Impact craters - morphology and formation along with some geological facts.

1. IMPACT CRATERS & SHOCK
METAMORPHISM
PRITHVI THAKUR
Integrated Mtech,
Geological Technology 4th year
Enroll. No. 11410021

2. A PECULIAR PROCESS:
Why impacts are different?
RARITY large meteorites are rare, even on a
geological timescale – there has been no
historic example – this lack of direct human
experience sets them apart from earthquakes,
volcanoes, etc.
IMMENSE ENERGY kinetic energy=1/2mv2 ,
very high even for small objects. The impact
energy is only limited by m & v and not the
Earth’s internal properties (volcanoes,
earthquakes).
INSTANT EFFECTS A 1km diameter crater (e.g.
Barringer meteor crater- Arizona) forms in a
few seconds. A 200km diameter structure(e.g.
Sudbury- Canada or Vredefort- S.A.) forms in
less than 10 mins.
CONCENTRATED ENERGY RELEASE Internal
terrestrial energy – Sub-continental to global
in extent. Small Impact- effects are largely
local. Large impacts(~100kms diameter)
accompanied by catastrophic environmental
effects on a global scale.
EXTREME PHYSICAL CONDITIONS Typical
impact velocities(5-8 km/s) results in shock
waves – intense stresses in target rocks –
large scale melting and vaporisation.
UNIQUE DEFORMATION EFFECTS e.g.
melting, Mineral deformation(PDF’s in
quartz), Selective mineral melting, etc. These
features are distinct from the normal
geological features.

3. FORMATION OF IMPACT CRATERS
1.Contact/Compression Stage
• K.E is converted into Shock Waves
• One set – transmitted from the interface into the target rock.
• A complementary set – reflected back into the projectile
(release wave).
• The impact point is surrounded by concentric, hemispherical
shock zones.
• Near the crater rim – shock waves become elastic/seismic
waves. This is the region of fracturing and brecciation.
• Release wave results in melting/ vaporization of the projectile
(vapor plume).
Fig 1

4. FORMATION OF IMPACT CRATERS
2. Excavation Stage
• Upward ejection (spalling) of large near surface fragments and
smaller ejecta curtain.
• Subsurface flow of the target material to form the transient
crater (bowl-shaped depression).
• Excavated Zone – material is fractured, excavated & ejected
beyond the transient crater rim.
• Displaced Zone – target material is driven downward &
outward coherently and does not reach the surface.
Hat = final transient crater depth
Hexc = depth of excavation
Fig 2
Theoretical transient crater

5. FORMATION OF IMPACT CRATERS
3. Modification Stage
• The excavation stage ends when the transient crater has grown to its maximum size, and the subsequent modification stage begins immediately.
• Factors responsible for modification are gravity and rock mechanics (~ restoring forces).
• The modification processes of uplift and collapse merge gradually into the normal processes of geological mass movement, isostatic uplift, erosion and
sedimentation.
• The extent to which the transient crater is modified depends on its size and properties of the target rock.
• On this basis, impact structures can be classified as: Simple Craters, Complex Craters andMultiring Basins.

6. SIMPLE CRATER COMPLEX CRATER
Fig 3

7. TYPES OF IMPACT CRATERS
Simple Crater s
• Bowl shaped depressions.
• Less than a few kms across
• Original transient cavity –shape & dimensions preserved
• During modification, crater immediately filled to half its
original depth – mixture of redeposited ejecta & debris
from walls and rims.
• This crater filling unit, called Breccia lens, is a mixture of
rock fragments & impact melt.
D = final crater diameter (10-20% greater than premodified
transient crater)
dt = true depth of the final crater
da = apparent depth of the crater
Fig 1

8. TYPES OF IMPACT CRATERS
Complex Crater s
• Characterized by central uplift region.
• Formed in larger structures (>4km - crystalline rocks, >2km
- sediments)  these values only applicable to earth.
• Late stage modification – complex interactions b/w
shock-wave effects, gravity, strength & structure of target
rocks.
• Rocks around periphery – collapse downward & inward
along concentric faults (ring grabens) and a series of
terraces along the outer margin.
• Centre of the transient crater – forms central uplift.
• The amount of Stratigraphic uplift (S.U) is about 1/10th
the final diameter (D).
Fig 4. Dr= diameter of the complex crater

9. TYPES OF IMPACT CRATERS
Mul ti r ing Bas ins
• Composed of multiple concentric uplifted rings and
intervening down-faulted valleys.
• They have 2 or more interior rings in addition to the outer
rim.
• The existence of multi-ring basin has not yet been
demonstrated on earth.
• The few possible candidates are Manicouagan (Canada,
100 k.m.), Vredefort (South Africa, >200 k.m.), Sudbury
(Canada, >200 k.m.)
• It is not yet clear whether the transition b/w complex
crater and multi-ring basin is purely based on size or
some other special conditions are required.
Fig 5. Multi-ring Basin, Moon

10. STAGES OF SHOCK METAMORPHISM
• < 2 G P a - f r a c t u r i n g a n d b r e c c i a t i o n , w i t h o u t d e v e l o pme n t o f u n i q u e s h o c k f e a t u r e s .
• > 2 G P a t o < 3 0 G P a - S h a t t e r c o n e s . A t l o w p r e s s u r e s ( < 1 0 G P a ) , o c c u r s w i t h o u t d i s t i n c t mi c r o s c o p i c
d e f o rma t i o n f e a t u r e s . A t h i g h e r p r e s s u r e s , c o n t a i n s d i s t i n c t i v e mi c r o d e f o rma t i o n f e a t u r e s .
• ~ 8 G P a t o 2 5 G p a - Mi c r o s c o p i c p l a n a r d e f o rma t i o n f e a t u r e s i n i n d i v i d u a l mi n e r a l s , e s p e c i a l l y q u a r t z
a n d f e l d s p a r .
• > 2 5 G P a t o 4 0 G P a - T r a n s f o rma t i o n o f i n d i v i d u a l mi n e r a l s t o amo r p h o u s p h a s e s ( d i a p l e c t i c g l a s s e s ) .
• > 3 5 G P a t o 6 0 G P a - S e l e c t i v e p a r t i a l me l t i n g o f i n d i v i d u a l mi n e r a l s , t y p i c a l l y f e l d s p a r s . I n c r e a s i n g
d e s t r u c t i o n o f o r i g i n a l t e x t u r e s .
• > 6 0 G P a t o 1 0 0 G P a - C omp l e t e me l t i n g o f a l l mi n e r a l s t o f o rm a s u p e r h e a t e d r o c k me l t .
• > 1 0 0 G P a - C omp l e t e r o c k v a p o r i s a t i o n . No p r e s e r v e d ma t e r i a l s f o rme d a t t h i s s t a g e . ( v a p o r i z a t i o n a n d
s u b s e q u e n t c o n d e n s a t i o n t o g l a s s y ma t e r i a l h a v e b e e n i d e n t i f i e d s o f a r )

11. Shatter Cones Carbonate Phosphate Impact Melt
Diaplectic Glass
2 sets of decorated PDFs
Quartz multiple sets of PDFs

12. REFERENCES
• Traces of Catastrophe – NASA
• www.mit.edu – MIT open course Impact Craters
• www.en.Wikipedia.org
• www.images.google.com
• www.flickr.com
• www.tumblr.com