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.
The idea that the earth is 4.6 billion years old came from the work James Hutton. Hutton observed the small changes taking place on his farm and using scientific methods he drew conclusion based on his observations. Hutton theorized that the forces changing the landscape of his farm were the same forces that had changed the earth’s surface in the past. He believed that by studying the present, people could learn about the earth’s past.
Geologists refined Hutton’s theory and said that the same processes are occurring but the rates at which they occur may vary.

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
Layers of rocks, called strata, are like the pages in book, detailing the sequence of events that took place in the past.
Various kinds of rocks form layers. Igneous rocks form layers when successive lava flows stack up. In some metamorphic rocks like marble layers are visible. To

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
Relative age indicates only that one rock layer is younger or older than another rock layer. In order to learn more about the earth scientists often need to find the absolute age, or actual age of a rock layer.

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
Average is the key word as events like flooding can cause a large of amount of sediment to be deposited and also the rate of deposition can change drastically over time…as a result the absolute age of is not very accurate when using the rate of erosion and sediment deposition

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
You might have heard of dating trees by counting the rings inside the trunk. Scientists have derived a similar method more estimating the aged of certain sedimentary deposits. Some sedimentary deposits show definite layers.
During the summer, when snow and ice melt rapidly, a rush of water can carry a large amount of sediments into a lake. Most of the coarse particles settle quickly to form a layer on the bottom of the lake. With the winter when the surface of the lake begins to freeze, find particles still suspended in water will slowly settle to form a thin layer on top of the coarse layer of sediments that has already been deposited. So each set of winter and summer layers is one varve.
By counting the varves, scientists can estimate the age of the sediments.

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.
Remember that isotopes of an element are atoms with a different number of neutrons in their nuclei. Rocks often contain radioactive isotopes.

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
Fossils are an important source of information for establishing both the relative and absolute ages of rocks. Fossils also provide clues to past events, climates, and evolution.
Fossils are almost always found in sedimentary rock. Sediments generally small and lightweight do not damage a dead organism but cover it. Sediments also prevent dead organisms from being destroyed by other organisms. The covering sediments may also slow down or stop the decaying process.

13. Fossil Record Kinds of Fossils Bones teeth Shells
Rarely an entire organism
Replicas
Imprints
Fossils form in many different ways. Usually only the hard parts of organisms, such as bones, shells, and teeth are preserved. In rare cases, an entire organism may be preserved. In some fossils, only a replica of the original organism may be preserved. Other fossils merely provide evidence that life existed.

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
Normally dead plants and animals are eaten by other animals or decomposed by bacteria. Left unprotected, even hard parts such as bones decay, leaving no trace of the organism. Only dead organisms that are buried quickly or protected from decay can become fossils
Mummification- most bacteria cannot survive without water, the mummified organism does not decay.
Amber- delicate features such as legs and antennae have been preserved, even DNA has been recovered from amber

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
Tar seeps- often covered by water. Animals that came to drink the water become trapped in the sticky tar. Other animals preyed upon the trapped animals and became trapped. The bones of thousands of animals that lived about 15,000 years ago have been found trapped within the La Brea Tar Pits in southern California. In tar seeps in Poland, entire woolly rhinoceroses have been found with their flesh and fur mostly intact.
Freezing- the frozen bodies of mastodons & woolly mammoths have been found in Siberia and Alaska, woolly rhinoceroses have also been found in frozen arctic soil

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
Organisms are usually preserved through petrification. In this process, mineral solutions such as groundwater remove the original organic materials and replace them with new materials. Some common petrifying minerals are silica, calcite, pyrite.
There is a petrified forest in arizona, it contains petrified logs of cone bearing trees

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
Sometimes no part of the original organism survives in fossil form. However, trace fossils, such as tracks, footprints, boring, and burrows, can still provide information about prehistoric life. The fossilized traces of ancient organism are often remarkably detailed.

18. Fossil Record Trace Organisms cont… Coprolites Gastroliths
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
Coprolites- some ancient worms, snails, and crabs are known to have existed because fossil remains of their waste material have been found. After the food was digested the sand or mud was excreted as castings or waste material, these castings were found as fossils in some marine sediments. They can be cut into thin sections and observed through a microscope. The materials identified in these sections reveal feeding habits of ancient animals such as dinosaurs.
Gastroliths- can be recognized by their smooth, rounded, and polished surfaces. However identification of these stones is certain only if found within the remains of a dinosaur.

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
The fossil record provides scientists with many clues to the geologic history of the earth. For instance, scientists can use fossils to find the relative ages of rocks.

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
Paleontologists can use fossils to establish the relative ages of rock layers in which the fossils are located. Fossils of organisms that lived during more recent time periods are found in younger rocks than those that lived in earlier time periods. Scienctists can use index fossils to give absolute ages to specific rock layers because index fossils lived ruing short spans of geologic time, the rock layer can be dated very accurately.
Scientists can use index fossils to date rock layers in different widely separated areas. Geologists also use index fossils to help located oil & natural gas deposits.

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