Some very straightforward principles are used to determine the age of fossils.

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Angular unconformity – tilted rocks are overlain by flat-lying rocks b. William Smith (late1700s–early 1800s) noted that sedimentary strata in widely separated areas could be identified and correlated by their distinctive fossil content 2.

Disconformity – strata on either side are parallel c. Principle of fossil succession – fossil organisms succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content a.

Nonconformity – older metamorphic or intrusive igneous rocks in contact with younger sedimentary strata 1. Given enough time, remains may be petrified (literally “turned into stone”) 4. The percentage of radioactive atoms that decay during one half-life is always the same: 50 percent b.

The remains of relatively recent organisms – teeth, bones, etc. However, the actual number of atoms that decay continually decreases c.

Comparing the ratio of parent to daughter yields the age of the sample 1.

Radiometric dating is a complex procedure that requires precise measurement 2.

Rocks from several localities have been dated at more than 3 billion years 3. The grains composing detrital sedimentary rocks are not the same age as the rock in which they occur b.

Students not only want to know how old a fossil is, but they want to know how that age was determined.

This activity on determining age of rocks and fossils is intended for 8th or 9th grade students.

It is estimated to require four hours of class time, including approximately one hour total of occasional instruction and explanation from the teacher and two hours of group (team) and individual activities by the students, plus one hour of discussion among students within the working groups.

Explore this link for additional information on the topics covered in this lesson: This activity will help students to have a better understanding of the basic principles used to determine the age of rocks and fossils. Objectives of this activity are: 1) To have students determine relative age of a geologically complex area.