impact events.
• example) K-T boundary, 65 mya, 70 of all species
  went extinct (including the dinosaurs).
• extinctions allowed other species (mammals) to
  thrive and evolve.

CLASSZONE.COM