Time and Geology
Physical Geology 12/e, Chapter 8
Define the following terms: Geology textbook page 194-204
Geologic Time Terms:
Uniformitarianism
Numerical age
Relative time
Original Horizontality
Superposition
Lateral Continuity
Cross-Cutting Relationships
Inclusions
Unconformity
Disconformity
Angular Unconformity
Nonconformity
Correlation
Definition
Quick Sketch
Zircon Article
• Read the article
• On the same paper as your terms, tell at
least 5 facts from the reading that explain or
describe earth’s geologic history.
– Whether finished or not all work is due today.
Relative vs. Numerical Age
• Relative age - the order of events or objects, from first
(oldest) to last (youngest)
– Determined by applying simple principles, including original
horizontality, superposition, lateral continuity, cross-cutting
relationships, inclusions, unconformities, and correlation of rock
units and fossils
• Numerical age/ Absolute Age - the age of events or objects,
expressed as a number or numbers
– Determined using radiometric dating (determining how much
radioactive decay of a specific element has occurred since a rock
formed or an event occurred)
The Law of Superposition
• In any undisturbed
sequence of strata, the
oldest layer is at the
bottom of the
sequence, and the
youngest layer is at the
top of the sequence.
The Cross-Cutting Law
• Any feature that cuts
across a body of sediment
or rock is younger than the
body of sediment or rock
that it cuts across.
• NOTE:
• • A fracture is a crack in
rock.
• • A fault is a fracture along
which movement has
occurred.
Law of Inclusion
• If one rock body
contains fragments of
another rock body it
must be younger than
the fragments of rock
it contains. OR…The
inclusions are older
than the rocks which
contain them.
Principles associated with Relative Dating
The comparing of rock units to decipher their age relative to
one another
• Principle of Original Horizontality
Sedimentary.
If layers are folded, episode of deformation must have
occurred after rocks formed. Age of folding is younger than
youngest deformed rocklayers are deposited in
approximately horizontal sheets unit.
Relative Age Determination
• Contacts - surfaces separating
successive rock layers (beds)
• Formations - bodies of rock of
considerable thickness with
recognizable characteristics
allowing them to be distinguished
from adjacent rock layers
• Original horizontality - beds
of sediment deposited in water are
initially formed as horizontal or
nearly horizontal layers
Relative Age Determination
• Superposition - within an
undisturbed sequence of
sedimentary or volcanic
rocks, layers get younger
from bottom to top
• Lateral continuity - original
horizontal layer extends
laterally until it tapers or
thins at its edges
Relative Age Determination
• Cross-cutting relationships - a
disrupted pattern is older than the
cause of the disruption
– Intrusions and faults are younger than the
rocks they cut through
• Baked contacts - contacts between
igneous intrusions and surrounding
rocks, where surrounding rocks have
experienced contact metamorphism
• Inclusions - fragments embedded in
host rock are older than the host rock
Unconformities
• Unconformity - a surface (or
contact) that represents a gap in
the geologic record
• Disconformity - an unconformity
in which the contact representing
missing rock layers separates beds
that are parallel to each other
• Angular unconformity - an
unconformity in which the contact
separates overlying younger layers
from eroded tilted or folder layers
Unconformities
• Nonconformity - an unconformity
in which an erosional surface on
plutonic or metamorphic rock has
been covered by younger
sedimentary or volcanic rock
– Plutonic and metamorphic rocks
exposed by large amounts of erosion
– Typically represents a large gap in
the geologic record
Correlation
• Correlation - determining the time-equivalency of rock
units
– Within a region, a continent, between continents
• Physical continuity
– Physically tracing a continuous
exposure of a rock unit
– Easily done in Grand Canyon
• Similarity of rock types
– Assumes similar sequences of rocks
formed at same time
– Can be inaccurate if very common
rock types are involved
• Correlation by fossils
– Fossil species succeed one another through
the layers in a predictable order (faunal succession)
– Similar fossil assemblages (groups of different fossil species) used
Numerical Age Dating
• Numerical dating - puts
absolute values (e.g., millions
of years) on the ages of rocks
and geologic time periods
– Uses radioactive decay of
unstable isotopes
– Only possible since radioactivity
was discovered in 1896
– Radioactive isotopes decay in
predictable manner, depending
on the half-life (time it takes for
a given amount of radioactive
isotope to be reduced by half)
Fossils
Earth Science Textbook page 343-45 and
Fossils Handbook page 10-15
•
•
•
•
•
•
Direct fossils
Indirect fossils
Petrified
Replacement
Mold
Cast
•
•
•
•
•
Carbonization
Impression
Amber
Tracks
Coprolites
• Petrified
• Replacement
• Mold & Cast
• Carbonization
• Impression
• Amber
• Tracks
• Coprolites
Conditions for Fossil
Preservation
(Earth Science Textbook page 345)
A)
B)
Combining Relative and
Numerical Ages
• Radiometric dating gives numerical
time brackets for events with known
relative ages
– Individual layers may be dated directly
– Radiometric dating of units above and
below brackets age of units in between
• Geologic Time Scale
– Divided into four Eons
• Hadean, Archean, Proterozoic, Phanerozoic
– Precambrian (all time prior to Phanerozoic)
represents 87% of geologic time)
Geologic Time Scale
• Standard geologic time scale
– Worldwide relative time scale
– Subdivides geologic time
based on fossil assemblages
– Divided into eons, eras,
periods, and epochs
• Precambrian - vast amount
of time prior to the Paleozoic
era; few fossils preserved
• Paleozoic era - “old life”
– appearance of complex life;
many fossils
Geologic Time Scale
• Mesozoic era - "middle life"
– Dinosaurs abundant on land
– Period ended by mass extinction
• Cenozoic era - "new life"
– Mammals and birds abundant
– We are currently in the Recent
(Holocene) Epoch of the
Quaternary Period of the
Cenozoic Era
– Most recent ice ages occurred
during the Pleistocene Epoch of
the Quaternary Period
Age of the Earth
• Numerical dating gives
absolute age for Earth of
about 4.56 billion years
– Oldest age obtained for
meteorites, believed to have
been unchanged since the
formation of the solar system
– Earth and rest of solar system
very likely formed at this time
• Geologic (deep) time is vast
– A long human lifetime (100
years) represents only about
0.000002% of geologic time
Geologic Time Pre-Post Test
Practice
• Each person needs a scratch piece of
paper
• Number 1-20
• Answers only
• Work as a group/table partner
Geologic Time Review
Test Review
Divisions of Geologic Time
• Eras are subdivided into periods...periods are
subdivided into epochs.
Era
Period
Epoch
E + P = EP
•
Today…
Today we are in the Holocene Epoch of the
Quaternary Period of the Cenozoic Era.
Which unit is the largest?
Which unit is the smallest?
Geologic Time
How do geologists determine how old rocks are?
1. Relative dating -- determine whether the rock is older or younger
than other rocks
2. Absolute dating -- use radiometric dating techniques to determine
how long ago the rock formed in the exact number of years
*Not all rocks can be dated absolutely, so combinations of techniques
are used.
Relative Age Dating
The Law of Superposition
• In any undisturbed
sequence of strata, the
oldest layer is at the
bottom of the
sequence, and the
youngest layer is at the
top of the sequence.
The Cross-Cutting Law
• Any feature that cuts
across a body of sediment
or rock is younger than the
body of sediment or rock
that it cuts across.
• NOTE:
• • A fracture is a crack in
rock.
• • A fault is a fracture along
which movement has
occurred.
Law of Inclusion
• If one rock body
contains fragments of
another rock body it
must be younger than
the fragments of rock
it contains. OR…The
inclusions are older
than the rocks which
contain them.
2
A
D
C
B
E
Superposition/Cross-Cutting
3
A
B
D
E
C
Superposition/Cross Cutting
4
H
H
G
G
C
E
D
F
F
E
M
A
B
C
D
C
A
B
M
Numerical Age Dating
• Numerical dating - puts
absolute values (e.g., millions
of years) on the ages of rocks
and geologic time periods
– Radioactive isotopes decay in
predictable manner, depending
on the half-life (time it takes for
a given amount of radioactive
isotope to be reduced by half)
Correlation
• Correlation - determining the time-equivalency of rock
units
– Within a region, a continent, between continents
• Physical continuity
• Similarity of rock types
• Correlation by fossils
End of Chapter 8