Lesson 3
Learn the two principles essential for geological interpretation, The Law of Original Horizontality and The Law of Superposition.
We've now seen what volcanic and tectonic features look like on sonar images of the seafloor. Our next task is to interpret them geologically. We want to determine whether the most of volcanism occurs through fissure eruptions, by building pillow lava mounds, or whether the seafloor is really just lots and lots of seamounts. Most importantly, we will want to construct a model which explains how these features were made, e.g., which feature occurred first, and which second, what the seafloor might have looked like in the beginning, and what physical conditions must have been present for that type of volcanism to occur. We begin this research by making a geologic map of the seafloor from all the sonar and bathymetric data, and continuing to follow the scientific method. So far, we've posed questions about the Puna Ridge, and now we're at sea collecting the data to answer the questions. In the meantime, however, let's learn about the geologic principles so we'll be able to interpret the data.
There are two fundamental principles that guide geologists when they interpret field data. To those of us who are geologists, these two principles seem as old as Earth itself, and are an integral part of the science of geology. The two principles are The Law of Original Horizontality and The Law of Superposition. We'll discuss these in today's lesson.
The Law of Original Horizontality states that when strata such as sediments (sand, silt, or clay) are formed, they are laid down in horizontal, flat-lying layers. If you pour sand into a tub of water, the layers will build up more or less horizontally and will be basically flat. Lava from volcanoes such as Kilauea also follows this law for the most part, though true horizontality is violated in some cases when lava flows down hills and form layers on the flanks of cones. In this case, the 'horizontal' plane is really a slope.
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| This figure shows both the Principle of Superposition (younger layers form on top of older layers) and the Principle of Original Horizontality (layers are deposited horizontally) |
The Law of Superposition is the backbone of what field geologists attempt to do in most of their work: to unravel the sequence of events that occurred to form an area. This law states that younger layers will be laid down on top of older layers. If we were then to dig a hole, we'd see that the oldest layers are found at the bottom and youngest layers at the top of the section. In another context, this also means that if you see a section of sedimentary rock or lavas in a road cut, and if the section hasn't been disturbed, then the oldest layers will be at the bottom and the youngest will be highest in the section.
Of course, we know that the Earth is not a static, unchanging body. Plate tectonics produces earthquakes, volcanoes, continental collisions, and many other forms of dynamic interactions that disturb the surface of the Earth that we can observe. Layers that were originally horizontal can be folded and overturned by squeezing, stretching, and other compressional forces on many scales, and older layers can end up on top of younger layers by actions of thrust faults and overturned folds, so geologists have to be very observant and use as many scientific tools as possible to study an area.
Some ways that we can get tricked in the field are shown here.
The figure to the left
shows a thrust fault where the layers on the left are pushed upward relative to the layers
on the right. This results in Old and Middle-Aged layers (O and M) being positioned next
to Young layers (Y). The geologist must be aware of the fault (F) in order to interpret
this section.
The figure to the right
shows folded layers of rock that have also resulted in old layers (O) being above younger
layers (Y). If the area in the white box was exposed in the field, a geologist might think
the upper layers were the youngest. He or she would have to look to look at a larger
cross-section, and for other clues like fossils or weathering to help him decipher this
problem.
Next time we'll apply these principles to the side scan sonar records we've been collecting. We'll see how we can determine the sequence in which the volcanic or tectonic events occurred by carefully studying where feature overlap one another.