Bell Ringer• What is the theory of evolution?

• How do fossil records prove this theory? Give two examples of animal fossils found that give evidence to support the theory of evolution.

Chapter 9.3Relative Age

of RocksMr. Perez

Important Vocabulary• Sedimentary rock• Principle of superposition• Relative age• Unconformity

Oldest Rocks on the Bottom

• According to the law of superposition, in undisturbed layers of rock, the oldest rocks are on the bottom and the rocks become progressively younger toward the top

Rock Layers• Why do younger rock layers pile up on older rock

layers???• BECAUSE…

o Sediment accumulates in horizontal beds, forming layers of sedimentary rock

o The first layer that forms is at the bottomo The next layer forms on top of the previous oneo And so on…

• HOWEVER…o Forces generated by mountain formation can turn layers

over• When this happens, other clues in the rock layer can help

us determine their original positions and relative ages.

Relative Ages• The relative age of something is its age in comparison to the

ages of other things• Geologists can determine the relative age of rocks by examining

their places in a sequence.• Relative age does not tell you the age of rock in actual years…

you only know if a rock layer is older or younger

Other Clues Help• Finding the relative age is easy, if the rock

haven’t been faulted or turned upside down.• In layers that have been disturbed you may have

to find fossils and other clues to date the rockso Example- If Mr. Perez finds a fossil in the top layer of rock that

is younger than a fossil in the bottom layer, we can hypothesize that the layers have been turned upside down by folding during mountain building

Unconformities• Unconformities are gaps in a sequence of rock

layers that is due to erosion or periods without any deposition (new sediments falling)

• Three types of unconformities:1. ANGULAR UNCONFORMITY2. DISCONFORMITY3. NONCOMFORMITY

Angular Unconformity

• Angular Unconformities Form when…1. Horizontal layers of

sedimentary rock are tilted and uplifted

2. Erosion and weathering wear down tilted layer

3. Younger sediment are deposited on top of tilted layer

Disconformity• Disconformity forms when…

1. Sedimentary rock layers have been formed2. Erosion occurs, not completely removing a layer- No deposition occurs

as time passes3. Younger rocks form above the erosion surface when deposition of

sediments begins again

• This causes a gap to form in the rock record.

Nonconformity• Nonconformities form

when…1. Metamorphic or igneous

rock are uplifted2. Erosion occurs3. Sedimentary rocks are

deposited on top of this erosion surface

• Sometimes rock fragments from below are incorporated into sediments deposited above

Matching up Rock Layers

• Evidence used for correlation:1. Fossils- if the same types of fossils are found in the same type of

rock layer in two different locations2. Other dating methods- testing of rocks in the lab

Resources• Florida Science Grade 7

Glencoe Science & McGraw Hill Publishing• Google Images