What are mountains made of ? They are made of rock of course, but what are rocks made of ? There are 93+ elements and these must form the mountains as mixtures of substances, but which one predominate ? The conventional geological approach concentrates on the physics of rock formation, not the chemistry. This gives a variety of names for many types of rock, all of which seem a bit arbitary. A more fundumental approach is to start with the elements which make up the Earth's crust, then using elementary chemistry deduce what types of rock have to form the basic rocks of the Earth's crust. With this knowledge, the common rocks and the differences caused by the physical processes becomes easier to understand.

The starting data is the relitive quantities of the elements which form the Earth's crust. These are listed in the table below. On the left of the table are listed the elements of the Earth's crust from the most common to the less common. The amounts as percentages of the total mass of the Earth's crust are shown beside them. Look at these columns first.

..........................Mass %...............Atomic Wt............Valency..........% Oxygen used in oxide..........%Silicates
Oxygen..............49.13%....................16.........................2
Silicon................26.0 %.....................28.........................4..............................29.7............................ 6.8
Aluminium............7.45%....................27.........................3................................6.6............................38.9
Iron.....................4.2 %.....................36........................2/3..............................1.9............................13.0
Calcium...............3.25%....................40..........................2...............................1.3.............................9.4
Sodium................2.4 %.....................23..........................1...............................0.8.............................6.4
Potassium............2.35%....................39.1.......................1...............................0.5.............................4.6
Magnesium..........2.35%.....................24.3......................2...............................1.5.............................9.7
Hydrogen............1.0%.......................1............................1...............................8.0
All others............1.87%
.......................100.0........................................................................................50.3

The first amazing result is that oxygen percentage is 49.13%, that is very nearly half the Earth's crust by weight is oxygen. Silicon forms the next most common element, such that three-quarters of the Earth's crust is oxygen and silicon. Most of the remaining quarter comprises a group of 6 metals, and in a quantitive sense you can ignor all the rest of the elements, except maybe hydrogen. What about carbon? This element although vital for life, is comparitively rare as part of the Earth's crust. A property of life however is to find and concentrate those elements which it requires, such that the rarity of carbon is only an inconvenience.

As half the mass of the crust is oxygen and a quarter is silicon, we might expect the oxide of silicon to be very common. Suppose all the silicon combined with oxygen to form silicon dioxide, how much of the available oxygen would this use up? Silicon has an atomic weight of 28 (as shown in the third column of the table) and this combines with two oxygen atoms of atomic weight 16 to form silicon dioxide (SiO2). Thus the mass of oxygen required is 2x16/28 times the amount of silicon (the percentage mass of which from the table is 26%), giving the total mass of oxygen in the Earth's crust required to oxidise all the silicon to be 29.7%. If this were the predominent reaction it would still only use up about 60% of the 49.13% of available oxygen. So does this silicon oxidation occur to form most of the rock of the Earth's crust? Silicon dioxide is silica (sand) or quartz, and although there is a lot of it about, it does not form nearly the amount of rock we might expect from the above and we must look a little further into the chemistry.

The approximate atomic weights and valencies of the elements are listed in the third and fourth columns of the table. The percentage of oxygen required to form the oxides not only for silicon as above, but for all the metals and hydrogen is calculated from these values and shown in column 5. We see the oxygen required to oxidise the silicon is shown (as calculated previously) to be 29.7% with the other values being calculated similarly. Summing the oxygen requirement shown in this column would give the oxygen requirement to turn all the significant elements into their oxides. We see amazingly that there is nearly enough oxygen available to oxidise everything. We might then anticipate that the Earth's crust would be substanually a mixture of these oxides. However this is not the end of the chemistry, for if such oxides ever existed in quantity, the conditions of temperature the mixture would be subjected to would cause some of the metal oxides to fuse with silica to give silicates. For example, combining a molecule of aluminium oxide with three molecules of silicon dioxide gives a molecule of aluminium silicate.

If we assume this type of reaction occurs for all the metal oxides, turning them into silicates, then the percentages by mass of the various silicates (and the 6.8% of silicon dioxide remaining) is shown in column 6 of the table. Now things look entirely different. This suggests that rocks are made of a mixture of metal silicates with aluminium silicate predominating. The remaining silicon dioxide is then only 6.8% of the total. Clearly, both the oxidation and the production of silicates is an over simplification of the chemistry involved and the process will not be as neat or as complete as assumed. However for all its simplicity this does indicate what rocks are made of. For example granite is mainly a mixture of quartz and felspar. Quartz is silicon dioxide and felspar is mainly aluminium silicates. In geological terms we must expect minerals not made from the elements listed above to be comparitively rare, except perhaps the oxide of hydrogen, that is water. Even though hydrogen comprises only 1% of the Earth's crust by weight, because hydrogen is so light, when combined with 8% by mass of oxygen it forms 9% of the Earth's crust by mass, which is a huge amount of water. Some of this water is absorbed and combines with the silicates in the rocks, but the remainder, because it is not very dense, slops about on the surface (as the oceans).

Here starting from what the Earth's crust is made of, we have deduced what we might plausibly expect to find as rocks. The influence of the stability and properties of the silicates will control the type of mix, which brings us back to the physics of the standard geological approach with a better understanding of the constituents.