IMPACT CRATER EJECTA
by: Charles O’Dale
3. Ejecta types
- strewen fields
- Darwin Glass
- Ivory Coast
- Libyan Desert Glass
- North American tektites
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).
AUTOCHTHONOUS – (formed in place);
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) Of rocks whose primary constituents have not been formed in situ (formed elsewhere and clearly moved to their current location).
PARAUTOCHTHONOUS – (moved but appear to be in place)
Ground which has been disturbed by impact, thrust or nappe displacement, but where the displacement is small enough that the rocks are still in contact with their source (moved but appear to be in place).
3. EJECTA TYPES
Solid, liquid and vaporized material ejected from an impact crater during its formation.
Impact ejecta found at distances greater than 5 crater radii from the rim of the source crater, as opposed to proximal ejecta, which are found closer than 5 crater radii from the crater rim, and which make up about 90% of all material thrown out of the crater during the impact event.
All ejecta that are found up to 5 crater radii from the rim of the impact crater; 90% of all ejecta are found within this region. Note that the limit of proximal ejecta scales with the crater size. Ejecta found at greater distances are called distal ejecta.
An ejecta blanket is a generally symmetrical apron of ejecta that surrounds an impact crater; it is layered thickly at the crater’s rim and thin to discontinuous at the blanket’s outer edge.
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.
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.