Continental vs Oceanic Asteroid Impacts

Paul S De Carli1,2, Adrian P Jones2, and G David Price2

1 SRI International, Menlo Park, CA94025, USA paul.decarli@sri.com2University College London, Gower St, London WC1E 6 BT, United Kingdom

2004 GSA; Denver, Co; 8 Nov., T-74, 82-13

Why Worry About Impacts?

• Consider the surface of the Moon

• For each lunar crater of diameter X, there have been ~ 20 terrestrial craters of that size. (Earth’s gravity helps)

• Impacts must be considered as possible drivers of geologic events on the Earth, particularly on the early Earth, when large impacts were more frequent.

Possible Effects of Large Impacts

• Formation of large igneous plumes via decompression melting of mantle after excavation of overburden (controversial, but not impossible)

• Modification of plate motion (Speculative, no quantitative studies to date)

Impact Statistics

• Lunar studies indicate decrease in impact flux ~ 3.5 GA ago.

• Relatively constant flux of impacts since then.

• Terrestrial large (200 km dia) crater forming events are rare, ~ 1/200 MA? Frequency is highly uncertain because corresponding Lunar impact statistic would be ~ 1/4 GA.

Effects of Large Impacts

• Impact energy enormous, >10 MT (~1017 J)

for the ~1 km diameter Meteor Crater.

• Semi-empirical estimates of crater size, etc., extrapolated from cm-scale lab experiments and nuclear explosions.

• Hydrocode calculations of large impacts. Validity of calculation constrained by modeling and computational limitations.

Continental vs Oceanic Impacts

• Dallas asked “Could there be a difference?”

• Possibly. One might expect the shock wave to attenuate more rapidly in Continental crust as a consequence of the hysteresis of the phase transitions of tectosilicates to denser phases. Because of the steep release adiabat, rarefaction waves would be faster in Continental than in Oceanic crusts.

Granite Modeling Problems

• Hugoniot and release adiabats differ; known differences (µs-duration experiments) depend on peak pressure. Shock duration (s- duration cratering events) effects on phase transition behavior are unknown.

• As a first approximation use a compromise Hugoniot, intermediate between load and unload curves.

Modeling Continental Crust

• Experimental data show qtz (granite) in dense structure (6-3 glass?) during release down to ~7 GPa.

• We use modified Hugoniot to model loading and release behavior.

0.56 s after Impact

0.56 s after Impact

~1.2 s after Impact

~3.2 s after Impact

~5 s after Impact

~12 s after Impact

Close-in Pressure-time HistoriesTarget Points for histories

0.15 km below surface,

r = 0.15 km, 2.15 km, 4.15 km

Mantle Pressure-time Histories

Conclusions and Caveats I

• Peak Pressure >2X higher in mantle (32 km depth) for Oceanic impact than for Continental impact.

• Impulse (area under Pressure-time curve) appears larger for Continental impact.

• CAVEAT- The differences in Peak Pressure and Impulse might be smaller if one used detailed Hugoniot and Release Models.

Conclusions and Caveats II

• Importance of using appropriate material models is demonstrated by these calculations.

• Present work was hardware and software limited; ~ 100 hrs/calculation on fast PC.

• CAVEAT- A supercomputer result will be no better than the material models used.

Conclusions and Caveats III

• Conservative interpretation of present results- Suggestive of a possible large difference between Oceanic and Continental impacts.

• Present results too crude to be relevant to Hagstrom’s observations of antipodal craters or volcanism.

Acknowledgements

Chris Quan, Richard Clegg and Bence Gerber of Century Dynamics for technical assistance.

PSD thanks Century Dynamics for supporting his use of Autodyn™.