Resources for a sustainable future

10/3/2010 BBC  Deforestation has revealed what could be a giant impact crater in Central Africa, scientists say.The 36-46km-widefeature, identified in DR Congo, may be one of the largest such
structures discovered in the last decade.
Italian researchers considered other origins for the ring, but say these are
unlikely.
They presented their findings at the recent Lunar and Planetary Science
Conference in Texas, US.
The ring shape is clearly visible in the satellite image by TerraMetrics Inc
reproduced on this page.
Only about terrestrial 25 impact craters are of comparable size or larger,
according to the web-based Earth Impact Database.
Giovanni Monegato, from the University of Padova, said the feature was revealed
only after trees were cleared from the area over the last decade.
The Unia River flows around the ring structure, underlining its round shape. The
central part of the Wembo-Nyama feature is irregular and about 550m in
elevation.
This is about 50-60m higher than the depression where the river flows. Although
this might sound counter-intuitive, experts say that impact craters can
sometimes lift up dense rocks. The surrounding rocks may subsequently erode,
leaving a dome.
Confirmation needed
The putative crater lacks a well-defined outer ridge, though the University of
Padova team says this could be explained by deep weathering and erosion in the
tropical climate.
They add that the drainage pattern in the ring is very similar to those found in
large impact craters in humid environments.
LARGEST IMPACT CRATERS
The Chicxulub crater is buried under Mexico’s Yucatan Peninsula
Vredefort: S Africa, 300km-wide, 2 billion years old
Sudbury: Canada, 250km-wide, 1.8 billion years old
Chicxulub: Mexico, 170km-wide, 65 million years old
Popigai: Russia, 100km-wide, 35.7 million years old
Manicouagan: Canada, 100km-wide, 214 million years old
Mr Monegato said the team would now have to travel to the region to carry out
field studies. Researchers would examine rocks from the site for tell-tale signs
associated with space impacts.
These might include shocked quartz – a form of the mineral which occurs where
rocks have been hit suddenly by a massive force. It is found only at nuclear
explosion sites and at asteroid impact sites.
Finding such evidence will be crucial to confirm an impact origin over other
processes which might explain the structure.
The researchers have considered whether volcanism or salt diapirism (a process
where evaporite minerals intrude vertically into surrounding rock, forming
dome-like structures on the surface) could be responsible for the annulus.
But Mr Monegato and his colleagues say the known geology of the region – along
with other features of the structure (for example, no known salt diapirs reach
such a great size) – appear to rule out such explanations.
“I am quite optimistic about an impact crater origin for this ring,” Mr Monegato
told BBC News.
If it is an impact structure, the scientists estimate it could have been punched
into the crust by a space rock measuring about 2km across.
Further studies will be required to accurately determine an age for the ring,
but it appears to post-date the Jurassic Period.
Go to: http://news.bbc.co.uk/go/pr/fr/-/1/hi/sci/tech/8526093.stm