Continental vs Oceanic Asteroid
Impacts
2004 GSA; Denver, Co; 8 Nov., T-74, 82-13
Paul S De Carli1,2, Adrian P Jones2, and G David Price2
1 SRI International, Menlo Park, CA94025, USA
paul.decarli@sri.com
2University College London, Gower St, London WC1E 6
BT, United Kingdom
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 Histories
Target 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™.