Donald Feist, of Helensburgh, asks :-

If all the elements in planet Earth other than the lightest two or three were formed within earlier generations of stars, and planet Earth was later formed from the remains of these stars, how is it that the metals used in modern technologies are not evenly distributed through the lithosphere, but tend to be concentrated in particular places?

Alan Gilmore, an astronomer at the University of Canterbury`s Mt John Observatory, responded.

Several stages of material sorting have resulted in the Earth as we know it. First the gas and dust from earlier stars condensed into a cluster of stars of which the sun was one. Some leftover dust and gas formed a disk around the proto-sun, as we presently see around many young stars. The dust and ices clumped into small planets. These eventually gathered into bigger planets. Meteorites show that the early small planets had nickel-iron alloys, sulphides, phosphides, many oxides and silicate minerals.

Early sorting of Earth's minerals happened through repeated melting of Earth's crust by volcanism and asteroid impacts. Chemical reactions with water in the early oceans also helped. In the first billion years around 500 new minerals appeared.

Plate tectonics added more. When a continental plate of wet, chemically diverse rocks slides under another plate, it melts and further concentrates scarce elements. Such processes made some 1500 minerals in Earth's first two billion years.

Then life got into the act. Bacteria originated 3.5 billion years ago but didn't have much effect at first. Then algae began producing oxygen two billion years ago. Oxidation of rocks led to the majority of Earth's 4,400 known minerals.

Starting 800 million years ago the Earth went through a series of temperature fluctuations ranging from ice ages to hotter than now. During the hot times heavy rain eroded clay minerals from rocks. Carbonate minerals were precipitated from shallow seas.

Around 460 million years ago oxygen made the ozone layer. That shielded the land from lethal ultra-violet light. Mosses were able to move onto land and begin the evolution of plants and trees. Plants and fungi increased the weathering rates on land. So by 400 million years ago Earth had the mineralogical diversity we see today.

For more details of this fascinating subject see an article by Robert Hazen in Scientific American, March 2010. It can be seen on-line at http://www.scientificamerican.com/article.cfm?id=evolution-of-minerals