Re: Why does the earths core remain molten after millions of years?

Dear Matthew,

The answer to your question is that there is still ample heat in the 
Earth’s interior to maintain a liquid outer core.  The Earth’s core is 
only partially molten.  The outer core is composed of molten iron and 
nickel, and the inner core is solid.  The inner core has grown at the 
expense of the outer core over the 4.5 billion years of the Earth’s 
history.  Ultimately the core will entirely solidify, but this event will 
occur only in the far geological future.

The Earth has two sources of internal heat.  One is the heat of accretion, 
which is heat converted from gravitational energy as the materials which 
formed the Earth fell together under gravity in the early Solar System.  
This heat is now estimated to make up about 20% of the total heat flow 
from the Earth’s interior.  That fact is an indication of how long it has 
taken the Earth to vent this heat to space.

The other source of heat is the decay of radioactive isotopes of uranium, 
thorium and potassium, incorporated into the Earth at its accretion.  This 
radiogenic heat is the principal explanation of why the Earth’s interior 
is still so hot after billions of years.  Without radiogenic heat the 
Earth would by now have cooled down to the point where the core would 
probably be solid.

A final factor to consider is the Earth’s size.  The Earth is the largest 
of the rocky planets in the Solar System, and the thickness of the mantle 
acts as a blanket.  Heat is conveyed to the surface by conduction and by 
convection, the process that drives plate tectonics.  The rate of heat 
loss governed by the Earth’s size and composition, balanced against the 
production of heat by radiogenic decay in the mantle and core and the 
remaining heat of accretion, explains why the Earth’s outer core is still 
molten.

The Earth is not a blob of liquid.  The thin outer crust sits on the solid 
mantle, which is 2900 km thick.  The liquid outer core beneath the mantle 
is 2255 km thick, and the solid inner core is 1215 km in diameter.  
Therefore, and ignoring the thin crust, the liquid outer core comprises 
only 35% of the Earth’s diameter.  But even if the Earth were largely 
liquid it would still retain the shape of a globe under the force of 
gravity, albeit it would be more easily deformed by the centrifugal force 
of its spin and by the tidal pull of the Sun and other massive objects in 
the Solar System.  The situation of a planet composed of volatile 
materials is realized in the cases of Jupiter, Saturn and the other giant 
planets.  Jupiter is composed mainly of hydrogen and helium, which is a 
gas in the outer regions of the planet and probably a metallic liquid at 
the enormous pressures in its interior.  Jupiter keeps its shape because 
the enormous force of gravity pulls all of its matter towards the centre.

I hope this answers your question.

Best wishes,

David Scarboro