A New Supercontinent
Geologic studies of bits of ancient continental crust offer a wealth of information about Earth's age and the makeup and structure of our planet during its formative years. Such studies are also
causing several leading scientists to question long-held views about early continents and plate tectonics.
Ancient Rocks
Recent decades have seen the development of new radiometric dating techniques, which have allowed scientists to more accurately pinpoint the ages of ancient rocks. During the 1970s, geologists working
in South Africa discovered rocks that were more than 3.5 billion years old. The formation is referred to as the Barberton Greenstone Belt for the distinctive green color of the rocks. Shortly after this
discovery, even older rocks were found near Isua on the southwestern coast of Greenland. Rocks there dated back to approximately 3.75 billion years. Western Australia also has rocks that are nearly as
old.
The oldest rocks found thus far, however, are located near the Acasta River, deep in the rugged backcountry of Canada's Northwest Territories. In the 1980s, geologist Samuel Bowring of the Massachusetts
Institute of Technology worked with Australian scientists to analyze gneisses from the Acasta area. Initial research indicated that the rocks were 3.96 billion years old. Additional studies conducted
throughout the following decade, however, dated some of the rocks as 4.03 billion years old. Even though older crystals have been found elsewhere, they were eroded from ancient rocks and now exist in
younger sedimentary rocks. Thus, the Acasta gneisses are officially considered the oldest rocks on Earth.
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ancient crust.
Continental Clues
The Geological Survey of Canada (GSC) has been mapping the Acasta region in an attempt to decipher its complex geology. GSC geologists theorize that the Acasta region may be a remnant of a supercontinent
that existed before Rodinia. Rodinia is widely accepted as the first supercontinent to form on Earth's surface. However, some scientists hypothesize that an earlier supercontinent formed and broke apart
one billion years before Rodinia. To support their hypothesis, the GSC geologists point out that the oldest, lowest layers of the Acasta Formation are weathered, and thus were likely exposed to surface
processes at some point. In addition, the area has an unconformity, or gap, in the rock record. The position of the ancient layer below this unconformity is a clue that the rock was not intruded later,
millions of years after the unconformity was eroded, but rather may represent the core of an ancient supercontinent.
The Role of Plate Tectonics
The GSC geologists are also questioning whether plate tectonics played the same major role in the early Archean as it does today. Many geologists assume that plates were actively moving billions of
years ago. This assumption is supported by evidence—some ancient formations exhibit signs of tectonic activity, such as sea-floor spreading.
Other geologists, however, raise doubts about whether continental plates could form in the early Archean—Earth's surface was simply too hot, they argue. This hot surface would also be buoyant and thus
unlikely to sink. Recall that sinking plates are a crucial component of plate tectonics.
The GSC geologists are not finding evidence of plate tectonics in the oldest rocks they've examined. They do not discount the theory as a whole, but they do stress that a new model may be needed to
explain the processes that were taking place when Earth's crust first formed.
Activity
As this feature shows, there are many unresolved questions in science, and scientific thought is constantly evolving as new evidence is discovered. Choose a topic in Earth science that interests you.
Research conflicting opinions about the topic. Then write a report stating which view you find most valid. Back up your opinion with scientific facts.
