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