For millennia mankind has inhabited the third planet from the sun; the planet that supports life and is know 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 colour and taste 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 elementsX. 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 granitic 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 categorisation to the Earths 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.
Fig 1 The sliced view of the Earth identifying the layers and spheres.+
Now that the Earth s interior has been identified and separating into regions the question to be ask is how was it determined?
One way that we can find out what is under the surface is to drill. Many countries have attempted to drill wells down into the Earth s crust but attempts come into trouble around the 7km depth. The deepest well belongs to the Soviets (Russian) and it is located on the Northern Kola Peninsula. It is the result of a twenty-year effort to drill to a target depth of 15km but fell short at 12km in 1989. It took 5 years to drill 7km; 9 years to drill the next 5km and at the bottom of the hole the temperature was 190|C. From doing this information about the Earth s interior can be determined at roughly 12km, now what about deeper?
Volcanic activity brings up materials in magma such as Xenoliths that are pieces of mantle in the lava, example: coarse-grained olivine (peridotite) xenoliths in basaltic lava.
This is only useful to depth of about 200 km. Well that takes care of most of the mantle (lithosphere and asthenosphere) but what about the core?
The best evidence of core composition comes from samples from the solar system, ie. iron meteorites. These meteorites are believed to be the core of small protoplanets that were shattered in the early life of the solar system. There composition is thought to be extremely similar to Earth s and the other terrestrial planets.
The last and most widely used method is that of seismology. (Seismos is a Greek word meaning shock)|. Seismology deals with seismic waves, which are vibrations in the body of the Earth. Scientists categorize seismic movements into four types of diagnostic waves that travel at speeds ranging from 3 to 15km per second. Two of the waves travel around the surface of the Earth in rolling swells and are called surface waves. The other two, Primary (P) or compression waves and Secondary (S) or shear waves, penetrate the interior of the Earth. Primary waves compress and dilate the matter they travel through (either rock or liquid) similar to sound waves. They also have the ability to move twice as fast as S waves. Secondary waves propagate through rock but are not able to travel through liquid. Both P and S waves refract or reflect at points where layers of differing physical properties meet. They also reduce speed when moving through hotter material. These changes in direction and velocity are the means of locating discontinuities. These Seismic discontinuities aid in distinguishing the divisions of the Earth into inner core, outer core, mantle etc.
Fig 2 – illustration of the difference between P-waves and S-waves. These seismic waves (which are generated naturally by earthquakes, volcanoes and impacts, and may be produced artificially by explosions and mechanical device) tell us about the interior in several general ways.|
Beginning some three centuries ago with Isaac Newton s calculations, we have formed a detailed model of what the internal structure of the Earth looks like. We have looked at some of the advances in technology, the greatest of these being seismology and we are now left to decided where to go next could it be DOWN.
- Zhann K Jochinke 2001
1 USGS Website
The Interior of the Earth
Eugene C Robertson
2 The Blur Planet (2nd edition)
An introduction to Earth System Science
BJ Skinner, SC Porter, DB Botkin
3 OJTA Website
The Interior of the Earth