SOCORRO, N.M. – New Mexico Tech geophysics professor Dr. Kent Condie has been making headlines around the world in recent weeks.
Condie is the first author of an academic paper in Earth and Planetary Science Letters that purports to show evidence that tectonic activity slowed to a crawl for more than 200 million years, creating a scenario some scientists call “snowball Earth.” Condie’s paper has two co-authors: Dr. Craig O’Neill of Macquarie University in Sydney, Australia and Dr. Rick Aster, also of New Mexico Tech. The trio analyzed data from many sources in geology, geochemistry and geochronology.
After the initial publication in the academic journal, several mass media outlets have reported on the study, including USA Today, Discovery Channel, New Scientist magazine and two news websites in India
Dr. Kent Condie displays his most recent text book, "Mantle Plumes and Their Record in Earth History."
“I’m surprised at the attention,” Condie said. “I’ve presented this research at scientific meetings but haven’t gotten such an overwhelming response.”
A professor at New Mexico Tech since 1970, Condie has contributed several books on plate tectonics. He first published Plate Tectonics and Crustal Evolution in 1976. Now in its fourth printing, the text is widely used in academia. His most recent book, Earth as an Evolving Planetary System, is an updated version of Plate Tectonics published in 2006.
“Plate tectonic theory was being born when I worked on my Ph.D. in the early 1960s at Scripps Institution of Oceanography (UC San Diego),” he said. “I’ve seen significant evolution of the theory over the years.”
The distribution of isotopic ages of igneous rocks shows a sparsity of ages between about 2.4 billion and 2.2 billion years ago, suggesting a “crustal age gap” at that time. However, many scientists assumed the gap would vanish as more samples were collected and dated. Condie has amassed more than 40,000 isotopic ages of zircons and the gap has not been filled, reinforcing the theory that plate movements and volcanism nearly came to a halt for 200 million years.
Zircon (zirconium silicate, or ZrSiO4) is especially useful in radiometric dating because of its ubiquitousness in crustal rocks in the continental lithosphere. Zircon contains thorium and uranium, two radioactive isotopes with half lives of billions of years. By comparing the ratio of uranium (or thorium) to lead, scientists can determine the age of zircons and the igneous rock hosted by zircons. Zircons are omnipresent in rocks formed at subduction zones. They also occur in sedimentary rocks and can be used to date the source of the sediments.
Condie and colleagues suggest that plate tectonics slowed tremendously for 200 million years (between 2.4 and 2.2 billion years ago) when the Earth went into a “stagnant lid cooling regime.” While convection continued in the Earth’s deep interior, the outer plates virtually stopped moving. The planet is currently in a “mobile tectonic regime,” characterized by the intrinsic relationship between internal convection and movement of the plates.
The implications of the model are far-reaching. The Earth’s atmosphere 2.5 billion years ago was mostly methane with an average temperature estimated at more than 50 degrees C. During the 200 million year age gap, volcanic sinks for oxygen were greatly reduced in number and, for the first time, oxygen was released into the atmosphere.
“Also we’re suggesting that when submarine volcanism significantly shut down for 200 million years, ferrous iron stopped coming into the oceans and banded iron formation stopped being deposited,” Condie said. “That also freed up oxygen to go into the atmosphere.”
Earth scientists have ample evidence of atmospheric oxygenation and global glaciation at about 2.4 billion years ago.
“The only life on Earth at that time were microbes,” Condie said. “These microbes were either producing oxygen or they were the types that produced methane or lived off methane. The growth of oxygen in the atmosphere destroyed methane, and led to an atmosphere capable of sustaining higher life forms. As reflected in the fossil record, the first advanced cells – or eukaryotes – also appeared about 2.4 billion years ago and by 2 billion years ago, multicellular organisms had appeared on Earth.”
Despite the mounting evidence, Condie isn’t ready to declare that the “stagnant lid” theory indisputable. “For over 15 years, scientists have known about the age gap in the geologic record,” he said. “As we dated more samples, the gap still hasn’t filled in. It still could be that the crust of this age is hiding in the Amazon jungles or under the Antarctic ice sheet. We can’t eliminate that possibility yet.”
– NMT –