Jacob Samorodin wrote:

> Igneous
> Sedimentary
> Metamorphic.

Now, with planetary science having attained the maturity it now has, are there any additions to those three?

Alex writes:
No, these are still the three basic rock types, though each has subgroups (e.g. clastic sedimentary, contact metamorphic, etc.).

Jacob Samorodin wrote:
[1] A large asteroid/comet impact will generate limited/local igneous rock within or near the formed impact crater

[2] The impact shockwave [if it's large enough] will deform sedimentary layers, converting it into "metamorphic rock forms" near the impact zone.

[3] And descending & settling asteroidal/cometary dust may [depending on the frequency of impacts in the area] form nearby "sedimentary-type" layers over a long period of time. Agree?

Alex writes:
Addressing each in turn:

[1] Are you referring to impact melt? If so, then it has been well-known that impactors can generate considerable shock heating during central uplift and deposition of energy into the subcrater basement, as well as uplifting geothermal gradients. I'm not sure what you mean by "converting it into 'metamorphic rock forms'". One can examine a breccia, which by definition is highly shocked, but sometimes one can identify within the matrix the basic rock types and minerals. The impact may disrupt regional stratigraphy, but horizontal continuity relationships over macroscales should be preserved, which should help in identification.

[2] See above.

[3] Again, it is possible, though certainly a function of the atmospheric density at the time of deposition. Interestingly, Malin and Edgett posited this as an alternative explanation to liquid water to account for their ancient sedimentary layers, though they favor the former for other reasons.

Jacob Samorodin wrote:
By the way, Dr. Blackwell, what do you think of my revised and refined crude calculations for the newly-discovered NEO "asteroid-pair" 1994 KW4?

Alex writes:
In all honesty, I haven't looked at 1999 KW4 issue or your calculations so I cannot really offer a cogent opinion.

Alex R. Blackwell
University of Hawaii