Our planet is a complex object. Its composition is varied, and the processes which drive change are unpredictable. The distance from the surface of the earth to its centre is over 6000 kilometres and mankind has succeeded in drilling just 0.2 percent of this distance into its depths. Consequently, the study of the internal structure of the earth is heavily reliant on secondary evidence such as measurements of seismic waves and the constantly changing magnetic field. Despite these difficulties, scientists have reached a consensus on what is happening deep within the planet.
At the simplest level, the earth can be divided into three separate layers: the crust on which we live, the viscous mantle beneath, and the core at the very centre. In reality, there are sufficient differences between regions within these layers to identify more divisions than just these three.
~ The Inner Core ~
At the very centre of the planet is a hot ball of solid iron and nickel, a little over 2,400 kilometres in diameter. Its outer surface is at a depth of about 5,000 kilometres. The temperature of the inner core is estimated at about 5700K, well above the normal melting temperature of iron at the surface of the earth, and similar to the surface of the sun. However, pressures at this depth exceed three million atmospheres - raising the melting point of iron significantly. The heat of the core is a remnant of the planet's creation - generated by the collision and gravitational compression of the material which formed the earth billions of years ago.
~ The Outer Core ~
Beyond the inner core sits a layer of liquid iron and nickel. It begins 2,900 kilometres below the surface of the earth, making it about 2000 kilometres thick. Temperatures are still high, but the pressue is sufficiently low to allow the iron-nickel mixture to melt and flow. The composition and behaviour of the fluid in the outer core is vital to the existence of life on the planet. The circling metals in the outer core are responsible for the earth’s magnetic field, which protects all life from the solar wind.
Before 1936, scientists were only aware that the earth had a core. The separation into inner and outer core was discovered by studying the reflection of seismic waves, caused by earthquakes, from the core boundaries. It is believed that the inner core is gradually growing and the outer core shrinking, as molten material in the outer core solidifies on the boundary between the two. This is a natural consequence of the earth cooling.
~ The D" Layer ~
The core is separated from the mantle by a thin, and little understood layer labelled the D’’ (D prime prime) layer. This layer is only a few hundred kilmetres thick, and it still hasn’t been definitively established that it is significantly distinct in composition from the mantle above. However, seismic studies do suggest that there is a chemical difference. The D'' layer may consist of material which was heavy enough to sink through the mantle, but too light to penetrate the outer core.
~ The Lower Mantle ~
The lower mantle is the single heaviest layer of the earth, accounting for about 50 percent of the entire mass of the planet. It is a viscous solid, about 2,250 kilometres thick. The composition of the lower mantle is 46 percent silicon dioxide, and 38 percent magnesium dioxide, making oxygen, silicon and magnesium the most common elements. Small quantities of other metals are present.
Temperatures in the lower mantle can still reach 4,000C. Although the material is unable to melt, the pressure being too high, it does move like a liquid over extremely long timescales. Variations in temperature produce convection currents which intrude on the upper mantle. As hot material rises through the mantle, this can produce ‘hot spots’ on the surface of the earth such as those in Yellowstone and Iceland. Movement of these hot spots is observed over time, as the tectonic plates of the crust travel over the slower moving mantle material.
~ The Upper Mantle ~
Compositionally, the upper mantle is very similar to the lower mantle. It covers most of the remaining distance from the edge of the lower mantle to the crust, ending 10 to 15 kilometres below the surface on average. It is studied separately from the lower mantle due to the difference in the way seismic waves travel through the layer. Because the temperature is cooler, the upper mantle is less plastic than the deeper areas of the mantle.
Attempts have been to drill down into the mantle, but no hole has reached a depth of more than about 12 kilometres, well short of the mantle on the continental crust. Success in this area of scientific study is likely to come from devices digging from the ocean bed, where the crust is less thick than on land.
~ The Crust ~
The earth’s crust is the layer with which we are familiar. It is the thinnest layer of the planet, varying from about 5 to 10 kilometres on the ocean beds to 30 to 50 kilometres beneath continental land masses. The solid crust floats on the more plastic mantle causing the phenomenon of continental drift. The elemental composition of the crust is more varied than the deeper layers, although silicon dioxide still accounts for nearly two thirds of all compounds.
The crust was generated, and is continually refreshed, albeit very slowly, by geological processes such as subduction and crust formation at mid-oceanic ridges. In this way, material is constantly moved from crust to mantle and back, over the course of billions of years. It is the crust’s solidity, compared to the relative plasticity of the mantle, which leads to earthquakes. The crust is unable to flow, and, instead, cracks or slips catastrophically when pressure builds. The resultant earthquakes are the most destructive natural events on the planet.