Chapter 11 Fossils

11.1

Fossil: evidence such as the remains, imprints, or traces of once living organisms preserved in rocks Many times dead plants & animals are destroyed due to scavengers & microorganisms Fossils can tell us when, where, & how organisms lived

Necessary Conditions Body of a dead organism must be protected by sediments to keep out scavengers & microorganisms Organisms with hard parts such as bones, shells, or teeth are more likely to become fossilized

Petrified Remains Hard, rocklike remains where some or all of the original materials are replaced by minerals Examples: bones, wood

Carbonaceous Films Tissues of organisms are made of carbon As sediments pile up on the organism, it becomes exposed to pressure & heat This forces liquids & gases from the organism

After the gases & liquids leave the organism, a thin film of carbon is left. This is called a CARBONACEOUS FILM

Molds & Casts Mold: –An object or organism is buried in rock and sediments –Holes in the rock allow air & water in causing the organism or object to dissolve –All that remains is the outer shell of the shape of the organism (hollow inside)

Molds & Casts Cast: –The hollow area of the mold fills in with other sediments –This is a cast of the organism –Page 307, Fig 11.5

Original Remains When the actual organism or parts of the organism are found Example: fly in amber, tar pits, glaciers

Trace Fossils Fossilized tracks and other evidence of animal activity Example: Dinosaur prints

Index Fossils Species that lived on Earth for short periods of time, were abundant, and were widespread geographically Used to tell the age of rock layers

11.2 Relative Ages of Rocks

Principle of Superposition For undisturbed layers of rock, the oldest rocks are on the bottom and the rocks become younger towards the top

Relative Dating Used to determine the order of events and the relative age of rocks by examining the position of rocks in a sequence Tells how old something is in relation to something else DOES NOT give exact age

Unconformities Gaps in rock layers Develop when erosion removes existing rock layers 3 types

Angular Unconformities Horizontal layers of rock are tilted and uplifted, erosion, new layers are deposited on top of the tilted layers

Disconformities All layers are horizontal, erosion, new layers are deposited on top

Nonconformity Metamorphic or igneous rocks are uplifted and eroded Sedimentary rock layers are deposited on top Nonconformity is the surface between the two types of rocks

Sandstone on granite

Time To Draw!

Correlating Rock Layers Rock layers which are the same in two different locations To prove that two layers of rock are the same in two different locations, scientists look at fossils

If they have the same fossils, there is a good chance that they are the same… AGE!

11.3 Absolute Ages of Rocks

Absolute Dating Method used to determine the age, in years, of a rock or other object Uses properties of atoms in rocks and other objects to find their ages

Radioactive Decay Isotopes: –When the same element has atoms with different numbers of neutrons in their nuclei –Example: C-14 and C-16

Radioactive decay occurs when isotopes lose neutrons and gain protons This process forms a new element Example: –Uranium-238 is radioactive—parent material  DECAY  –Lead-206 is not radioactive—daughter material

Because the lead-206 is NOT radioactive it will NOT decay anymore

Half-Life: –The time it takes an isotope to have half of its atoms decay –Example: C-14  N-14 –Half-life = 5730 years

Radiometric Dating As time goes on, the amount of parent material decreases and daughter material increases By measuring the amounts of parent and daughter materials in a rock & by knowing the half-life of the parent, a geologist can calculate the absolute age of the rock (radiometric dating)

Scientists must decide which parent and daughter materials to study