2Time is… Either relative or absolute Time is: a measured period during which an event, process, or condition exists or continues.
3So….How do scientists tell how old rocks are anyway? Scientists use fossils to help determine the age of rock layers.The age of a fossil can be determined by carbon dating:Carbon exists in all living thingsCarbon decomposes at a specific rate that can be calculatedScientists can measure how much carbon a fossil has leftin it in order to figure out how long it has been dead.4. This is an example of absolute age (dating).Some fossils can be used as index fossils:These fossils are used to determine the approximate age of thelayer of rock that they are found in.In order to be an index fossil they have to:Have only existed for a certain short amount of time.Have to have lived in many different places across the globe.
4Dating TechniquesOne ways in which geologists look at time: Relative Dating
5Relative Dating is..…the measure of a sequence of events without knowing the exact date at which the events occurred.What happened first?What happened next?
6UNIFORMITARIANISM Relative Age: The age of a rock layer compared to the other rock layers around it. ( Younger or older )Absolute Age: The age in years (definite age….14 years old)The rock layer thatforms first is found onthe bottom of the pileand is the oldest layer.The rock layer that ison top of the pile is theyoungest.This is called:The Law ofSuperposition.SANDSTONEYoungest LayerSHALELIMESTONEOldest Layer
7Principle of Superposition Q#1Which rocks are older?Which rocks are younger?
8The Law of Cross-cutting Relationships When a layer cutsacross another layerin the sequence, it hasto be younger than thelayer that it cuts apart.LimestoneShaleThis igneous intrusionis younger than theconglomerate, sandstoneand shale because itcuts through each ofthose layers.SandstoneGraniteIntrusionConglomerateThis is called:The Law of Cross-cutting Relationships
9Principle of Cross-Cutting Q#2What is younger?The rocks or the fault line?
11Sediments laid down in layers over time lithified into rock Tilting by Erosion of exposedUplifted areasUplift184.108.40.206.220.127.116.11.18.104.22.168.4.New layers of sedimentlaid down on topof the tilted anderoded layers3.2.1.
12Principle of Inclusions Rock layers which contain other rocks are younger than the inclusion.Theinclusion(the otherrocks) is older!
13Principle of Inclusions Q#3Which is older? The rock layer or the rocks included in the layer?
14Principle of Unconformities Unconformities are surfaces that represent gaps in the geologic record.
15Principle of Unconformities missing rock layer = missing time