Fossils and the Rock Record

The Rock Record Rocks record geological events and changing life forms of the past Planet Earth was formed 4.6 billion years ago Earth’s surface and interior have been changed by the same geological processes throughout Earth’s history

Why study fossils? Fossils are preserved remains, imprints, or traces of once living organisms which are preserved in rocks. We can piece together unknown puzzles of the past by studying fossils

Almost all fossils are found in sedimentary rock!!! WHY?

To become a fossil, a dead body must be protected from scavengers and microorganisms Sediments can cover and not damage a dead organism Sediments protect dead organisms from being destroyed by other animals May also slow down or stop the decaying process

Usually fossils are the hard parts of organisms, such as: bones, shells, and teeth This is because these parts preserve better

Ways to Become a Fossil 4 main ways Unaltered remains- remains have not been changed much over time Mummification Altered remains- remains are changed over time Carbonization Mold & Cast Petrification

Petrification Petrified remains are hard and rock like The original material is replaced with minerals. Commonly Petrified Objects… Wood Bones

Carbonization When lots of sediments build up, the body deteriorates and is left as only a thin film of carbon residue. Residue forms an outline of the organism, a fossil imprint. This can be of fish or plants. This kind of fossil is called a carbonaceous film.

Carbonization

Molds & Casts Air and water reach something buried in sediments and decay it It will leave behind an empty space in the rock called a mold Later other sediments fill in the cavity and harden This produces a cast of the original object

Molds/Casts

Unaltered Remains Sometimes actual remains may be found preserved in some way Amber- Hardened tree sap Frozen ground Ice Tar pits

Tar Pits

Mummification Drying Often found in desert caves or buried beneath the desert sand Most bacteria cannot survive without water, no waterorganism does not decay!

Other Fossil Evidence Trace fossils – evidence of animal activity Tracks Burrows Tell us about the lifestyles of the animals.

Trace Fossils

Index Fossils From species that were on Earth for a short amount of time but were abundant Fossils used to determine age of rock layers Fossils can also be used to determine ancient environmental conditions If a scientist found the same index fossil in two different areas of the world this indicates the layers were made at the same time

Index Fossils

Relative Dating Used to determine order of events and the relative age of rocks by their position in a sequence. Doesn’t tell you the exact age Tells you age in relation to other rocks Gaps in the rock record are called unconformities.

Telling the relative age of rock beds Law of Superposition- In an undisturbed layer of rock, the oldest rocks are on the bottom and the rocks become progressively younger toward the top. Principle of original horizontality-layers of sediment are generally deposited in a horizontal position Principle of cross cutting relationships- states that if a rock layer cuts through another the layer that cuts through must be younger than the layer it cuts

Telling the relative age of rock beds Which layer is the youngest? Which layer is the oldest?

Unconformities-Breaks in the rock record Angular Unconformity –Horizontal layers are tilted and lifted upwards

Angular Unconformity

Disconformity -A gap in the record caused by rocks being exposed and eroded. How it works? Sediment was deposited then eroded and new sediment was deposited on top leaving a gap in the record.

Disconformity

Nonconformity –sedimentary rock forms above another rock type (igneous or metamorphic). The other rocks are uplifted and eroded.

Nonconformity

Absolute Dating Method used by geologists to determine age, in years, of a rock or other object. Uses the atoms in rocks and other objects.

Radioactive Decay Radioactivity- Spontaneous decay of the nuclei of unstable atoms When the number of neutrons in an atom change, it becomes a new element

Half Life Time it takes for half the atoms in the isotope to decay Ex: Carbon-14 has a half-life of 5730 yrs. Every 5730 half of the C-14 will decay into N-14

Dating When C-14 decays into N-14, C-14 is considered to be the parent material and N-14 is the daughter product C-14 can be used to date objects up to 70,000 years By measuring the amounts of parent & daughter material, geologists can calculate the age of a rock. This is called radiometric dating

Radiocarbon Dating Carbon 14 can be used to date fossils, bones, and wood as old as 70,000 years Organisms take carbon to build tissues and once they die, it decays away Used mostly for igneous and metamorphic rocks…can not be used for sedimentary rocks

Uniformitarianism Principle that states that the processes occurring today are similar to the processes from the past. Used this theory to argue against those who thought the Earth had only been around for a few thousand years.