What are Fossils?• Fossils are the evidence or remains of once-living plants

or animals• Fossils provide evidence of past existence of a wide

variety of life-formso Body Fossils are parts of the bodies of organisms that have

been preserved as fossilso Trace Fossils are fossilized evidence of life, such as burrows,

tracks, droppings, and nests• The total number of fossils that exist is known as the

fossil record. The fossil record provides evidence that a population has undergone changes in their environment, which is referred to as evolution

Learning from Fossils• Throughout history, people have found

fossilized remains of ancient organisms and tried to learn from them

• Many myths, such as those of dragons and griffins, resulted from finding dinosaur bones

• Paleontologists are scientists who study the Earth’s history and use fossils to learn about past climates and geologic conditions

• Paleo- means “very old”

Preservation of Original Remains• When all the organic material has been removed

and the hard parts of a plant or animal have been changed by mineral replacement or by recrystallization

• When pore spaces are filled with mineral substances, it is referred to as permineralization.

• Most common method of fossilization• Example: Petrified Wood

Molds and Casts

Mold – hollowed-out impression of the shell that was left in sedimentary rock

Cast – When the cavity of the hollowed-out impression becomes filled with minerals or sediments

Preserved Remains• Preserved remains – actual organisms preserved in

fossils• Trapped in amber (ancient tree sap) or frozen in ice• Scientists can study the organism’s DNA• Most useful fossils, but also the rarest

Why Study Fossils?

• The study of fossils provide us with clues about past climates, the motions of plates, and other geological events

– Describe how organisms have changed through time

– Show evidence of ancient environmental conditions

– Help scientists find patterns and cycles that can be used to predict future phenomena

– Allows scientists to interpret and describe Earth’s history

Index Fossils Remains of plants or animals that can be used by geologists to correlate rock layers over large geographic areas or to date a particular rock layer

Easily recognized & Abundant

Widely distributed geographically

Must have lived during a short period of time

Law of Fossil Succession

• Principle that states that fossils from different time periods show the nature of life during that time period

• Fossils are found in a definite and determinable order due to life changing over time

Geologic Dating

Relative Dating

• Compares the age of a rock (or fossil) to the age of another rock

• Tells which is older but does not give actual age

Steno’s Laws

Original HorizontalityAs sediments are deposited, they form flat, horizontal layers

Lateral ContinuitySediments are deposited in continuous sheets extending as wide as the basin in which they form

Steno’s Laws (cont)

Law of SuperpositionBecause sedimentary rocks are deposited on top of each other, the oldest rocks are at the bottom and youngest at the top

Cross-Cutting RelationshipsA rock formation cutting across another is younger than the rocks it disturbs

Unconformity

A gap in the rock record due to erosion

Key Bed

• A distinctive layer of rock that can be found in most places around the world

• A clay bed containing iridium is present around earth. It is dated around 66 million years ago and marks the mass extinction of the dinosaurs

Correlation

Matching rock layers in different regions of Earth to create a common timeline

Uniformitarianism

• The processes that form Earth’s features today act the same way now as they did in the past

• It would takes millions of years to form Earth’s landscape

• Erosion occurs slowly now, and likely did in the past

Absolute Dating

• Gives actual age measured in years or other unit of time

• Typically measured in years, but there are several ways to measure time in nature

Tree Rings

• Trees grow more quickly in spring and summer than in the winter, creating alternating thick lighter bands (summer) and thin dark bands (winter)

• By counting the number of dark bands you can estimate the age of a tree

• By analyzing the width of each ring scientists can study climate change

Ice Cores

• A cylinder of ice is extracted from a glacier or ice sheet

• Layers form within glaciers because there is less snowfall in the summer which allows a dark layer of dust to accumulate on top of winter snow

• Scientists drill deep into ice sheets to produce core samples and analyze them to determine climate change and to measure atmospheric gases

Radiometric Dating

• Process that uses radioactive decay to determine absolute age of a material

• Uses several different isotopes to check accuracy of the ages that have been calculated

Chemistry Review

Half - Lives

• Radioactive decay eventually splits the atom

• When this happens, the original (now unstable) atom is called the parent material and the resulting (stable) atom is called the daughter material

• As time passes, the proportion of radioactive isotopes will decrease and the proportion of daughter isotopes will increase

• The average amount of time it takes for half of the atoms in a sample to decay is referred to as the Half-Life

• Each year, the sample is divided in half– 50% parent / 50% daughter (1)– 25% parent / 75% daughter (2)– 12.5% parent / 87.5% daughter (3)

Limitations to radiometric dating

• Can only be used on certain materials – Organic Remains– Older Rocks (+100,000 years)

• Works best with igneous rocks