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Rock Record Spring 2013. Determining Relative Age Geologists estimate that the earth is 4.6 billion years old James Hutton – Principle of uniformitarianism-

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Presentation on theme: "Rock Record Spring 2013. Determining Relative Age Geologists estimate that the earth is 4.6 billion years old James Hutton – Principle of uniformitarianism-"— Presentation transcript:

1 Rock Record Spring 2013

2 Determining Relative Age Geologists estimate that the earth is 4.6 billion years old James Hutton – Principle of uniformitarianism- current geologic processes are the same processes that were at work in the past This principle is one of the foundations of geology.

3 Determining Relative Age Relative age- indicates that one layer is older or younger than another layer based upon its position – Scientists can determine the order in which rock layers were formed Commonly done in sedimentary rocks

4 Determining Relative Age Law of Superposition – States that an underformed sedimentary rock layer is older than the layers above it and younger than the layers below it. Unconformities- this is when layers of rock are not in their original position – Nonconformity- stratified rock lies on stratified rock – Angular Unconformity- boundary between the tilted layers and the horizontal layer – Disconformity- boundary between the older eroded surface, and the younger overlying layers is nearly horizontal

5 Determining Relative Age Crosscutting Relationships – Law of crosscutting relationships States that a fault or an intrusion is always younger than the rock layers it cuts through

6 Determining Absolute Age Rates of Erosion & Deposition – Studying rates of erosion allows scientists to determine how old a feature is Ex: Niagra Falls– the banks of Niagra Falls are eroding at about 1.3 m/year so it is estimated that it is 9000 years old This allow good approximations only for features10,000 to 20,000 years old

7 Determining Absolute Age Rates of Erosion & Deposition – The rate of sediment deposition can be calculated using data collected over a long period of time. On average 30 cm of sedimentary rock are deposited over a period of 1000 years

8 Determining Absolute Age Varve Count – Varve- an annual layer of sedimentary deposit on a lake bed Consist of light colored and a dark colored band – A coarse summer layer & a fine winter layer form one varve – Usually formed in glacial lakes

9 Determining Absolute Age Radioactive Decay – Radioactive isotopes have nuclei that emit particles and energy at a constant rate. The natural radioactivity found in rocks gives scientists an accurate way to find the absolute age of rocks. – As an atom emits particles and energy, the atom changes into a different isotope of the same element or an isotope of a different element. – Scientists measure the concentrations of the original radioactive isotope & the newly created isotopes & then compare the proportion of the two to determine the age of the rock.

10 Determining Absolute Age Half-Life – The time it takes for half the mass of a given amount of a radioactive element to decay into its daughter elements Ex: Uranium-238 decays into Lead-206: uranium is the parent element, lead is the daughter element

11 Determining Absolute Age Carbon Dating – Carbon dating is used when rock samples are recently formed Rocks contain carbon-14 Carbon-14 decays into carbon-12

12 Fossil Record Paleontologists- scientists that study fossils – Fossils- remains or traces of animals or plants from a previous geologic time. Provide clues about events, climates, & evolution Evolution- change of living things over time. Paleontology- study of fossils Fossils are almost always found in sedimentary rock

13 Fossil Record Kinds of Fossils – Bones – teeth – Shells – Rarely an entire organism – Replicas – Imprints

14 Fossil Record Preservation of Organisms – Only dead organisms that are buried quickly or protected from decay can become fossils. – Mummification- drying Mummified organisms are often found in desert caves and buried beneath desert sands – Amber- hardened try sap Insects are preserved this way when they get trapped in the sap and it hardens

15 Fossil Record Preservation of Organisms cont… – Tar Seeps – formed by thick petroleum oozing to the earth’s surface Animals become trapped in the sticky tar and tar eventually covers them preserving them – Freezing Low temperatures of frozen soil & ice protect & preserve animals because most bacteria cannot survive freezing temperatures

16 Fossil Record Petrification – Petrification- mineral solutions remove original organic materials and replace them with new materials – Often results in a near perfect replica of the original organism

17 Fossil Record Traces of Organisms – Trace fossils- tracks, footprints, borings, & burrows Formed when sand or silt fills the hollowed out area and hardens – Imprints, Molds, & Casts Imprints are usually of leaves, stems, flowers, & fish and are formed from soft mud or clay that has hardened Molds empty cavities left by the shells of snails & parts of trees ; retains the surface markings & shape of the original organism Casts form when a mold fills with mud or sand; it is a replica of the original organism

18 Fossil Record Trace Organisms cont… – Coprolites coprolites- fossilized dung or waste materials from ancient animals – Gastroliths Gastroliths- stones that were found in the digestive systems of dinosaurs to help grind their food

19 Fossil Record Interpreting the Fossil Record – Provide clues to the relative ages of rocks – Reveal environmental changes & how they have affected organisms through the geologic history of the earth

20 Fossil Record Index Fossils – Fossils found exclusively in rock layers of a particular geologic age – To be considered an index fossil… Must be present in rocks scattered over a wide area of the earth’s surface Index fossils must have features that clearly distinguish them from all other fossils Organisms from which the index fossil formed must have lived during a relatively short span of geologic time Must occur in fairly large numbers within the rock layers

21 Fossil Record Fossil Clues to the Past – Fossil tells us about different climate and environmental changes that occurred in the past. Ex: tropic plant and animal fossils have been found in both Canada and the Arctic


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