The Impact Cratering Process

[1dave]

H. J. Melosh broke the Impact Cratering Process into three parts:
Contact and Compression; Expansion and Excavation; and Modification.




Of primary importance is the composition, mass, and velocity of the projectile, and the composition of the target.

Water, ice, sedimentary rock, porous rock, crystalline rock, or iron; or some combination in either body alters possible outcomes.



The angle of impact is another important variable. The atmosphere and curvature of the earth assure that few impacts will be at zero or ninety degrees.

Impact angles less than 6̊ will either make a long trench or ricochet.

Angles less than 10̊ will produce elliptical craters

Angles above 10̊ will produce circular craters.

Angles below 45̊ (the most probable impact angle) tend to produce major down range jets containing large portions of the projectile.
If the projectile contained considerable amounts of precious metals, This is where you want to search!

The more direct the impact, the more the projectile energy is transferred to the target.



I.. Contact, Shock, Deformation, Compression







Shock waves travel much faster than the projectile. As the protagonists deform and compress, water is compressed into ever denser forms of ice, quartz is squashed into shistovite, rock is pressure-flaked into shatter cones. Sedimentary layers may be separated.

"Shatter cones are conical fracture patterns developed during the passage of a shock wave through rock (Dietz, 1960). The surface of the cones has distinctive striations which radiate from small parasitic horsetail-like half cones on the face of larger cones. The diverging pattern of the striations is distinct from the parallel grooves formed on slickensides. After making appropriate corrections for post-impact deformation, shatter cones have been shown to point toward "ground zero."
. . . "They occur solely in sandstones and underlying granitic basement gneiss."
- The Beaverhead Impact Structure, SW Montana and Idaho:
Implications for the Regional Geology of the Western U.S.
Peter S. Fiske and Robert B. Hargaves. 1995

This stage ends with the top of the projectile about at ground level, in an area about the volume of the original projectile, with most of the projectile’s energy transferred to the target. The underlying rocks are heated, compressed, and ready to be accelerated to high speed.

Interaction of harmonics augment and cancel each other, crushing here, melting there, vaporizing in some places, and leaving others untouched. Both materials may either melt or vaporize upon unloading from such pressures. The greater the energy, the more all moves toward ionization..

The contact and compression stage is the shortest of the three stages and ends after the projectile has unloaded from high pressure. The duration is longer in oblique impacts, but It lasts for as long as a second only with the very largest of impacts.

II. Expansion Excavation, Ejecta Deposits.
This stage begins almost at the moment of impact.



Molten jets of material spurt up and down range at highest speed, then slow.
The projectile shockwave reaches its rear surface and reflects as a release wave, unloading the projectile, expanding it into the molten or vapor stage that leaves the crater as a plume.



Deep compressed minerals in the target may require thousands of years to unload and rebound, but most expand rapidly, like the ice that instantly turns to steam and expands to over 2,000 times its original volume.
These pressures set particles in motion that rapidly open the crater that becomes many times larger in diameter than the projectile. A mixture of projectile and target may be jammed into openings between sedimentary layers.

Excavation tubes develop, like ocean rip tides, that send impact "rays" away from the crater, often in a "butterfly pattern" as was found from the 1908 Russian tree blow down. The ground above the edges may be flipped over, leaving the crater rims tipped up, surrounded by huge tipped back boulders.





The ejecta curtain rises and begins to blanket the surrounding area.



The excavation stage lasts much longer than the first stage, taking seconds or minutes to reach completion, depending upon the crater size.

III. Modification
Modification begins after the crater is fully excavated. During modification, the bowl shaped walls of the excavation “transient crater” generally collapse under gravity. Loose debris slides down the steep walls to the floor of small craters. Slump terraces form on the walls of large craters and central peaks or rings rise in the interiors.



Isostatic rebound may eventually lift and level the crater floors.



Crater Scaling involves major mathematics computations to decide what multiplier to use to predict what size projectile to use to get what size crater. Let the computer do it for you at:

www.purdue.edu/impactearth

[1dave]

The diameter numbers by impact craters don't sound like much, but . . .


13 meters Carancas, Peru


0.89KmNordingen, Germany
(EDIT: Not the 25Km I was first told.)


1 km Barringer Crater, Arizona


10 Km Upheaval Dome west of Moab, Utah



40Km Chesapeake Bay

[1dave]

Zingers from Space

I ran a simulation at:
www.purdue.edu/impactearth

for an object 50 Km diameter, with a density of sedimentary rock (3,000 Kg/m-cubed) hitting into sedimentary rock at 72 Km/ second. (the highest speed they will allow)

Energy:

Energy before atmospheric entry: 5.09 x 10^26 Joules = 1.22 x 10^11 MegaTons TNT

The average interval between impacts of this size is longer than the Earth's age.
Such impacts could only occur during the accumulation of the Earth, between 4.5 and 4 billion years ago.

Crater Dimensions:

Transient Crater Diameter: 422 km ( = 262 miles )
Transient Crater Depth: 149 km ( = 92.7 miles )
Final Crater Diameter: 932 km ( = 579 miles )
Final Crater Depth: 2.32 km ( = 1.44 miles )
The crater formed is a complex crater.
The volume of the target melted or vaporized is 3200000 km^3 ( = 768000 miles^3 )
Roughly half the melt remains in the crater, where its average thickness is 22.9 km ( = 14.2 miles ).

Ejecta 4,000 Km away:

The ejecta will arrive approximately 21.5 minutes after the impact.
At your position there is a fine dusting of ejecta with occasional larger fragments.
Average Ejecta Thickness: 4.43 meters ( = 14.5 feet )
Mean Fragment Diameter: 383 microns ( = 15.1 thousandths of an inch )

Global Damage:

The Earth is not strongly disturbed by the impact and loses negligible mass.
The impact does not make a noticeable change in the tilt of Earth's axis (< 5 hundredths of a degree).
Depending on the direction and location of impact, the collision may cause a change in the length of the day of up to 781 milliseconds.
The impact does not shift the Earth's orbit noticeably.

There are some very interesting FACTS above!

I have some REAL PROBLEMS with their GUESSES.

The average interval between impacts of this size is longer than the Earth's age.
Such impacts could only occur during the accumulation of the Earth, between 4.5 and 4 billion years ago.

1. There are larger projectiles sailing around out there at those kinds of speeds. It is pure luck that we haven't been hit by one recently.
Hale-Bopp is 19 to 25 miles (30 to 40 kilometers) in diameter.
When it was at its closest point to the Sum, Comet Hale-Bopp was moving at 156,000km/h (almost 100,000mph) with respect to the Solar System.
Other comets are moving even faster: if a body approaches closer to the Sun, the additional gravitational pull will give it a larger speed. For instance, comet Hyakutake passed very close to the Earth, moving at 282 000km/h (175 000mph) at perihelion.
2. The universe is 13.7 billion years old, the Milky Way Galaxy is about 13.2 Billion years old. Scientists have discovered galaxies average one supernova about every hundred years, so we are surrounded by around 132 million supernova remnants.
3. There is nothing to indicate that the number of supernovas is slowing down. Each supernova sends billions of cometary knots in all directions. The galactic number of comets must be increasing instead of decreasing.

4. The unthinkable:
We are just beginning to realize that most stars have planetary systems.
Supernova ejecta travels at about 1/4 the speed of light.

Chunks of those planets must be rocketing through space in all directions at around a THOUSAND times that speed, 75,000 Km/sec or 46,500 Miles per second!

What happens when THEY collide with planets?

[1dave]

Jan 5, 2015 5:41:36 GMT -7 @snowmom said:

www.impact-structures.com/the-controversy-the-spanish-impact-structures-and-competing-models-of-an-endogenetic-origin/
how far, how fast... once, the world was flat. Now we know more.

The controversy: The Spanish impact structures and competing models of an endogenetic origin
Introduction

At the beginning of the sixties, scientists opened the discussion about meteorite impact as a remarkable geologic process on Earth and other solid planets and their satellites, and at the end of the seventies, the late Eugene Shoemaker characterized impact as the possibly most important geologic process in our planetary system. The discussions at that time are well reflected by the books: Shock Metamorphism of Natural Materials (French, B.M. and Short, N.M., eds.), Mono Book Corp., Baltimore, 1966 and Impact and Explosion Cratering (D.J. Roddy, R.O. Pepin, R.B. Merrill, eds.), Pergamon Press, 1977.

On the other hand, many geoscientists, mainly from geology, strictly refused acceptance and considered impact structures rather obscure objects. In the 1953 vol. 37 of the respected Bull. Am. Assoc. Petrol. Geol., the geologist D. Hager published a 30 pp. paper on the Barringer Meteorite Crater in Arizona. In this article, the generally accepted impact crater is again regarded as an endogenetic structure and an “explosion” origin denied. Instead, a graben-like sinkhole formed by evaporite dissolution is postulated – a common feature in the regional geology there. The many iron meteorites spread around the crater are considered to be from a later meteorite shower and purely incidential. – The evaporite dissolution model will come across us again with the Spanish regional geologists refusing the Azuara impact event.

In an article published at the beginning of the eighties in the journal “Die Naturwissenschaften”, a professor of geology from New Zealand claimed an endogenetic origin for most of the impact structures established at that time, the Barringer crater in Arizona included. The iron meteorites around the crater were in this case regarded by him as material originating from the deep mantle.

In his 1964 textbook on “Sedimentology and ore genesis, vol. 2 (Elsevier)”, Professor G.C. Amstutz wrote the remarkable sentence:

” … as recently seen when the myth of flying saucers and of meteor impact craters swept around the world and even affected the scientists” (!!!)

At the same time, an intense controversy about the origin of the 25 km-diameter Ries crater in southern Germany arose, when Shoemaker and Chao had established the coesite and stishovite SiO2 high-pressure modifications in suevite samples of that ring structure. After 100 years of a volcanic-origin model, the so-called Ries problem or Ries enigma seemed to have been solved and an impact origin very probable. The opposition of most geologists was vehement. Their arguments were based on the peculiar features of the regional geology: The Miocene Ries crater is located in the southern Germany volcanic province of Upper Tertiary age, a tectonic lineament is running through the structure, it is situated in the vertex of a basement uplift and at a regional facies border. This and much more was stated again and again, even when there were no doubts left about an extraterrestrial origin of the Ries structure.

Once more in 1987, on occasion of an impact workshop held in Parys (South Africa), the opposition against the Ries as an impact structure and the regional-geology arguments were articulated. Prof. Nicolaysen from Johannesburg, not exactly an unknown scientist, contributed a paper about the Ries and Steinheim Basin paired impact structures, again pointing to the pecularities of the regional geology and the apparent association with tectonic features.

But the story does not end here. In 2003, at the Geological Society of America (GSA) meeting in Seattle, R.A. Zimmerman claimed again an enodogenetic, diatremic origin for the Ries crater:

gsa.confex.com/gsa/2003AM/finalprogram/abstract_60058.htm

We see, in the game impact versus endogenetic origin, again and again and world-wide, the regional-geology card is played, however without any worth. Impacting a planetary target is a purely statistical process, and a cosmic projectile impacting the earth does not show any consideration for regional geology. Moreover, there is hardly any point on the earth’s surface, where there is not a geologic anomaly in the form of regional gravity or geomagnetic special features, tectonics with crossing faults, a regional facies border, a dome, a basin, volcanism, salt diapirism …

Poor meteorite – where to fall and not to confuse geologists?!

The vehement opposition of many geologists (especially from the Zaragoza university and (few) from the Center of Astrobiology, Madrid) against the impact origin of the Azuara and Rubielos de la Cérida structures in Spain and the regional-geology argumentation show that the nearly 50 years old learning process is still going on.

Here, we will present the most important complexes of the controversy, we will discuss the observations in relation to the impact cratering process, and we will point to the completely different way of looking at the situation by the Zaragoza (and few Madrid) geologists.

Unfortunately and unexplicable in every detail, a few obviously influential persons of the so-called impact “community” (whatsoever it is) have cast their lot with the Spanish deniers of the Spanish impacts culminating in the elimination of Azuara from the Canadian impact database at the University of New Brunswick (managed by J. Spray). Also this odd thing will closely be addressed here on our Controversy page.

[1dave]

Jan 26, 2015 14:40:19 GMT -7 @gemfeller said:

Here's one for your collection Dave; a possible catastrophic strike just 4,800 years ago in the Indian Ocean:

Thanks Rick!

www.nytimes.com/2006/11/14/science/14WAVE.html?pagewanted=all&_r=1&

Chevrons

At the southern end of Madagascar lie four enormous wedge-shaped sediment deposits, called chevrons, that are composed of material from the ocean floor. Each covers twice the area of Manhattan with sediment as deep as the Chrysler Building is high.

On close inspection, the chevron deposits contain deep ocean microfossils that are fused with a medley of metals typically formed by cosmic impacts. And all of them point in the same direction — toward the middle of the Indian Ocean where a newly discovered crater, 18 miles in diameter, lies 12,500 feet below the surface.

The explanation is obvious to some scientists. A large asteroid or comet, the kind that could kill a quarter of the world’s population, smashed into the Indian Ocean 4,800 years ago, producing a tsunami at least 600 feet high, about 13 times as big as the one that inundated Indonesia nearly two years ago. The wave carried the huge deposits of sediment to land.

Most astronomers doubt that any large comets or asteroids have crashed into the Earth in the last 10,000 years. But the self-described “band of misfits” that make up the two-year-old Holocene Impact Working Group say that astronomers simply have not known how or where to look for evidence of such impacts along the world’s shorelines and in the deep ocean.

Scientists in the working group say the evidence for such impacts during the last 10,000 years, known as the Holocene epoch, is strong enough to overturn current estimates of how often the Earth suffers a violent impact on the order of a 10-megaton explosion. Instead of once in 500,000 to one million years, as astronomers now calculate, catastrophic impacts could happen every 1,000 years.