Post by 1dave on Dec 15, 2014 16:14:11 GMT -5
An extension of Geology For Rockhounds.
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.
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
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
. . . "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