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

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.

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.

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.

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.

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.

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.

Law of Superposition

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.

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.

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.

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.

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.

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

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.

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.

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

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