December 17, 2000
Earth's continental land masses created in short, fast bursts
Findings challenge multi-million year continental crust theory

by Janet Wong

Dec. 6, 2000 -- Scientists believe they have unraveled one of geology's
most enduring mysteries about how the Earth's continental crust was built,
and they say it happened in a relative blink of an eye.

According to Alexander Cruden, associate professor of geology at the
University of Toronto and second author of the paper to appear in the Dec.
6 issue of Nature, the way that granite forms - a rock that makes up about
70 to 80 per cent of the Earth's continental crust - is not the sluggish,
multi-million year process that scientists previously believed. In fact,
Cruden and his co-authors argue that the process occurs in rapid, dynamic
and possibly catastrophic events that take between 1,000 and 100,000
years, depending on the size of the granite intrusion. And that's changing how
scientists look at the formation of the Earth's continents.

Cruden conducted the research with Nick Petford of Kingston University and
Ken events this week  McCaffrey of the University of Durham, both in England,
and Jean-Louis Vigneresse of events by day  the Centre National de la
Recherche Scientifique in Nancy, France.

"In the past, we thought that granite magma - which cools and crystallizes
to form very large granite intrusions - moved up through kilometres of
crust as large, solid blobs at rates of about a metre per year. Because
the continental crust is largely made up of these intrusions, the prevailing
view was that the continents grew slowly and steadily over millions of
years. But we've found that magma actually has quite low viscosity and is
relatively runny," says Cruden. "Because it is runny, it's able to channel
its way from the mantle and lower crust through fractures and cracks that
are as small as one metre thick. This way, the magma can ascend 20 to 30
kilometres into the upper crust quite rapidly."
 

                     A cliff on a mountainside in Greenland. The cliff is
                     about 800 m high (i.e. two CN Towers) and is made up
                     of granite (brown rocks) and "rafts" of older rock
                     that were intruded by this granite (grey rocks).
                     Photo: John Grocott, Kingston University, England

Therefore, says Cruden, a 50 km wide intrusion of granite, in say
Greenland or the Canadian Shield, that geologists would have once estimated to have
taken millions of years to form may have actually taken only 50,000 years.
Smaller intrusions that are 10 km across may form in as little as 1,000
years. And from a geological point of view, that's extraordinarily fast,
he adds.

The researchers used experimental studies that involved melting rock
samples to understand how granite magma initially forms in the upper mantle and lower
crust and how fast it can move. That data was then applied to theoretical
models to determine its method and rate of ascension. New models for the
emplacement stage - where the granite is intruded into older rock in the
upper crust - are based on a combination of theoretical studies and
fieldwork in areas such as the Canadian Shield, Sweden, the Sierra Nevada
of California, Greenland and the Andes of South America. A unique aspect
of the research is that the three main stages of granite formation -
generation, ascent and emplacement - are regarded together as a system.
Historically, these processes have been studied by different geological
specialists in isolation from each other.

Cruden likens the granite formation process to subterranean volcanic
eruptions. Like Lego blocks built on top of one another, large parts of
the Earth's continental land masses were created by tens of thousands of quick
eruptions or bursts of molten magma that were transferred rapidly from the
mantle and lower-most crust and then injected as large horizontal sheets
into the upper crust. These sheets then cooled and crystallized to form
the large granite intrusions that we see exposed at the surface of all
continents today, he says.

The Earth's continents began forming approximately four billion years ago,
Cruden explains. "This research has important implications for how we
understand the basic physics and chemistry of crust formation processes as
well as the formation of economic ore deposits - gold and copper, for
example - many of which are associated with granite intrusions."

This study was funded by Natural Environmental Research Council of the
United Kingdom, the Natural Sciences and Engineering Research Council of
Canada and the Centre National de la Recherche Scientifique of France.

Janet Wong is a news services officer with the Department of Public
Affairs.

CONTACT:
Alexander Cruden, U of T Department of Geology, ph: (416) 946-7284; email:
cruden@geology.utoronto.ca
U of T Public Affairs, ph: (416) 978-6974; email: jf.wong@utoronto.ca

Oryginal: http://www.newsandevents.utoronto.ca/bin1/001206a.asp

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