Topics: Geology, Stratigraphy, Fossil Pages: 47 (817 words) Published: July 31, 2014
Telling Geologic Time
Putting it in perspective

Determining geological ages
• Relative age dates – placing rocks and
events in their proper sequence of formation
• Numerical dates – specifying the actual
number of years that have passed since an
event occurred (known as absolute age

Figure 1-15 (p. 16)
Geologic Time Scale.


The physical,
biological and
principles acting
on the Earth in
the present also
acted on the
Earth in the past.

Getting Dates
Relative Dating

Law of Superposition
• Developed by Nicolaus
Steno in 1669
• In an undeformed
sequence of sedimentary
rocks (or layered igneous
rocks), the oldest rocks
are on the bottom

Superposition in the Grand Canyon

Principles of relative dating
• Principle of original horizontality
• Layers of sediment are generally deposited
in a horizontal position
• Rock layers that are flat have not been

Original Horizontality (d)
Lateral Continuity (e)

Figure 1-6 (p. 5)
Illustration of original lateral continuity.

Law of Cross-cutting

•An inclusion is a piece of rock that is
enclosed within another rock
•Which layer is younger?


Baked Contacts

Contact Metamorphism

Lava flow

Geologic History
• A cross-section through the earth reveals the variety of geologic features. View 1 of this animation identifies a variety of geologic features; View 2 animates the sequence of events that produced these features, and demonstrates how geologists apply

established principles to deduce geologic history. [by Stephen Marshak]
• Play Animation Windows version >>
• Play Animation Macintosh OS9 version >>
• Play Animation Macintosh OSX version >>

Gaps in the Record:

Angular unconformity, Siccar Point, Scotland

Erosional Surfaces

• Unconformities represent gaps in the
geologic record because of surface erosion

Figure 1-8 (p. 8)
Siccar Point,
Eastern Scotland.
It was here that
James Hutton first
realized the
significance of an

Formation of
an angular

Development of
a nonconformity

Development of a

• Four types of
(A) Angular
unconformity. (B)
Nonconformity. (C)
Disconformity. (D)

Sub-bottom seismic profile

Gaps in Time in
the Grand

Types of Unconformity
• This animation shows the stages in the development
of three main types of unconformity in cross-section,
and explains how an incomplete succession of strata
provides a record of Earth history. View 1 shows a
disconformity, View 2 shows a nonconformity and
View 3 shows an angular unconformity. [by Stephen
• Play Animation Windows version >>
• Play Animation Macintosh OS9 version >>
• Play Animation Macintosh OSX version >>

• Top: Block diagram

• Bottom: Geologic

Figure 1-11 (p. 11)
An example of how the sequence of geologic events
can be determined from cross-cutting relationships
and superposition.
• From first to last, the
sequence indicated in
the cross-section is first
deposition of D, then
faulting to produce
fault B, then intrusion
of igneous rock mass C,
and finally erosion
followed by deposition
of E. Strata labeled D
are oldest, and strata
labeled E are youngest.

Interpreting the
Sequence of Events

Principle of
Faunal Succession
• Correlation often relies upon fossils
• Principle of fossil succession – fossil
organisms succeed one another in a definite
and determinable order, and therefore any
time period can be recognized by its fossil

Determining the ages of
rocks using fossils

Getting Dates
By the numbers:
Absolute Dating

Using radioactivity in dating
• Radioactivity
• Spontaneous changes (decay) in the
structure of atomic nuclei

Radioactive Decay
• Isotopes
– Elements...
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