CHAPTER 9: Clues to Earth’s past Section 3: Relative Ages of Rocks

Principle of superposition—process of reading undisturbed rock layers 1. oldest rocks in the bottom layer 2. younger rocks in the top layers

B. How old something is in comparison with something else is its relative age. 1. The age of undisturbed rocks can be determined by examining layer sequences. 2. The age of disturbed rocks may have to be determined by fossils or other clues.

C. Unconformities—gaps in rock layers that form when rock eroded and new sediment is deposited on top of the erosion surface. 1. Angular unconformity—rock layers are tilted, and younger sediment layers are deposited horizontally on top of the eroded and tilted layers. 2. Discontinuity A layer of horizontal rock once exposed and eroded before younger rocks formed over it. 3. Nonconformity—sedimentary rock forms over eroded metamorphic or igneous rock pictures of unconformities

angular unconformity animation.htm An angular unconformity develops when new horizontal layers of sedimentary rock form on top of older sedimentary rock layers that have been folded by compression. An example of an angular unconformity at Siccar Point in southeastern Scotland is shown above.

In a nonconformity, horizontal layers of sedimentary rock overlie older igneous or metamorphic rocks. A nonconformity in Big Bend National Park, Texas, is shown above.

A disconformity develops when horizontal rock layers are exposed and eroded, and new horizontal layers of rock are deposited on the eroded surface. The disconformity shown below is in the Grand Canyon.

D. The same rock layers can be found in different locations; fossils can be used to correlate those rock layers.

Figure A Identify the type of unconformity shown. Is it possible that there were originally more layers of rock than are shown? Describe how the rocks above the fault moved in relation to rocks below the fault. Hypothesize how the hill on the left side of the figure formed. Figure B 4. Is it possible to conclude if the igneous intrusion on the left is older or younger than the unconformity nearest the surface? 5. Describe the relative ages of the two igneous intrusions. How did you know? 6. Hypothesize which two layers of rock might have been much thicker in the past.