Geological events that occurred long
ago can be arranged in the relative
order in which they occurred.
History of the Earth
Older at the bottom. Why?
– In an undisturbed layer of rock,
the oldest rocks are on the bottom
and the rocks become
progressively younger toward the
top
– This happens because sediments
are often deposited in horizontal
beds, forming layers of
sedimentary rock
Relative
Dating
• Relative dating is a strategy
of estimating the Earth’s age
by comparing rock layers and
geologic features to each
other
• Early geologists did not have
high tech, like radiometric
dating, which will be
discussed later
• Relative dating does not imply
an exact amount of time
– It points out that one event
happened before another event
– It does not explain exact
lengths of time between
geologic events
Relative Dating Principles
•
There are six principles that guide
relative dating
– Principle: a standard, belief, rule, or law;
not principal, like Principal Galafaro
1. Principle of Superposition
2. Principle of Original Horizontality
3. Principle of Lateral Continuity
4. Principle of Faunal Succession
5. Principle of Crosscutting Relations
6. Principle of Inclusions
Before We Begin: 3 Types of Rocks
Can Create Rock Layers
1. Igneous: form from lava or
magma
2. Metamorphic: form by
intense heat and pressure
3. Sedimentary: form by
compacting sediments
together
– Most common rock layers
– Most likely to have fossils
(To be discussed in more
detail later in the school year)
(1) Principle of Superposition
• In an undisturbed layer of rock, the
oldest rocks are on the bottom and
the rocks become progressively
younger toward the top
• This happens because sediments
are often deposited in horizontal
beds, forming layers of sedimentary
rock
(1) Principle of Superposition Diagram
• Which layer is the oldest?
– THICKER gray layer at the bottom
• Which layer is the youngest?
– THINNER gray layer at the top
• Is that possible? Why or why not?
– Yes, the same gray type of material was deposited at
different times.
Younger
Older
As little sediments fall to
the sea floor, they gather
in flat layers. What would
you name this principle?
(2) Principle of Horizontality
(2) Principle of Original
Horizontality
• This principle states that layers of
sediment are originally deposited
horizontally under the action of
gravity
• The Principle of Original
Horizontality was proposed by
the Danish geological pioneer
Nicholas Steno (1638–1686)
What principle might this describe?
• (3) Principle of Lateral Continuity
(3) Principle of Lateral Continuity
• The principle of lateral continuity states that
layers of deposited sediments will first widen
laterally in all directions
• In the example, the rock layers that are now
separated by a valley or other erosional
features, can be assumed to be originally
unbroken.
– Even though the layers in the previous slide
appear “bent” they continued sideways from and
to each other.
What principal might this represent?
(4) Principle of Faunal Succession
• Although sedimentary rock
material is different material
Area divided
by several kilometers
marked
by different
colors, the
period in Earth’s history can still
be identified as the same by the
specific fossils
– Usually index fossils
(4) Principle of Faunal Succession
• States that sedimentary
rock layers contain
particular types of
fossilized flora (plant-life)
and fauna (animal-life)
– Fossils of the index nature
may link time periods across
broken and sometime
different layers of rock
– Fossil forms were also
thought to succeed each
other in a specific and
predictable order …not so
much anymore…
What principal might this represent?
(4)Principle of Crosscutting Relations
Charles Lyell in Principles of Geology (1830), the principle of
cross-cutting relationships states that the geologic feature which
cuts another is the younger of the two features.
(5) Principle of Crosscutting Relations
• The Principle of Crosscutting
Relations states that:
• Faults occur after the
sedimentary beds have been
deposited
– Igneous intrusions may occur after
the sedimentary beds have been
deposited
– What does that mean?
• Lava might pour through the cracks of
the rocks
(6) Principle of Inclusion
• States that fragments of rocks are included
within other rock deposits:
• In sedimentary material, rock fragments are
compacted and cemented together
– Called clasts
• In igneous material, the magma travels, picks
up pieces of the rocks and the rocks are
found inside the cooled magma
– Called xenoliths
(6) Principle of Inclusion
clasts
xenoliths
Principles In Action over Time:
Uniformitarianism vs. Catastrophism
• Currently, the geologic processes that
shape the planet are considered to be
slow and gradual. Uniformitarianism has
been accepted by scientists for the last 150 years.
• Uniformitarianism states that the slow and gradual
speed of past geological processes is the same as the
present slow and gradual speed
– i.e. Today, severe earthquakes and floods occur
occasionally across the globe. This matches how geologic
processes occurred in the past.
• Catastrophism explains that past geologic processes
occurred fast and suddenly
– i.e. In the past, there were multiple global earthquakes and
a single flood that shaped the earth.