Chapter 4.2 Notes (Geologic time scale)

Scientists have learned how to "read" the record of time that is preserved
in the rocks.

Rock Layering
Layers of sedimentary rocks form one on top of the other.  Older rocks are
found under younger rocks.  So going down into deeper layers of rock is
like going back in time. This allows scientists to determine the relative age
of rocks.

Relative age - We know a certain layer of rock is younger or older than
another layer by comparing its position to other layers.
 

Unconformities - a break in the layering of rock. Layers of rock tilted at an angle are covered by horizontal layers.

                                                                                                                                                              (click picture to enlarge)

Absolute age  is determining the approximate age of rocks in years.
The main method is through radiometric dating.  (See chapter 13)

Through absolute and relative dating, scientists have pieced together
earth's history.

Divisions of geologic time
There are four large units (ERAS) of geologic time.  Each era represents
a major stage in earth's history.
    Precambrian Era - first era, longest era, includes formation to 640 million
                                    years ago.  It ended when many newlife forms began
                                    to appear.

    Paleozoic Era - 400 million years long.  Plants and animals begin to live
                                    on land.  This era ended with the extinction of many
                                    its organisms.

    Mesozoic Era - The time of dinosaurs.  Earth was warm and wet.  This
                                    era ended when most dinosaurs died out.

    Cenozoic Era - From the end of the Mesozoic Era until today.  Climate is
                                cooler and drier.  Humans have only been here for a
                                small part of this era.

Each of these eras is divided into smaller units called Periods.
The Periods are divided into Epochs.
Links to geologic time scales:  http://vulcan.wr.usgs.gov/Glossary/geo_time_scale.html
                                                                   http://www.ucmp.berkeley.edu/help/timeform.html
Science and Society - The earth is our clock.  A day is the time it takes the
earth to rotate on its axis.  A year is the amount of time it takes the earth to
revolve around the sun.  The age of the earth is the total number of revolu-
tions it has made around the sun.