EARTH AND SPACE SCIENCE

1. EARTH AND SPACE SCIENCE Chapter 8 The Rock Record 8.1 Determining Relative Age

2. 8.1 Determining Relative Age Objectives • State the principle of uniformitarianism. • Explain how the law of superposition can be used to determine the relative age of rocks. • Compare three types of unconformities. • Apply the law of crosscutting relationships to determine the relative age of rocks.

3. Introduction • The Earth is estimated to be about 4.6 billion years old. • James Hutton (left), an 18th century Scottish physician and farmer, suggested the Earth was far older (billions of years) than most people at the time thought. • Hutton used scientific method to study a variety of aspects about Earth.

4. Uniformitarianism • Hutton theorized that the same natural forces that changed the landscape of his farm had changed the Earth’s surface in the past. • Uniformitarianism states that current geologic processes, such as weathering and erosion, are the same processes that have shaped the Earth in the past. • Uniformitarianism allows scientists to learn about Earth’s past by studying its present. • Though rates of these processes may vary in time and space, this is one of the founding principles of modern geology.

5. Uniformitarianism • In Hutton’s time, many people thought that the Earth was only 6,000 years old and that all geologic features had formed at the same time. • Hutton’s observations that natural forces caused slow changes on the landscape over time raised questions. • Hutton’s observations led him to believe that the Earth was much older than people thought and other scientists began to explore this possibility.

6. Relative Age • Rock layers, strata, show a sequence of events in the past. • Relative age is the age of an object in relation to the ages of other objects. • When determining the relative age of rock layers, one does not necessarily have to address the absolute age of the rock in years. • All rock types can form layers. • Igneous rocks, for example, can form layers as one lava flow tops a previous flow or existing rocks.

7. Law of Superposition • Sedimentary rocks form after deposition and lithification of deposited sediments. • Sedimentary rock units form layers called beds which are separated by bedding planes. • The law of superposition states that a sedimentary rock layer is older than the layers above it and younger than the layers below it.

8. Law of Superposition

9. Principle of Original Horizontality • The principle of original horizontality states that sedimentary rock layers form in horizontal layers and remain that way unless disturbed. • If sedimentary rock layers are not horizontal, it may be assumed that they have been tilted or deformed. • Tectonic forces can push older rock layers over younger ones or overturn a group of rock layers. • If older rock layers have been thrust over or turned up over younger rock layers, evidence is found to explain the anomaly.

10. Principle of Original Horizontality • Graded bedding is the arrangement of layers in which larger, heavier particles are located in the bottom layer. • If larger particles are located in the top layer of the same rock unit, it may indicate that the rock layers have been overturned. • The shape of the bedding plane also provide information about the formation of the rock units. • As sand is deposited, it forms curved beds at an angle to the bedding plane called cross-beds.

11. Principle of Original Horizontality • The top of these cross-beds are usually eroded before new layers are deposited. • In cross-beds, the sediment appears curved at the bottom and cut off at the top. • The shape of cross-beds can also provide evidence of original position. • Small waves that form on the surface of sediments due to water or wind action are called ripple marks.

13. Principle of Original Horizontality • The crest of ripple marks face upward in undisturbed sedimentary layers. • Once a scientist establishes original orientation of rock layers, he can determine relative age of the strata by using the law of superposition.

14. Unconformities • Rock layers are commonly exposed and eroded. • New rock layers can form in the place of the eroded layers. • A break in the geological record occurs at the site of the missing rock layers. • An unconformity is a break in the geologic record created when rock layers are eroded or when sediment is not deposited for a long period of time.

15. Unconformities • There are three types of unconformities. • Nonconformity – stratified rock rests upon unstratified rock • Angular unconformity – boundary between a set of tilted layers and a set of horizontal layers • Disconformity – boundary between two horizontal layers of old sedimentary rock and younger, overlying layers that are deposited on an eroded surface

17. Unconformities • Determining relative age of rock units in the real world may be difficult due to rock units being disturbed by faults or intrusions. • A fault is a break in rock along which movement has occurred.

18. Unconformities • An intrusion is a mass of igneous rock that forms when magma invades cracks in rock and cools and solidifies. • The law of crosscutting relationships states that a fault or crosscutting igneous intrusion is always younger than the rock layers it cuts through. • If a fault or intrusion cuts through a fault or unconformity, then it is younger than the unconformity and any rock unit which it affects.

19. References • James Hutton - http://www.drl.tcu.edu/Scotland/NorthernLights/hutton.html • Law of Superposition - http://www.csun.edu/~psk17793/ES9CP/ES9%20fossils.htm • Cross-beds - http://www.geo.vu.nl/~palmorph/staff/pallavi/HolPics.htm • Ripple Marks - http://capital2.capital.edu/faculty/tlahm/download/Andros%202001

20. References • Unconformities (left) - http://www.colorado.edu/GeolSci/courses/GEOL1020-2/1.htm • Unconformities (right) - http://www.outreach.canterbury.ac.nz/resources/geology/page14.shtml • Fault - http://www.fs.fed.us/r4/boise/field_trip/lookout/faults.html