An Introduction to the Anatomy of the Earth
The Earth can be divided into crust, mantle and core but what are the differences between these layers and how do we know this?
The crust is the outermost part of the physical Earth and the analogy of the skin of an apple is a good one. There are two kinds of crust – continental and oceanic:
- Continental crust varies in thickness but its average thickness is approximately 35km (it can be as thick as 70km in some areas). Continental crust has a huge variety of rock types.
- Oceanic crust is much thinner and averages at approximately 8km. It is made up almost exclusively of basalt that sits below a thin layer of sediment. The youngest oceanic crust is found at Mid Ocean Ridges.
The boundary that marks the transition from crust to mantle was first discovered by Andrija Mohorovicic. It was thereafter named the Moho boundary or the Moho discontinuity (discontinuity because of the change in earthquake wave speeds).
The mantle is the largest layer of the Earth and is made up mostly of rocks abundant in magnesia and iron oxides. There is a further discontinuity within the mantle at approximately 670km and this is thought to be a marker for a more iron oxide/magnesium rich and silicate depleted make-up (some experts argue that it is not a lack of silicates but that the silicates themselves are denser). This discontinuity separates the upper mantle from the lower mantle.
In the upper mantle, below the crust, down to approximately 350km, shock waves velocity reduces and this region is called the asthenosphere. It is characterised by increasing temperatures with depth. This results in causing some rock to melt. Despite popular belief, the whole mantle is not molten magma but is mostly solid with pockets of liquid magma.
This area is regarded as a layer of weakness that lies below a brittle layer that includes the uppermost part of the mantle and the crust. This area is called the lithosphere and is the region of the Earth where plate tectonics occur.
The core is the deepest internal part of the Earth and less is known about it than either the mantle or the crust. What we do know about it comes from several sources: earthquake waves, meteorites, magnetic field and density investigations and the relative abundance of elements. Indeed, earthquake waves have aided in providing a rough estimate for the size and physical conditions of the core itself. It is believed that the area known as the outer core meets the mantle at around 2,900km from the Earth’s surface, the inner core being somewhere in the region of 5,140km from its surface.
Given that the densities of the mantle and crust are below that of the mean density of the Earth, it is safe to assume that the majority of the density comes then from the core. This is where meteorites have proven useful. The composition of some meteorites is that of iron and nickel and it is believed that if the Earth’s core was made up similarly, it would account for this density deficiency. It is theoried that the core (outer) is possibly liquid.
To summarise, the solid Earth comprises of a lithosphere, asthenosphere, outer core and inner core, each with marked boundaries and transitions of density and chemical make-up.
Source: Holmes’ Principles of Physical Geology by D. Duff