Lecture 34 – Meteorite Impact Events
1) Why are impact events studied?
a) Have visibly affected the surfaces of most _________ and ___________ in
our solar system
b) Impacts have recently been recognized as important geologic events both
__________ and ___________. Geologists and physicists can gain
information about_______________________, cratering mechanics,
environmental/biological effects, and nature of ________________over time.
c) reservoirs for _____________.
d) associated with ___________________.
e) Related to large amounts of energy.
2. Studies of the Moon’s craters indicate intense bombardment of meteorites from 4.23.9 Ga. These impacts likely sterilized the surface of the earth and caused its
atmosphere and possibly early oceans to be vaporized.
3) Types of craters
a) ____________ = a small _____________________ with a raised rim, less
than _____________ kms in diameter.
b) ______________ = characterized by a________________ , relatively large
______________, and an _____________________. Diameters range from
______ to ________ of kms.
c) ______________ = diameters from _______________ of kms, central uplift is
a _______________ of mountains surrounded by multiple _______________
__________________ and____________________ . Similar to giant bullseyes. These have not been positively identified on Earth yet.
4) Stages of Impact
a) Stage One:__________________________– occurs when the projectile first
_______________________________. Kinetic energy of the projectile is
converted to ________________that travel through the target rock ________
from the point of impact. Shock waves are also transmitted ___________the
projectile often causing ________________ of the projectile. Peak shock
wave pressures can exceed _________ (1GPa = 100kbar). For most impact
events, this stage lasts ______________ second.
b) Stage Two: _______________ – actual impact crater is opened. Rock located
at shallow depths moved _________ and ___________. At deeper depths,
rock dominantly moves ___________ and ___________. These movements
produce a___________________. A ________ impact crater can be
excavated in _____________ minutes.
b) Three: ________________– crater differentiation occurs. If projectile is
relatively _________, a simple crater will form due to ________________
and______________________. If the projectile is______, a complex crater
will form due to ____________________(forming the central peak). Simple
definition... “when things stop falling.”
35 m
Energy (J)
2.1 x 1012
Frequency of
Comparable Terrestrial
Impact (#/m.y.)
Minimum damaging
earthquake, M=5
1.1 km
55 m
6.2 x 1016
Mt. St. Helens eruption,
blast only
6.9 km
350 m
1.5 x 1015
Largest recorded
earthquake, M=9.6
31 km
1.5 km
1.3 x 1021
Total annual heat flow
from the Earth
200 km
10 km
3.7 x 1023
Largest known impact
structure on the Earth
5) Where are impact craters located?
a) More than ________________have been recognized on the Earth. Notice
the concentration of craters in N. America, Europe and Australia. This is not
because objects preferentially impacted in those areas!! Identification of
impact craters on the Earth is limited by:
1. ______________ (jungles!)
2. burial by ______________
3. _________________.
b) There are _________________ located in_______________. Probably best
known in North America is the _________________ (aka Meteor Crater) in
Arizona. Diameter = 1.186km, age = 0.049 million years.
c) The smallest crater on the Earth is the ___________________ in Kansas.
Diameter = 0.015km (49.8 ft!), age = <0.001 million years (~1000 years.)
d) The largest crater on the Earth is _______________ in South Africa.
Diameter = 300km (186 miles), age = ~2 billion years.
6) Effects of an impact: Local
a) rock/mineral deformation both on a large and small scale
a. large scale _______________ and_______________. 2-6 GPa
b. shattercones = cones with distinctive____________________. Occur
in fine-grained rocks like limestone. 2-30 GPa.
c. __________________ in minerals = quartz! Features include:
1) grain___________. 5-7 GPa
2) __________________________________________ (PDFs) – fine
lineations along crystallographic axes. 8-25 GPa
3) Conversion to ____________________ polymorphs (coesite and
stishovite). 2-10 GPa
d. Microspherules formed from __________________ of droplets of
molten rock
e. Ir - platinum group element occurs in great abundance only at or near
impact events, its only common source is extraterrestrial (i.e.
b) _____________________ of target rock. 30-100 GPa
7) Effects of an impact: Global, these are controversial!
a) extinction events
a. ______ in atmosphere causing________________. Volcanic ash has
been linked to global dust clouds.
b. ________________ of carbonate rocks may cause release of _______
causing global ______________.
***Note! There are many discrepancies!!
Example: _______________ and the _______________, Yucatan Peninsula,
b) hot spots, flood basalts, continental rifting
a. good ideas but no scientific evidence to prove it!
7) Recognition and identification of craters: What to look for!
a) a _________________ (if not covered by vegetation, sediment, or destroyed).
Sometimes craters are filled with water. But (!!) how would you tell this from
any other hole in the ground (sinkholes, volcanic calderas, etc...)?
a. look for ____________________ minerals,
_________________________ (PDFs) and shatter cones
b. _____________ fragments.
b) if the crater was destroyed?
a. ______________ (only from extraterrestrial sources)
b. ________________ in rock layers
Example: ___________________, Nevada contains shocked quartz but
so far a crater has not been identified.