# The Relative Age of Rocks

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The Relative Age of Rocks
Chapter 8-2

Relative Age of Rocks (Look at the bottom of 137)
(1) Relative age- a rocks age compared with the ages of the rocks around it. (2) Absolute age- absolute age is the number of years since the rock formed. It is difficult to gage the absolute age of rocks so geologists have to make educated guesses based on the conditions of the rocks around the one being studied.

Geologists and their Rocks (Flip Page Over)
Geologists are scientists who study rocks. (3) Law of superposition- Geologists use this law to gage the relative age of sedimentary rocks. According to the law of superposition, in horizontal sedimentary rock layers the oldest rock layers are at the bottom.

It is Grand!

The Grand Canyon (4) The walls of the Grand Canyon in Arizona illustrate the law of superposition very clearly, as the deeper into it you go the older the rocks are. (False in the Handout)

Rock layers and the Age of Rocks
The oldest layers are always at the bottom with the younger layers on the top.

Relative Age of Rocks Questions (Flip Page back to Front Page)
What does the position of rock layers reveal? What is the relative age of rocks? What is the absolute age of rocks?

Answers (Front Page) 1. The position of rock layers reveals the age of the rocks in the layers. 2. The relative age of rocks is the age of the rock as compared to other rocks around it. The absolute age of rocks is the years since the rocks formed.

What Are Other Clues in Determining the Age of Rocks
“ There’s only two things that scare me and one is nuclear war”. “What’s the other”? “Other what” “Things that scares you” “The true age of rocks, haven’t got a clue how you find the age of those things”.

Other Clues in determining the Relative age of Rocks?
“ COULD IT BE LIQUID HOT MAGMA!”

Liquid Hot Magma is a Big Clue in Determining the Age of Rocks
Age of Rocks- Geologists also study extrusions and intrusion of igneous rock to help determine the relative age of rocks. Igneous rock- forms when magma or lava hardens. Extrusions- lava that hardens on the surface is called an extrusion. Intrusions- lava that harden in the cracks of igneous rock and becomes igneous.

Faults, Gaps, Extrusions and Intrusions

(5) Extrusion Extrusion
How it Forms- lava that hardens on the surface of the earth is called an extrusion What Geologist learn- Because the extrusions are always younger then the rocks around them geologist study them to determine the relative age of the rocks that surround them.

(5) Intrusion Intrusion
How it Forms- When magma harden into a mass of igneous rock an intrusion forms What Geologist learn- Because geologist understand that the intrusions are always younger then the rocks around them they can determine the relative age of the rocks that surround them.

(5) Fault Fault How it forms- a break in the earths crust. Forces inside the earths crust cause movement of rock on opposite sides of the fault to spread the outer crust apart. What Geologist learn- It is difficult to find the relative age of rocks that are cut by fault lines that are moving.

A Famous Fault

Faults and Intrusions (6) If a fault cuts through an extrusion the extrusion has to be older. “A” is the fault and “D” is the extrusion in the picture below.

Gaps in the Geologic Record
The geologic record of sedimentary rock layers is not always complete. When older rock erodes newer rock is exposed leaving gaps in the study of the geologic past.

Unconformity ( 7) Unconformity- The surface where new rock meets much older rock layers is called an unconformity, which is also a gap in the geologic record. Unconformity also shows where some layers of rock have been lost do to erosion.

Unconformity

Inclusions Inclusions- an inclusion is a piece of rock that is contained in another rock. Inclusions start off as solid rock and break off, becoming imbedded in another rock. (8) The inclusion is always older then the rock in which it is imbedded.

Fossils Geologists also use fossils to help date rocks.
(10) Index fossils- are many fossils that represent one organism that existed only briefly. They are useful because they tell the relative age of the rocks in which they occur.

Index Fossils (11) Must- 1. Be found in many different areas.
2. Different types of ammonites are great index fossils.