IES-04 Terrestrial Impact Structures (1)
Transcript of IES-04 Terrestrial Impact Structures (1)
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Terrestrial Impact Structures:
Observation and Modeling
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Impact craters are found on anyplanetary body with a solid surface
Mars
Moon
Mercury
Ida-243
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Earths Known Impact Structures
Earth retains the poorest record of impact craters amongst terrestrial planets
Why? Plate tectonics - ErosionSedimentation - LifeOceans are relatively young and hard to explore
Many impact structures are covered by younger sediments, others are highly erodedor heavily modified by erosion. Few impact craters are well preserved on the surface
~160
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Manicouagan, Canada (62mi)
Roter Kamm, Namibia (1.6mi)
Brent, Canada (2.4 mi)
Wabar, Saudi Arabia (0.072mi)
Vredefort, South Africa
(125-185mi)
Meteor Crater, AZ (0.75mi)
Wolfe Creek, Australia (0.55m
Spider, Australia (8.1mi)
Popigai, Russia (62 mi)
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Meteor Crater a.k.a. Barringer Crater
Meteor Crater, Arizona, is one theworlds most well known crater.
Less than 1 mile across, it wascreated about 50,000 years ago.
Formed by an iron asteroid.Lots of melted droplets and solidpieces of an iron-nickel material have
been recovered in the area.
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First-recognized impact crateron Earth:
Meteor Crater1891:Grove Karl Gilbert organizes an expedition to Coon
Mountain (old name of Meteor crater) to explore the impacthypothesis. He soon concluded that there was no evidence forimpact, and attributes it to volcanism.
1902:Daniel Moreau Barringer secures themining patents for the crater and the landaround it.
1906 & 1909: Barringer writes papers attributingthe crater to an impact event. Drilling andexploration continued at great expenses.
1928: Meteor crater becomes generally accepted as an impact crater.An article from National Geographic attributes the impacthypothesis to Gilbert, and fails to mention Barringers work.
1929: Investors decline to provide more funding to continue drilling. Barringer diesof a massive heart attack.
1946:The crater becomes officially Meteor Crater. The Meteoritical Societydefines the proper scientific name as the Barringer Meteor Crater.
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Impact Observations
Physical:
shape, inverted stratigraphy, material displaced
Shock evidence from the rocks:
shatter cones, shocked materials, melt rocks,
material disruptionGeophysical data:
gravity & magnetic anomalies
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Observational: Physical
Shape:circular features Moltke Tycho
(2.7 mi) (53 mi)
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Mystery structure #1
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Gosses Bluff crater, AustraliaComplex crater with a central peak ring
(143 million years old)
Crater diameter:22 km
Mostly eroded awayonly spotted by thedifferent color of the
vegetation
Inner ring:5 km
Round bluff that is
fairly easy to spot.
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Mystery structure #2
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Aorounga crater, ChadComplex crater with a central peak ring
Crater diameter:12.6 km
Buried under rocksand sand for a long
time, it has been
uncovered again byrecent erosion.
Possible crater
Aorounga may be part
of a crater chain
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Mystery structure #3
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Richat Structure, Mauritania
Structure diameter:30 miles
Formed by volcanic
processes.
Not every circular feature on Earth is an impact crater!It is necessary to visit the feature on the ground to observe its structural features and
obtain rock samples. Only then we can be sure of what it is.
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Mystery structure #4
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Clearwater, Canadatwo craters, both 290 Ma
Clearwater West:22.5 miles
Complex structure
Clearwater East:16 miles
Probably they were made by a double asteroid, like Toutatis
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Mystery structure #5
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Chicxulub Structure, Mexico65 Myr old (end of dinosaurs!)
Structure diameter:106 miles
Crater is not reallyvisible at the surface
First indication from world wide distribution of ejecta
Only field work, drilling, and geophysical data could identify it.
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Observational: Physical
Shape:circular features Moltke Tycho
(2.7 mi) (53 mi)
Inverted Stratigraphy: Meteor Craterfirst recognized by Barringer(only for well preserved craters)
Material displaced:Solid material broken up and ejectedoutside the crater: breccia, tektites
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Observations: Shock Evidence
Shatter cones:conical fractures with typicalmarkings produced by shockwaves
Shocked Material:shocked quartz
high pressure minerals
Melt Rocks:melt rocks may resultfrom shock and friction
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Observations: Geophysical data
Gravity anomaly:based on density variations of materialsGenerally negative (mass deficit) for impactcraters
Magnetic:based on variation of magnetic properties
of materials
Seismic:sound waves reflection and refractionfrom subsurface layers with differentcharacteristics
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Seismic Reflection and RefractionSound waves (pulses) are sent downward. They are reflected or refracted by layers with
different properties in the crust. Different materials have very different sound speeds.
In dry, unconsolidated sand sound speed may reach 600 miles per hour (mi/h).
Solid rock (like granite) can have a sound speed in excess of 15,000 mi/h.
The more layers between the surface and the layer of interest, the more complicated the
velocity picture.
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Impact Modeling
Numerical modeling (i.e., computer simulations) is the best method to
investigate the process of crater formation and material ejection
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Formation of Impact Craters
Depth of transient craterfunction of the energy of
impact and the propertiers of
the target material
DDth
Dth= Threshold diameter for transition from simple to
complex craters (around 4 km on Earth)
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Verification by numerical model
Formation of a simple crater
Formation of a complex crater
Simulations from Kai Wnneman, University of Arizona)
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Modeling Examples
Formation of the Chesapeake structure:
material behavior: crater collapse and finalshape
Origin of tektites:
expansion plume (vaporized material), solid and
melted (e.g., tektites) ejecta
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Chesapeake Crater, VA
Marine impact event, about 35 Myr old, with typical inverted sombrero shape due tomulti-layer nature of target region: soft sediments + hard rock
Its existence explains several geological features of the area including the saline
groundwater and higher rate of subsidence at the mouth of the Chesapeake Bay.
Inner basin (the head of the sombrero) is about 25 miles wide - Outer basin (the brim
of the sombrero) extends to about 53 miles.
Soft sediments
Hard
rock
Simulation from Gareth Colins, university of Arizona (2004))
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Chesapeake Crater
Simulation from Gareth Colins, university of Arizona (2004))
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Tektites
Silicate glass particles formed by the melting of terrestrial surface sediments byhypervelocity impact.They resemble obsidianin appearance and chemistry.
Few inches in size, black to lime green in color, and aerodynamically shaped.Concentrated in limited areas on the Earths surface, referred to as strewn fields.
Four tektite strewn fields are known:North American @34 Ma (Chesapeake crater)Central European (Moldavites) @ 14.7 Ma (Ries crater)
Ivory Coast @ 1 Ma (Bosumtwi crater)Australasian @ 0.77 Ma (unknown crater)
CentralEuropean
Australasian
NorthAmerican
IvoryCoast
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Understanding tektites
1788:Tektites are first described as a type of terrestrial volcanicglass.
1900:F.E. Suess, convinced they were some sort of glassmeteorites, coined the term tektite from the greek word
tektos, meaning molten.
1917: Meteoriticist F. Berwerth provides the first hint of aterrestrial origin of tektites by finding that tektites werechemically similar to certain sedimentary rocks.
1948: A Sky & Telescope article by H.H. Nininger sustains thehypothesis of a lunar origin of tektites
1958: An impact origin for tektites is discussed in a paper by J.S.Rinehart.
1963-1972: The Apollo program returns samples of the Moon to Earth, disproving
the connection tektites-Moon.
1960:J.A. OKeefe enters the dispute, in favor of the lunar originhypothesis.
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Modeling Tektite Formation
Potentialtektites
Solidtarget
Meltedimpactor
Simulation from Natalia Artemieva, Russian Academy of Science, Moscow (2003)
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Modeling Tektite Ejection
Simulation from Natalia Artemieva, Russian Academy of Science, Moscow (2003)
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Tektite Formation: Moldavites
0 100 200 300 400 500
-200
-100
0
100
200
Distanceacross
trajectory(km)
Distance along tra jectory (km)
Tektites form in typical medium-size impacts in areas with surface sands
They tend to be distributed downrange of the impact point
Their low water content is due to the thermal evolution of the melt droplets
Stffler, Artemieva, Pierazzo, 2003
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In summary:
Impact craters are everywhere, even on Earth!
Not every circular structure is an impact crater
Terrestrial impact structures tend to be eroded, buried or
modified by geologic processes
By combining remote and ground observations,laboratory experiments, and theoretical studies we can
learn what happens in a large impact event1
and to recognize impact structures