Week 7: Geologic Time
This week's activities:
- Relative Time (p.166-169)
- Deep Time (p.170-173)
Relative Time
Geologists examine time in two ways:
- numerical (or absolute) dating
- relative dating (you have not idea of the exact age in years, instead you determine which is older and which is younger relative to each other)
Today in class we will discuss how to figure out the relative sequence of events, using the following principles:
PRINCIPLE OF SUPERPOSITION
The oldest layer of sedimentary and extrusive igneous rocks is always at the bottom
PRINCIPLE OF ORIGINAL HORIZONTALITY
Sedimentary and extrusive igneous rocks are originally laid down in horizontal layers and any change from that is viewed as a disturbance of those layers (something happened later) - folding, tilting (occur during mountain-building events, for example)
sedimentary and igneous extrusive rocks that are folded or tilted must have first been deposited in horizontal layers before being folded or tilted
PRINCIPLE OF CROSS-CUTTING RELATIONSHIPS
Anything that cuts across a layer must have happened after that layer was already there. This principle is used in three instances:
- igneous intrusion: magma intrudes other rocks by forcing its way through fractures and any other avenue to make its way upward (magma is usually less dense than solid rock). Thus, the rocks had to be there first before the magma pushed its way across them
- faulting: faults break across rocks, therefore, the rocks had to be there first before the fault can break them.
- erosion: erosion will cut the top off of underlying layers. The underlying layers had to be there first before erosion can cut across them.
Deep Time
This activity involves understanding the vastness of geologic time by taking the geologic events listed on p.173 and placing them into different metaphors for time. To fit into the metaphors, calculations must be done - a calculator would be helpful today. The geologic time scale with its absolute dates is presented on p.170.
If you miss week 7:
Read through these notes and look over the activities in your manual to get a feel for the material covered this week. Read the material in your manual for each activity with care so that you understand what you missed, talk to a classmate to find out more. Contact Dr. Munn to arrange to make up the quiz that you missed (this must be done sometime before the next class). Worksheets collected for this lab will be listed on the Lab Schedule page and must be turned within a week. Click here for the syllabus information about missing class. Worksheets collected for this lab will be listed on the Lab Schedule page and must be turned within a week. Here are more specifics about the worksheets that were collected:
- Relative Dating Worksheet (see p.167 for the principles and read these lab notes for more information about these principles).
- California and the Next Generation Science Standards - this is a short introduction to NGSS with a couple questions related to the activities we did in lab - Relative Time is on p.166-169 and Deep Time is on p.17-173