1. Relative Dating
This week, you will look at rock units symbolized as block diagrams. Using geologic principles and laws, you will determine the sequence of events; in other words, what happened first, second, third, etc.
The first important law to note is the law of superposition. Basically, this law states that rocks on the bottom are older than the rocks on top. Look at the top block diagram, Figure 1.1. It makes perfect sense that layer A had to have been deposited before B simply because B rests atop it. Layer B could not be atop A if A was not already there when B was deposited. Therefore, A must be older than B and B must be younger than A.
Figure 1.1: The Law of Superposition
The second law is the law of horizontality. It states that, due to gravity, all rocks are originally deposited horizontally. A lava flow will spread out horizontally due to gravity and sediments being deposited in a lake or the ocean will also spread out horizontally. Look at Figure 1.2. Notice that these layers are not horizontal. This means that they must have been folded or faulted in order to become tilted as they are. You can still tell that A is the oldest and E is the youngest based on superposition.
Figure 1.2: The Law of Horizontality
The third useful principle is known as cross-cutting relationships. It states that anything that cuts into or affects in anyway a layer(s), must be younger than the layer(s) it cuts into. This, too, is common sense, because one thing can't affect another thing that is not there. In the bottom box diagram, notice that H is an intrusion that has cut across layers A‐F. Intrusions are areas where magma has cut into the preexisting rock. We know that H must be younger than those layers because those layers had to be there for H to intrude into anything! What about G? We can't place G in the sequence because it is not affected by H. We don't know if layers A-G were deposited and then H intruded, or if H intruded layers A-F and then G was deposited later.
Examine the layers in Figure 1.3. You should now be able to determine that layer A is the oldest layer, based on superposition, and that layers A-G are folded based on original horizontality. Unlike the tilted layers that we saw earlier, these layers don't reach the surface; they are interrupted by layer H. Notice that there is a squiggly line at the base of H. This is an unconformity line and it represents erosion. This is known as an angular unconformity because the rocks below the unconformity are at a different angle than the rocks above.
There are two other major types of unconformities. Nonconformities occur where sedimentary rock overlies igneous or metamorphic rock, and disconformities occur between two horizontal layers.
In Figure 1.2, the magmatic intrusion (red) is cut by erosion, and a sedimentary layer (light blue) is deposited above. This is a nonconformity.
Figure 1.3: Original Horizontality
In Figure 1.3, there was erosion between two horizontal, sedimentary layers forming a disconformity.
Figure 1.4: Events Placement
Let's place the events of the illustration Figure 1.4 in order from oldest to youngest. The best place to start is at the bottom. We have to have something to fault, fold, layer, or erode. Layer A is on the bottom so its deposition must be the oldest event. Notice that F, G, B, and D are all horizontal and are affected by the fault. They must be part of a unit. Now we must decide if the unit or the fault came next. Obviously, the fault cuts through the unit, so layers F, G, B, and D must come next. Remember that, according to cross‐cutting relationships, anything that affects something else must be younger than what it affects. The fault must be younger than the layers within the unit. Notice that layer D is missing from the right side of the fault. That means that it must have eroded away. The line that marks the base of E must be an unconformity; it cuts the fault so it had to happen after the fault. Because the layers below E are horizontal as E is, this would be a disconformity. Lastly, E and C were deposited. We would list this as follows, from oldest to youngest, bottom to top:
9. Deposition of C
8. Deposition of E
7. Unconformity/erosion
6. Fault
5. Deposition of D
4. Deposition of B
3. Deposition of G
2. Deposition of F
1. Deposition of A
What you have been doing is referred to as relative dating. You are ordering units and events based on how they relate to each other; i.e., A is older than B, D is younger than C, the fault is younger than the fold, etc. Now, you will be applying actual dates those rocks and events; e.g., A is 424 million years old, D is 15 million years old, the fault is 70-64 million years old, etc.
2. Absolute Dating
To date layers, we use radioisotopes. Radioisotopes are alternate, less stable forms of an element. They are unstable because they have a different number of neutrons in the nucleus than the stable form. Because of this instability, they will break down or decay. This decay progresses at a very consistent and predictable rate. Eventually, the parent isotope, the unstable form, will decay into another element, the daughter isotope, which is stable.
Figure 2.1: Isotope Decay
In Figure 2.1, we start out with 100% of the parent isotope, an unstable form of uranium (U), and 0% of the daughter isotope, a stable form of lead (Pb). This would be the concentration of the two in newly formed igneous rock. Notice that through time, the uranium concentration is being reduced while the lead is increasing in concentration. When the amount of the parent isotope, uranium, reaches 50%, we say that one half-life has passed. Each time the parent concentration is reduced by half, another half‐life has passed.
Figure 2.2: Half-Lives Timeline
In Figure 2.2, notice that we designate a half-life every time that the parent has been reduced by 50%. Uranium 238 has a half‐life of 4.5 billion years, so, because this decay is so precise, we know that 4.5 billion years has passed if we analyze a rock with only 50% of the parent remaining.
As you can imagine, one must be careful to make certain that the correct dates are determined. To date a rock, it must have been undisturbed since its formation and must not have been exposed to the atmosphere. The rock must remain uncontaminated by outside isotopes until it can be analyzed.
One scientist doesn’t come up with a date from one analysis that is immediately accepted by all other scientists. That scientist will run dozens of tests on several rocks to eliminate error. In addition, other scientists will run tests on the same rock and similar rocks from other areas. When all of the data corroborate, we are confident that we have an acceptable date for the rock. Modern dating techniques have lowered the error in many isotopes to less than 1%. That means that we can formulate a range in age for a given rock. A 100 million year old rock would date to 99‐101 million years with a 1% error.
Figure 2.4: Dated Sandstone
Even with the error, we can achieve more precise dates.
In Figure 2.4, a geologist has dated the rocks above and below the sandstone on the left. We now know that the sandstone must be between 100 and 102 million years old (ma).
Another scientist finds that the sandstone is between 102 ma and 104 ma. Because the two dates have 102 ma in common, we can be reasonably sure that the sandstone is 102 million years old. With more units dated, that number can become more concise and we effectively eliminate the 1% error. This is a very simplistic example, but it is easy to see how these units can be dated so precisely.
Figure 2.5: Half-Lives Plotted
To determine the age of a rock, two things must be known; we must know the number of half‐lives that have passed and what a half‐life represents. Let's say that you find that you have found a rock that contains 33% of the parent material and you know that the parent isotope has a half‐life of 200 million years. All you have to do now is find the number of half‐lives that have passed.
Figure 2.5 shows half-lives and the percentage of parent isotope remaining. From this graph, we can see that about 1.7 half-lives have passed when 33% (0.33) of the parent remains. Now we have all that we need. If 1.7 half-lives have passed, and a single half‐life lasts 200 million years, we just multiply 1.7 x 200 million to get an age of 340 million years.
Figure 2.6: Sample Block Diagram
Figure 2.6 is one of the block diagrams. Let's say that a geologist has dated layer D at 435 million years and layer E at 390 million years. Can we determine the age of the fault?
Unfortunately, we cannot. All that we know is that it had to have occurred between 435 ma and 390 ma because it occurred after the deposition of D and before the deposition of E.
Figure 2.7: Correlation in United States, France, and China Diagram
Once we work out a sequence, we can compare it to another sequence at a different location. This is known as correlation. In Figure 2.7, we see the same sequence in the United States and France. After further investigation, we learn that the rocks are identical. Perhaps, they were separated by the breakup of Pangaea. We draw dashes between the two to represent the rocks that are missing and to confirm that we recognize the units across the Atlantic as the same rock. We see a very similar unit in China, but it seems to be missing Lava B in China. From that, we learn that the lava flow seen in the United States and France did not make it to China. Does that mean that the sandstone that is 120 ma in the United States and France is also 120 ma in China? We assume so, but we can know for sure.