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GEOLOGIC TIME & THE EARTH’S HISTORY
Part I
The Rock Record
Relative Age Principle of Uniformitarianism
James Hutton 18th 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
Daughte
r Ele
ment
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 organism2. Amber Preserved – tree sap traps an
insect and hardens3. 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 fossil1. Must be present in a wide area of earth’s
surface2. Must have features that clearly identify
them from all other fossils3. 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