Avatar: Laws of Stratigraphy

Slide 1

In the mid‐1600's, the Danish scientist Nicholas Steno studied the relative positions of sedimentary rocks. He noted that large particles settled more easily and more quickly than smaller particles. This tendency produces beds or layers as the obvious features in rock formations. The formations form one on top of another thus the layer on the bottom is older than the layer on top. This simple but useful concept is called the Law of Superposition. It helps determine the relative age of rocks and the fossils found within them.

Slide 2 Layers of rock form when particles (or sediments) settle from water, air and, as is the case with sand, sandstone. The Law of Original Horizontality states that most sediment layers, when originally formed, were laid down horizontally. This again is helpful in determining the order in which sedimentation formed, with the oldest layers forming first. However, we often find layers of sediments that are not horizontal. This indicates that they formed under some extreme and strange situations or, which is more often true, that they formed horizontally and then were moved by the forces of tilting such as found in mountain building events. Rock layers are also called strata, and, as we learned earlier, the study of these layers is called Stratigraphy.

Slide 3

The law of superposition and the law of horizontality simply say that when looking at layers of sediment you will find the oldest material was laid down first. If you find the layers on an angle, they were likely originally laid down horizontally. Within the layers, there are also clear signs of reference to which way was originally up and which was down. As you can see, these two principles are extremely helpful in establishing relative age sedimentary profiles.

Slide 4 When looking at layers in a sedimentary rock profile, a geologist may find areas where igneous rock cuts across the sequence. It became obvious to geologists that the intrusion of igneous rock must have come after the sediments were laid down as it cuts through the pre‐formed layers. The Law of Cross‐Cutting states any geologic feature (igneous intrusion, fault, etc) that cuts into or across rock layers must be younger than these layers.

Slide 5

This diagram illustrates how a lava flow of basalt demonstrates the Law of Superposition. The Basalt flow is younger than the beds below it and older than the beds above it. Also notice how

© KC Distance Learning the molten rock of the volcanic flow baked the rock underneath it. Above the basalt flow a bed was deposited long after the flow had cooled and hardened and has not been baked. Absolute or quantitative ages for the flow and dike can be determined and relative ages from the fossils in the surrounding rocks contribute to the geologic time scale.

Slide 6

In this picture we see how the granite, which is a mass of rock that cuts across the structure of the rocks around it, demonstrates the Law of Cross‐Cutting. Granite cuts across the preexisting rocks and is younger than all the rocks that it cuts across and older than the rocks above it that it does not cut. Also notice that the contact between the rocks around it has been baked by the heat of the molten granite giving more evidence of the sequence of events.

Slide 7

As we shall see later in this course, when we explore the universe scientist often have to make certain assumptions. If these assumptions always prove to be correct then they can be made into a scientific law. The Law of Gravity and the laws described earlier are good examples of scientific laws. In fact, understanding of the law of gravity explains the Law of superposition and the Law of horizontality. If, for example, there was a time on Earth when gravity did not function as it does today it would be impossible to explain what we see in rock layers today.

Slide 8 Finally, one major assumption scientists who study the past make is that the laws of physics that are at work today are the same as the laws of physics at work in the past. The evidence that seems to support this is considered a scientific law. The Law of Uniformity as stated by James Hutton (18th century Scottish geologist) means that processes we observe in today's world likely operated throughout the past, back even billions of years ago. He concluded, after studying rocks at many outcrops, that each layer represented a specific interval of geologic time. Later on, we will use this law to prove the age of the Earth.

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