Reading
Rocks: What Rocks Can Tell Us about Time
Geologists
think of rocks as windows to the past. Geologists can determine how old rocks
are, for example, by using instruments that measure the percentage of certain
radioactive elements found in rocks. This process is called radioactive dating
and is a time-consuming, expensive process that cannot be done in the field. But
geologists can also determine a lot about a rock’s past simply by observing it
carefully. Many rocks contain clues that reveal conditions about their formation
and their relative ages.
Relative
dating is simply figuring out the order in which a series of rock layers formed.
Geologists use three laws to do this:
1.
The
law of superposition:
This law states that the oldest rock layer is the layer on the bottom and the
youngest layer is the layer on top. Unless rocks have been overturned by extreme
mountain building, the layers are in the same order that they formed. (Picture a
camper putting together a s’more’s
bar at a cook-out. First he (she) places the graham cracker on the plate. Then
he sets half a chocolate bar directly on the graham cracker. (The chocolate has
not been on the plate as long as the graham cracker. It is younger.) Finally, the camper tops the s’more with a toasted
marshmallow. The marshmallow is on top of the other two layers, has been there
for the least amount of time, and is the youngest
layer. Try to think of some other examples to make this law clear in your
mind. How about the mattress, sheets, blankets and bedspread on a bed, for
example? It is helpful to remember an obvious analogy for this law because
students who think they understand this law tend to get confused on tests!)
Figure 1. A model of the three laws of relative dating.
2.
The
law of included fragments:
This law states that included fragments in a rock are always older than the rock
itself. Just as the chocolate chip in a chocolate chip cookie must be older than
the cookie (The chocolate chip was sitting in a package at the grocery store
long before the cookie dough was mixed and baked to make the cookie), a pebble
that is part of a conglomerate rock is older than the rock. The pebble had to
exist first in order to become part of the conglomerate when it formed.
3.
The
law of cross-cutting relationships:
Magma under pressure squeezes into nearby rock whenever fractures form in the
rock. Then the magma slowly cools and becomes rock itself. This newly formed
igneous rock is, of course, younger than the rock it intruded. In the same way,
fractures and faults are younger than the rock that they cross. Faults cannot
occur in rock that does not already exist. The rock must be present first, which
means it will be older than the fault, when the fault occurs.
Scavenger
hunting for clues in the rocks at Bluff Point State Park:
Bluff
Point State Park is a nearly rectangular peninsula near Groton, Connecticut.
Extending from its southwest corner is a long curved sandbar that extends (or,
at times almost extends, to Pine Island.) When the sandbar completely bridges
the mainland to Pine Island it is referred to as a tombolo. Otherwise it is a
spit.
The
southernmost end of Bluff Point State Park is a rocky cliff with glacially
plucked boulders at its base, some of which extend into Long Island Sound. Some
of the boulders show interesting features that enable us to learn about the
history of the rocks.
Figure
2. Bushy Point Beach at Bluff
Point State Park, Groton
Hike
along the southern coast of Bluff Point State Park with your camera, notebook,
hand lens (if you have one), a ruler and a pen. Observe as carefully as you can
everything that relates to your knowledge of rocks and rock features. Take
photographs (or make illustrations) of everything you see; then describe these
observations in your notebook according to the instructions below. Be sure to
identify your comments with numbers that match up with the pictures you have
taken.
Before
reaching the bluff on the southern end of Bluff Point State Park:
1.
Use the
geologic map of Connecticut to determine what kind of rock you would expect to
find at Bluff Point State Park. Record the names and descriptions of any rocks
you expect to find.
2.
Rocks in
some parts of Connecticut formed from marine sediments of the ancient Iapetos
Ocean that became shales, and later schists, when they were put under great
pressure during the stage of compression in Connecticut’s geologic history.
Refer back to your study of the history
of Connecticut geology. How do you think this
part of Connecticut formed and from what did it form?
Figure
3. The rocky
part of Bluff Point State Park along the Sound.
Questions
to answer as you hike across the point:
3.
Are the bluff and surrounding rocks made of the same kind of
rock? Make a list of all the different kinds of rocks you find; then describe
each. Use your rock key to name as many of the rocks as you can.
4.
What is the name of the most prominent rock? Use the geologic
map to give the precise name, such as a Stony
Creek Granite Gneiss, for example.
Scavenger
Hunt:
Look
for a rock that has a stripe going
through it. What looks like a stripe is really
the location where magma intruded (squeezed into) a fracture in the rock and
then cooled to become igneous rock. If the magma cooled slowly, giving the
crystals enough time to grow to a size of 1 cm or larger, the intrusion is called a pegmatite. Pegmatite is a common rock in some parts of Connecticut.
5.
Look for intrusions in the rocks near the bluff. Photograph
and/or sketch them; then describe their texture in your notebook. Are the
crystals so small that they are difficult to see or are they large enough to
measure? If they cannot be measured easily, they have a fine-grained texture. If
they can be measured, record the average crystal diameter and indicate whether
the intrusion is a pegmatite or not. Also
measure the width of each intrusion you study.
6.
Look for more complex patterns. Find a rock that has two or
more joints (fractures which have not had movement along them) cutting across
the rock in different directions. Use the law of cross-cutting relationships to
determine the relative age of each. Illustrate the rock with its joints and
label its parts from oldest to youngest.
Figure
4
. Cross-cutting vein at Bluff Point.
7.
Faults are fractures in rocks along which movement has
occurred. When fractures form, layers within the rock still match up on either
side of the fracture. But when pressure causes the rock to move on one side,
large features no longer match up. Geologists say the features are discontinuous.
This means that you can follow a feature in the rock until you get to the
fault. The feature on
the right side of the fault would be found above or below the position it is
found on the left depending on the direction the rock moved when the fault
occurred. Sometimes so much movement occurred that the matching feature cannot be
found on the other side of the fault.
Try to find a rock that shows a fault. This will be more difficult than
searching for joints. If you think you have found a fault, take a picture
of it (or
sketch it) and show the rock to your teacher.
Analysis
and Conclusions:
Other
Observations: