1. RELATIVE DATING
Process of placing events in the sequence in which they occurred; it does NOT identify actual dates
Original Horizontality
--Concept that most Sedimentary rocks are deposited as a horizontal LAYER
Superposition
--Concept that in an undisturbed sequence of sedimentary strata, the oldest rock layer will be at the BOTTOM and the youngest on TOP
Cross-cutting
--Concept that an igneous intrusion or fault is always YOUNGER than the rock it cuts across
Embedded Fragments
--Concept that rocks embedded in another rock must be OLDER than rock in which it is found
Uplift
--Over time, rocks can be tilted or folded by regional or local metamorphism.
--If rocks are TILTED/FOLDED then the metamorphism that caused it is younger than the rocks.
--New, younger rocks will form on top of the tilted or folded strata
UNCOMFORMITY – gap in the geologic record caused by erosion. (Look for a change in direction of rock layers)
Relative Dating with Fossils - help geologists study Earth’s past & determine…
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Approximately when life began

What plants & animals were 1 st to live on land
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When organisms appeared/disappeared
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How organisms live
Fossils can be in the form of:
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REMAINS
 IMPRINTS
 BONES
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
TRACES
TEETH
How Fossils form:
 The body must be PROTECTED from scavengers & bacteria
 HARD PARTS have a better chance of becoming fossil

Fossils are found in SEDIMENTARY rocks

Index Fossils
 Certain fossils can help find the RELATIVE AGE of rocks.
 Since each organism comes from a specific time period, we can use a fossil to determine the age of the rock it is found in.
 These special fossils are called INDEX FOSSILS.
 They are abundant, widely spread and must have lived only during a short part of earth's history.
Ex: GRAPTOLITE - an ancient plant -alive 350-450 mya.
Ex: TRILOBITE - an ancient animal -alive 500-600 mya.
2. ABSOLUTE DATING
Determining the ACTUAL dates for events; Give ages of rock in number of years
Counting the Years
 TREE RINGS (up to 10,000 years)
Radiometric Dating
 VARVES (glacial layers)

The most ACCURATE method for determining age is RADIOMETRIC dating which uses radioactive elements to measure age.
 Radioactive elements give of particles and energy as they decay and new elements form.

The original unstable isotope is called the PARENT isotope

The more stable form is called the DAUGHTER isotope.
 Scientists know exactly how long it takes for radioactive elements to decay, so they are like natural clocks allowing scientist to determine an absolute age.
Most absolute dating is done by measuring the amount of… o CARBON – 14: left over in plants & animals less than 50,000 years old. C-14 decays into nitrogen. o URANIUM – 238: left over in rocks older than 50,000 years old. U-238 decays into lead.
Radioactive decay
C-14 & U-238 are radioactive ISOPTOPES which have extra NEUTRONS in its nucleus making it unstable.
 Different radioactive materials take different amounts of time to decay.

Scientists use the idea of a half-life to describe this.
 The half-life of a radioactive material can be very short (less than a second) or very long (thousands of years) or anywhere in between.
 The time it takes for half the radioactive atoms in a sample to decay to a stable product is called its
HALF-LIFE.

The instrument that measures radioactivity is a GEIGER COUNTER.