Chapter 8 The Rock Record

1. Chapter 8The Rock Record Unit 5

2. LT 1: I can state the principle of uniformitarianism. • UNIFORMITARIANISM – a principle that geologic processes that occurred in the past can be explained by current geologic processes. • Developed by James Hutton • “If you study Earth’s present, you can learn about Earth’s past” • Basic foundation of the science of geology • Processes are the same, but the rates of the processes vary over time.

3. LT 2: I can explain how the law of superposition can be used to determine the relative age of rocks. • RELATIVE AGE – the age of an object in relation to the ages of other objects. • Indicates that one layer is older or younger than another layer but does not indicate the rock’s age in years. • LAW OF SUPERPOSITION – that an undeformed sedimentary rock layer is older than the layers above it and younger than the layers below it. • Beds – sedimentary rock layers • Bedding plane – boundary between two beds • Strata – layers of rock

4. LAW OF SUPERPOSITION The Grand Canyon strata

5. QUICK LAB

6. Principle of Original Horizontality • PRINCIPLE OF ORIGINAL HORIZONTALITY – sedimentary rocks left undisturbed will remain in horizontal layers. • When layers are disturbed, scientists look for clues for the original position and then apply the LAW OF SUPERPOSITION • GRADED BEDDING – heavy particles settle to the bottom of a lake or river faster than smaller particles • CROSS-BEDS – sand slides down the slope of a large sand dune, the sand forms slanting layers; which can appear curved at the bottom • RIPPLE MARKS – when waves move back and forth on a beach; when sand becomes sandstone, the ripple marks are preserved.

7. GRADED BEDDING CROSS-BEDS RIPPLE MARKS

8. LT 3: I can compare three types of unconformities. • UNCONFORMITY – a break in the geologic record • TYPES • NONCONFORMITY – stratified rock rests upon unstratified rock • ANGULAR UNCONFORMITY – the boundary between a set of tilted layers and a set of horizontal layers • DISCONFORMITY – the boundary between horizontal layers of old sedimentary rock and younger, overlying layers that are deposited on an eroded surface • Law of Superposition states that all rocks beneath an unconformity are older than the rocks above the unconformity.

9. UNCONFORMITIES ANGULAR UNCONFORMITY NONCONFORMITY DISCONFORMITY

10. LT 4: I can apply the law of crosscutting relationships to determine the relative age of rocks. • LAW OF CROSSCUTTING RELATIONSHIPS – the principle that a fault or body of rock is younger than any other body of rock that it cuts through • Fault – a break or crack in Earth’s crust along which rocks shift their position • Intrusion – a mass of igneous rock that forms when magma is injected into rock and then cools and solidifies. INTRUSION

11. LT5: I can summarize the limitations of using the rates of erosion and deposition to determine the absolute age of rock formations. • ABSOLUTE AGE – the numeric age of an object or event • Absolute Dating Methods • Rates of erosion • Only practical for geologic features that formed within the past 10,000 – 20,000 years • Example: Niagara Falls • Rates of Deposition • Not always accurate and just provides an estimate; flooding can affect the rates of deposition of sediment • Varve – a banded layer of sand and silt that is deposited annually in a lake, especially near ice sheets or glaciers • Its like counting growth rings in a tree trunk, scientists count the varves of a lake.

12. LT 6: I can explain how the process of radioactive decay can be used to determine the absolute age of rocks. • RADIOMETRIC DATING – a method of determining the absolute age of an object by comparing the relative percentages of a radioactive isotope (parent) and a stable isotope (daughter). • Half-life: the time required for half of a sample of radioactive material to break down by decay to form stable isotope. • Scientists look at the following radioactive isotopes: • Carbon – 14 • Argon • Potassium – 40 • Rubidium • Uranium – 235 • Uranium – 238 • Thorium

13. Carbon Dating • Used for younger rocks that contain organic material • The ages of wood, bones, shells, and other organic remains that are included in the layers and that are less than 70,000 years old • Carbon – 14 dating • Decays into stable nitrogen - 14

14. QUICK LAB