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