IMPACT CRATERS & SHOCK METAMORPHISM

PRITHVI THAKURIntegrated Mtech,

Geological Technology 4th yearEnroll. No. 11410021

A PECULIAR PROCESS: W h y i m p a c t s a r e d i f f e r e n t ?

RARITY large meteorites are rare, even on ageological timescale – there has been nohistoric example – this lack of direct humanexperience sets them apart from earthquakes,volcanoes, etc.

IMMENSE ENERGY kinetic energy=1/2mv2 ,very high even for small objects. The impactenergy is only limited by m & v and not theEarth’s internal properties (volcanoes,earthquakes).

INSTANT EFFECTS A 1km diameter crater (e.g.Barringer meteor crater- Arizona) forms in afew seconds. A 200km diameter structure(e.g.Sudbury- Canada or Vredefort- S.A.) forms inless than 10 mins.

CONCENTRATED ENERGY RELEASE Internalterrestrial energy – Sub-continental to globalin extent. Small Impact- effects are largelylocal. Large impacts(~100kms diameter)accompanied by catastrophic environmentaleffects on a global scale.

EXTREME PHYSICAL CONDITIONS Typicalimpact velocities(5-8 km/s) results in shockwaves – intense stresses in target rocks –large scale melting and vaporisation.

UNIQUE DEFORMATION EFFECTS e.g.melting, Mineral deformation(PDF’s inquartz), Selective mineral melting, etc. Thesefeatures are distinct from the normalgeological features.

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

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 depthHexc = depth of excavation

Fig 2

Theoretical transient crater

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 andsedimentation.

• 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 and Multiring Basins.

SIMPLE CRATER COMPLEX CRATER

Fig 3

TYPES OF IMPACT CRATERSS i m p l e C r a t e r s

• Bowl shaped depressions.

• Less than a few kms across

• Original transient cavity –shape & dimensions preserved

• During modification, crater immediately filled to half itsoriginal depth – mixture of redeposited ejecta & debrisfrom walls and rims.

• This crater filling unit, called Breccia lens, is a mixture ofrock fragments & impact melt.

D = final crater diameter (10-20% greater than premodifiedtransient crater)

dt = true depth of the final crater

da = apparent depth of the crater

Fig 1

TYPES OF IMPACT CRATERSC o m p l e x C r a t e r 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 targetrocks.

• Rocks around periphery – collapse downward & inwardalong concentric faults (ring grabens) and a series ofterraces 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

TYPES OF IMPACT CRATERSM u l t i r i n g B a s i n s• 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 complexcrater and multi-ring basin is purely based on size orsome other special conditions are required.

Fig 5. Multi-ring Basin, Moon

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 p m e 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 m i c r o s c o p i c

d e f o r m a 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 m i c r o d e f o r m a t i o n f e a t u r e s .• ~ 8 G P a t o 2 5 G p a - M i c r o s c o p i c p l a n a r d e f o r m a t i o n f e a t u r e s i n i n d i v i d u a l m i 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 r m a t i o n o f i n d i v i d u a l m i n e r a l s t o a m o 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 m e l t i n g o f i n d i v i d u a l m i 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 o m p l e t e m e l t i n g o f a l l m i n e r a l s t o f o r m a s u p e r h e a t e d r o c k m e l t .• > 1 0 0 G P a - C o m p l e t e r o c k v a p o r i s a t i o n . N o p r e s e r v e d m a t e r i a l s f o r m e 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 m a 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 )

Carbonate Phosphate Impact MeltShatter Cones

Diaplectic Glass

2 sets of decorated PDFs

Quartz multiple sets of PDFs

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