Given the radius of the Earth to be 6371Km, Jules Verne's assumption of the core temperature being approximately 200,000 ° Centigrade or 360,032 ° Fahrenheit at the centre of the Earth is untrue. This is not so due to the very effective convection in the liquid outer core that keeps the temperature gradient lower there, and so the temperature of the inner core is estimated at approximately 4700 °C. If the above were correct, the Earth would be totally liquid a few hundred km below and the surface plates and crust of the Earth would be very unstable on top of the fictional violent magma. Looking at global averages for thermal gradients, from deep mines or boreholes, 35ºC/Km is a more likely estimate. The core of the Earth is not composed of ‘incandescent gas’; the inner core is solid and is composed of predominantly Nickel and Iron. The inner core is hotter than the outer core as the temperatures are high enough here to allow the molecules of this layer to be in a liquid state but the intense pressure caused by thousands of kilometres of overlying rock, forces the molecules into a "solid" configuration and to further this the inner core transmits shear waves, confirming seismically that it is solid in arrangement.
Continental Crust has an average thickness of 32 km and the conditions below the crust can be revealed by lavas and rocks erupted in volcanoes. The data indicates that temperatures near the base of the lithosphere are approximately 1300°/1600º F. These temperatures are very near the melting point of mantle rocks but vary from about 900 °C at the base of 30 km thick crust and about 2700 °C at the base of 90 km thick crust. To reach an 'impassable limit' at a certain depth is not feasible proposition as the temperature still increases with depth until the core is reached whereupon the maximum temperature is reached. Some regulation is present preventing exponential increases of the internal temperature in the mantle. Again this is achieved by convection averaging the temperature differences and therefore preventing such proliferate temperatures.
Jules Verne correctly indicates that 'the interior of the globe neither gas or water, nor any of the heaviest minerals known, for in none of these cases would the Earth weigh what it does.' The inner core is not a gaseous or liquid province but encompasses a predominantly impure iron-nickel region with a composition including minor amounts of some lighter element such as oxygen and sulphur. In laboratory simulations it is proved that this mixture is of the correct density to clarify the known mass of the core.
Another factual statement proposed by J. Verne is that 'the number of volcanoes has diminished since the first days of creation and that the central heat is in a process of diminuition.' Despite radiogenic heating the planet has been slowly losing net heat since it was created. As this heat is lost, more and more of the liquid outer core becomes encompassed as the solid inner core. This slow gradual cooling is relative in the respect of net heat over time but the temperature still increases according to the geotherm the closer to the core you travel. The number of hot spots is increasing in areas such as Hawaii due to the constant movement of oceanic plates passing over the mantle plumes or hot spots, consequently melting the lithosphere causing small volcanic arcs. However research indicates that these are reducing due to the decreasing source of available basaltic material that fuels such plumes.
As mentioned previously the inner core is in fact a solid due to immense pressure and Jules Verne acknowledges this later with Humphry Davy stating that 'it could not be in a liquid state.' The endorsement however for this point is however incorrect, the forces of gravity which are brought about by the increasing pressure on material within the Earth, are so immense that the effects of lunar attraction are negligible. Experiments show that the expansion of a rock due to an increase of 300°C will increase its density by 1% and on the scale in question, the mass of the mantle is about 4 billion trillion tonnes, thus creating an immense gravitational pull.
Following the revelation that the liquid structure of the terrestrial nucleus was indeed erroneous Verne declares that 'The Earth has been heated by combustion on its surface.' This is clearly incorrect, as it is known that the Earth's heat originates from a mixture of two sources with the first being the initial heat from the formation of the planet. As the smaller bodies combined to form the Earth, their gravitational potential energy was converted to heat. And the second source is the regressive decay of radioactive elements in the Earth's mantle and core. Many elements present in the Earth have radioactive isotopes and a few of these, Potassium, Uranium and Thorium, have a few extra neutrons. These extra neutrons result in the element becoming unstable and undergo radioactive decay, producing heat.
Correctly J. Verne indicates that the Earth's crust does contain Sodium and Potassium as well as other metal elements, but the proposal that atmospheric vapours caused numerous volcanoes by water penetrating into the fissures of the crust, braking into fresh combustion with explosions and eruptions is untrue. The quantities of such elements are small in their amounts (Sodium 3.1 an potassium 2.9 [average composition in granite - andesite]) within the crust and it is known that sea floor spreading causes such volcanic eruptions, the subduction of lithospheric plates at plate boundaries and mantle plumes creating hot spots. However he is partly correct that water does increase the intensity of such events, but the water concerned here is primarily from the ocean in recycled oceanic crust whereby water remains trapped between the sediment grains and is slowly released as the temperature increases causing the rocks to melt at lower temperatures than usual, increasing the gaseous content, causing volcanic arcs.
An ‘Exciting Discussion about an Unparalleled Enterprise’ highlights many of the misconceptions and lack of understanding at the time, portrayed here by Jules Verne’s observations. Primarily Verne’s theories are either partially correct yet out of context, or, for the most part, incorrect. It seems that the majority of the misconceptions about the interior of the globe are based on flaws relating the to geotherm as an erroneous constant throughout the Earth and the formation thereof.
