Lecture 23 geologic history and time

Chapter 12: History Before History (Part 1)


Class Announcements:

 Midterm exam results should
be ready on Wednesday. Updated
grade record will be posted then.
 Exam key posted in glass case
outside lecture hall.
 Field trips this week. Be sure to read
instructions on website and bring
a pencil and eraser!
 Extra Credit exercise on “The Core”
is due Friday.
 An updated course schedule on
website after class today.


TODAY’s LECTURE:
Chapter 12:

Intro. to Historical Geology
1)
2)
3)
4)
5)

Geologic Time Scale
Relative age dating principles

Unconformities
Correlation of rock units
Absolute dating using
radioactivity
6) Development of the Geologic
Time Scale


Where did the
geologic time
scale come from?

Built up
over time by
correlation and
relative dating
of rocks
from around
the world!


Relative Dating: Key principles

Telling time in Geology:
❖ Earth’s history is recorded in rocks
of the crust…
❖ Geology seeks to unravel this history!
❖Two basic time keeping methods:
Relative dating - Place events in their proper
order.

Absolute dating - Determine, in years before the
present, when event actually occurred.


Relative Dating: Main Principles
Law of superposition
Principle of lateral
continuity
Principle of original
horizontality
Principle of cross-cutting
relationships
Rule of inclusions


Relative Dating: Main Principles

Law of superposition: In a sedimentary sequence
that
has
notisbeen
oldest
rock
A bed
of rock
older overturned,
than that above,the
younger
than
that units

below
are always at the bottom.
Original lateral continuity
Original horizontality
Cross-cutting relationships
Rule of inclusions


Fig. 12.04c
W. W. Norton


Relative Dating:
Dating: Main
Main Principles
Principles
Relative
Youngest

Oldest
Principle of Superposition


Law of superposition

Principles of Relative Dating

Principle of original lateral continuity: Most
sedimentary rock units were originally deposited
over a large area, but later became isolated by

erosion or faulting
Principle of original
horizontality:
Principle of
cross-cutting
relationships
Rule of Inclusions



Fig. 12.04fg
W. W. Norton


Fig. 12.11c
W. W. Norton
Note that geologic units can also
disappear between localities
because of a lateral transition
in environments if deposition.


Law of superposition
Principle of original
lateral continuity

Principles of
relative dating

Principle of original horizontality: Sedimentary layers

are originally deposited lying flat (i.e. ~parallel to
the Earth’s surface.
Principle of
cross-cutting
relationships
Rule of Inclusions


Sedimentary units deposited ~ flat
and parallel to the earth’s surface.

Original horizontality


So, what
happened
here?

Principle of Original Horizontality


What happened here?

Principle of Original Horizontality


Principles of Relative Dating
Law of superposition
Lateral of original

continuity
Principle of original
horizontality

Principle of cross-cutting relationships:
If one rock unit cuts across another, it is younger!
Rule of Inclusions


If something cuts something else,
it has to be younger than what it cuts.

Fig. 12.04h
W. W. Norton

Principle of cross-cutting relationships.


Which vein is youngest, ‘A’ or ‘B’?

‘B’

‘A’

Principle of Cross-cutting relationships


Principles of Relative Dating
Law of superposition
Lateral of original continuity

Principle of original horizontality
Principle of cross-cutting relationships

Rule of Inclusions: If a unit contains a clast,
or inclusion of another rock unit, the clast
or inclusion is older.


If a rock unit contains a piece of
another rock unit, the piece is older.

Fig. 12.04i
W. W. Norton
Rule of Inclusions


Principles of Relative Dating
Rule of Inclusions:

Fragments of one rock included in another.

Example
from
Papago
Park
fieldtrip…


Unconformities are buried
surfaces of erosion or

non-deposition…
Modern erosion surface

Fig. 12.04e
Stephen Marshak


Unconformities dominate
the geologic record…
Most of the rock
record consists
of unconformities,
or time gaps
in the record.
That is… most of
geologic history
was unrecorded,
or was recorded
and later lost
by erosion.