The earth has not always looked the way it looks today. In other words, the United States one billion years ago was in a totally different location than it is today!! How does this happen? And why does this happen? Let's take a look. In order for us to get some understanding of how the earth has changed over time, we first need to understand some of the things that took place, and are still taking place, in the earth. What about the internal structure of the Earth? Our best clues about the interior come from waves that pass through the Earth's material. When earthquakes shake and shatter rock within the Earth, they create seismic waves which travel outward from the location of the quake through the body of the Earth. Seismic waves are disturbances inside the Earth that slightly compress rock or cause it to vibrate up and down. The velocity and characteristics of the waves depend on the type of rock or molten material they traverse.
Studies of seismic waves have revealed two important types of layering in the Earth: chemical and physical. Compositional layering refers to layers of different composition. Physical layering refers to layers of different mechanical properties, such as rigid layers verses "plastic" or fluid layers. Compositional layering was the first type of layering recognized. Seismic and other data indicate that the Earth contains a central core of nickel-iron metal. The core is surrounded by a layer of dense rock, called the mantle, that extends most of the way from the core to the surface. Near the surface, the densities of the rocks are typically lower. The crust is a thin outer layer of lower density rock about 3 miles thick under the oceans and about 18.5 miles thick under the continents. he core-mantle-crust structure gives us important clues about the history of the Earth and other planets. First, it shows the importance of differentiation processes - processes that separate materials of different composition from one another. Most...
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