chapter 11 fossils. 11.1 fossil: evidence such as the remains, imprints, or traces of once living...

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