Regents Earth Science – Unit 12: Geologic HistoryEarth’s History

Fossil - the remains or evidence of a living thing

• only hard parts fossilize

• found in sedimentary rock Official NYS fossil: Eurypterid

Types of FossilsFossils can form in the following ways:

• Petrification – replacing living tissue with minerals

• Preserved Fossils – frozen or amber

• Casts and Molds – imprints of tracks, etc.

• Drying – mummies

• Others

FossilsFossil Formation:

1. Organism dies

2. soft parts decay or are eaten

3. hard parts are buried in sediment

4. sediment becomes rock

5. uplift, weathering and erosion expose the fossil

Fossils

Fossils Indicate Four Factors in Earth’s History:

1. life changed/evolved over time

2. Earth's surface has changed

• marine fossils are found at high elevation

3. Earth's climate has changed

• tropical coral fossils are found today in NYS

4. appearance and activity of past life

• teeth give clues to what an animal ate

• bones give clues to how the animal looked and move

Earth’s History

Two Processes help us determine the age of earth’s rocks and events:

• Relative Dating – determining if something is older or

younger

• Absolute Dating (Radioactive Dating) – determining an

approximate age in years

Relative Dating

Relative Age - the age of something compared to something else

• Geologic Column - an ideal sequence of rock layers created by combining data from all known rock sequences at various locations

Relative Dating

• Law of Uniformitarianism - processes that change Earth today are the same as those that changed it in the past

• "The Present is the Key to the Past"

• Principle of Original Horizontality - sedimentary rocks form in horizontal layers (sedimentary rocks not in horizontal layers have been changed since they formed)

• Principle of Superposition - in any undisturbed sequence of rocks, the oldest rocks are at the bottom, youngest at the top

oldest

youngestRules of Relative Dating:

Rocks are older than the events that happen to them (because the rocks exist first)

• Tilting, Folding, Faulting, and Intrusions are all younger than the rocks they effect

Tilting Folding Faulting Intrusion

Unconformity - buried erosional surface, represents a gap in time in Earth's history

• Formation of an

Unconformity:

Relative Dating – Sequence of Events

Sequence of Events – putting events in order of age from what happened first to what happened last

Sequence of Events:

1. deposition of limestone

2. deposition of sandstone

3. deposition of shale

4. deposition of limestone

5. deposition of sandstone

6. faulting of rock layers

7. intrusion of igneous rock

Sequence of Events that formed the landscape cross-section at the left:

1. Deposition of layers 1-5

2. Intrusion of #6

3. Tilting of layers 1-5 and Intrusion of #6

4. Erosion of rocks

5. Deposition of layers 9,10

6. Erosion of rock layers

relagecycle.gif (29947 bytes)

Relative Dating - CorrelationCorrelation – matching rock layers from around the world

Index Fossil - a fossil of an organism that lived for a short, well defined period of time and lived over a large geographic area

• used to date the rock layers in which it is found

index fossil

Index Fossils:

Relative Dating – Correlation

• Index Fossils of New York State are found at the bottom of the Reference Tables p. 8-9

• the letter on the index fossil at the top of the page is located on the time lines at the center of your chart

• where the letter is located is the time when that fossil existed

• the length of the line represents the entire time that any fossils of that animal species may have existed

Relative Dating – Correlation

• Volcanic Ash - can also act as an index to correlate rocks of different regions

• ash blown high into the atmosphere encircles the Earth and settles world wide as a thin layer of sediment

• this sediment lithify and become a thin layer of sedimentary rock

Absolute Dating• Radioactive Dating - method used to determine the absolute (actual) age of a

rock or event

• certain elements spontaneously decay (change) into different elements

• radioactive (parent) element - has an unstable nucleus that decays

• decay (daughter) element - stable element that is the result of radioactive decay

Reference Tables p. 1

• Half-Life - the time it takes for one-half of a radioactive element to change into a decay element

• the rate of decay is NOT effected by:

1. heat

2. pressure

3. chemical action

• Radioactive Decay Graph:

% Elem

ent

Absolute Dating

• Potassium-40 is used to date rocks/events millions to billions of years old

• Uranium-238 rarely used - uranium not found in most rocks

• Carbon-14 is used to date rocks/events up to ~50,000 years old

• radioactive decay is accurate for dating up to 5-7 half-lives:

Carbon 14

(half-life = 5,700 yrs.)

Geologic Time

• the Geologic Time scale divides Earth's history into sections of time (from largest to smallest):

1. Eon

2. Era

3. Period

4. Epoch

• boundaries between time periods

represent changes in the life on

Earth due to evolution and mass

extinctions

• Earth formed 4.6 billion years ago

• Reference Tables p.8-9

Geologic Time

• Reference Tables p. 8-9

• the time scale is divided into units of time called Eons

• Eons are subdivided into Eras

• Eras are subdivided into Periods

• Periods are subdivided into Epochs

• the absolute ages of the rocks for each time period are listed

Geologic Time

• Reference Tables p. 8-9

• the life that existed on Earth is listed when they lived on Earth

• when major life-forms became extinct is also listed

Geologic Time

• Reference Tables p. 8-9

• the bedrock that existed in NYS is shown by a dark line

• ex.: NYS has a complete rock record of rocks from the

Cambrian to Devonian

Geologic Time

• Reference Tables p. 8-9

• the major geologic events in NYS and what the landmasses of Earth looked like are given at their proper time in Earth’s history