IES-04 Terrestrial Impact Structures (1)

7/29/2019 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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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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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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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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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

IES-04 Terrestrial Impact Structures (1)

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