Evidence for what may be a large and relatively recent impact crater has been found off the coast of Antarctica.
Researchers from the Lamont-Doherty Earth Observatory in New York have been studying a 100km-wide depression, known as Bowers Crater, under the Ross Sea.
Team members examined cores drilled from around the area to look for evidence of an impact.
In the cores, they found microscopic glassy grains shaped like teardrops, spheres and dumbbells which are collectively known as tektites.
The evidence points to a space rock some 5km across having crashed into the Ross Sea about three million years ago.

Source


Also pertinent to the impact, (but with a better time correlation with Australites (780,000 years ago),

“An asteroid between 5 - 11 km across had broken up in the atmosphere and five large pieces had hit the Earth, creating multiple craters over an scatter ellipse area in Antarctica.
Scatter ellipses such as this accompany all such multiple impact sites, except that the Antarctic ellipse is the largest known on Earth. Of the five new impact craters, three of them are on the continental land mass and two more are in the Weddell Sea. The largest of these craters is about 322km by 322km.
The Antarctic scatter ellipse is of enormous size by Earth standards, measuring some 2,092 kilometres by 3,862 kilometres. Melted rocky debris, blasted from such meteoroid craters upon impact and explosion, and known as tektites, may have been carried thousands of kilometres from the impact site. Such tektites, called australites, are found in large strewn fields in Australia some 5600 kilometres from the largest proposed impact sites in the Ross Sea region.

The impacts occurred roughly 780,000 years ago during an ice age. When the impacts hit, they would have melted through the ice and through the crust below.”

Read more (PDF)

Another Weblink:

Weblink:

I wont mention Darwins crater, in Tasmania...

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Very interesting. This part of the article cracked me up:
oh, if only we had a better name for "ten-millionths of a metre" what a wonderful world it would be.

Somehow hominids managed to survive the impact of a 5-11km asteroid!

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Not surprised this hit was limited in effect being far from equator. But anomalies like the rise of the Altaplano bordering Bolivia/Peru may be related to impact more than a pole reversal as Professor hypothesizes.

@Ara Pacis
Hum,
These impacts occurred roughly 780,000 years ago during an ice age.
The extinctions that happened then, would be difficult to pin on either cause.

While it is probably true that the asteroid was the same size as the one blamed for killing off the dinosaurs 65 million years ago, the impact region was totally different.
The impacts would reflect the chemical content of the rocks around the impact craters.
The 780000 event seared a hole in the ice sheet roughly 322km by 322km that would have melted about 1% of the ice sheet and raised water levels by 60cm. The KT event happened in a shallow sea that was underlain by large sulphur containing deposits.

But of course there is the assumption that these big impacts, such as the Chicxulub impact, are a big hazard for life on the planet; the real culprit may have been volcanism...

(interestingly, new research has shown there was not a decline in dinosaur numbers before the KT event)

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Title: Source of the Australasian tektite strewn field - A possible off-shore impact site
Authors: Schnetzler, C. C.; Walter, L. S.; Marsh, J. G.

Although there is a preponderance of evidence that tektites were formed by asteroid impacts on the earth, no source crater has been found for the largest and youngest of the strewn fields - the Australasian strewn field. A combined Seasat/Geos 3 altimeter data set of sea surface heights in the northern portion of the Australasian strewn field has been examined for negative gravity anomalies on the continental shelf and slope which might be related to the source crater for these tektites. A large negative anomaly called the Qui Nhon Slope Anomaly is a sea surface depression of approximately 1.5 meters over an area of 100 km diameter. It corresponds to a gravity anomaly of about -50 mgal. It is proposed that this anomaly may be due to the impact structure that produced the Australasian strewn field.

Source

<Attachment> (27kb, 804 x 566)
Location map overlain with gravity anomaly map

Latitude:13.78 Longitude: 110.62


The Tonle Sap, 35x100 km, structure was proposed as the origin of the tektites. The structure is associated with -3 to -4 mgal gravity anomaly.
Possibly the same as Qui Nhon.
Latitude:12.80 Longitude: 104.12

References:
Moilanen J. (2004) List of probable and possible impact structures of the World. 29 October 2004.
Source (829kb, XLS)


Title: Occurrence, distribution, and age of Australian tektites
Authors: Chalmers, R. O.; Henderson, E. P.; Mason, B.

Geographic distribution, relative abundance, physical properties, and chemical composition of australites (Australian tektites) were studied. Tektites were found in late Pleistocene or early Recent sandy aeolianite; the presence of delicate surface features on many tektites suggests that these tektites have not been subjected to terrestrial forces which would have relocated them. The stratigraphic findings indicate an age of 7000 to 20,000 years before the present, although K-Ar and fission track data are consistent with an age of about 700,000 years. Distribution within the strewn-fall is irregular and is attributed to original nonuniform fall, burial by recent deposition, or removal by erosion. Variations in chemical composition and size with respect to geographic distribution are also discussed.

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