The Blur Planet (2nd edition) "An introduction to Earth System Science" 1998.
BJ Skinner, SC Porter, DB Botkin For millennia mankind has inhabited the third planet from the sun; the planet that supports life and is known to us as Earth. Although there has been life on Earth for quite sometime, it is only in the past few centuries that man has come to learn about what makes up the interior of this planet.
The English scientist, Isaac Newton, can be seen as a pioneer in regards to learning about the Earth's interior, as he calculated from his studies of planets and the force of gravity, that the average density of the Earth is twice that of surface rocks and therefore that the Earth's interior must be composed of a much denser material. Our knowledge of what's inside the Earth has improved immensely since Newton's time, but his estimate of the density remains essentially unchanged.
'A round sphere with many layers, all varying in thickness, each having it's own color and tasteÃ¢ÂÂ¦' this is a description of a gob-stopper and also bares a close resemblance to the internal structure of the Earth; a sphere divided into three layers, differing in density, composition, strength, and state.
The densest of these layers is the core, which is composed largely of metallic iron, with small amounts of nickel and other elements. The less dense mantle then covers this layer, being composed of magnesium and iron silicates. The outermost layer is that of the crust, it has the lowest density of all the layers and can be separated further as its thickness varies greatly from place to place, with the difference being distinguished by land and sea and also its composition. For this reason the core is subdivided into the continental crust (average thickness 45km with a granite composition) and the oceanic crust (average thickness 8km with a basaltic composition). Similarly the core can also be subdivided, but the difference is not one between compositions but one by physical state. The inner core of the Earth is solid iron; this is because it is under such high pressure, so high that temperature has no bearing on its state. The outer core has a balance between temperature and pressure so it's iron composition is in the molten state.
Rock strength can also add further categorization to the Earth's interior, bringing in the sphere layers: the mesosphere, asthenosphere, and lithosphere. The strength of a solid is controlled by both temperature and pressure; when heated a solid looses strength and when under pressure a solid gains strength. This is what divides the mantle and the crust into these three sphere layers. At the lower part of the mantle (depth 2883km-350km) there exists a region of high temperature and high strength, this is known as the mesosphere or middle sphere, then at a depth of 350-100km there is a sphere with balance of temperature and pressure (leaving rocks with little strength) called the asthenosphere or weak sphere which has a plastic nature, and finally there is the region from 100km to the Earth's surface called the lithosphere. The rocks in the lithosphere are more rigid, cooler and stronger than the plastic asthenosphere.