So the switch to volcanoes that erupt above water might have allowed oxygen to build up.
A Planet Like Indonesia
At the time of the tectonic upheaval 2.5 billion years ago, much of the planet resembled Indonesia today.
"It was flooded, with fairly shallow seas and some islands above the water, but with lots of volcanoes erupting underwater," Kump said.
Then the Earth's crust thickened and became more buoyant, forming the first large continents. With more of the Earth above water, far more volcanoes erupted into the air.
In this week's issue of the journal Nature, Kump and Barley compiled information about volcanic rocks up to 3.5 billion years old—close to the age of the oldest rocks on Earth. Nearly all of the oldest rocks formed underwater.
"Sure enough, we found pretty good evidence for an increase in volcanoes erupting on land," Kump said.
"I think it is a provocative idea," said James Farquhar, a geologist at the University of Maryland. Farquhar's own work on the chemistry of ancient minerals helped determine when atmosphere began filling with oxygen.
But the change in atmosphere may have been subtle.
Tim Lyons, a geochemist at the University of California in Riverside, noted that the balance among gases would only have needed to shift slightly to match the changes geologists see in the minerals from the time.
The oxygen weathered rocks and reacted with minerals and metals to create micronutrients, he said. That may have been enough to fuel the development of more complex life.
"Even though it could have been a very small step, it really changed the nature of the world," Lyons said.
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