How rocks evolve
Nov 13th 2008
From The Economist print edition
It is not just living organisms that evolve. Minerals do too, and much of their diversity has arisen in tandem with the evolution of life
EVOLUTION has come a long way since Charles Darwin’s time. Today it is not only animals and plants that are seen as having evolved over time, but also things that involve the hand of humans, like architecture, music, car design and even governments. Now rocks, too, seem to be showing evolutionary characteristics.
Rocks are made from minerals, which like all matter are composed of individual chemical elements. What makes minerals special is the way that the atoms of those elements are arranged in lattices which create unique crystalline structures and shapes. Today more than 4,000 different minerals can be found on Earth. When the planet began to be formed, however, few existed.
Curious as to how this great variety came about, Robert Hazen of the Carnegie Institution in Washington, DC, and a team of colleagues set out on their own voyage of discovery. Their study, just published in American Mineralogist, explores the history of minerals by identifying how much of the diversity was created by the rocks alone and how much of it was created by the evolution of life.
Before the formation of the sun and planets, all the chemical elements found on the periodic table were floating around in the primordial dust of the Solar System. Some elements were more common than others, but everything from argon to zinc was there. Minerals, however, were almost entirely absent. Only a handful, like diamonds and olivine, could be found, having been formed far away in exploding stars.
Out of the void
Minerals form from chemical elements in only a few ways. They can crystallise when molten lava becomes a solid; they can be left behind as residues when water carrying dissolved elements evaporates, or they can be deposited in water when concentrations of dissolved elements get too high. In one way or another, atoms of different elements are brought close together to form their crystalline lattices. And once some minerals are formed, more can be created chemically. Usually this happens by exposing minerals to agents that rip away electrons, just as oxygen does during oxidation.
As the planets congealed, gravitational forces and particle collisions created the high temperatures necessary to melt metals floating around in space, and minerals began their diversification. Once the sun and planets took shape, heat became readily available in the form of solar radiation, asteroid impacts, and gravitational pressure. Magma appeared in abundance and in some locations formed vast molten oceans. Dr Hazen and his team speculate from meteorite and early rock studies that several hundred mineral species, including zircon, quartz, clay and halite salt crystals, formed during this early period of mineral evolution.
Although Mercury and the Moon never progressed beyond the planetary-congealing stage, volcanic activity on Mars, Venus and Earth created powerful forces that caused further mineral diversification. Much of it resulted from volcanoes blasting out gases like carbon dioxide and water vapour, which allowed ice (also a mineral) to form near the poles of Earth, and possibly on Mars too.
Plate tectonics, the formation of faults and the moving of the continental crust over a molten interior, is a characteristic unique to Earth and it meant that minerals evolved far beyond those on any other planet in the Solar System. These epic processes melted and reprocessed vast amounts of rock and concentrated chemical elements in new ways. Massive ore deposits of copper, lead, zinc, silver, gold and other metals formed in this way.
Over 1,500 minerals are thought to have formed on Earth before the beginnings of life some four billion years ago. Obviously, minerals do not have genes and thus cannot mutate as living things do. Nevertheless, when life appeared, the evolution of minerals and the diversity of life became entwined.
Microscopic algae, the earliest living organisms, drew carbon dioxide from the atmosphere and expelled oxygen. Over millions of years this created an oxygen-rich atmosphere that rapidly removed electrons from minerals near the surface, creating rust out of iron and forming thousands of new minerals from other metals like nickel, copper and uranium.
Living stones
In the oceans, as animals with hard parts evolved, their bodies mineralised shells and skeletons for protection and support. Corals too started combining the calcium and carbonate that was floating freely in the water to construct reefs. All of the materials that animals made started to litter the sea floor and this vast accumulation of bone, shell and coral got pressed together into a mineral known as calcite.
On the land, plants produced acids around their roots that converted minerals of volcanic origin, like mica, feldspar and pyroxene, into clay minerals that ultimately formed intensely rich soils. This explains why volcanic islands like Hawaii are so lush.
New minerals created by living things continue to turn up. One of the most recent discoveries was by Hexiong Yang, who named it Hazenite as a tribute to Dr Hazen, his former teacher. Hazenite is a mineral formed by microbes in the highly alkaline Mono Lake in California.
Understanding just how dramatically life shapes minerals will play an important role in the exploration of the universe, says Dr Hazen. Knowing which minerals form at different stages of a planet’s evolution, and which depend upon life to be present, are crucial to understanding the mineralogy of other planets and moons.
With NASA’s Messenger probe now going into orbit around Mercury, Dr Hazen predicts that it will find only 300 or so minerals on the planet. If there are 500-1,000 detected, then it will suggest that there is a lot more to Mercury than anyone originally thought. And if minerals that depend upon life for their formation show up, then researchers will be flummoxed. The same is true for Mars and other planets—including the exoplanets that have been known about but which have just been seen for the first time orbiting stars outside the Solar System (see article). Dr Hazen argues that considering minerals in evolutionary terms is a powerful way to help identify how far a planet has developed geologically. Moreover it can tell you whether life was present at some point—and even whether it is present now.
