Which layer of earth can be directly observed?

Answer 1

Only the CRUST can be directly observed by scientists How do we know this? Has anybody ever been down there? Have we drilled holes into the center of the Earth and retrieved samples? The answer is no on both counts. It's too hot and the pressures are too high. But we are not stuck in absolute ignorance. Just as when we pondered the universe, it pays to make assumptions and to see if they are supported by the facts. Newtonian mechanics allows us to determine the overall mass of the Earth, and because we know the diameter (12,756km) and thus the volume of the Earth, we can calculate its average density as 5.515 g/cm3. If the Earth were more or less homogeneous, rocks found at the Earth surface should be in density around 5 to 6 g/cm3. When we measure them, however, we find that their density is on the order of 2.6 and 2.7 g/cm3. Obviously, if the density is below average at the surface, the density must be above average on the inside. Tinkering back and forth with the densities of many minerals and the relative abundance of elements in the solar system it is unlikely that any silicate mineral at depth can account for the necessary high inner density. The only materials of suitable density are heavy metals, such as iron, nickel, cobalt, copper, etc. Of the heavy metals, iron is cosmically most abundant, and thus a perfect candidate. In the past there have been the suggestion that certain structures in iron-nickel meteorites are indicative of an origin in the interior of a planet, and suggested that iron cores should be common in the interior of planets. These structures (Widmanstaetten Structure) in a mineral intergrowth named (octahedrite) supposedly indicated pressures that only were to be found in the interior of planets. We know now that these structures are more a reflection of cooling history, and probably could not have formed in planetary bodies that were larger than about 850 km (cooling too slow). The idea that the asteroid belt of the solar system may contain the remnants of a former planet is based on the assumption that octahedrite forms in the interior of earth size planets. This line of reasoning thus reduces the likelyhood that the Asteroid Belt consists of the fragments of a former planet.

Other evidence about the layered nature of the Earth comes from the observation and measurement of sound waves that travel through the Earth. These waves (seismic waves) are created naturally by earthquakes, and artificially by huge explosions (e.g. nuclear tests). As they travel trough the Earth they reveal its internal structure. This branch of Earth Sciences is also known as seismology. The next page illustrates how seismic helps to understand the Earth's structure. Basically, our knowledge of the position of the various boundaries (inner/outer core, core/mantle, crust/mantle) seen in the above figure, as well the physical properties of the various layers (density, velocity of sound) are largely due to seismic studies.

Obviously, something happened to change our Proto-Earth from the "dirty snowball" nature of a comet to the solid planet we know today. We know already that because the Earth was comparatively close to the sun, it lost most of its volatiles as the Sun started to heat up. Thus, it is a very dense planet (5.515 g/cm3) when compared to a gas giant like Jupiter (1.33 g/cm3). Now we have to ask how the materials that made up the early earth could have "unmixed" to form the currently observed structure.

This kind of "unmixing" or segregation is often described as differentiation by geologists, and differentiationcaused the heavy metals (iron, nickel and related elements) to be concentrated in the core of the earth, whereas the light elements (oxygen, silicon, aluminum, potassium, sodium, calcium etc.) were enriched in an outer layer of the earth that is now termed the upper mantle and the crust. Gravity, however, is not the only process that drives differentiation. Chemical affinities can also play an important role. Uranium and Thorium, for example, are very heavy elements, and contrary to expectation they are concentrated in the crust (primarily) and mantle. The reason for this aberration is the circumstance that ion size and chemical affinities of U and Th prevent them from being incorporated in the dense, tight crystal structures that are stable at the high pressures encountered in the earth's core. Because they can fit much more easily into the more open crystalline structures of silicate and oxide minerals, they are enriched in crust and mantle.

After the establishment of the internal structure depicted above, the earth reached approximate thermal equilibrium (heat generation balanced by heatflow through the earth's surface). Heat can be transported and transmitted in a variety of ways, such as conduction (through a copper rod), radiation (feeling the heat of a fire), and convection (hot water rising in a pot). Convection is the most efficient of these, and is found to play a role in many geologic processes. It is also the main process by which heat is moved to from the interior to the exterior regions of the Earth. Convection implies fluid behavior where hot material rises due to its lower density, and cold material sinks due to its higher density.

Just as many other things that go on in the Earth's interior, convection in its interior can not be observed directly. Fortunately, however, the conveyor belt motion that accompanies convection is evident in the way the Earth's crust moves. The theory that describes these motions is known as Plate Tectonics, and is the theory that brings together observations from many branches of earth science into a coherent whole.

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I'd like to get away from earth awhile

And then come back to it and begin over.

May no fate willfully misunderstand me

And half grant what I wish and snatch me away

Not to return. Earth's the right place for love:

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From: "Birches" by Robert Frost, 1915

Answer 2

Inner Core , Outer Core , Mantle posted by Ms. Chambers Science

Answer 3

The answer is the mantle, outer core, the inner core.

Answer 4

crust

Answer 5

crust