Transcript Document

Terrestrial Impact Structures:
Observation and Modeling
Mars
Mercury
Impact craters are found on any
planetary body with a solid surface
Moon
Ida-243
Earth’s Known Impact Structures
~160
Earth retains the poorest record of impact craters amongst terrestrial planets
Why? Plate tectonics - Erosion – Sedimentation - Life
Oceans are relatively young and hard to explore
Many 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
Roter Kamm, Namibia (1.6mi)
Spider, Australia (8.1mi)
Brent, Canada (2.4 mi)
Meteor Crater, AZ (0.75mi)
Wabar, Saudi Arabia (0.072mi)
Manicouagan, Canada (62mi)
Wolfe Creek, Australia (0.55mi
Vredefort, South Africa
(125-185mi)
Popigai, Russia (62 mi)
Meteor Crater a.k.a. 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
on Earth:
Meteor Crater
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: Daniel Moreau Barringer secures the
mining patents for the crater and the land
around it.
1906 & 1909: Barringer writes papers attributing
the crater to an impact event. Drilling and
exploration continued at great expenses.
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: Investors decline to provide more funding to continue drilling. Barringer dies
of a massive heart attack.
1946: The crater becomes officially “Meteor Crater”. The Meteoritical Society
defines the proper scientific name as the Barringer Meteor Crater.
Impact 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: Physical
Shape:
circular features
Moltke
(2.7 mi)
Tycho
(53 mi)
Mystery structure #1
Gosses Bluff crater, Australia
Complex 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 #2
Aorounga crater, Chad
Complex 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 #3
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.
Mystery structure #4
Clearwater, Canada
two 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 #5
Chicxulub Structure, Mexico
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: Physical
Shape:
circular features
Moltke
(2.7 mi)
Inverted Stratigraphy:
first recognized by Barringer
(only for well preserved craters)
Material displaced:
Solid material broken up and ejected
outside the crater: breccia, tektites
Tycho
(53 mi)
Meteor Crater
Observations: 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 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 Refraction
Sound 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 Modeling
Numerical modeling (i.e., computer simulations) is the best method to
investigate the process of crater formation and material ejection
Formation of Impact Craters
D<Dth
D>Dth
Depth of transient crater
function of the energy of
impact and the propertiers of
the target material
Dth= Threshold diameter for transition from simple to
complex craters (around 4 km on Earth)
Verification by numerical model
Formation of a simple crater
Formation of a complex crater
Simulations from Kai Wünneman, University of Arizona)
Modeling 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, 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 sediments
Hard
rock
Simulation from Gareth Colins, university of Arizona (2004))
Chesapeake Crater
Simulation from Gareth Colins, university of Arizona (2004))
Tektites
Central
European
North
American
Ivory
Coast
Australasian
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)
Understanding tektites
1788: Tektites are first described as a type of terrestrial volcanic
glass.
1900: F.E. Suess, convinced they were some sort of glass
meteorites, coined the term “tektite” from the greek word
tektos, meaning “molten”.
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: A Sky & Telescope article by H.H. Nininger sustains the
hypothesis of a lunar origin of tektites
1958: An impact origin for tektites is discussed in a paper by J.S.
Rinehart.
1960: J.A. O’Keefe enters the dispute, in favor of the lunar origin
hypothesis.
1963-1972: The Apollo program returns samples of the Moon to Earth, disproving
the connection tektites-Moon.
Modeling Tektite Formation
 Potential
tektites
 Solid
target
 Melted
impactor
Simulation from Natalia Artemieva, Russian Academy of Science, Moscow (2003)
Modeling Tektite Ejection
Simulation from Natalia Artemieva, Russian Academy of Science, Moscow (2003)
Tektite Formation: Moldavites
Distance across trajectory (km)
Stöffler, Artemieva, Pierazzo, 2003
200
100
0
- 100
- 200
0
100
200
300
400
500
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
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