As life has evolved into all its abundance and diversity, Earth’s rocks have come along for the ride. Two-thirds of the kingdom of minerals – the building blocks of rocks – can be traced to the emergence of life, say geologists.
This new perspective could shake up our picture of Earth’s geological history, and might even help us find life on other planets.
“Rocks and life evolved in parallel,” says Robert Hazen of the Carnegie Institution’s Geophysical Laboratory in Washington DC. “It’s so obvious – you wonder why we geologists didn’t think of it before.”
All rocks are made up of constituent minerals. For example, granite contains quartz, mica and feldspar. Geologists tend to classify such minerals in a methodical way, using factors such as their colour, hardness, and composition.
Hazen and his colleagues decided to take a fresh approach by exploring how the diversity, abundance and associations within the mineral kingdom have evolved over time. “What we have done differently is recognise that each mineral has its own story.” he says.
Breath of life
According to Hazen, the story begins with a mere 12 minerals that existed in the dust swirling around in the clouds that would eventually form the solar system – minerals like diamond, created in the fury of supernova explosions. When the Sun ignited, the heat from this event boosted the number to around 60.
The formation of the Earth and subsequent geochemical processes upped that to around 500, and the switching on of the conveyor belt of plate tectonics to around 1500.
“But it was life, which made its first appearance about 4 billion years ago, that made the biggest difference,” says Hazen. “It boosted the number of minerals to more than 4000.”
Life brought profound changes to Earth’s atmosphere and ocean chemistry.
Photosynthesising organisms created abundant atmospheric oxygen. Under this oxygen-rich environment, the chemical processes of oxidation and weathering generated a swathe of new species of metal-rich minerals, such as iron.
“Four billion years ago, metals on the surface like iron and copper remained pure and shiny,” says Hazen. “But the new atmosphere oxidised them, creating a host of new minerals.” Approximately half of the 4300 known mineral species are down to oxidation or weathering.
Search for ET
Around two billion years later, the nature of the mineral kingdom evolved again with the emergence of hard-bodied marine organisms. These created thick deposits of carbonate minerals, such as aragonite, when they died and settled on the seabed.
Plants also played a part by decaying and providing the compounds for the organic minerals that make up rocks like coal or black shale.
Even the amounts of precious metals may have been influenced by life – microbes are thought to trigger the deposition of gold from hot waters inside rocks.
Hazen says the new perspective could aid the search for extraterrestrial life. He reckons probes sent to planets like Mars should be designed to look for particular mineral features such as oxidised zones in cliff faces.
“If we find certain minerals, they will point uniquely to certain organisms,” he says.
Gary Ernst at Stanford University describes the study as “breathtaking” and says the new perspective could revolutionise the way geologists describe the mineral kingdom.
Journal reference: American Mineralogist (vol 93, p 1693)
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