CHAPTER 8-THE ROCK RECORD GRAND CANYON VIDEO GRAND CANYON VIDEO How can we determine ages of rocks and the Earth?

Uniformitarianism: a principle that geologic processes that occurred in the past can explain current geologic processes- presented by James Hutton in the 18 th century Section 1: Determining Relative Age

Geologists estimate that the Earth is about 4.6 billion years old Hutton’s work forced geologists to revise the estimated age-it was once believed to be only about 6,000 years old. Earth’s Age

Rocks form in layers (strata) Indicates age of rock relative to other layers of rock Relative Age

Undeformed rock layers of sedimentary rock- older layers on bottom Law of Superposition

Sedimentary rocks left undisturbed will remain in horizontal layers Principle of Original Horizontality

Graded bedding-sediments settle according to particle size-if large particles are on top, the layers have been disturbed Cross beds-slanted layers can be used to determine the original position of layers Ripple marks-formed by wind or water and preserved in rock. Ripples point upwards and indicate original position of rock Other clues about age, see pg. 204 in text

A break in the geologic record due to movement in the earth’s layers. Types: Nonconformity Angular unconformity Disconformity Unconformities, see pg. 205

A fault or a body of rock that cuts through another rock layer is younger than that layer (it formed after the layer it cuts through) Crosscutting Relationships Relative Age Video Clip