Chapter 17 The Rock Record

Page 322

BELLWORK: Order the following rock layers from oldest youngest

How old is the Earth?????

• 4.6 billion years old• How do we know???

– By the geologic record

§17-1 Determining Relative Age

• Objectives:

1. State the principle of uniformitarianism.

2. Explain how the law of superposition can be used to determine the relative age of rocks.

3. Compare the 3 types of unconformity.

4. Apply the law of crosscutting relationships to determine the relative age of rocks.

Uniformitarianism

• originated with Scottish geologist James Hutton in 1785

• history could be interpreted in terms of processes currently observed.

Relative Dating of Rocks

• In sedimentary rocks, Age is based on position

• “Law of Superposition”

Law of Superposition

Unconformities

• Gaps in the geologic record.

• Periods of erosion

• Unknown time of creation.

Nonconformity

• igneous intrusions into the sedimentary rock layers.

Disconformity

• Hard to recognize because all layers are horizontal.

Cross-Cutting Relationships

Here are some real life examples of cross-cutting relationships:

§17-2 “Determining Absolute Age”

• Objectives:1) Summarize the limitations of using the

rates of erosion and deposition to determine the absolute age of rocks.

2) Describe the formation of varves.3) Explain how the process of radioactive

decay can be used to determine the absolute age of rocks.

Relative Age = little sister, big brother, mom & granddad

• Absolute age => – Little sister is 2 Years old

– Big brother is 6 years old

– Mom is 30 years old

– Granddad is 65 years old

Relative Age based on Rates of Erosion

• Only works for the past 10-20,000 years.

• Example:– Niagara Falls is 9900 years

old based on current erosion of the edge of 1.3 m per year

Relative Age based on rates of deposition

• 30 cm every 1000 years for sedimentary rock

• BUT if there is a flood or other event this is not very accurate.

Varve Count

• Varve = yearly layers of sediment

• A lot like counting tree rings.

• Each year has:– a light colored, course layer

of sediment &

– a dark,fine layer of sediment.

Dating using Radioactive Decay

• Uranium- 238 is the most widely used element.

• It breaks down at a predictable rate to Lead-206:– ½ every 4.5 billion years

Gizmo on Radioactive Decay

Explorelearning.com

Explorelearning.com

Activity today is:

§17-3 The Fossil Record

• Objectives:

1) Describe 4 ways in which entire organisms can be preserved as fossils.

2) List 4 examples of fossilized traces of organisms.

3) Describe how index fossils can be used to determine the age of rocks.

Paleontologists – study fossils

• Fossils can only be found in sedimentary rock

                           

                                

Preservation is the Key !!

Remains Instantly Covered by– Mud

– Sand

– Silt/clay

– Other organisms

Mummification Occurs in deserts

Amber = pine sap

• Actually preserves insects perfectly including the DNA

Tar Seeps

• Like the La Brea Tar Pits in California.

Freezing

• Like “Ice Man” & the mammoth found in Siberia.

                            

                                      

Petrification

• Example:– The Petrified Forest in

Arizona

Trace Fossils

Very Gross trace fossils• Coprolites

– Fossilized poop

– Show scientists what the animals ate.

• Gastroliths – Stomach Stones used by

dinosaurs to grind their food

– Like birds do today.

Interpreting the fossil record

• Used to date rocks• How environments

have changed– Remember Wegener?

• How animals were affected & adapted to the environment– Remember Darwin?

Index Fossils

Example: Trilobites