Transcript File - Ms. D. Science CGPA

Earth’s History – Course 2 - Mrs. Bloch
4.2
The Relative Age Of Rocks
Essential Question: How do Scientist study Earth’s past?
How Old are Rock Layers?
How Can Rock Layers Change?
Pgs. 146-151
My Planet Diary for Florida pg. 146
Florida Mines Many different kinds of rocks are mined in Florida.
The state has one of the most active mining industries in the
country. In addition to providing the minerals needed to pave roads
and construct buildings, the quarries that the rocks come from also
hold fossils. These fossils could be used to figure out how old the
rocks surrounding them are.
Communicate: Rock quarries
can be used by both miners
and scientists. What other
natural resources have
multiple purposes?
How Old Are Rock Layers? Pg. 146
2 Ways to determine the age of a rock
Relative Age vs. Absolute Age
The Relative Age of a rock is its
age compared to the ages of other
rocks.
The Absolute Age (Numerical Age)
of a rock is the number of years that
have passed since the rock formed.
Rock Layers pg. 147
The Law Of Superposition
Geologists use the law of
superposition to determine the
relative ages of sedimentary rock layers.
According to the law of
superposition, in horizontal sedimentary
rock layers the oldest layer is at the
bottom.
 Each higher layer is younger than the
layers below it.
Law of Superposition
The law of
superposition
states that the older
layers of rock are on
the bottom and the
younger layers of
rock are on the top.
Clues From Igneous Rock pg. 148
Geologists study extrusions, intrusions, faults, and index fossils to
determine the relative ages of rocks.
An extrusion is formed by lava
that hardened on the Earth’s
surface and formed igneous rock.
An intrusion is magma,
Which cools and hardens into
a mass of igneous rock below
the Earth’s surface.
A fault is a break in Earth’s crust.
Forces inside Earth cause movement of the rock on opposite
sides of the fault. A fault is always younger than the rock it cuts
through. To find its relative age of the fault, geologist study the
relative age of the youngest layer cut by the fault.
APPLY IT!! Pg. 148
The Relative Age of Rocks
The diagram shows rock layers found at a site. Identify the area on the diagram that
shows an intrusion. What is the oldest layer of rock in the diagram?
1. Circle the area on the diagram that shows an intrusion.
2. Shade the oldest layer on the diagram.
3. Infer. What can you infer about the relative ages of areas B and E?
The Relative Age of Rocks pg. 149
Index Fossils
Fossils that are widely distributed and that represent organisms that existed for a
geologically short period of time could be used as index fossils.
Index fossils
are useful because they tell the relative ages of the rock layers in which they
occur.
Scientists use index fossils to match rock layers.
Match the rock layers in locations 2, 3, and 4 with first area shown. Which
fossils can be used as index fossils?
Assess Your Understanding pg. 149
1a. Explain In an area with several different rock layers, which is
oldest? Explain. ______________________________________
___________________________________________________
b. Infer How could a geologist match the rock layers in one area
to rock layers found in another area? _____________________
___________________________________________________
___________________________________________________
I get it! Now I know that you can find the relative age of rocks by
___________________________________________________
___________________________________________________
How Can Rock Layers Change?
Pg. 150
The geologic record of sedimentary rock layers is not complete. Most
of our planet’s geologic record has disappeared through erosion.
Gaps in the geologic record and folding can change the
position in which rock layers appear. These changes make it
more challenging for scientists to identify accurately the ages of
some rocks. (motion along faults can also change rock layers line up)
An unconformity is a gap in the geologic record. An unconformity
shows where rock layers have disappeared due to erosion.
Folding is when forces inside the Earth fold rock layers so much that
the layers are turned over completely and the youngest layers may
be on the bottom!
Folding and faulting in the Earth's crust
The Relative Age of Rocks – pg. 150
Unconformities
and Folding
•What are the oldest and
youngest layers in the
last two diagrams?
•Identify the unconformity.
Which is the part of the
fold that is overturned?
Apply It! Pg. 151
1. What does the photo show?
(an unconformity / folding)
2. What evidence do you see for your answer?
_____________________________________
_____________________________________
3. Challenge. What can you infer about the
History of this area?
_____________________________________
_____________________________________
Assess Your Understanding pg. 151
2a. List Name two ways rock layers can change.
___________________________________________________
b. Explain. How does folding change rock layers?
___________________________________________________
___________________________________________________
c. Draw Conclusions. Two locations include a layer of rock with
a particular index fossil. In one location, the layer occurs in a
higher position than in the other. What can you conclude about
the history of the two areas.
___________________________________________________
___________________________________________________
Radioactive Dating
Marie Curie
In 1896, French scientists named
Marie and Pierre Curie heard
about experiments that had been
done by another scientist, Henri
Becquerel. Marie Curie later
described what happened: