The Earth is of course a planet… and we kind of know what it looks like on the outside – and I remember seeing pictures from my school days of what it looks like on the inside – a core… a crust… and stuff in-between… now what was inbetween…

The following review is derived from combined with Science Basics 04 Year 10 by Joanne Macown (2008) Nelson Cengage Learning, and USGS Inside the Earth – found at

Here we go!!!

Well firstly – obviously with a planet, the further into it you go the hotter it is going to be and the higher the pressure.  The earth is made up of 3 main parts a bit like an egg – the core (yolk), the mantle (white) and the crust (shell).

The inner core is made up of iron and nickel(about 2600km thick) – despite being well above the melting temperature of iron and nickel at around 5000C, the inner core is solid because of the great amount of pressure that exists in the center of the planet.  The solid inner core is surrounded by a liquid outer core around 2500km thick – this is made from molten iron and nickel.

As the earth rotates the molten metal spins and this is thought to be responsible for the earths magnetic field.

Picture USGS Inside the Earth

The mantle is the middle layer of the earth – it is a 2,900km dense layer of mostly semi-solid rock organised into sublayers – the very outer most sublayer of the mantle, just underneath the earths crust, is quite solid and rocky and is divided into distinct plates – this layer, along with the earth’s crust, is often referred to as the Lithosphere (from Greek Lithos = stone).  The plates sit directly on top of another mantle sublayer called the asthenosphere (Greek asthenos = weak).  The asthenosphere is a thinnish layer of semi sold hot-molten rock.  Scientists believe the rigid lithosphere “floats” slowly over the top of the asthenosphere.

The earth’s crust is the outermost and thinnest layer – it is ridged but brittle in comparison to the other layers – and is prone to cracking.    Beneath the oceans, the crust is about 5km thick and beneath the continents it averages about 30km thick – this of course varies a bit – under some mountain ranges it can be 100km thick.

The Moho or Mohorovicic discontinuity is the boundary between the crust and the upper mantle – it has distinct seismic activity so is thought to be made from a different rock composition to the crust.

This is interesting and like a refresher course… I did have trouble in this section when compiling information about lengths of different layers –  i.e. some have the core being smaller than 2,500km and others have it being larger… It is worth considering that there is some discrepancy there if students have 2 different reference points for this information.  There are also many sites that state the mantle is solid rigid rock… but of course the mantle would have to have fluid properties (even semisolid rock has fluid properties) to fit into the rest of plate tectonic theory… So… need to make sure students are pointed to references which are consistent with this fluid model if they are provided with any references or resources for their learning.  

Again… I think this refresher on the earth is probably best included in the engage session