Did Tectonics Get an Early Start?
A recent discovery near the Great Wall in China adds new support to the theory that plate tectonics began very early in the Earth’s history. The finding not only will help in understanding the geological processes of ancient Earth, but it could also provide some clues about the development of early life.
It has long been known that plate tectonics – the motion of oceanic and continental plates – dates back at least 1.9 billion years. But Timothy Kusky, professor of geology at St. Louis University, believes the plates began moving much earlier. While in China last summer, Kusky discovered the oldest complete section of sea floor on the planet. This section of sea floor is 2.5 billion years old, more than 500 million years older than any previously discovered sea floor sample. These rocks date back to the Archean, a period of time when the Earth’s continental plates first began to form.
|The Earth’s tectonic plates. The discovery of ancient ophiolites lends support to the theory that plate tectonics operated during the Archaen.
Credit: UC Berkeley
For decades, geologists have debated whether plate tectonics operated during the Archean. Those who have argued against that theory have cited the lack of any Archean ophiolites as their main line of evidence. Ophiolites are rock structures that form at oceanic spreading ridges. These areas on the sea floor are where oceanic plates are forced apart by the addition of new magma from below. These regions are mixing zones where lava and fluids from the Earth’s crust and mantle interact with seawater. The rock structure that results from this mixture is called an ophiolite sequence.
The ancient rocks discovered by Kusky in Dongwanzi, China have a structure very similar to that of much younger volcanic rocks created through sea floor spreading.
"This discovery shows that the plate tectonic forces that create oceanic crust on the Earth today were in operation more than 2.5 billion years ago," says Kusky. "The rocks are similar in that the entire sequence includes the same sequence of rocks found in younger oceanic crust and mantle, although one significant difference is that the ophiolite sequence appears thicker in the Archean Dongwanzi example. More work needs to be done on this preliminary conclusion, including more field mapping."
The research, a collaborative effort involving Peking University and Washington University in St. Louis researchers, was published in the May 11, 2001, issue of the journal Science. The dating process is described in detail in the Science paper.
"The rocks were dated by U/Pb [uranium/lead] geochronology on zircon, the most accurate method for dating rocks of this age," says Kusky. Uranium/lead geochronology looks at how much uranium-238 in a rock has decayed into lead 206. It takes about 4.5 billion years for half of any quantity of U-238 atoms to decompose. The ratio of lead to uranium atoms in a rock sample can indicate how long ago the rock crystallized.
|Tim Kusky measuring and sampling an outcrop of cumulate ultramafic rocks near the village of Gushanzi in the Dongwanzi ophiolite
Credit: Tim Kusky
Kusky dated the rocks with the assistance of Robert Tucker, associate professor of earth and planetary science at Washington University in St. Louis.
Stephen Mojzsis, assistant professor of geological sciences at the University of Colorado at Boulder and member of NASA’s Astrobiology Institute, says this is an interesting finding that confirms what had been suspected of the Earth’s tectonic activity.
"It has long been assumed that plate tectonics was operative in the Archean," says Mojzsis. "This work provides some more direct evidence of that."
Kusky and Dr. Jiang-Hai Li of Peking University in Beijing made the discovery while they were investigating geological structures located a few miles away from the Great Wall of China.
"I was in China giving a lecture series at Peking University on the early history of the Earth," says Kusky, "and then we planned two weeks of field work examining the many varied geological units of the North China craton [a portion of continental crust that has been stable for a long period of time]. On the third day of the trip I came across an outcrop of sheeted dikes in a greenstone belt, and decided that it was more important to make a field map of the geological relationships around the sheeted dikes than to continue our trip around North China."
Kusky says that the rocks are exposed, so excavation was not necessary. They only had to hammer fresh samples off the mountain slopes and other outcrops.
"My intent was at first to obtain a better understanding of the geology of the North China craton, but after discovering the Archean ophiolite, my intent was to document its relationships as completely as possible, and to collect a representative suite of samples for geochronology, geochemistry, and to search for signs of early life in the sulfides and chert units," says Kusky.
Kusky believes the finding could have far-reaching implications for theories about the development of complex life on Earth. Many scientists believe life during the Archean period consisted mainly of single-celled organisms in the oceans. Just when they evolved into more complex organisms has been contested for years.
|Hydrothermal Vent. Life may have developed and diversived around hydrothermal vents as plate tectonics began.
"Because hot volcanic vents on the sea floor may have provided the nutrients and temperatures needed for life to flourish and develop, it’s possible that life developed and diversified around these vents as plate tectonics began," Kusky said.
Kusky says he plans to investigate the ancient rock for signs of fossilized life.
"We have initiated a study of sulfides, the cherts, and related rocks and we are currently looking at the sulfur isotopes and searching for any other traces or isotopic signatures of early life," says Kusky. "If we find evidence for ancient life in these rocks it could prove important for our understanding of how and where life diversified on Earth, as we can fairly certainly say that we are dealing with sea floor hydrothermal vents in this case, and we know the age of the samples are [approximately] 2505 million years old. This time period is critical in the evolution of life, and our work may lead to being able to better calibrate the evolutionary clock, including perhaps helping bracket some of the divergences between various eukaryotes, archaebacteria, eubacteria, and anaerobic prokaryotic ancestors."
According to Mojzsis, the discovery of the 2.5 billion-year-old ophiolites can also tell us something about the Archean environment, as well as the development of the continents.
"For the period 2.5 billion years ago, this was the crucial time in which atmospheric oxygen levels were increasing and changing the chemistry of the surface environments of the Earth," says Mojzsis. "The discovery of a 2.5 billion-year-old ophiolite might provide us with much-needed information about crustal heat-flow during the Archean. The Earth loses most of its internal heat through hotspots and by seafloor spreading. Seafloor spreading ultimately results in [tectonic] plate movements. It is generally thought that this is the primary mechanism for the generation of the kinds of crusts that eventually form the continents."
Kusky and his team are in the process of submitting several proposals to the U.S. National Science Foundation to characterize and study the petrological, chemical, and structural aspects of the Dongwanzi ophiolite. He is also submitting proposals to NASA to investigate and interpret the signs of life in the rocks, and to determine the implications for the evolution of early life on earth.
"There is a lot of work yet to be done on these rocks," says Kusky. "We anticipate spending many years with various colleagues working on the many different aspects of this remarkably well preserved window into the early Earth."