GEOLOGIC TIME & THE EARTH’S HISTORY Part I. The Rock Record Relative Age Principle of Uniformitarianism James Hutton 18 th century Scottish physician.

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

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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