Lecture 2

Principles of Stratigraphy, Geologic Time Scale, Relative Dating -

Focus Questions:  What are the major families of rocks?

   How can we identify the order of events in the rock record?

1)      Rock cycle

Igneous rocks-assemblages of mineral grains that have crystallized out of molten material (e.g., magma, lava) coming from great depths inside the Earth.  Examples are granite and basalt

                Extrusive-means the molten material reached the earth’s surface (lava) and crystallized

                                Craters of the Moon National Monument

                Intrusive-means the molten material crystallized at depth (magma) in the Earth

                                Mount Rushmore National Monument, SD             

       Sedimentary rocks-assemblages of mineral or rock grains that are cemented together (lithified)

formed from loose sediment (examples are clay, silt, sand)

                Cuyahoga Valley National Park, OH

Or formed from shell fragments, or from minerals that precipitate from water (examples are calcium carbonate, salt, gypsum) 

     Metamorphic rocks-rocks that have been altered by temperature and/or pressure (examples are

Marble, gneiss (pronounced “nice”), schist, slate

                Great Smoky Mountains National Park, TN and NC

2)       Principles of Stratigraphy

Sedimentary rocks-accumulate in layers called strata (singlular = stratum) or beds.  The study of these strata, of their relationships, and the fossils and minerals they contain is called Stratigraphy

              There are 3 Principles of Stratigraphy developed by Nicholas Steno, 17th century

 

and one more principle

 

There are often breaks in the rock record, where rock has been eroded away-these are Unconformities       

                          Three types of unconformity are common:

3)  What is Relative Dating?  Sorting out the relative age of units using the principles of stratigraphy-we can see which rock layer is older or younger than another, but not exactly how old they are, just their relative ages.

1)      Review of the Principles of Stratigraphy  can help us solve these relative age problems.  We can also add to the toolbox-FAULTS and FOLDS. 

Fault-a surface along which rocks have broken and moved.  Displacement can often be seen in outcrop.

Folds-rocks can fold when under tremendous pressure  with/without increased temperature. 

    SYNCLINES, ANTICLINES    Synclines “smile” (the fold is like this “U”)

                                                  Anticlines are  folded the other way, like the letter “A”

 

2)       Grand Canyon  example of “layer cake stratigraphy”

We can use these principles to sort out the relative ages of the layers exposed in the Grand Canyon.

 

3)       A Final Principle of Stratigraphy-the Principle of Faunal (Fossil) Succession

 

4)       William “Strata” Smith, discoverer of faunal succession

What is Time in Geology?

1)      James Hutton-late 18th century - first to understand we are looking at “deep time” – unconformities

 

2)      Correlation-identifying corresponding geologic units that are geographically distant

 

Time correlation-correlation of rock units of the same age

Lithologic correlation (based on features of the rock units, including fossils)

3)      The Geologic Time Scale-

Periods-pure time , Systems-rocks with time significance

Example:  Rocks of the Jurassic system were deposited in the Jurassic Period

Reasoning for this distinction-not every place has all the rocks.  Time is not missing, rocks can be missing.  Example-Ohio.  Ohio has lots of Paleozoic Era rocks (Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, Permian) but no Mesozoic Rocks and only a small amount of late Cenozoic layers.  Time isn’t missing, rocks are.

4)  Geologists define time as a single, irreversible continuum.  That means, time is like an arrow-no breaks, no gaps, no repeats 

Next Class, Radiometric Dating and Sedimentary Processes