How Continents Grow from Rocky Roots
Diamond-loaded rocks are now shedding light on how continents might grow from older microcontinents.
The zones where these continental pieces come together and move apart could help explain belts of seismic activity that lie within continents, such as the New Madrid seismic zone in the middle of the United States, researchers added.
Continents are usually thought to grow from young pieces of crust (the outermost layer of the planet) mashing against each other. However, a team of researchers in Australia and the United States has discovered evidence that the upper surfaces of continents have far older roots beneath them.
Scientists investigated Arkansas, which as part of south-central North America has a surface consisting of relatively young crust formed 1 billion to 1.6 billion years ago. To learn more about what might lie beneath, they focused on diamond-bearing volcanic rocks that erupted in Arkansas 100 million years ago, which carried up fragments of the upper mantle from 60 to 90 miles (100 to 150 kilometers) below the surface.
Sulfide minerals in these samples revealed that parts of the Earth's mantle between Arkansas and Louisiana formed more than 3 billion years ago. The composition of these rock fragments links them with ancient continental roots, much like those found under much of Canada .
Seismic imaging of the area under southern Arkansas and Louisiana previously showed a block of rigid rock nearly the size of Texas existing 60 to 110 miles (100 to 175 km) below the surface known as the "Sabine microcontinent." The investigators suggested the rock fragments they analyzed came from here.
"Sometimes younger houses are built upon or moved onto older foundations, and this is what has happened in the Sabine block," researcher Karl Karlstrom at the University of New Mexico told OurAmazingPlanet.
The roots of the Sabine microcontinent are buoyant compared with the rest of the upper mantle they extend into. The scientists think the older rocks of the microcontinent served as a kind of life raft for the younger rocks atop them until the latter merged with the growing North American continent .
The Sabine microcontinent perhaps "was added to North America during a plate collision about 1.6 billion to 1.5 billion years ago," Karlstrom said. "Alternatively, the Sabine block may have collided during the Appalachian convergence about 250 million to 300 million years ago to fill the place left by the rifting away of the Argentine Precordilleran block."
Re-opening old faults
These findings suggest that older microcontinents dramatically influence the evolution of continents. The boundaries of where these pieces of continents merge together "can get reactivated as zones of increased seismic hazard such as the New Madrid seismic zone," Karlstrom said. "If you have a cold or rigid block against a block of a different composition and character, stresses can build up."
"Early zones of weakness -- old faults and flaws in the architecture of the continent -- can get reactivated as new faults," he added. "That is the way continents eventually split apart. Like people, early developed faults and flaws sometimes never quite heal up."
Further research into how continents are segmented is needed to better understand their structure and evolution. Data from the EarthScope Transportable Arra y of seismic stations "will help us see what's really down there and how it may affect future tectonism," Karlstrom said.
The scientists detailed their findings online April 12 in the journal Geological Society of America Bulletin.
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