Absolute age: is the EXACT (or absolute) age of an animal, fossil, person, etc. Absolute age can be determined by using radiometric dating. Example: You are 16 years old. Relative age: is the RELATION of the age comparing to something else, not the exact age. It's never an "exact" number. Example: Fossil is anywhere from 5,000 to 8,000 years old. |
Absolute Age
For finding Absolute Age in :
Sedimentary rock is not good because it is the age of certain pieces, not the whole rock.
Igneous rock is good because you use the age of crystalization
Metamorphic rock is not good, except high grade metamorphism.
Sedimentary rock is not good because it is the age of certain pieces, not the whole rock.
Igneous rock is good because you use the age of crystalization
Metamorphic rock is not good, except high grade metamorphism.
There are multiple laws that aid geologists in determining the age of a rock layer
Rules
Law of Superstition : The older rock is located on bottom and the younger rock is located on top
Law of original horizontally : This law states that layers of sediment were originally deposited horizontally under the action of gravity.
Cross-Cutting rule : Any event that disturbs a rock layer must be younger than the layer it disturbs.
Law of Inclusions : If one rock body contains fragments of another rock body it must be younger than the fragments of rock it contains. OR…The inclusions are older than the rocks which contain them.
Law of Superstition : The older rock is located on bottom and the younger rock is located on top
Law of original horizontally : This law states that layers of sediment were originally deposited horizontally under the action of gravity.
Cross-Cutting rule : Any event that disturbs a rock layer must be younger than the layer it disturbs.
Law of Inclusions : If one rock body contains fragments of another rock body it must be younger than the fragments of rock it contains. OR…The inclusions are older than the rocks which contain them.
Unconformities
An unconformity is a surface that show no deposition or erosion and separate younger starta from older, a gap in the geologic time record. If you unconfrom, you don't follow the pattern.
Some may ask, what are the 3 types of unconformities ?
1. Disconformity - lack of conformity
2. Angular Unconformity - An erosional surface on tilted or folded layers with younger, flat-lying layers over top
3. Nonconformity- An erosional surface between sedimentary rock and non-sedimentary rock. It can look like an igneous intrsion, but sedimentary rocks are younger.
Some may ask, what are the 3 types of unconformities ?
1. Disconformity - lack of conformity
2. Angular Unconformity - An erosional surface on tilted or folded layers with younger, flat-lying layers over top
3. Nonconformity- An erosional surface between sedimentary rock and non-sedimentary rock. It can look like an igneous intrsion, but sedimentary rocks are younger.
The geologic column
Over the past 150 years detailed studies of rocks throughout the world based on stratigraphic correlation have allowed geologists to correlate rock units and break them into time units. The result is the geologic column which breaks relative geologic time into units of known relative age. The geologic column was created and fairly well known before geologists had a means of determining numeric ages.
Faunal Succession
Once geologists had worked the relative ages of rocks throughout the world, it became clear that fossils that were contained in the rock could also be used to determine relative age. It was soon recognized that some fossils of once living organisms only occurred in very old rocks and others only occurred in younger rocks. Furthermore, some fossils were only found within a limited range of strata and these fossils, because they were so characteristic of relative age were termed index fossils. With this new information, in combination with the other principles of stratigraphy, geologists we able to recognize how life had changed or evolved throughout Earth history. This recognition led them to the principle of fossil succession, which basically says that there is a succession of fossils that relate to the age of the rock.