Answer to Question 1
The three major concentric layers are the crust, mantle, and core.
The crust is the thin, solid surface layer, and includes compositionally different oceanic and continental crust. The mantle is the thickest layer and is also mostly solid but includes the less rigid asthenosphere. The core is metallic, with an inner solid part and an outer liquid part.
Answer to Question 2
The principle states that observations of both a process and its result can apply to the interpretation of other, similar results where the process was not observed. In other words, if you can understand the geologic processes that are responsible for materials and features you presently see in nature or in the laboratory, then you can infer that similar materials and features found in ancient rocks and landscapes are the result of these same processes. If uniformitarianism guides interpretation of geologic history, then can it also be used to forecast future events? In a general way, yes, because scientists infer that the same natural processes operating today operated in the past and will operate in the future. This does not mean, however, that scientists can predict exactly when or where a geologic event will occur. For example, geologists can study the behavior of past eruptions of a volcano by observing the resulting lava flows and layers of volcanic ash, and then hypothesize that future eruptions will feature similar behaviors. This application of uniformitarianism does not, however, mean that the time and severity of the next eruption can be forecast far in advance.
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David P. Bradley (White Earth Oijbwe, and Mdewakaton Dakota), Indian Country Today.
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