Satellite Applications for Geoscience Education
Based on a geological cross-section, identify the oldest and youngest formations. The fossils formed within the rocks as a result of mysterious forces. If you know the relative ages of two rock layers, (1) Do you know which. relative dating practice 1. C. an abundance of fossils. D. a region of A geologic cross section for a portion of Earth's crust is shown. Letter A is a. In other words, fossil shells, bones, and teeth were never a part of a living creature! The relative age of a rock is its age in comparison with other rocks. Figure Cross-section of sedimentary layers: (A-C) igneous intrusion, (D) cross-section, Often described as the founder of modern geology, Hutton formulated a.
Thus we know that the fault is younger than the limestone and shale, but older than the basalt above. Principle of Inclusions If we find a rock fragment enclosed within another rock, we say the fragment is an inclusion.
If the enclosing rock is an igneous rock, the inclusions are called xenoliths. In either case, the inclusions had to be present before they could be included in the younger rock, therefore, the inclusions represent fragments of an older rock. In the example here, as the basalt flowed out on the surface it picked up inclusions of the underlying sandstone.
So we know the sandstone is older than the basalt flow.
Similarly, the overlying rhyolite flow contains inclusions of the basalt, so we know that the basalt is older than the rhyolite. This principle is often useful for distinguishing between a lava flow and a sill.
Recall that a sill is intruded between existing layers. In the case shown here, we know that the basalt is a sill because it contains inclusions of both the underlying rhyolite and the overlying sandstone.
This also tells us that the sill is younger than the both the rhyolite and the sandstone. Principle of Chilled or Baked Margins When a hot magma intrudes into cold country rock, the magma along the margins of the intrusion will cool more rapidly than the interior.
Rapid cooling of magma results in fine grained rock or glassy rock and if this occurs along the margins of the intrusion, we will see the effects of rapid cooling along the margins. Since slower cooling will occur farther away from the margin the rock farther away will be coarser grained. Thus, if we see chilled margins, we know that the intrusions must be younger that surrounding rock because the surrounding rock had to have been there first in order to cause the cooling effect.
When magma comes in contact with soil or cold rock, it may cause the soil or rock to heat up resulting in a baked zone in the surrounding rock near the contacts with the igneous rock. Such margins indicate that the igneous rock is younger that the soil or rock that was baked.
Application of the Principles of Stratigraphy Figure Although we will go over this in lecture, you should study the methods and reasoning used so that you could determine the geologic history of any sequence of rocks. Fossil Succession Once geologists had worked the relative ages of rocks throughout the world, it became clear that fossils that were contained in the rock could also be used to determine relative age.
It was soon recognized that some fossils of once living organisms only occurred in very old rocks and others only occurred in younger rocks. Furthermore, some fossils were only found within a limited range of strata and these fossils, because they were so characteristic of relative age were termed index fossils. With this new information, in combination with the other principles of stratigraphy, geologists we able to recognize how life had changed or evolved throughout Earth history.
This recognition led them to the principle of fossil succession, which basically says that there is a succession of fossils that relate to the age of the rock. Unconformities - Breaks in the Stratigraphic Record Because the Earth's crust is continually changing, i. When sediment is not being deposited, or when erosion is removing previously deposited sediment, there will not be a continuous record of sedimentation preserved in the rocks.
DETERMINING AGE OF ROCKS AND FOSSILS
We call such a break in the stratigraphic record a hiatus a hiatus was identified in our trash pit example by the non-occurrence of the Ceramic Cups layer at the Zoo site. When we find evidence of a hiatus in the stratigraphic record we call it an unconformity.
An unconformity is a surface of erosion or non-deposition. Three types of unconformities are recognized. Angular unconformities are easy to recognize in the field because of the angular relationship of layers that were originally deposited horizontally.
Nonconformities occur where rocks that formed deep in the Earth, such as intrusive igneous rocks or metamorphic rocks, are overlain by sedimentary rocks formed at the Earth's surface. The nonconformity can only occur if all of the rocks overlying the metamorphic or intrusive igneous rocks have been removed by erosion. Disconformity Disconformities are much harder to recognize in the field, because often there is no angular relationship between sets of layers.
Disconformity are usually recognized by correlating from one area to another and finding that some strata is missing in one of the areas. The unconformity recognized in the Zoo trash pit is a disconformity. Disconformities can also be recognized if features that indicate a pause in deposition, like paleosols ancient soil horizonsor erosion, like stream channels are present.
Variation in Unconformities The nature of an unconformity can change with distance.
The same procedure of shaking, counting the "survivors", and filling in the next row on the decay table should be done seven or eight more times. Each time represents a half life. Each team should plot on a graph Figure 3 the number of pieces of candy remaining after each of their "shakes" and connect each successive point on the graph with a light line.
AND, on the same graph, each group should plot points where, after each "shake" the starting number is divided by exactly two and connect these points by a differently colored line. After the graphs are plotted, the teacher should guide the class into thinking about: Is it the single group's results, or is it the line based on the class average?
Introduction to Geology
U is found in most igneous rocks. Unless the rock is heated to a very high temperature, both the U and its daughter Pb remain in the rock. A geologist can compare the proportion of U atoms to Pb produced from it and determine the age of the rock.
The next part of this exercise shows how this is done. Each team is given a piece of paper marked TIME, on which is written either 2, 4, 6, 8, or 10 minutes. The team should place each marked piece so that "U" is showing. This represents Uranium, which emits a series of particles from the nucleus as it decays to Lead Pb- When each team is ready with the pieces all showing "U", a timed two-minute interval should start.
During that time each team turns over half of the U pieces so that they now show Pb This represents one "half-life" of U, which is the time for half the nuclei to change from the parent U to the daughter Pb A new two-minute interval begins.
Continue through a total of 4 to 5 timed intervals. That is, each team should stop according to their TIME paper at the end of the first timed interval 2 minutesor at the end of the second timed interval 4 minutesand so on.
After all the timed intervals have occurred, teams should exchange places with one another as instructed by the teacher. The task now for each team is to determine how many timed intervals that is, how many half-lives the set of pieces they are looking at has experienced.
The half life of U is million years.
Both the team that turned over a set of pieces and the second team that examined the set should determine how many million years are represented by the proportion of U and Pb present, compare notes, and haggle about any differences that they got. Right, each team must determine the number of millions of years represented by the set that they themselves turned over, PLUS the number of millions of years represented by the set that another team turned over.
Pb atoms in the pegmatite is 1: Using the same reasoning about proportions as in Part 2b above, students can determine how old the pegmatite and the granite are. They should write the ages of the pegmatite and granite beside the names of the rocks in the list below the block diagram Figure 1. This makes the curve more useful, because it is easier to plot it more accurately.
That is especially helpful for ratios of parent isotope to daughter isotope that represent less than one half life. For the block diagram Figure 1if a geochemical laboratory determines that the volcanic ash that is in the siltstone has a ratio of U If the ratio in the basalt is 7: Students should write the age of the volcanic ash beside the shale, siltstone and basalt on the list below the block diagram.
Why can't you say exactly what the age of the rock is? Why can you be more precise about the age of this rock than you could about the ages of the rock that has the trilobites and the rock that contains acritarchs and bacteria? Based on cross-cutting relationships, it was established that the pegmatite is younger than the slate and that the slate is younger than the granite.
Therefore, the slate that contains the acritarch and bacteria is between million years and million years old, because the pegmatite is million years old and the granite is million years old. The slate itself cannot be radiometrically dated, so can only be bracketed between the ages of the granite and the pegmatite.
The trilobite-bearing limestone overlies the quartz sandstone, which cross-cuts the pegmatite, and the basalt cuts through the limestone. Therefore the trilobites and the rock that contains them must be younger than million years the age of the pegmatite and older than million years the age of the basalt.Relative Dating - Example 1