During the first few billion years of our planet's existence, Earth was practically one giant ocean. That changed when land first cropped up about 2.4 billion years ago, giving rise to what we now know as continents. While this was first thought to be a slow and steady process, a new study suggests that the birth of continents was a far more abrupt event.
In a study published in this week's issue of the journal Nature and covered in a EurekAlert news release, a team of researchers led by University of Oregon geologist Ilya Bindeman wrote about their discovery of "archival quality" traces of rainwater in close to 300 shale samples. This helped the researchers determine when recently surfaced land had started weathering, as the team also examined the changes in the ratios of various oxygen isotopes found in the shale.
Based on the analysis, it was about 2.4 billion years ago when Earth's landmass first reached about two-thirds of what's seen in current times. A report from the Daily Mail also noted that that was the time when the chemical weathering started, as the newly emerged land was first exposed to carbon dioxide.
The study also suggested that the birth of Earth's continents happened much faster than once thought, as changes in oxygen isotope levels in the shale samples matched the timing of the land collisions that helped form Kenorland, our planet's first known supercontinent. According to the Daily Mail, researchers had once thought that the process was a gradual one that happened from 1.1 billion to 3.5 billion years ago.
"Crust needs to be thick to stick out of water," Bindeman explained in a statement.
"The thickness depends on its amount and also on thermal regulation and the viscosity of the mantle. When the Earth was hot and the mantle was soft, large, tall mountains could not be supported. Our data indicates that this changed exponentially 2.4 billion years ago. The cooler mantle was able to support large swaths of land above sea level."In addition to learning more about the birth of our planet's continents, the researchers also discovered that the emergence of land from water changed Earth's albedo, or the ratio of radiation reflected by a surface. Bindeman said that Earth would have originally been "dark blue with some white clouds," had it been possible to observe it from space all those billions of years ago, but the world's first few continents helped enhance its reflection.
"Today we have dark continents because of lots of vegetation," he continued.
While the birth of continents resulted in the chemical weathering of the newly emerged land, the researchers wrote that this phenomenon might have been responsible for a series of glacial episodes that took place between 2.4 billion and 2.2 billion years ago, as carbon dioxide and other greenhouse gases "[disrupted] the radiative balance" of our planet. Bindeman noted that the emergence of large continents caused light to be bounced back into space, leading to "runaway glaciation" and what could have been Earth's first-ever snowfall.