Dating the Earth Study Guide
The earth, as we've seen, is about four and a half billion years old. Some rocks exist for all ages, except for the very oldest times. The geological cycles have created and destroyed mountains and even continents during the saga of plate tectonics, changing igneous rocks into sedimentary rocks, and both of those into metamorphic rocks, which in turn can again become sedimentary rocks. But how do we know about the ages of the continents, the mountains, and indeed the earth itself?
The striking thing about sedimentary rock is that it often occurs in layers, which are called beds or strata by geologists. Sometimes, you see beds when a hill has been sliced through using dynamite so a road can pass. You can also see beds on a massive scale in the Grand Canyon where rock 2,000 meters thick was deposited, on and off, during an interval of 300 million years. Beds are the distinct layers of rock on a small scale. Often, in places such as the Grand Canyon, beds of a similar color and type of rock are built into even larger layers called formations. If beds are the layers of a layer cake, a formation is the cake itself.
Thus, beds are thinner than the formations as they make up the actual formations. Both beds and formations are types of strata, which is the general term used by geologists for the layers of rock. Igneous and metamorphic rock can occur in strata as well, such as when lava overflows a layer of sedimentary rock, adding a stratum (the singular of strata) of igneous rock to the sedimentary strata. The geological study of strata is called stratigraphy, and the scientists are called stratigraphers.
The first principle used by stratigraphers is what they call the law of original horizontally. This law applies to sedimentary rock layers, which for the most part, were formed when sediments were laid down under water. The seabed where the sediments are deposited is usually the shallow offshore area called the continental shelf, which is under water but actually part of the geology of continents. The continental shelf is nearly horizontal. In other cases, sedimentary rock was laid down when sea level was higher and there were large expanses of regions that are now land but which long ago were covered with a shallow sea. In the United States, for example, much of what is now Texas and Oklahoma was under water. Sediments deposited there were laid down horizontally as well.
Thus, according to the law of original horizontality, sedimentary rock began as horizontally deposited layers. That means that wherever we see sed imentary rock whose strata are tilted, we know that tremendous geological forces have been at work that raised up and tilted massive volumes of rock. Plate tectonics is such a force.
Another guiding scientific rule that is important to stratigraphers is the principle of superposition. This rule states that more recent (younger) sedimentary layers of rock were laid down on top of older layers. As you hike, for example, down into the Grand Canyon (whose layers are still fairly horizontal), you descend back in time as you go downward into the strata.
Of course, because of the mighty forces of plate tectonics, one must apply the principle of superposition with care. What if the strata are vertical? That means the layers have been tilted by 90°. Then which is the oldest? To the right or to the left? You cannot tell just from looking. Furthermore, because complete folding sometimes occurs in the layers, it is even possible for an older layer to now sit on top of a younger one. Imagine, for example, folding a sandwich in half. Assume you had originally layered the lettuce on top of the ham. In a portion of a folded sandwich, the lettuce is now under the ham.
To find younger layers completely under older ones is relatively rare. For the most part, the principle of superposition works fine, because rocks are tilted only slightly, or at least not up to 90° or completely over.
Another concern for stratigraphers is the concept of conformity. Conformity involves the following question: If you see strata, how continuous was their formation? In other words, do they represent an unbroken interval of deposition? If so, over how many thousands, millions, or hundreds of millions of years? Stratigraphers are excited to find a region of rock with a high degree of conformity, because then they can study an unbroken record of geological history.
However, numerous breaks can be found in the rock record of various regions of the world's strata. Such breaks in sedimentation are called unconformities, which are classified into three basic types. Study Figure 8.1 and then read the more detailed descriptions when you return to the text.
An angular unconformity occurs when a relatively horizontal stratum sits on top of a number of tilted strata, which seem to have been sliced off horizontally. What happened? At some point back in time, a formation of horizontal strata was tilted at angle. Then it was subjected to erosion, which wore it down across the layers. Next, it perhaps ended up again under water, at which time new layers of sediment were deposited. This situation results in what is known as an angular unconformity. A large interval of time passed between the deposition of the tilted layers and the deposition of the horizontal layer.
The second type of unconformity is called a disconformity. Nothing is very difficult about this type; seimentation simply stopped for a long time interval and then resumed. For example, if the sea level falls (which happens, because of plate tectonics), the layers of sediment that had been under water will now be above water and will no longer be receiving what would have been their next bed of sediment. Then if the sea rises again, sedimentation resumes. The disconformity makes life tough for geologists, because they have no information about the missing interval of time.
A nonconformity is the third type of unconformity. A nonconformity has igneous or metamorphic rock underneath sedimentary strata. For example, as you go down deeper into the continents, what you tend to find is more and more igneous rock. The place where sedimentary rock ends and igneous rock begins is a nonconformity. You cannot tell exactly what happened at that spot and what the time interval was step by step like you can when the sediments have been laid down in conforming layers.
For another example of how nature creates a nonconformity, imagine underwater sedimentary strata that are then lifted up to become continental land. Next, volcanism spreads lava over the surface to form a thick layer of igneous rock. Eventually, the surface is again under water and new sedimentation begins on top of the igneous bed. That's a nonconformity. The sedimentation was not continuous.
Stratigraphers search across large geographical regions for what they call correlations. Correlations occur when a stratum of rock is clearly the same over hugely separated regions. If the geologists can figure out something about the stratum in one region—its age, for example—then they know the age of the stratum in other places. For example, over the last two million years, there have been widespread ash layers from tremendous volcanic explosions, which covered large regions of the western United States. Because the same ash layer is found in different places, and we know that the ash layer was deposited at the same time in all these places, we can use the ash layer to start unraveling the stratigraphic story at all the places just before and just after the eruption.
Another way to find correlations is to seek the same fossils in similar-looking beds of sedimentary rock. Particularly important to geologists are what is known as index fossils. A extinct ancient species can be an index fossil if the species was highly biologically distinctive (unique) and it existed on Earth for only a relatively brief and confined period of time. For example, assume it can be determined that a particular ancient species of trilobite (a crab-type creature) existed for only ten million years, say, from 500 to 490 million years ago. Then wherever you find rock with that species of trilobite, you know the rock's approximate age
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