Relative Rock Age - Superposition

1. 
FOSSILS and STRATIGRAPHY
Relative Geologic Dating

2. FOSSILS
READ / HIGHLIGHT / SUMMARY
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4. Some Questions to Consider
 How old are you relative to your parents?
 Younger than your parents
 Relative to Mr. Winter?
 Younger than Mr. Winter
 Relative to Mr. Winter’s kids?
 Older than Mr. Winter’s kids
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5. Relative Age
 How old is the Earth?
 How did we arrive at this number?
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4.5 Billion Years Old
 Danish scientist Nicolaus Steno (17th Century)
spent time observing the different layers of
rock that had formed in Tuscany, Italy.
 His observations helped to establish the basis
for relative dating and started a new branch
of geology called stratigraphy – the study of
rock layers.
 The concept of relative dating is that layers
of rocks are ordered chronologically, so
their ages can be compared.

6. Two Laws of Relative Dating
 Law of Original Horizontality
 All sedimentary rock layers initially form in horizontal layers.
 Any change from this position is due to the rock being disturbed
later.
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7. Two Laws of Relative Dating
 Law of Superposition
 In sedimentary rock, the older layers of rock are deposited first.
 Newer layers are deposited after the older layers.
 Therefore, newer layers are formed on top of older layers of rock.
 Observing an undisturbed layer of rock, you can assume that the
older rocks are at the bottom and the youngest are at the top.
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8. The Position of Rock Layers
 According to the law of superposition, in horizontal sedimentary
rock layers, the oldest layer is at the bottom.
 Each higher layer is younger than the layers below it.
- The Relative Age of Rocks

9. Finding the age of Earth
 Steno’s laws revolutionized the way people viewed the age of Earth.
 In the 1800s, scientists measured the approximate thickness of all
sedimentary rock layers at Earth’s surface.
 These measurements ranged from about 25–112 m (15–70 mi).
 They then observed and measured the rate at which sedimentary
layers form, and calculated it to be around 0.3 m (1 ft) every 1,000
years.
 Using these rates, scientists then calculated how long it would take
for all of the sedimentary layers in the world to build up.
 Based on these calculations, these scientists determined that Earth
had to be perhaps a hundred million years old!
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10. Gaps in the rock layers
 100 million years was still an underestimated age of Earth.
 The revised age did not take into account processes, such as
weathering, erosion, and underground geologic activities, that
cause change at the surface.
 These processes are gradual and sometimes are not noticeable
for extremely long periods of time. It could take millions of
years for weathering and erosion to unearth underground
structures.
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11. Gaps in the rock layers
 The folding and twisting of
rock layers resulting from
pressure below may also take
millions of years.
 For example, geologic forces
can exert enough pressure
to cause horizontal layers of
rock to fold.
 The forces that cause these
features can often bring old
layers of rock to the surface,
where they are exposed to
the agents of weathering
and erosion.
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12. When the layers fold downwards and
form a bowl-like shape, it is called a
syncline.
When the layers of rock fold upward like an
arch, it is called an anticline.
Anticline and Syncline
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14. Unconformity
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 Over time, parts of the older
layers in an anticline can be
weathered and eroded away.
New, horizontal layers can
then form on top of this
eroded surface.
 These gaps in the geologic
record, due to layers of rocks
lost to weathering and
erosion, are called
unconformities.

15. Formation of an unconformity
 An unconformity occurs where erosion wears away layers of
sedimentary rock. Other rock layers then form on top.
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16. Formation of an unconformity
 An unconformity occurs where erosion wears away layers of
sedimentary rock. Other rock layers then form on top.
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17. Formation of an unconformity
 An unconformity occurs where erosion wears away layers of
sedimentary rock. Other rock layers then form on top.
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18. Formation of an unconformity
 An unconformity occurs where erosion wears away layers of
sedimentary rock. Other rock layers then form on top.
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20. Cross-Cutting Relationships
 To determine relative age, extrusions and intrusions of igneous
rock, faults, and gaps in the geologic record can be studied.
- The Relative Age of Rocks

21. Relative Intrusions
 Analyzing the positions of rock layers is not the
only method to find the relative age of rocks.
 Clues from igneous rock formations can provide
information as well.
 Molten rock cools and solidifies, it forms igneous
rock.
 Igneous rock usually forms as slabs of giant
rock.
 However, hot molten rock can sometimes
pierce through layers of other rock.
 When this hot molten rock cools and solidifies
within the pre-existing rock, it forms an
intrusion.
 An intrusion is always younger than the layers
of rock that it pierces.
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22. Relative Extrusion
 An extrusion is also an igneous
formation.
 However, an extrusion forms when
lava cools and solidifies on top of
older rock formations.
 In undisturbed areas, extrusions are
always younger than the rock layers
below them.
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23. - Radioactive Dating
Relative Intrusion and Extrusion
 The age of a sedimentary rock layer can be determined relative
to the absolute age of an igneous intrusion or extrusion near
the sedimentary rock.

24. Clues from Faults
 A fault is a break in Earth’s crust
usually caused by geologic forces
within Earth.
 These forces move or shift opposite
sides of a fault and provide insight into
the relative age of the rock layers.
 Since the layer of rock had to be
present in order to break, the fault is
always younger than the youngest
layer of rock cut by the fault.
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25. How a Fossil Forms
- Fossils
 Most fossils form when living things die and are buried by
sediment. The sediment slowly hardens into rock and
preserves the shapes of the organisms.

26. How a Fossil Forms
- Fossils
 Most fossils form when living things die and are buried by
sediment. The sediment slowly hardens into rock and
preserves the shapes of the organisms.

27. How a Fossil Forms
- Fossils
 Most fossils form when living things die and are buried by
sediment. The sediment slowly hardens into rock and
preserves the shapes of the organisms.

28. How a Fossil Forms
- Fossils
 Most fossils form when living things die and are buried by
sediment. The sediment slowly hardens into rock and
preserves the shapes of the organisms.

29. Using Fossils to Date Rock
 Scientists can estimate a rock layers age by searching
for certain organisms fossilized within the rock layers.
 These fossils have been previously dated by other
scientists and are called index fossils.
 If an index fossil is found within a layer of rock,
scientists will have a good estimate of when that
layer of sediment was deposited to form the rock.
 What makes a good index fossil?
 Ideally, these fossils were ancient organisms that
were fairly common and easily fossilized.
 Most index fossils are organisms with a hard shell,
because they do not decompose as quickly as soft
tissue.
 Index fossils often have distinctive features that
allow them to be easily distinguished from closely-
related organisms.
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30. Using Fossils to Date Rocks
 Index fossils are useful because they tell the relative ages of the
rock layers in which they occur.
- The Relative Age of Rocks

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32. Identify the relative age of each layer
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33. Identify the relative age of each layer
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34. Identify the relative age of each layer
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35. Identify the relative age of these areas
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