Passage adapted from Kate Wong, “Building Earth’s Continental Crust.” ©2000 by Scientific American
Four billion years ago, Earth’s continents began taking
shape. Scientists long held that this process unfolded gradually.
It would have taken millions of years for granite, the primary
component of the continental crust, to form in the mantle and
migrate to the upper crust, they reasoned. But new research
suggests that these events may have proceeded at rather a
different pace. According to a report published in the journal
Nature, the emergence of granite occurred by way of swift,
dynamic and possibly catastrophic events lasting from 1,000 to
100,000 years.
Geologist Alexander Cruden of the University of Toronto
and his colleagues turned to experimental studies—melting
rock samples, for example—to explore how granite magma
forms and how fast it can move. Their results proved
surprising. “In the past we thought that granite magma, which
cools and crystallizes to form very large granite intrusions,
moved up through kilometers of crust as large, solid blobs at
rates of about a meter per year,” Cruden says. “But we’ve
found that magma actually has quite low viscosity and is
relatively runny. Because it is runny, it is able to channel its
way from the mantle and lower crust through fractures and
cracks that are as small as one meter thick.”
According to this model, granite intrusions in Greenland or
the Canadian Shield, depending on their size, would have taken
only thousands of years to form, which is extraordinarily fast
from a geological point of view, Cruden notes.
Four billion years ago, Earth’s continents began taking
shape. Scientists long held that this process unfolded gradually.
It would have taken millions of years for granite, the primary
component of the continental crust, to form in the mantle and
migrate to the upper crust, they reasoned. But new research
suggests that these events may have proceeded at rather a
different pace. According to a report published in the journal
Nature, the emergence of granite occurred by way of swift,
dynamic and possibly catastrophic events lasting from 1,000 to
100,000 years.
Geologist Alexander Cruden of the University of Toronto
and his colleagues turned to experimental studies—melting
rock samples, for example—to explore how granite magma
forms and how fast it can move. Their results proved
surprising. “In the past we thought that granite magma, which
cools and crystallizes to form very large granite intrusions,
moved up through kilometers of crust as large, solid blobs at
rates of about a meter per year,” Cruden says. “But we’ve
found that magma actually has quite low viscosity and is
relatively runny. Because it is runny, it is able to channel its
way from the mantle and lower crust through fractures and
cracks that are as small as one meter thick.”
According to this model, granite intrusions in Greenland or
the Canadian Shield, depending on their size, would have taken
only thousands of years to form, which is extraordinarily fast
from a geological point of view, Cruden notes.