Post on 23-Jan-2016
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Terrestrial Impact Structures: Terrestrial Impact Structures: Observation and ModelingObservation and Modeling
Impact craters are found on any Impact craters are found on any planetary body with a solid surfaceplanetary body with a solid surface
MarsMars
MoonMoon
MercuryMercury
Ida-243Ida-243
Earth’s Known Impact StructuresEarth’s Known Impact Structures
Earth retains the poorest record of impact craters amongst terrestrial planets
Why?Why? Plate tectonics - Erosion – Sedimentation - LifePlate tectonics - Erosion – Sedimentation - Life Oceans are relatively young and hard to exploreOceans are relatively young and hard to exploreMany impact structures are covered by younger sediments, others are highly eroded or heavily modified by erosion. Few impact craters are well preserved on the surface
~160
Manicouagan, Canada (62mi)Manicouagan, Canada (62mi)
Roter Kamm, Namibia (1.6mi)Roter Kamm, Namibia (1.6mi)
Brent, Canada (2.4 mi)Brent, Canada (2.4 mi)
Wabar, Saudi Arabia (0.072mi)Wabar, Saudi Arabia (0.072mi)
Vredefort, South Africa Vredefort, South Africa (125-185mi)(125-185mi)
Meteor Crater, AZ (0.75mi)Meteor Crater, AZ (0.75mi)
Wolfe Creek, Australia (0.55mi)Wolfe Creek, Australia (0.55mi)
Spider, Australia (8.1mi)Spider, Australia (8.1mi)
Popigai, Russia (62 mi)Popigai, Russia (62 mi)
Meteor Crater Meteor Crater a.k.a.a.k.a. Barringer Crater Barringer Crater
• Meteor Crater, Arizona, is one the worlds most well known crater.
• Less than 1 mile across, it was created about 50,000 years ago.
• Formed by an iron asteroid. Lots of melted droplets and solid
pieces of an iron-nickel material have been recovered in the area.
First-recognized impact crater First-recognized impact crater on Earth:on Earth:
Meteor CraterMeteor Crater1891:1891: Grove Karl Gilbert organizes an expedition to Coon
Mountain (old name of Meteor crater) to explore the impact hypothesis. He soon concluded that there was no evidence for impact, and attributes it to volcanism.
1902:1902: Daniel Moreau Barringer secures the mining patents for the crater and the land around it.
1906 & 1909:1906 & 1909: Barringer writes papers attributing the crater to an impact event. Drilling and exploration continued at great expenses.
1928:1928: Meteor crater becomes generally accepted as an impact crater. An article from National Geographic attributes the impact hypothesis to Gilbert, and fails to mention Barringer’s work.
1929:1929: Investors decline to provide more funding to continue drilling. Barringer dies of a massive heart attack.
1946:1946: The crater becomes officially “Meteor Crater”. The Meteoritical Society defines the proper scientific name as the Barringer Meteor Crater.
Impact ObservationsImpact Observations
Physical: shape, inverted stratigraphy, material displaced
Shock evidence from the rocks: shatter cones, shocked materials, melt rocks, material disruption
Geophysical data: gravity & magnetic anomalies
Observational: PhysicalObservational: Physical
Shape: circular features Moltke Tycho
(2.7 mi) (53 mi)
Mystery structure #1Mystery structure #1
Gosses Bluff crater, AustraliaGosses Bluff crater, AustraliaComplex crater with a central peak ring
(143 million years old)
Crater diameter: 22 km
Mostly eroded away only spotted by the different color of the
vegetation
Inner ring: 5 km
Round bluff that is fairly easy to spot.
Mystery structure #2Mystery structure #2
Aorounga crater, ChadAorounga crater, ChadComplex crater with a central peak ring
Crater diameter: 12.6 km
Buried under rocks and sand for a long
time, it has been uncovered again by
recent erosion.
Possible crater
Aorounga may be part of a crater chain
Mystery structure #3Mystery structure #3
Richat Structure, MauritaniaRichat 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.
Mystery structure #4Mystery structure #4
Clearwater, CanadaClearwater, Canadatwo craters, both 290 Matwo 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
Mystery structure #5Mystery structure #5
Chicxulub Structure, MexicoChicxulub Structure, Mexico65 Myr old (end of dinosaurs!)65 Myr old (end of dinosaurs!)
Structure diameter: 106 miles
Crater is not really visible at the surface
First indication from world wide distribution of ejecta
Only field work, drilling, and geophysical data could identify it.
Observational: PhysicalObservational: Physical
Shape: circular features Moltke Tycho
(2.7 mi) (53 mi)
Inverted Stratigraphy: Meteor Crater
first recognized by Barringer (only for well preserved craters)
Material displaced: Solid material broken up and ejected
outside the crater: breccia, tektites
Observations: Shock EvidenceObservations: Shock Evidence
Shatter cones: conical fractures with typical markings produced by shock waves
Shocked Material: shocked quartz high pressure minerals
Melt Rocks: melt rocks may result from shock and friction
Observations: Geophysical dataObservations: Geophysical data
Gravity anomaly: based on density variations of materials Generally negative (mass deficit) for impact craters
Magnetic: based on variation of magnetic properties of materials
Seismic: sound waves reflection and refraction from subsurface layers with different characteristics
Seismic Reflection and RefractionSeismic 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.
Impact ModelingImpact Modeling
Numerical modeling (i.e., computer simulations) is the best method to Numerical modeling (i.e., computer simulations) is the best method to investigate the process of crater formation and material ejectioninvestigate the process of crater formation and material ejection
Formation of Impact CratersFormation of Impact Craters
Depth of transient craterDepth of transient craterfunction of the energy of function of the energy of
impact and the propertiers of impact and the propertiers of the target materialthe target material
D<Dth
D>Dth
Dth= Threshold diameter for transition from simple to complex craters (around 4 km on Earth)
Verification by numerical modelVerification by numerical model
Formation of a simple crater
Formation of a complex crater
Simulations from Kai Wünneman, University of Arizona)
Modeling ExamplesModeling Examples
Formation of the Chesapeake structure: material behavior: crater collapse and final
shape
Origin of tektites: expansion plume (vaporized material), solid
and melted (e.g., tektites) ejecta
Chesapeake Crater, VAChesapeake Crater, VA
Marine impact event, about 35 Myr old, with typical “inverted sombrero” shape due to multi-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 sedimentsSoft sediments
Hard Hard rockrock
Simulation from Gareth Colins, university of Arizona (2004))
Chesapeake CraterChesapeake Crater
Simulation from Gareth Colins, university of Arizona (2004))
TektitesTektites
Silicate glass particles formed by the melting of terrestrial surface sediments by hypervelocity impact.They resemble obsidian in appearance and chemistry.
Few inches in size, black to lime green in color, and aerodynamically shaped.
Concentrated in limited areas on the Earth’s 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)
Central European
Australasian
North American
Ivory Coast
Understanding tektitesUnderstanding tektites
1788:1788: Tektites are first described as a type of terrestrial volcanic
glass.
1900:1900: F.E. Suess, convinced they were some sort of glass meteorites, coined the term “tektite” from the greek word tektos, meaning “molten”.
1917:1917: Meteoriticist F. Berwerth provides the first hint of a terrestrial origin of tektites by finding that tektites were chemically similar to certain sedimentary rocks.
1948:1948: A Sky & Telescope article by H.H. Nininger sustains the hypothesis of a lunar origin of tektites
1958:1958: An impact origin for tektites is discussed in a paper by J.S. Rinehart.
1963-1972:1963-1972: The Apollo program returns samples of the Moon to Earth, disproving the connection tektites-Moon.
1960:1960: J.A. O’Keefe enters the dispute, in favor of the lunar origin hypothesis.
Modeling Tektite Formation
Potential tektites
Solid target
Melted impactor
Simulation from Natalia Artemieva, Russian Academy of Science, Moscow (2003)
Modeling Tektite EjectionSimulation from Natalia Artemieva, Russian Academy of Science, Moscow (2003)
Tektite Formation: MoldavitesTektite Formation: Moldavites
0 100 200 300 400 500
-200
-100
0
100
200
Dis
tan
ce a
cro
ss tr
aje
ctor
y (k
m)
Distance along trajectory (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
Stöffler, Artemieva, Pierazzo, 2003
In summary: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