IMPACT CRATER EJECTA
by: Charles O’Dale
When the crater formation process ends, the resulting circular structure and the surrounding area is covered by an ejecta blanket. The factors affecting the appearance of impact crater ejecta are the geology of the target surface and the size and velocity of the impactor. Another factor, the impact angle, will modify the pattern of the ejecta blanket. A study at the Ames Vertical Gun Ballistic Range confirmed this effect.
The studies found that the ejecta pattern remains more or less linear around the impact site until the impact angle is <45° (measured from horizontal). At shallower angles the crater becomes increasingly elongated in the direction of projectile travel, and the ejecta patterns undergo even more pronounced changes. When the impact angle is <15°, the ejecta pattern becomes elongated in the downrange direction and an exclusion zone, where no ejecta appears, develops in the uprange direction. Exotic ejecta patterns like this can be found on the Moon, as well as elsewhere in the solar system (Wood, 2003).
1 STOKES LAW: If the particles are falling in the viscous fluid by their own weight due to the Earth’s gravity, then a terminal velocity, also known as the settling velocity, is reached when this frictional force combined with the buoyant force exactly balance the gravitational force. The resulting settling velocity (or terminal velocity) is given by:
Vs = ( 2 (ρp – ρf ) / 9 η ) g R2 where:
- Vs is the particles’ settling velocity (m/s) (vertically downwards if ρp > ρf, upwards if ρp < ρf ),
- R is the radius of the spherical object (in metres),
- g is the Earth’s gravitational acceleration (m/s2),
- ρp is the mass density of the particles (kg/m3),
- ρf is the mass density of the fluid (kg/m3), and
- η is the fluid’s viscosity (in [kg m-1 s-1]).
There are three stages to the impact cratering process, contact & compression, excavation and modification. The excavation stage is further subdivided into two distinct processes:
- upward ejection of large near-surface fragments and smaller ejecta (ejecta curtain);
- subsurface flow of target material to form the transient crater.
autochthonous – (formed in place),;
parautochthonous – (moved but appear to be in place); and,
allochthonous (formed elsewhere and clearly moved to their current location). Allochthonous impactites can be further subdivided into those within and around the final crater (proximal) and those some distance from the final crater
(distal). The latter are always ejecta, including air – fall deposits. (Grieve / Therriault 2013)
On our planet, erosion will quickly remove this blanket and destroy any surviving meteorite fragments, with the result that crater ejecta remains in only the youngest and best-preserved impact structures. This ASTER Infra Red image documents the pattern of the ejecta blanket around the relatively young Barringer Crater. The pattern of the ejecta blanket, although it has been modified by 49K years of erosion, implies an impact from the south west. The majority of the ejecta blanket forms in the north east, downrange, direction.
Some of the Barringer Crater ejecta curtain can been seen here in visible light as illustrated in this image. I took this image of the crater from about 1000′ above the ground. The “lighter” coloured sand is the remnants of the ejecta curtain. The pattern of the ejecta implies that the direction of the impactor was from the upper left of this image. If I was flying here at the time of impact, 49 thousand years ago, I would not know what hit me!!
The Manicouagan impact may not have had a significant effect on life. No extinctions are recorded at the documented time of impact, 214 million years ago. The fireball generated by the impact probably expanded as far as the present location of New York City.
The impact also triggered powerful seismic events and ejected material out of the atmosphere. The ejected material was sent on a ballistic trajectory around the earth. Like the Chicxulub impact, the Manicouagan impact left behind a global geochemical signature in the rock record.
As a result of the Manicouagan impact, molten rock and dust from this bedrock left a thin layer of glass beads and shattered mineral grains in a rock deposit in the United Kingdom.
Dating of this UK deposit connected it to Manicouagan precluding most of the European impact sites nearby as the source of the deposit layer.
In an article published today (November 14, 2002) in the journal Science researchers led by Dr. Gordon Walkden of Aberdeen University have reported the discovery of a 214 million year old impact layer in the rocks of the west of England. The 2 cm thick layer consists of millimetre-sized green spherules that were formed as molten droplets of rock in the impact of a large asteroid or comet with the Earth. The droplets formed by condensation from gasses generated by vaporisation of rocks at enormous temperatures and were scattered over the entire Earth surface.Julian Parker, from Aberdeen, studied the spherule layer with Walkden and discovered quartz grains that had been deformed by intense pressures. “The orientation of the distorted planes through the grains showed they had been shocked,” said Walkden, “and proves the layer was formed as debris thrown out from a giant collision.” Dr. Simon Kelly, from the Open University, measured the age of the spherule layer using the decay of radioactive potassium that is found in all potassium-bearing minerals. The age of 214 million years is the same as the 100 km wide Manicouagan impact structure in Canada which is, therefore, the likely source of the impact layer. Kelly, however, suspects that a number of craters that have similar ages may have formed the same time as a string of impacts.
Walkden, Kelley, Parker, Thackrey (2006), THE ANATOMY OF A NEW IMPACT DEPOSIT: THE LATE TRIASSIC SPHERULE LAYER, SW. ENGLAND European Space Agency: [SELECTED QUOTES]The documented late Triassic spherule layer of SW England deposit (illustrated here) contains an abundance of spherules, common shocked quartz and a suite of accessory minerals believed to have been derived direct from the impact site. These include garnets, ilmenites, zircons and biotites. Garnets and ilmenites are highly fractured, and biotites show prominent kink bands indicative of shock.
The time of the deposit was determined by noting the decay of radioactive potassium in the spherule layer and dates at 214 ± 2.5Ma. This is comfortably within the date of the late Triassic Manicouagan impact event.
Single grain K-Ar ages for the biotites are in the range 700-1100Ma which are superficially similar to the Grenville rocks around the Manicouagan impact site. Given the geometry of the contemporaneous Triassic continental assembly, the contact point of the impactor was only 20 crater diameters from the site of deposition of this ejecta layer. (Thackrey et al 2006) [SELECTED QUOTES]
A distal impact deposit has recently been reported from a location near Bristol, England. The 0-15 cm thick, discontinuous layer is of Late Triassic age and occurs in a sequence of red calcareous mudstones deposited unconformably on Carboniferous limestone, in a semi-arid, continental environment. It consists of closely packed, millimetre sized green spherules, composed of a green clay surrounding a calcitic or hollow core. They have been inferred to be diagenetically altered type I spherules, formed from quenched impact-melt droplets, deposited aerially from an expanding impact ejecta curtain. The stratum, with a reported 39Ar – 40Ar age of approximately 214 Ma, is coincident with the age of two known impact craters from the Late Triassic, Manicouagan in Canada and Rochechouart in France. The focus of the present study is to determine if a meteoritic component and projectile type can be identified from this impactoclastic air-fall bed and adjacent strata, using a combination of geochemical analyses – chromium isotope systematics, siderophile and platinum group elemental ratios. In particular, we have developed a new method for the isolation and concentration of chromium, following sample dissolution. Chromium isotope ratios are determined using a multiple collector, inductively coupled plasma, mass spectrometer (MC-ICP-MS) for which the conditions have been established to obtain high precision ratio measurements. ( AMOR et al 2005).
Abstract–The 1.85 Ga Sudbury impact structure is one of the largest impact structures on Earth. Igneous bodies—the so-called “Basal Onaping Intrusion”—occur at the contact between the Sudbury Igneous Complex (SIC) and the overlying Onaping Formation and occupy ~50% of this contact zone. The Basal Onaping Intrusion is presently considered part of the Onaping Formation, which is a complex series of breccias. Here, we present petrological and geochemical data from two drill cores and field data from the North Range of the Sudbury structure, which suggests that the Basal Onaping Intrusion is not part of the Onaping Formation. Our observations indicate that the Basal Onaping Intrusion crystallized from a melt and has a groundmass comprising a skeletal intergrowth of feldspar and quartz that points to simultaneous cooling of both components. Increasing grain size and decreasing amounts of clasts with increasing depth are general features of roof rocks of coherent impact melt rocks at other impact structures and the Basal Onaping Intrusion. Planar deformation features within quartz clasts of the Basal Onaping Intrusion are indicators for shock metamorphism and, together with the melt matrix, point to the Basal Onaping Intrusion as being an impact melt rock, by definition. Importantly, the contact between Granophyre of the SIC and Basal Onaping Intrusion is transitional and we suggest that the Basal Onaping Intrusion is what remains of the roof rocks of the SIC and, thus, is a unit of the SIC and not the Onaping Formation. (Anders et al 2015)
Discovery of distal ejecta from the 1850 Ma Sudbury impact eventWilliam D. Addison, Gregory R. Brumpton, Daniela A. Vallini, Neal J. McNaughton, Don W. Davis, Stephen A. Kissin, Philip W. Fralick, Anne L. Hammond GEOLOGY, March 2005
ABSTRACT A 25–70-cm-thick, laterally correlative layer near the contact between the Paleoproterozoic sedimentary Gunflint Iron Formation and overlying Rove Formation and between the Biwabik Iron Formation and overlying Virginia Formation, western Lake Superior region, contains shocked quartz and feldspar grains found within accretionary lapilli, accreted grain clusters, and spherule masses, demonstrating that the layer contains hypervelocity impact ejecta. Zircon geochronologic data from tuffaceous horizons bracketing the layer reveal that it formed between ca. 1878 Ma and 1836 Ma. The Sudbury impact event, which occurred 650–875 km to the east at 1850 ± 1 Ma, is therefore the likely ejecta source, making these the oldest ejecta linked to a specific impact. Shock features, particularly planar deformation features, are remarkably well preserved in localized zones within the ejecta, whereas in other zones, mineral replacement, primarily carbonate, has significantly altered or destroyed ejecta features.
AMOR, Kenneth, HESSELBO, Stephen P., and PORCELLI (2005), GEOCHEMICAL ANALYSIS OF A LATE TRIASSIC DISTAL IMPACT EJECTA LAYER FROM SW ENGLAND. Donald Department of Earth Sciences, University of Oxford.
Denise ANDERS, Gordon R. OSINSKI, Richard A. F. GRIEVE, and Derek T. M. BRILLINGER (2015); The Basal Onaping Intrusion in the North Range: Roof rocks of the Sudbury Igneous Complex. Meteoritics & Planetary Science 50, Nr 9, 1577–1594 (2015)
Richard A. F. Grieve and Ann M. Therriault. (2013); Impactites: their characteristics and spatial distribution. Earth Sciences Sector, Natural Resources Canada
Kofman R.S., Herd C.D.K., Froese D.G., (2010); The Results of the Investigation of the Whitecourt Crater (Alberta, Canada), GeoCanada 2010 – Working with the Earth
Thackrey, Walkden, Kelley, Parker, (2006); THE ANATOMY OF A NEW IMPACT DEPOSIT: THE LATE TRIASSIC SPHERULE LAYER, SW. ENGLAND European Space Agency:
Wood, C.A. 2003. The Modern Moon, Sky Publishing Corp.