Terrestrial impact structures

Information about Terrestrial impact structures

Published on January 16, 2008

Author: Vilfrid

Source: authorstream.com

Content

Terrestrial Impact Structures: Observation and Modeling:  Terrestrial Impact Structures: Observation and Modeling Impact craters are found on any planetary body with a solid surface:  Impact craters are found on any planetary body with a solid surface Mars Moon Mercury Ida-243 Earth’s Known Impact Structures:  Earth’s Known Impact Structures 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 ~160 Slide4:  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 (0.55mi) Spider, Australia (8.1mi) Popigai, Russia (62 mi) Slide5:  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. Slide6:  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:  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:  Observational: Physical Shape: circular features Moltke Tycho (2.7 mi) (53 mi) Slide9:  Mystery structure #1 Slide10:  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. Slide11:  Mystery structure #2 Slide12:  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 Slide13:  Mystery structure #3 Slide14:  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. Slide15:  Mystery structure #4 Slide16:  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 Slide17:  Mystery structure #5 Slide18:  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. Slide19:  Observational: 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 Slide20:  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:  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 Slide22:  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.  Slide23:  Impact Modeling Numerical modeling (i.e., computer simulations) is the best method to investigate the process of crater formation and material ejection Slide24:  Formation of Impact Craters Depth of transient crater function of the energy of impact and the propertiers of the target material D<Dth D>Dth Dth= Threshold diameter for transition from simple to complex craters (around 4 km on Earth) Slide25:  Verification by numerical model Formation of a simple crater Formation of a complex crater Simulations from Kai Wünneman, University of Arizona) Slide26:  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:  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:  Chesapeake Crater Simulation from Gareth Colins, university of Arizona (2004)) Tektites:  Tektites 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 Slide30:  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. 1963-1972: The Apollo program returns samples of the Moon to Earth, disproving the connection tektites-Moon. 1960: J.A. O’Keefe enters the dispute, in favor of the lunar origin hypothesis. Modeling Tektite Formation:  Modeling Tektite Formation Potential tektites Solid target Melted impactor Simulation from Natalia Artemieva, Russian Academy of Science, Moscow (2003) Modeling Tektite Ejection:  Modeling Tektite Ejection Simulation from Natalia Artemieva, Russian Academy of Science, Moscow (2003) Tektite Formation: Moldavites:  Tektite Formation: Moldavites 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

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