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1 posted on 03/18/2003 9:33:33 AM PST by blam
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To: blam
Source: University Of Arizona
Date: 2003-03-18

Worried About Asteroid-Ocean Impacts? Don't Sweat The Small Stuff

The idea that even small asteroids can create hazardous tsunamis may at last be pretty well washed up.

Small asteroids do not make great ocean waves that will devastate coastal areas for miles inland, according to both a recently released 1968 U.S. Naval Research report on explosion-generated tsunamis and terrestrial evidence.

University of Arizona planetary scientist H. Jay Melosh is talking about it today (March 17) at the 34th annual Lunar and Planetary Science Conference in League City, Texas. His talk, "Impact-Generated Tsunamis: an Over-Rated Hazard," is part of the session, "Poking Holes: Terrestrial Impacts."

Given all life's worries, new evidence that asteroids smaller than a kilometer in diameter won't generate catastrophic tsunamis is welcome news, and not only for coast dwellers. It will save taxpayers the cost of financing searches for small Earth-approaching asteroids, a savings of billions of dollars, Melosh said.

(The current NASA-funded effort to search and map truly hazardous Earth-approaching asteroids -- those one kilometer or larger in diameter -- is now half done and on track to be finished by the end of the decade, Melosh noted. NASA funds NEAT, LINEAR and the UA Spacewatch programs in this effort.)

The idea that asteroids as small as 100 meters across pose a serious threat to humanity because they create great, destructive ocean waves, or tsunamis, every few hundred years was suggested in 1993 at a UA-hosted asteroids hazards meeting in Tucson.

At that meeting, a distinguished Leiden Observatory astrophysicist named J. Mayo Greenberg, who since has died, countered that people living below sea level in the Netherlands for the past millennium had not experienced such tsunamis every 250 years as the theory predicted, Melosh noted.

But scientists at the time either didn't follow up or they didn't listen, Melosh added.

While on sabbatical in Amsterdam in 1996, Melosh checked with Dutch geologists who had drilled to basement rock in the Rhine River delta, a geologic record of the past 10,000 years. That record shows only one large tsunami at 7,000 years ago, the Dutch scientists said, but it coincides perfectly in time to a giant landslide off the coast of Norway and is not the result of an asteroid-ocean impact.

In addition, Melosh was highly skeptical of estimates that project small asteroids will generate waves that grow to a thousand meters or higher in a 4,000-meter deep ocean.

Concerned that such doubtful information was -- and is -- being used to justify proposed science projects, Melosh has argued that the hazard of small asteroid-ocean impacts is greatly exaggerated.

Melosh mentioned it at a seminar he gave at the Scripps Institution of Oceanography a few years ago, which is where he met tsunami expert William Van Dorn.

Van Dorn, who lives in San Diego, had been commissioned in 1968 by the U.S. Office of Naval Research to summarize several decades of research into the hazard posed by waves generated by nuclear explosions. The research included 1965-66 experiments that measured wave run-up from blasts of up to 10,000 pounds of TNT in Mono Lake, Calif.

The experiments indeed proved that wave run-up from explosion waves produced either by bombs or bolides (meteors) is much smaller relative to run-up of tsunami waves, Van Dorn said in the report. "As most of the energy is dissipated before the waves reach the shoreline, it is evident that no catastrophe of damage by flooding can result from explosion waves as initially feared," he concluded.

The discovery that explosion waves or large impact-generated waves will break on the outer continental shelf and produce little onshore damage is a phenomenon known in the defense community as the "Van Dorn effect."

But Van Dorn was not authorized to release his 173-page report when he and Melosh met in 1995.

Melosh, UA planetary sciences alumnus Bill Bottke of the Southwest Research Institute and others agreed at a science conference last September that they needed to find the report.

Bottke found the title - "Handbook of Explosion-Generated Water Waves" - in a Google search.

Given a title, UA science librarian Lori Critz then discovered that the report had been published and added to the University California San Diego library collection in March 2002. Bottke also tracked it down, and had the report by the time Melosh requested it by interlibrary loan. Both made several photocopies.

Melosh said, "I since found out it was actually read into the Congressional Record as part of the MX Missile controversy."

BIOSKETCH: H. JAY MELOSH

Melosh, a professor in the UA planetary sciences department and Lunar and Planetary Laboratory, is well known for his work in theoretical geophysics and planetary surfaces. His principal research interests are impact cratering, planetary tectonics, and the physics of earthquakes and landslides. His recent research has focused on studies of the giant impact origin of the moon, the K/T boundary impact that extinguished the dinosaurs, the ejection of rocks from their parent bodies, and the breakup and collision of comet Shoemaker-Levy 9 with Jupiter. Melosh also is active in astrobiological studies that relate chiefly to the exchange of microorganisms between the terrestrial planets.

Melosh earned his doctorate from Caltech in 1973 and joined the UA faculty in 1982. He is on the 12-member science team for Deep Impact, a $279 million robotic mission that will become the first to penetrate the surface of a comet when it smashes its camera-carrying copper probe into Comet Tempel 1 on July 4, 2005.

2 posted on 03/18/2003 9:59:20 AM PST by blam
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To: blam

Deep Impact - The Vredefort Dome

One hundred kilometers south west of Johannesburg are the towns of Vredefort and Parys, the latter lying next to the Vaal river. The towns are a short deviation from the N1 highway to Cape Town. In an arc to the north and west of the towns is a partial ring of hills, some 70 kilometres in diameter. The hills are centred on Vredefort, which is shown by the letter "V" in the photograph below, taken from the Space Shuttle. The ring of hills and the area within them is known as the "Vredefort Dome".


NASA Johnson Space Center (Image STS008-35-1296)

From high altitude these hills bear a strong resemblance to the larger, near-circular ring of hills north-east of Vredefort, whose northern side, the Magaliesberg hills, cuts through Pretoria (letter "P") and whose southern side, the "Witwatersrand" ridge, cuts through Johannesburg (letter "J"). These surround the uplifted area known as the "Johannesburg Dome". The white dots running in a band along the Witwatersrand are not clouds, they are the waste dumps from the gold mines along the south side of the Witwatersrand. West of Johannesburg and due north of Vredefort can be seen the gold mine dumps of Carletonville. The gold mines of Klerksdorp and Welkom lie north-west and south-west of Vredefort, and their dumps are also easily seen. Is this half ring of gold mines around Vredefort a coincidence?


NASA Johnson Space Center (Image STS51I-33-56AA)

Pictured above is a higher resolution view of the Vredefort hills from space. The Vaal river meanders through the hills, flowing from north-east (upper left) to west (centre right). Parys (the Paris of South Africa!) lies on the loop of the river protruding within the ring of hills. Vredefort is close to the geometric center of the hills.

Only the north western half of the structure remains visible. The south eastern half was flooded by the sediments of the Karoo Supergroup, which cover the Free State province.

What Created the Vredefort Dome?

The Magaliesberg - Witwatersrand feature is the result of natural upliftment from below of sedimentary sandstone layers - what was once the bottom of a sea - so that the Magaliesberg rock layers slope down to the north, while the Witwatersrand rock layers slope down to the south. The "Johannesburg Dome" in the center this feature is occupied by the archean granitic crust, some 3000 to 3400 million years old.

The same rock layers seen in the Witwatersrand are found in the Vredefort dome hills, but here they are found standing nearly vertically - the result of extreme upliftment.

Evidence has been found by geologists that the cause of this upliftment was an extreme impact event, caused by an asteroid some 10 kilometres in diameter. The ring of hills we see now are the eroded remains of a dome created by the rebound of the rock below the impact site after the asteroid hit. The original crater, now eroded away, is estimated to have been 250 - 300 kilometres in diameter. Some 70 cubic kilometres of rock would have been vaporised in the impact.

The Vredefort structure is currently regarded the biggest and oldest clearly visible impact structure on Earth. It just beats the Sudbury impact structure in Canada for this ranking. The Sudbury structure is some 200 km in diameter and is estimated to be 1.85 billion years old.

Evidence for four impacts even older than than Vredefort, that occurred 3.2 to 3.5 billion years ago, has been found in the greenstone rocks around Barberton in South Africa and corresponding rocks in the eastern Pilbara block of Western Australia. However, these impacts are no longer recognizable as structures on the surface like Vredefort's.


NASA Lunar Orbiter image

The central mountains in the 85 kilometre diameter lunar crater Tycho show what the Vredefort dome is the remains of - the central rebound peak, NOT the crater itself, which at Vredefort has long since eroded away. The erosion processes occuring on Earth are of course missing on the Moon, hence the "as new" appearance of Tycho.

What is the evidence for the impact at Vredefort?
The pictures below tell some of the story.

The Earth Heaved and Melted...

Within the ring of hills at Vredefort is found granitic gneiss rock - as in the Johannesburg Dome - which is dated at some 3200 million years old. Here a quarry at Leeuwkop near Parys reveals one of the indicators of the impact. The image above left shows the cut surface of the rock. The pale and medium greys on the left are the natural colours of the granite. But from the centre to the right edge is a wide dark grey band containing large and small fragments of the granite. This is "pseudotachylitic brecchia" - the dark grey is granite that was melted by the impact and flowed, carrying chunks of unmelted granite within the melt. The vertical face seen here is two to three metres in height.

The image above right is of a nearly horizontal exposure of pseudotachylite at the quarry. The width of the band is about one metre. Similar rock is found at elsewhere, in geological faults where rocks move against each other and melt at the interface, but this produces a melt band that is centimetres rather than metres wide as seen here.

The basement granite exposed within the ring of hills is estimated to have been seven to ten kilometres beneath the surface when the impact occurred, which is dated at 2000 million years ago. The date was established from zircon crystals found in the pseudotachylite and granophyre (below), and, more precisely, is 2023 +- 5 million years. All the covering rock has since been removed by erosion. Later the south-east part was covered by much younger Karroo rock formations.

...and Flowed and Solidified

The force of the impact produced deep fractures in the underlying rock. Rock melted by the impact flowed down into the cracks, producing what are now exposed as ridges of hard dark rock - the granophyre dykes. This contrasts with normal geological dykes, where molten rock from deeper in the earth has flowed upwards through cracks in the rock above.

The picture above left shows one of the dykes located close to the centre of the impact, at Daskop. Its width at the surface is about three metres, but it is hundreds of metres long where it is exposed here.

The boulder in the dyke shown above right shows the small, fractured inclusions of unmelted rock that are typically 1 - 3 cm in size. This is much smaller than the rocks embedded in the pseudotachylite at the quarry, which are up to metres in size. The small inclusions may have come from considerable distances - tens of kilometres away.

These rocks have also been the target of graffiti artists - in this case San hunters perhaps 2000 years ago. Shown above left is an engraved hippopotamus.

Pictured above right is an equally accurate engraving of a rhinoceros. Less obvious is the eland, the largest antelope, engraved inverted (from this perspective) in front of the rhino. It shows up more clearly in the large version of this image.

The Shatter Cones

Another signature of an impact is left by the passage of shockwaves through the underlying rock. This produces fractures with a characteristic "fir tree" pattern. In this particularly clear example found next to the Vaal river at Schoemansdrif, the shock wave passed through from upper left to lower right.

The Hills uplifted around the Dome

The hills surrounding the granite dome reproduce the layered rock formations seen in Johannesburg (the Witwatersrand). The Vaal river flows westwards through gaps in the hills.

The view above left is a panorama over the Vaal river. It was taken facing west, at Schurwedraai. The next ring of hills forms the horizon.

The rocks at the site where the photograph of the river was taken at Schurwedraai are quite different to the basement granite, but also show dark grey veins of pseudotachylite - melted rock. The pen gives the scale in the picture on the right.

This ring of hills comprises quartz conglomerates as found in the gold-bearing strata of the Witwatersrand reefs. The white quartz pebbles are evident. This was once the bed of a fast flowing water course which deposited grains of quartz and the pebbles. This area was mined for gold in the 1880's. However the concentration of gold was much poorer than at Johannesburg, and the diggings were soon abandoned. Old mine adits are still to be seen in the hills. This is the Amazon Reef.

The outermost ring of hills was home to a quite different group of people in the 1500's to 1700's. These were SeSotho/SeTswana-speaking farmers. This village at Askoppies was a defensive position on the crest of the hill, but it did not save the village from destruction, by the warriors of Mzilikazi. The view shown above left looks east, back in towards the inner rings of the Vredefort dome.

The stone walls of the village are shown above right. They are made of the fine-grained grey Ventersdorp lavas that comprise this ridge. These rocks are 2700 million years old.

Link to the original page.

5 posted on 03/18/2003 11:33:02 AM PST by farmfriend ( Isaiah 55:10,11)
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To: blam; 75thOVI; AndrewC; Avoiding_Sulla; BenLurkin; Berosus; CGVet58; chilepepper; ckilmer; ...
Catastrophism

7 posted on 06/24/2006 6:15:08 PM PDT by SunkenCiv (updated my FR profile on Wednesday, June 21, 2006.)
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To: blam
Careful. With a headline like that it won't be long before the more randy FReepers start posting pics of Grace Park.
8 posted on 06/25/2006 7:25:56 AM PDT by BenLurkin ("The entire remedy is with the people." - W. H. Harrison)
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· Catastrophism ping list · join · view topics · view or post blog · bookmark ·

9 posted on 01/01/2007 9:16:33 PM PST by SunkenCiv (Ahmedumbass and the mullahcracy is doomed. https://secure.freerepublic.com/donate/)
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