Concepts of relative dating
Thus, we can recognize a break in the depositional sequence at the Zoo site. Rapid cooling of magma results in fine grained rock or glassy rock and if this occurs along the margins of the intrusion, we will see the effects of rapid cooling along the margins. Faults, dikes, erosion, etc. This is the same principle used to determine relative age in the trash pits discussed previously. Thus, we can deduce that the mudstone and shale are older than the rhyolite dike. Once these age relations were worked out, another principle fell into place - the principle of fossil succession.
In the diagram to the right,the fault cuts the limestone and the sandstone, but does not cut the basalt. Principle of Original Continuity If layers are deposited horizontally over the sea floor, then they would be expected to be laterally continuous over some distance. When sediment is not being deposited, or when erosion is removing previously deposited sediment, there will not be a continuous record of sedimentation preserved in the rocks. Principles of Stratigraphy Stratigraphy is the study of strata sedimentary layers in the Earth's crust. For educational purposes, we shall start with a method which barely works at all under any circumstances. Because the Ceramic Cups layer is found at the Tulane site, but not at the Zoo site, the civilization that produced the Ceramic cups probably did not live in the Zoo area. With this new information, in combination with the other principles of stratigraphy, geologists we able to recognize how life had changed or evolved throughout Earth history. This also tells us that the sill is younger than the both the rhyolite and the sandstone. Fossil Succession Once geologists had worked the relative ages of rocks throughout the world, it became clear that fossils that were contained in the rock could also be used to determine relative age. We discuss the 7 principles of stratigraphy first and then see how these apply to fossils. This is the same principle used to determine relative age in the trash pits discussed previously. Principle of Inclusions If we find a rock fragment enclosed within another rock, we say the fragment is an inclusion. Given these conditions, we can find out how long the hourglass has been running. Thus rocks that we now see inclined or folded have been disturbed since their original deposition. Since slower cooling will occur farther away from the margin the rock farther away will be coarser grained. Variation in Unconformities The nature of an unconformity can change with distance. But, since the rhyolite dike does not cut across the shale, we know the shale is younger than the rhyolite dike. That there was no sand in the lower bulb when the process started. Similar cultures must have existed in both areas and lived at the same time. Or again, if there may have been sand in the lower bulb when it started running, but we have a good reason to think that there can't have been very much, then we also have a good reason to think that the figure which we get for the time can't be very wrong; and so on. The surface marking the break in deposition would be called an unconformity in geologic terms, and represents time missing from the depositional record. The geological processes that we'll be discussing work rather like that. So we know the sandstone is older than the basalt flow. Furthermore, some fossils were only found within a limited range of strata and these fossils, because they were so characteristic of relative age were termed index fossils. Application of the Principles of Stratigraphy Figure
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