![]() Careful geologists seek a preponderance of evidence before settling on an interpretation. The careful historical geologist will search for as many examples as possible to see if they agree with one another. Examples seen in the field can frequently be ambiguous. The way-up indicators shown on this page are hopefully straightforward, but nature is vast and varied. A note of cautionīe cautious about leaping to big, important conclusions from a single way-up structure. Higher-grade metamorphic rocks are so thoroughly recrystallized that any original geopetal structures would be destroyed. Metamorphic rocks develop no way-up structures as a consequence of metamorphism, but sometimes primary structures in a sedimentary or volcanic protolith can potentially survive as discernible patterns in lower-grade metamorphic rocks. in a few rare intrusions, primary structures including grading and cross-bedding(!).In igneous rocks, there are fewer options, but a few that are handy include: In sedimentary rocks, the following way-up structures can aid the historical geologist in figuring out the paleo-“up” direction: They always look different up-side-down compared to right-side-up. The key thing is that a way-up structure must be display some difference between its top and its bottom. So what are these geopetal tools, exactly? Without the way-up indicators, we would have been fooled. Therefore, the exposed layers are part of a larger-scale fold, and the upper (upright) portion of that fold has been removed by the forces of erosion. However, way-up structures in these three units point downward as the “up” or younging direction. Given superposition alone, we would assume the lowest one is oldest, and the uppermost one is youngest. They are folded into what appears to be a series of anticlines and synclines. Three layers are shown in outcrop at Earth’s surface, color-coded green, blue, and yellow. Consider this example: Geopetal structures that point in the paleo-“up” direction (red arrows) are of critical value in deciphering the story told by a sequence of strata. The welter of terminology shouldn’t turn us off: it’s an indication that geologists put a strong emphasis on finding and relying on way-up structures. (This is the same as paleo-“up” or “facing direction.”) ![]() The help us determine “younging direction,” the direction in which strata get younger. Way-up indicators are critical for figuring out the correct sequence of geologic units. We call these patterns that look different right-side-up compared to up-side-down by the general term “ way-up structures.” Some geologists also call them “geopetal structures.” In such situations, we need reliable tools in order to accurately interpret which direction was “up” when the rock originally formed. Way-up (or “geopetal”) structures allow us to tell the two apart. If the beds are up-side-down in such a tectonic inversion, it could really throw off the interpretations of the Historical Geologist: they would be reading Earth history backward! A tight, recumbent fold in the Sierra de Juarez, Mexico, shows both upright and up-side-down bedding. The older event can influence the more recent event, but not the other way around.Įven more extreme is when compressive stresses associated with convergent plate tectonic settings manage to fold beds into up-side-down positions. Did an ice age cause the extinction? Or did the extinction somehow trigger the ice age? In order to pose intelligent questions about causality, you need to know which one is older. In another layer, you find evidence of glaciation. In one layer, you find evidence of a mass extinction event. An exciting outcrop: but in order to interpret it correctly, we need to know which strata are older, and which are younger.Ĭonsider the hypothetical example illustrated in the image at right: you discover a vertical sequence of strata. If layered rocks have experienced mountain building, they may be rotated from their original horizontal positions into vertical orientations, potentially confusing geologists, since the principle of superposition no longer applies. With rocks, the answer is not always clear.
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