In this activity you will learn to determine the sequence of geologic events from cross-
sections of strata (rocks) in a given area. It is a bit of a mind-puzzler, so have fun!
Before absolute dating of rocks was developed in the 20th century, geologists had
to rely on relative age dating, which places geologic events in their order of occurrence. The
method begins with the careful drawing and description of strata (the geologic cross section or
profile). Relative age dating assumes that the lower layers in any particular cross section are
older than the upper layers in that cross section (the law of superposition) and that an object
cannot be older than the materials of which it is composed. Igneous rocks are dated according to
whether they caused metamorphism in the surrounding rock (proof that they intruded into the
preexisting rock), whether they cross cut preexisting rocks, or whether sediments were deposited
on them after they were formed. The profile from one location is then compared with profiles
from surrounding sites to determine the geologic history of a larger area. If fossils are present in
the rocks, they may also be used to correlate rock layers across large distances and, now that
absolute time has been established, to determine the age of the rocks.
In this process, you will study the rocks and events in a geologic cross section and put
them in the correct order from oldest to youngest. In order to do your best on this activity, you
must understand a few of the basic principles that are applicable to relative age relationships
between rocks:
in a sequence of undeformed sedimentary rocks, the oldest beds are
on the bottom and the youngest are on the top.
: sedimentary layers are horizontal, or nearly so, when
originally deposited. Strata that are not horizontal have been deformed by movements of
the Earths crust.
: groups of fossil plants and animals occur in the geologic record
in a definite and determinable order. A period of geologic time can be recognized by its
respective fossils.
geologic features, such as faults, and igneous intrusions are
younger than the rocks they cut.
a rock body that contains inclusions of preexisting rocks is younger that
the rocks from which the inclusions came from.
The easiest way to do relative age dating is to work from oldest to youngest. Try to find the
oldest rock (usually located near the bottom) in the diagram below and work your way up. Your
first example is the diagram below. Review the and the
and you will see that the only possible answer to this puzzle is that
layer A is the oldest and layer D is the youngest.
Figure A. Cross sections showing a simple example of undisturbed sedimentary rocks. A is the oldest, D is the
youngest.
Here are some additional hints that will help you with your diagrams:
Sedimentary rocks:
If rocks are folded, the folding is younger that the youngest rock affected.
If they are folded into a syncline (a U-shaped fold) the youngest rocks are in the core of
the fold (see figure B). The opposite is true for an anticline (a big dome-shaped fold).
Sedimentary rocks that contain fragments of another rock are younger than the rocks that
the fragments came from.
Figure B. Oldest rocks: A, followed by B, C and D. All four sedimentary layers were folded into a syncline.
Layers E and F were then deposited at a later time and are the youngest.
Igneous rocks are formed by the solidification of a liquid magma; the therefore can intrude into
preexisting rocks or be poured out onto the surface of the earth:
If an igneous body crosscuts another rock, the igneous rock is younger than that rock (see
Activity figures 1 & 3).
If a body of granite contains unmelted inclusions of another rock, the granite is the
younger rock.
Remember! Granites can intrude into other rocks, even though they may be on the bottom
of your geologic diagram. Look carefully for the granitic pattern (see below) and for
irregular contacts between the granite and the country (preexisting) rock (see Activity
figure 2). The granite may also metamorphose the country rocks:
Intrusive rocks produce contact metamorphism (shown as a starred pattern within the
country rock pattern, see Activity figures 2 & 3) along their contacts with the older rocks
they intrude into.
Lava flows may cause contact-metamorphism with the older rocks they lie upon.
Metamorphic rocks are preexisting rocks that have been metamorphosed (changed into different
rocks) by large amounts of heat and pressure in a region. These rocks have usually been
deformed by large, mountain forming events, and therefore if they are in contact with layered or
unmetamorphosed rocks, they are usually the oldest rocks in the sequence (considering that if
those rocks had been in place when the metamorphism occurred, they also would be
metamorphosed!). Always look for the metamorphic pattern (see below) to determine if there is
a metamorphic rock in your sequence.
Metamorphic rocks are older than sedimentary rocks deposited above them or with
igneous rocks that may intrude them.
Now, familiarize yourself with the rock patterns:
Sedimentary Rocks
Conglomerate Sandstone Siltstone Shale
Shale Limestone Limestone
Igneous Rocks Metamorphic Rocks
Granite Granite Basalt Gneiss Schist
Special Features:
Contact that is an unconformity (layers on either side of it are of differing ages)
Fault
Zone of contact metamorphism
:
Answer the following questions based on your reading.
1. Which of the principles apply to sedimentary rocks?
2. Are only sedimentary rocks used for relative age determinations? Explain:
3. Explain the relative age relationship of faults to the rocks they cut:
4. Look at the diagrams in Part 2.
a. In figure 1 below, could it be possible to determine an absolute age of these rocks? If
yes, explain in detail how you may be able to do this:
b. In figure 3 below, could it be possible to determine an absolute age of these rocks? If
yes, explain in detail how you may be able to do this:
5. How do you determine the relative ages of igneous rocks? (List the ways)
For each of the following cross sections, determine the relative age sequence of the
rocks. Place the answers in the spaces on the right. Remember, always start by looking for the
oldest rock first and working your way from oldest to youngest. Dont forget to consider all
intrusions and faults! The diagrams go from simplest to hardest to let you progressively improve
your skills.
Figure 1:
Figure 2:
Figure 3:
Figure 4:
Try to write the geologic history of this area:
Figure 5:
Try to write the geologic history of this area:
Figure 6:
O
Youngest ______
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Oldest ______
Try to write the geologic history of this area:
