Reading Rocks: What Rocks Can Tell Us about Time
General Introduction for Determining Relative 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
Investigation of the Rocks and Beach at Bluff Point State Park:
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.
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:
- Why are there so many boulders and rocks on the south edge of Bluff Point State Park?
- What evidence do you see in the rock that shows this area was geologically active at one time?
- Are the joints you studied approximately the same widths or do they vary? Why?
- Is Bluff Point State Park a good site to use law of superposition to determine relative ages of rocks? Explain why or why not.
- Did you locate any included fragments in the rocks you studied? If so, sketch and describe the sample. Then indicate the relative ages of the rock and its fragment. (Refer back to the explanation of the law of included fragments, if needed.)
- What direction must the water currents be traveling to have deposited the sand beach where it is found? Make a drawing of the sand beach and draw arrows to show those currents. Is the beach a tombolo or a spit? Do you think this can change? How?