TOPIC 13:
INTERPRETING GEOLOGIC HISTORY

The composition, structure, position and fossil
content of Earth’s rocks provide information about
Earth’s geologic history.

The geologic history dates back to about 4 billion
years.

The Geologic history is located in the ESRT on
page 8 and 9.
RELATIVE DATING

Relative Dating is the determination of the age
of a rock or even in relation to the age of other
rocks or events.

Relative age differs from absolute age.

Absolute age refers to the actual age of rock or
an event in years.
PRINCIPLE OF SUPERPOSITION
When observing layers of sedimentary rocks,
geologists assume the bottom layers is the
oldest. Each layer is progressively younger, so
the top layer is the youngest. This is called the
LAW OF SUPERPOSITION.
 This rule is the used as a method for relative
dating.
 An exception to this rule occurs, when the rocks
layers have been overturned or flipped upside
down.

DATING INTRUSIONS, EXTRUSIONS AND
INCLUSIONS
When molten rock-magma squeezes into
preexisting rocks hardens, it crystallizes to form
an igneous rock body called an intrusion.
 An intrusion is younger, in relative age than any
rock it cuts through.

DATING INTRUSIONS, EXTRUSIONS AND
INCLUSIONS


When molten rock-lava flows on the surface and
solidifies (hardens), it forms a mass of igneous rock
called an extrusion.
An extrusion is younger than any
rocks beneath it, and older
than rocks that may later
form on top of it.
DATING INTRUSIONS, EXTRUSIONS AND
INCLUSIONS

Often when magma rises toward the Earth’s surface,
pieces of the rock the magma is intruding (pushing
through) will fall into the magma. If the magma is cool
enough, those pieces will not melt, they will become
and inclusion.

An inclusion is a body
of older rock within
igneous rock.
DATING ROCK FEATURES

Structural features in rocks can be used in relative dating

For example, a rock is older than any fault, joint, tilting or fold
that appears in it. The reason is that rocks form without these
features, and are folded, tilted and cracked afterward.

Rocks also contain mineral deposits called veins. A vein forms
when a watery mineral solution fills a crack or a permeable
zone in the rock. Like an intrusion, a vein is younger than the
rock around it.

In sedimentary rocks, the sediments are older than the
sedimentary rock itself.

In igneous rocks, the individual crystals vary in age because the
form at different temperatures, reached by the magma as it
cooled and hardened.
RELATIVE DATING PRACTICE EXERCISES
Below is a cross section for use practicing the application of the principles
of relative age dating. For the cross section list the order of events that
occurred to form the rocks illustrated. Make sure to include when erosion
and deformation occurred. ** If there is a wavy line, erosion has occurred
and if layers are not horizontal, folding has occurred**
Answers:
*Granite intrudes
*Dike intrudes
*Erosion
*Deposition of Lower Sandstone
*Deposition of Lower Shale
*Deposition of Limestone
*Tilting and *Erosion
*Deposition of Upper Sandstone
*Deposition of Shale
*Erosion
RELATIVE DATING PRACTICE EXERCISES
Below is a cross section for use practicing the application of the principles
of relative age dating. For the cross section list the order of events that occurred
to form the rocks illustrated. Make sure to include when erosion and deformation
occurred. ** If there is a wavy line, erosion has occurred and if layers are not
horizontal, folding has occurred**
Answers:
*Gneiss forms
*Erosion
*Deposition of Sandstone (S.S.)
*Dike intrudes
*Erosion
*Deposition of Shale (Sh.)
*Dike II intrudes
*Erosion
*Deposition of Upper Sandstone
*Faulting and Folding
*Erosion
*Volcanics Deposited
*Erosion