Geology - Earth's Interior

INTRODUCTION

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 materialâ„-. Our knowledge of what's inside the Earth has improved immensely since Newton's time, but his estimate of the density remains essentially unchanged.

So what is this new knowledge of the Earth's interior?

PRESENT KNOWLEDGE

'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 elementsІ. 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)â„-. 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.â„-

Now that the Earth's interior has been identified and separating into regions the question to be ask is ' how was it determined?'

GEOLOGICAL OBSERVATIONS

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 190Ñ"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?

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