Chapter 6 review
1. UNIFORMITARIANISM:
-The processes that shape the
Earth today are the same
processes that occurred in the
geologic past
“The present is the key to the past”
2. ORIGINAL HORIZONTALITY
-Sedimentary rock layers are ALWAYS
laid down in horizontal layers, until some
other process alters them
3. SUPERPOSITION
-The bottom layer of horizontal
sedimentary rock layers is always the
oldest, UNLESS the layers have been
overturned or have had older rock
formed on top of it
*The lower layers must first be in place
before the next layer can be deposited
YOUNGEST
OLDEST
Extrusion:
When molten rock flows on the
Earth’s surface forming an igneous
rock
Intrusion:
Occurs when magma squeezes
between layers of pre-existing rock
beneath the Earth’s surface
-Causes surrounding layers to metamorphose
(Contact metamorphism)
FOLDS AND FAULTS
Fold:
Folds and Faults are always
younger than the original
rock layers
-bends in rock layers produced by
movements of the earth’s crust
Faults:
-breaks in the rock where shifting of
rock layers has occurred, often
associated with earthquakes
-Preserved remains or trace evidence
of a plant or animal living in the past
Fossils reveal
clues to ancient
environments
Mrs. Sharp
Index Fossils: Fossils that are found
geographically widespread and lived for
a short period of time
By comparing index fossils in various
locations on Earth, it is possible to
correlate (match) the relative ages
of the rocks in which they appear
“Geographically
widespread” means the
index fossils are found
in more than one rock column
Lived for a “short
period of time” means
the index fossils are
found in only one layer
What is Correlation?
When geologists try to match rock
outcrops in different locations to see
if they formed at the same time.
How to correlate (match) rock layers:
 Similarity of rock types
 Matching index fossils
 Volcanic ash layers used as time
markers
Relative Age of rocks: (sequence)
The age of a rock layer in
comparison to its surrounding layer
-Uses rock similarity, fossil
evidence, and volcanic time markers
to determine its order of occurrence
Absolute Age of rocks: (true age)
The age of a rock layer in years
-Uses radiometric dating to
determine age--radioactive decay
Radioactive decay is the process by
which the natural breakdown of
unstable atoms of an element occurs,
releasing particles and energy (heat),
and changes that element’s atoms into a
new element
Example: it takes 4.5 billion years for
uranium 238 to change into lead 206
Radioactive decay is NOT
AFFECTED by temperature
(heat) or pressure!
Radioactive decay
occurs at a constant
rate known as half-life.
HALF-LIFE:
Half-life is the rate (time) it
takes for one-half of the amount of
original material to decay
If we know the half-life of a radioactive
material, the age of the material can be
determined by measuring the amount of
decayed material in the sample.
ESRT pg. 1 Radioactive Decay Data chart
Some radioactive substances have a
Short half-life: Carbon 14
-Good for dating recent organic
remains (between 1,000-50,0000 yrs.)
238
206
Uranium decays to Lead
238
Long half-life: Uranium
-Good for dating much older rocks
(a very long half-life) - it takes 4.6
billion years for uranium to decay to lead
Example:
The amount of Carbon-14 remaining
in a fossil is 0.5 grams. How old is the
fossil? An equal sample of an existing
organism shows the original amount of
Carbon-14 was 2.0 grams.
(2.0g  _____  _____)
1.How many half-lives did the sample
undergo?
2.Multiply this by the half-life for
Carbon-14 (ESRT)
Example:
The amount of Carbon-14 remaining
in a fossil is 0.5 grams. How old is the
fossil? An equal sample of an existing
organism shows the original amount of
Carbon-14 was 2.0 grams.
(2.0g  1.0g  0.5g)
1.How many half-lives did the sample
undergo?
2.Multiply this by the half-life for
Carbon-14 (ESRT) Answer: 2 x 5,700 = 11,400 yrs.
Example:
The amount of Carbon-14 remaining
in a fossil is 0.5 grams. How old is the
fossil? An equal sample of an existing
organism shows the original amount of
Carbon-14 was 2.0 grams.
(2.0g  1.0g  0.5g)
1.How many half-lives did the sample
undergo? 2
2.Multiply this by the half-life for
Carbon-14 (ESRT)
Example:
The amount of Carbon-14 remaining
in a fossil is 0.5 grams. How old is the
fossil? An equal sample of an existing
organism shows the original amount of
Carbon-14 was 2.0 grams.
(2.0g  1.0g  0.5g)
1.How many half-lives did the sample
undergo? 2
2.Multiply this by the half-life for
Carbon-14 (ESRT)
Example:
The amount of Carbon-14 remaining
in a fossil is 0.5 grams. How old is the
fossil? An equal sample of an existing
organism shows the original amount of
Carbon-14 was 2.0 grams.
(2.0g  1.0g  0.5g)
1.How many half-lives did the sample
undergo? (how many times did it decay)
2 half-lives
2.Multiply this number by the half-life for
Carbon-14 (ESRT) Answer: 2 x 5,700 yrs. = 11,400 yrs.