GEOLOGIC TIME & THE EARTH’S HISTORY Part I

The Rock Record  Relative Age  Principle of Uniformitarianism James Hutton 18 th century Scottish physician and gentleman farmer Drew CONCLUSIONS based on OBSERVATIONS THEORIZED – same geologic processes (volcanism, erosion, etc.) changing the landscape of his farm, were the same forces that had changed the earth in the past

Ideas of the Past  Before Hutton most people thought the earth was only 6,000 years old  geologic features were formed at the same time

Uniformitarianism  Hutton’s Observations  Forces changing his farm were very SLOW  Hutton’s Reasoning  It must have taken millions of years to create the features of the earth’s crust  Hutton’s Conclusion  The earth MUST be older than previously thought

To Determine the Earth’s Past  Determine the order rock layers and structures were formed  These layers are called strata, which provide details about the order of geologic events that occurred in earth’s past

Relative Age  One layer or strata is older or younger than another.  This is NOT an exact age  Most scientists study strata of sedimentary rocks

Law of Superposition  Sediments are deposited horizontally  Layers not horizontal that are tilted or deformed did so AFTER the layers were formed  Sediments harden into rock beds  Bedding planes – boundary between two beds  It is how scientists determine the relative age of a layer of sedimentary rock  First they must find original arrangement of layers to apply the law of superposition

Clue 1- Size of Particles  These are clues scientists use to determine the original order of layers  Largest particles deposited near bottom of layer

Clue 2 – Shape of Bedding Plane  Cross-bedded layers  Curved sheets at an angle to bedding plane  The tops can be eroded before new layers are deposited

Clue 3 – Wind/Water Marks  Ripple marks from wave or wind action form peaks  Peaks point upward, telling scientists the original arrangement of the rock strata (layers)

Unconformities – break in the geologic record  Movements of the earth’s crust can lift and expose layers to erosion  They can also lower as eroded or sea level rises  All while new sediments are being deposited  Indicates: for a time, deposition stopped, then erosion occurred, then deposition began again.  Three types

1. Nonconformity  Metamorphic and igneous are normally unstratified  When stratified rock is on top of unstratified rock  Example: Granite can be lifted to the surface by crustal movements. Once exposed, it is eroded. Sediments are then deposited on top of the eroded granite. The boundary between the deposited sediment and eroded granite represent an unknown period of time.

2.Angular Unconformity  Horizontal layers are folded and tilted and then eroded  Erosion stops and new horizontal layer is deposited on the tilted layer

3.Disconformity  Layers of sediment on the ocean floor are lifted above sea level – with no tilting or folding  Once exposed, it is eroded  Falls below sea level again and deposition begins again  A large time gap exists where the upper and lower layers meet at the unconformity

Crosscutting Relationship  Tectonic Activity disturbs rock layers  Law of superposition is hard to determine  Law of crosscutting relationships  A fault or intrusion is younger than the layers it cuts through Fault – break/crack in rock Intrusion – igneous rock formed from contact with magma

Absolute Age  Actual age of a rock layer – Not Accurate  Rates of Erosion and Deposition  Erosion – scientists measure rate of erosion of a stream bed and can estimate the age of stream Features that formed 10,000 to 20,000 years ago  Deposition – Rate of deposition calculations require data over a long period of time; then geologists can estimate the avg. rate for sedimentary rocks. Floods can deposit a lot of sediment in just a day

Varve Count  Think of it like tree rings  It is annual (yearly) deposits of sediment  Light bands of coarse particles followed by dark banks of fine particles  Glacial Lakes  Summer – rapid ice melt carries large amount of coarse sediment  Winter – freezing surface of lake causes fine sediment in water to settle.

Radioactive Decay  Accurate method/Natural Clock  Radioactive Isotopes – atomic nuclei that emit particles (energy) at a constant rate  As particles are emitted, it changes into a different isotope of the same element

Radioactive Decay – How it works!  Scientists measure amount of original radioactive isotope  Then scientists measure amount of newly created isotopes  Then they compare proportions of original/new isotope to find absolute age

Radioactive Decay - Example  Uranium (radioactive element) U-238  Emits two protons and two neutrons (called RADIOACTIVE DECAY)  This decreases the atomic mass (n + p) and atomic number (# of protons) giving a new element Th-234  Eventually nonradioactive Lead (Pb-206) is produced Parent Element Daughter Element

Carbon Dating  Organic Materials contain CARBON  Some geologic materials contain remains of living things.  Scientists use the radioactive form of Carbon-14 (It is naturally occurring, absorbed by plants, consumed by animals through food chain)  Decays to form Nitrogen-14  Used to find the age of wood, bones, shells, and organic remains of early humans  Can be used on samples up to 70,000 years old.

The Fossil Record  Paleontologist – scientists that study fossils to learn about earth’s past  Paleontology – study of fossils  Fossils are found primarily in sedimentary rock

Kinds of Fossils (9 Types) 1. Mummification – drying of an organism 2. Amber Preserved – tree sap traps an insect and hardens 3. Tar Seeps – thick petroleum oozes to earth’s surface usually covered by water. Animals come to drink the water and become trapped in the tar. 4. Freezing – Organisms are frozen and preserved 5. Petrification – Organism materials are replaced by minerals by groundwater, forms a replica

Types of Fossils 6. Trace Fossils – Footprint, or trail left behind by animal 7. Imprints, Molds, and Casts – Imprints of leaves, empty cavities are molds, Casts are where molds have filled with clay and hardened 8. Coprolites – Fossilized dung of ancient animals 9. Gastroliths – stones that were in digestive tracts of some dinosaurs survive as fossils – only verified if found with dinosaur remains.

Index Fossils  Some fossils are of organisms from specific geologic time periods in earth’s history  Must meet following criteria to be an index fossil 1. Must be present in a wide area of earth’s surface 2. Must have features that clearly identify them from all other fossils 3. Organism that is fossilized must have lived in a relatively short geologic time span 4. Must occur in fairly large numbers in rock layers

Why are Fossils Important?  They tell us  How organisms have evolved (changed through time)  Climate through time  What organisms ate, where they lived, how they lived  Age of rock layers