Time is… Either relative or absolute Time is: a measured period during which an event, process, or condition exists or continues.
So….How do scientists tell how old rocks are anyway? Scientists use fossils to help determine the age of rock layers. 1.The age of a fossil can be determined by carbon dating: 1.Carbon exists in all living things 2.Carbon decomposes at a specific rate that can be calculated 3.Scientists can measure how much carbon a fossil has left in it in order to figure out how long it has been dead. 4. This is an example of absolute age (dating). 2.Some fossils can be used as index fossils: 1.These fossils are used to determine the approximate age of the layer of rock that they are found in. 2.In order to be an index fossil they have to: 1.Have only existed for a certain short amount of time. 2.Have to have lived in many different places across the globe.
Dating Techniques One ways in which geologists look at time: Relative Dating
Relative 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?
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 that forms first is found on the bottom of the pile and is the oldest layer. The rock layer that is on top of the pile is the youngest. This is called: The Law of Superposition. Youngest Layer Oldest Layer LIMESTONE SHALE SANDSTONE UNIFORMITARIANISM
Principle of Superposition Which rocks are older? Which rocks are younger? Q#1
Conglomerate Sandstone Shale Limestone Granite Intrusion When a layer cuts across another layer in the sequence, it has to be younger than the layer that it cuts apart. This igneous intrusion is younger than the conglomerate, sandstone and shale because it cuts through each of those layers. This is called: The Law of Cross-cutting Relationships
Principle of Cross-Cutting What is younger? The rocks or the fault line? Q#2
Sediments laid down in layers over time lithified into rock Uplift Tilting by Erosion of exposed Uplifted areas New layers of sediment laid down on top of the tilted and eroded layers
Principle of Inclusions Rock layers which contain other rocks are younger than the inclusion. The inclusion (the other rocks) is older!
Principle of Inclusions Which is older? The rock layer or the rocks included in the layer? Q#3
Principle of Unconformities Unconformities are surfaces that represent gaps in the geologic record.
Principle of Unconformities missing rock layer = missing time
Where's the missing sediment? Q#4 How do you know?
Where's the missing sediment? Q#5 How do you know?
What happened first? How do you know?
Where is the youngest rock? How do you know?
Where is the unconformity? How do you know?
What rock layer is the oldest? How do you know?
Which is younger? The fault or the rock layers? How do you know?
Does this diagram show an unconformity? How do you know?