Relative Dating & Fossils
What are the conditions necessary for the preservation of fossils?
1. Fossil should have a hard part like a shell 2. Fossil should be buried quickly 3. Low oxygen so decomposition is stopped 4. Rapid lithification of sediment 5. no scavengers
What makes an ideal index fossil?
1. must be found all over the world 2. easy to identify and recognize 3. lived for a short period of time
nonconformity
A place where an old, eroded rock surface is in contact with a newer rock layer.
Why are fossils found in sedimentary rock?
Igneous and metamorphic have undergone heat or pressure so fossils would not survive.
Law of Superposition
In undesturbed rock layers, the rock on the bottom is older than the rock on the top.
permineralization
Pore space in wood and bone filled in with materials.
Strata
Rocks - another name for rocks
index fossil
a fossil known to have lived in a particular geologic age that can be used to date the rock layer in which it is found
disconformity
a layer of horizontal rock once exposed and eroded before younger rocks formed over it
principle of inclusion
a rock body that contains inclusions of preexisting rocks is younger than the rocks which the inclusions came from
extinction
an entire species dies off and this helps explain the wide separation between the major groups of animals
mold
cavity or impression (hole) left behind when a fossil dissolves
If the sedimentary rocks are folded into a u shaped fold (syncline) the youngest rocks are in the ....
core
trace fossil
evidence of the fossil's existence is preserved (example: burrow or track)
principle of fossil succession
fossil organisms succeed one another in a definite and determinable order, and any time period can be recognized by its fossil content
principle of cross cutting relations
geologic features, such as faults and igneous intrusions are younger than the rocks they cut
What is relative age dating?
geologists used to estimate the age of things by placing geologic events in their order of occurrence
principle of faunal succession
groups of fossil plants and animals occur in the geologic record in a definite and determinable order
angular unconformity
horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers that may be either vertical or at an angle to the overlying horizontal layers
cast
if sediment fills the mold then it is possible to have a replica of the fossil in sedimentary rock
principle of superposition
in a series of stratified sedimentary rocks the lowest stratum is the oldest
replacement
minerals replace the materials of the organism (petrified wood)
metamorphic rocks in contact with layered or unmetamorphosed rocks are usually the oldest or youngest rocks in the sequence
oldest
principle of original horizontality
sedimentary layers are horizontal when originally deposited. Strata that are not horizontal have been deformed by movements of the earth's crust
law of unconformities
surfaces called unconformities represent gaps in the geologic record where layers were either not deposited for a time or it was eroded
original preservation
the organism is preserved in its whole part, fossils that have the soft and hard parts of plants and animals remains that have not undergone any kind of change since the organisms death
carbonization
the whole fossil is converted to carbon,. It occurs when an organism is buried under fine sediment. as time passes, pressure squeezes out the liquid and gaseous components and leaves behind a thin residue of carbon.
If a body of granite contains unmelted inclusions of another rock the granite rock is younger or older.
younger
an igneous rock is always (younger or older) than the rocks that it intrudes
younger
If sedimentary rocks are folded the folding is .younger or older than the youngest rock affected.......
younger than the youngest rock affected