The Rock Record Chapter 8 James Hutton  18 th Century Scottish physician  Observed geologic changes that took place on his farm  By studying the present, people could learn about the past

Uniformitarianism  The present is the key to the past  Geologic processes that occurred in the past can be explained by current geologic processes  Same processes, but rates can change  Volcanism, erosion  Opposite of catastrophism – changes occur abruptly

Earth’s Age  4.6 billion years  Hutton reasoned that change was very slow  Observed changes must have taken place over billions of years, not thousands  Supported Darwin’s Theory of Evolution

Relative Age  Age of an object related to another  Object 1 is older than object 2  Don’t know the actual age, just that one is older than the other  Original Horizontality –Sedimentary rocks left undisturbed will remain in horizontal layers

Law of Superposition  Sedimentary layer of rock is older than the layer above it, but younger than the layer below it  This is true as long as layers are NOT disturbed

Determining Original Rock Position  Graded Bedding –Large particles settle first, followed by smaller particles –If particle size doesn’t match up, there must have been some geologic activity

Determining Original Rock Position  Cross beds –Sand beds can form at angles as sand blows down a dune –Older ones erode as newer ones are formed –Get strange angles –Can violate Original Horizontality Rule

Determining Original Rock Position  Ripple Marks –Smalls waves on rock surfaces (like at the shore on sand) –Can match up ripple marks –Ripple marks should point up

Unconformities  Break in geologic record when rock layers erode or sediment is not deposited for a long time  Three types of unconformities 1. Nonconformity –Stratified rock sitting on a nonstratified rock –Sedimentary on igneous or metamorphic

Unconformities 2. Angular unconformity –Tilted layers set on horizontal layers 3. Disconformity –New layers deposited on old layer after erosion

Disconformity 1)Sediment deposition 2)Sediment erosion 3)More sediment deposition

Law of Crosscutting Relationships  A fault or rock intrusion is younger than the rock it cuts through  Intrusions can be dikes, sills, or magma (plutons)

Absolute Age  Numeric age of a rock formation  Absolute dating methods –Geological methods –Chemical methods

Age – Geological Methods 1. Rates of erosion –Practical only for features that have formed in the past 10,000 to 20,000 years –Won’t work for older things because rates of erosion change over time

Age – Geological Methods 2. Rates of deposition –Can estimate average rates of deposition –About 30 cm of sedimentary rock formed every 1000 years –Rates can change though (Floods) so age is estimated

Age – Geological Methods 3. Varve count –Varves – banded layer of sand and silt that is deposited annually in lakes –Each band represents one year –Annual events bring in the sand and silt to form the layers  Snow melt runoff in spring  Leaves in the fall

Age – Chemical Methods 1. Radiometric dating –Age determined by comparing relative percentages of a radioactive isotope and a stable isotope –Radioactive isotope = parent –Stable isotope = daughter –Using known decay rate, can calculate absolute age

Radiometric Dating  Half life –Time it takes half the mass of a given amount of a radioactive isotope to decay into its daughter isotope –Not affected by temperature, pressure or other environmental factors –C-14 half-life = 5,730 years –U-238 half life = 4.5 billion years

Radiometric Dating  Limitations –No parent or daughter isotopes can be lost or gained through leaking or contamination –If sample is too old, not enough parent isotope –If sample is too young, not enough daughter isotope –Must choose correct isotope

Carbon Dating  Young rock age determined by dating organic matter in the rock by means of Carbon – 14  Living organisms contain both C-14 (radioactive) and C-12 (not radioactive)  C-14 to C-12 ratio constant during life  C-14 decays to C-12 after death

The Fossil Record  Fossils –The remains of animals or plants that lived in a previous geologic time –Most commonly found in sedimentary rock  Paleontology –The scientific study of fossils

The Fossil Record  How fossils form –Mummification  Found in hot and dry places –Amber – hardened tree sap –Tar seeps  Animals fall in and can’t get out –Freezing – buried in frozen soil –Petrification  Minerals replace organic materials  Creates nearly perfect replica

The Fossil Record  Types of Fossils –Imprints –Molds & Casts  formed from empty shells –Coprolites  fossilized waste materials –Gastroliths  Stones in dinosaur digestive systems

The Fossil Record  Trace Fossils –Tracks, footprints, borings and burrows giving evidence of past animal movement  Index fossils –Fossil used to establish age of rock layers because it is distinct, abundant, and widespread –Only existed for a short span of geologic time