Earth is very old (4.6 billion years old) and has changed over
geologic time
The rock record provides evidence of geological events and
life forms of the past.
Relative-age dating tells the sequence of events
without exact dates. This is done by comparing one event or
rock layer to another. Stratigraphy is the study of rock layers.
States that in an undisturbed rock sequence, the
oldest rocks are at the bottom and as you travel
up the rock sequence the layers get younger
States that layers of sedimentary rocks are always laid
down horizontally (flat)
Tectonic forces (mountain building, uplifting,
erosion) may tilt or bend them in the future
States that anything cutting across a rock layer is
younger. The rock layer has to be there first.
Fault: fracture or break
in the rock that results
in a mass movement
Sometimes deposition is interrupted by erosion events called
unconformities.
• depositional ends
• erosion removes previously formed rocks
• deposition resumes
Therefore missing rock layers show missing time
Usually shown on rock sequence maps by drawing a wavy line
The 3 types of unconformities:
1. Angular
2. Disconformity
3. Nonconformity
Angular unconformity : on the bottom are tilted
eroded sedimentary layers with horizontal
sedimentary layers on top
horizontal sedimentary layers are interrupted by erosion
then more horizontal sedimentary layers are on top
This is a “gap” in the rock record. There is a period of
time missing due to either an erosion even or nondeposition of rock for a period of time.
igneous or metamorphic rocks on the bottom with
sedimentary rocks on top
This occurs when you are missing time because of
an igneous or metamorphic event.
To relative age date, you will place the following layers of
rock in sequence from oldest to youngest.