CHATPER 4
LESSON 2
THE
RELATIVE AGE
OF
ROCKS
http://xeontribe.com/weird-andbreathtaking-rock-formation/weird-
/
rocks-formation-01
HOW OLD ARE FOCK
LAYERS?
Most of the stone that is mined in Florida is
used for road construction.
http://www.dep.state.fl.us/geology/geologictopics/minerals.htm
A ROCK’S AGE COMPARED
TO THE AGE OF OTHER
ROCKS.
THE NUMBER OF YEARS
THAT HAVE PASSED SINCE
THE ROCK FORMED.
(ROCK CYCLE 3:16)
LAW OF SUPERPOSITION
(Good animation on DE – law of superposition)
According to the law of superposition ,undisturbed horizontal sedimentary rock layers
the oldest layer is at the bottom. Each higher layer is younger than the layers below it.
Ordering the Grand Canyon’s History The law of superposition can
be applied to the layers exposed in the Grand Canyon. Interpreting
Illustrations Which layer is the oldest? youngest?
Clues from Igneous Rock
Extrusion
Lava that hardens on the surface and forms igneous rock.
Intrusive
Magma may push into bodies of rock below the surface. There the
magma cools and hardens in to a mass of igneous rock called
intrusive. An intrusion is always younger than the rock layers around
and beneath it.
Fault
A break in Earth’s crust. Forces inside the Earth cause movement of
the rock on opposite sides of a fault.
http://www.nps.gov/history/history/online_books/geology/publicatio
ns/state/tx/1968-7/sec1.htm
FIG. 24. Sketch showing intrusions and extrusions in cross section and plan views. A, Volcanic
neck, the feeder pipe for a volcano. B, Volcanic neck after erosion has removed the cone. C, Sill,
sheet of igneous rock that has spread laterally along the beds. D, A sill exposed by erosion. E,
Intrusive plug that forced its way upward through bedded rocks. F, The top of a plug uncovered
by erosion with upturned beds exposed on the flank. G, Laccolith, a tackhead-shaped intrusion
that spread laterally along the bedding and arched the overlying rocks. H, Eroded laccolith
exposed at the surface. The dip of the flanking hogbacks is less than in the plug (E, F) that cuts
across the bedding. J, Dike, intrusive rock that fills a fissure that cuts across bedded rocks. This
fissure filling cuts across the bedding and thereby differs from a sill (C). K, Lava flow that came
from a dike feeder fracture, which is the source of most lava in the Big Bend.
San Andreas fault. Aerial photograph of the San Andreas fault crossing the Carrizo Plain of California, USA. The San Andreas fault is one of the
world's great seismic faults, forming the boundary between the Pacific and North American tectonic plates. It is marked by the valley-like
depression running from top to bottom of the picture. Ridges and rifts line either side. This fractured landscape is formed by the pressures
created by the opposing, lateral movements of the two tectonic plates. The San Andreas fault extends almost the full length of California and is
responsible for major earthquakes.
DE – fault
animation
http://www.scie
ncephoto.com/
media/167148/
enlarge
How do fossils show age?
Index Fossil
These fossils help geologists match rock layers. A fossil must be widely distributed and
represent an organism that existed for a geologically short period of time.
Index fossils are useful because they tell the relative
ages of the rock layers in which they occur.
How did you layer location 2, 3, and 4. What did you infer about the
history of location 4? (Click and find out)
Layer B must have eroded over time.
Unconformity
Rock layers erode away, an older rock surface may be exposed. Then
deposition begins again, building new rock layers. Unconformity is a gap in
the geologic record. It shows where rock layers have been lost due to
erosion.
Did you label and circle
correctly?
FOLDING
Sometimes, forces inside Earth fold rock layers are turned over completely.
1. What does the photo show? (an unconformity/
folding).
folding
2. What evidence do you see for your answer to
Question 1?
The layers are bent at an angle.
3.What can you infer about the history of this area?
After the rock layers formed, forces inside Earth raised and folded them.
The Relative Age of Rocks
1.What is the youngest rock layer?
Explain.
2.Is the extrusion older or younger
than rock layer B? Explain.
3.Is the fault older or younger than
rock layer A? Explain.
4.How could a geologist use the fossil
in rock layer B to date a
rock layer in another location? N
5. ___ fault
a. the number of years since a rock has formed
6. ___ extrusion
b. a break in Earth’s crust
7. ___ unconformity
c. the way to determine relative ages of rocks
8. ___ relative age
d. a hardened layer of magma beneath Earth’s surface
9 .___ law of superposition
10.___ intrusion
e. the age of a rock compared with the age of other rocks
f. fossils used to help geologists match rock layers
11.___ absolute age
g. the surface where new rock layers meet a much older rock
surface beneath them
12.___ index fossils
h. a hardened layer of lava on Earth’s surface
Answer Key
1.Layer C is the youngest because the law of
superposition says that a layer is younger than
the layers below it.
2.The extrusion is younger because extrusions
are always younger than the rock layers below
them.
3.The fault is younger than layer a because rock
layers are always older than the faults they
contain.
4.The fossil might be an index fossil.
Geologists
can use index fossils to match rock
layers at
locations that are far apart.
5.
b
6.
h
7.
g
8.
e
9.
c
10.
d
11.
a
12.
f