Earth’s History in Fossils
•Scientists use fossils to form a picture of the Earth’s past
•They show changes in populations, environment changes and adaptations/evolution
Fossils
•A fossil is the remains or evidence of a living thing. They can be a bone or an organism or an imprint in a rock
•Most common: bones, shells, pollen grains, and seeds. Usually not a complete organism
Fossil Formation
•A fossil can only form if the organism is buried in sediments soon after death
•Organisms that lived in or around water were preserved more than others
•Fossils are almost never found in igneous or metamorphic rocks
Petrification or Permineralization
•Where minerals and water seep into bones of decayed animals and turn the bones to stone after evaporation
•Example petrified Forest of Arizona
Molds and Casts
•The empty space that has the same shape of an organism is the mold
•If it is filled in by minerals the hardened form is a cast
Imprint: An imprint is formed in soft sediments and then when it hardened it is reserved as a fossil
Preservation of an Entire Organism
1. Freezing prevents decay, ex: wooly mammoth
2. Animal can fall in bogs or tar pits and be preserved
3. Mummification (desert environment)
4. Amber sap from evergreen trees hardens and traps insects inside
Trace Fossils
 Trace fossils reveal much about an animals appearance without showing any parts of the animal ex: tracks, trails,
footprints, and burrows
Interpreting Fossils
•Fossils indicate that many different life forms have existed at different times throughout Earth’s history
•When they are arranged according to age, they show that living organisms have evolved
•Fossils can give clues to the earth’s climate in the past
Ex: corals have been found in arctic regions, corals only live in
warm oceans
History of the Earth: Scientists use various methods to determine the age of rocks and the Earth. Following are the
various dating techniques scientists use.
1. Law of Superposition
•Fossils are almost always found in sedimentary rock which are made of layers
•Law of Superposition states that in a series of sedimentary rock layers, younger rocks normally lie on top of older rocks
•The law is used to determine whether a fossil or layer of rock is older or younger than another fossil or layer of rock
2. Index Fossils
•Fossils of organisms that lived only a short period of time
• Helps to determine the age of rock layers and be able to compare layers found in different parts of the world
3. Principle of Uniformitarianism
•The present is the key to the past
•The processes that shape the Earth today are the same processes that have been going on for billions of years
4. Unconformities
•Gap in the rock record, missing layers of sedimentary rock that eroded away or were never deposited
•Disconformity forms between layers of sedimentary rock
•Nonconformity forms between non sedimentary and sedimentary rocks
5. Principle of Cross Cutting Relationships
•A fault is a crack in the rock where one side can move, rock layers are always older than the fault
•Intrusion of magma has to have formed after the rock layers were present, therefore the intrusion is younger
•Extrusion is when magma is forced to the top of the rock layers, layers were there first, therefore the extrusion is
younger
•Cross-cutting relationships: Cross-cutting rocks such as igneous intrusions are younger than the rocks they cut through
6. Relative Dating
•Rate of Deposition: Can use the average amount of deposition, about 30mm a year to date fossils and rock layers
•The Principle of Superposition states that older beds are covered by younger beds so in a sedimentary sequence the
youngest unit is at the top.
•This is not an exact date, compare one layer to another
7. Absolute Dating
•Radioactive dating; the breakdown of a radioactive element into a decayed element occurs at a constant rate.
•This is used to determine the exact age of something
•It is determined that the Earth has rocks that are about 4 billion years old
•Half life is the amount of time it takes a radioactive element to have half decay into a non radioactive element
•Ex: Carbon 14 takes 5,170 years for a half life
•The oldest rocks on the moon date to 4.6 billion years old. A half billion years older that the oldest rocks on Earth.
Evidence suggests that the Earth and the Moon formed about the same time.
Other dating techniques
8. Tree rings count number and thickness
9. Seasonal climate changes deposition of light and dark colored sediments called varve
10. Key bed layers of rocks that become a time marker