geology

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
dating)

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

Uniformitarianism

The physical,
biological and
chemical
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
disturbed

Original Horizontality (d)
Lateral Continuity (e)

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

Law of Cross-cutting
Relationships

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

Inclusions

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:
Unconformities

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
historical
significance of an
unconformity.

Formation of
an angular
unconformity

Development of
a nonconformity

Development of a
disconformity

Unconformities
• Four types of
erosional
unconformities.
(A) Angular
unconformity. (B)
Nonconformity. (C)
Disconformity. (D)
Paraconformity.

Sub-bottom seismic profile

Gaps in Time in
the Grand
Canyon

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
Marshak]
• Play Animation Windows version >>
• Play Animation Macintosh OS9 version >>
• Play Animation Macintosh OSX version >>

• Top: Block diagram

• Bottom: Geologic
map

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
content

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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