The methods that geologists use to establish relative time scales are based on geologic laws and principles. A scientific law is something that we understand and is proven, and a principle is a guide we use to help us evaluate a system. Geologic laws and principles are generally easy to understand and simple. Geologists use stratigraphic principles – rules that help us interpret relationships between rocks – to describe and interpret relationships between layers and types of rock and determine the relative ages of rocks and geologic events (i.e., the order in which they formed or occurred).
To provide context for the stratigraphic principles, we’ll begin this section with a review of the different rock types. Sedimentary rocks (e.g., sandstone) are made from broken pieces of other rock that are eroded in the high areas of the earth and transported by wind, ice, and water to lower areas where they are deposited. Igneous rocks form through cooling and crystallizing of molten rock. Metamorphic rocks form when heat and pressure cause recrystallization within rocks, which can alter the mineralogy and/or texture. This distinction is important because these three rock types are formed differently and therefore, the events that lead to their formation are interpreted differently when assessing the rock record using geologic laws and principles.
To interpret stratigraphic relationships between geological units (types and layers of rock), geologists use geologic cross-section diagrams (e.g., Figure 6.1). These are drawings that illustrate the relationships between rocks if you cut into the earth and look at the layers of rock below the surface. A useful analogy for a cross-section is a piece of layered cake: if you cut a piece of the cake and remove it, viewed from the side you will see the top layer of icing, the cake layer beneath it, and then layers of filling and cake alternating as you go deeper into the cake (moving down the slice from the surface at the top of the cake). Geologists use data including maps based on rock outcrops at the surface, cores drilled from the rock, and geophysical data (e.g., seismic data) to help construct cross-sections.
There are 7 stratigraphic principles that geologists use to relatively date rock units:
law of superposition
principle of original horizontality
principle of lateral continuity
principle of cross-cutting relationships
principle of unconformities
principle of inclusions
principle of faunal succession
The law of superposition states that in an undeformed sequence of sedimentary rocks the oldest rocks will be at the bottom of the sequence while the youngest will be on top. Imagine a river carrying sand into an ocean, the sand will spill out and come to rest on top of the seafloor. This sand was deposited after the sand of the seafloor was already deposited, so it is deposited above the seafloor sediments. Over time, more layers can be deposited on top of the sand as sediments are carried into the ocean by rivers and deposited. We can then create a relative time scale of rock layers from the oldest rocks at the bottom (labeled#1 in Figure 6.1) to the youngest at the top of an outcrop (labeled #7 in Figure 6.1).
Explanation:
