When craters are formed by impact events,
they disappear relatively quickly with the rework- ing of the Earth’s crust—buried beneath sedi- ments, pulled under at subduction zones, folded into mountain ranges or eroded. Eroded, buried or otherwise modified impact craters are called impact structures. An asteroid can hit the Earth with a velocity
between 10 and 70 km/s [up to 250,000 km/h, or 160,000 mi/h], releasing large amounts of kinetic energy.3
Crater formation is a rapid process that
can be divided into three stages: contact and compression, excavation, and postimpact crater modification. Structural alterations and phase
changes in the target rocks occur during the first stage, and the morphology of the crater is deter- mined in the later stages. Upon contact, the projectile pushes target
material out of its path, compressing and acceler- ating it. The target resists penetration and decel- erates the projectile. Following contact, the projectile stops almost immediately, traveling a distance approximating one or two times its diameter into the target rock. The kinetic energy is converted into heat and shock waves that pen- etrate both the impacting body and the target rock. A large asteroid can produce shock pres- sures in excess of 100 GPa [14.5 million psi] and
temperatures greater than 3,000°C [5,400°F] in the impact volume, causing fracturing, shock metamorphism, mineral phase changes, melting and vaporization.4
compression stage depends on the projectile’s size, composition and velocity; for all but the largest impacts, this stage lasts only up to a few seconds. During the excavation stage, which lasts a few
seconds to minutes, a hemispherical shock wave propagates into the target, generating high pres- sure in the material. The expanding shock wave causes irreversible changes in the shocked volume. After the shock wave passes, the high pressure is released by a rarefaction, or pressure- release wave. This pressure wave leads to cre- ation of a mass flow that opens the crater. The material excavated by the impact is ejected from the crater and can be thrown great distances. The larger the impactor, the more likely it will be completely vaporized and ejected as well. Solid, liquid and vaporized debris are thrust back up along paths that form a cone-shaped “ejecta cur- tain.” Ejected material follows ballistic trajecto- ries upward and back down to Earth. Some debris may travel beyond the atmosphere and then reenter it thousands of kilometers away. Ejecta size ranges from vapor and dust to
giant blocks. Molten rock and vaporized matter that condenses during flight can form round, sand-sized particles called spherules. Such spherules, often glassy, can land thousands of kilometers from the impact site and form distinc- tive layers in the sedimentary record.5
In the Glassy impact-melt inclusions
Chicxulub, Mexico, impact of 65 million years ago, ejecta distribution reached global propor- tions, with some spherules traveling as far as New Zealand.6
Spherule layers often outlast the
crater itself and may be the only evidence of an ancient impact, as is the case for some 3.5 billion- year-old impact deposits in South America and Australia. The thickness of the spherule layer typically decreases with distance from the impact site. Nearer the crater and inside it, ejected mate-
> Rocks formed by impact. Suevite (top) is a glass-bearing impact breccia. This whole-rock specimen from the Bosumtwi impact structure, in Ghana, shows a variety of irregular rock clasts plus frothy glass inclusions in a fine-grained, clastic matrix. A close-up of a drill core (bottom) from the suevite outside the northern rim of the Bosumtwi crater also shows inclusions of glass. The wide dimension of the upper sample is 25 cm [9.8 in.], and the core diameter in the lower figure is 5 cm [2 in.]. (Photographs courtesy of Christian Koeberl.)
rial is deposited as coarsely sorted breccias— mixtures of angular fragments of target rock and basement rock in a fine-grained matrix of pulverized material. Clastic breccias that contain mixed rock types, including shock- metamorphosed fragments, impact melt or glass, are called suevites (left).7
In impact-melt brec-
cias, the matrix cementing the fragments is crystallized from molten rock. Rocks affected by impact are known collectively as impactites.
Duration of the contact and
16
Oilfield Review
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64