IMPACT CRATER

What is an impact crater?

• can be applied to any depression, natural or manmade, resulting

from the high velocity impact of a projectile with a larger body. In

most common usage, the term is used for the approximately

circular depression in the surface of a planet, moon or other solid

body in the Solar System, formed by the hyper-velocity impact of a

smaller body with the surface.

• and they range from small, simple, bowl-shaped depressions to large, complex, multi-ringed impact basins. Meteor

Crater is perhaps the best-known example of a small impact crater on the Earth.

HISTORY

Daniel Moreau Barringer (1860 – 1929)

• a geologist best known as the first person to prove the existence of a meteorite crater on the Earth, the Meteor Crater in Arizona.

The site was formerly known as the Canyon Diablo Crater, and the meteorite that created the crater is officially called

the Canyon Diablo Meteorite, the name that is on all officially labeled fragments of the meteorite. Scientists refer to the

crater as Barringer Crater

• He studied the impact dynamics of Barringer Meteor Crater,

located near Winslow, Arizona. To understand the dynamics,

Shoemaker inspected craters that remained after underground

atomic bomb tests at the Nevada Test Site at Yucca Flats. He found

a ring of ejected material that included shocked quartz (coesite

), a form of quartz that has a microscopically unique structure

caused by intense pressure.

Eugene Merle Shoemaker

Crater formation involves high velocity collisions between solid

objects, typically much greater than the velocity of sound in those objects. Such hyper-velocity impacts produce physical effects such as melting and vaporization.

Impacts at these high speeds produce shock waves in solid materials, and both impactor and the material impacted are rapidly compressed to high density. Following initial compression, the high-density, over-compressed region rapidly depressurizes, exploding violently, to set in train the sequence of events that produces the impact crater

Three distinct stages of processes of crater formation:

initial contact and compression

Excavation

modification and collapse

impactor first touches the target surface

(This contact accelerates the target and decelerates the impactor)

Contact and compression

both the impactor and the target close to the impact site are

irreversibly damaged

and it continues moving away from the impact behind the

decaying shock wave.

Contact, compression, decompression, and the passage of the shock wave all occur within a few

tenths of a second for a large impact. The subsequent excavation of the crater occurs more

slowly, and during this stage the flow of material is largely sub-sonic. During excavation, the crater grows as the accelerated target material moves

away from the impact point. The target's motion is initially downwards and outwards, but it becomes

outwards and upwards. The flow initially produces an approximately hemispherical cavity. The cavity

continues to grow, eventually producing a paraboloid (bowl-shaped) crater in which the centre has been pushed down, a significant volume of material has

been ejected, and a topographically elevated crater rim has been pushed up. When this cavity has

reached its maximum size, it is called the transient cavity.

Excavation

In most circumstances, the transient cavity is not stable: it collapses under gravity. In small craters, less than about 4 km diameter on Earth, there is

some limited collapse of the crater rim coupled with debris sliding down the crater walls and drainage of impact melts into the deeper cavity. The resultant structure is called a simple crater, and it remains

bowl-shaped and superficially similar to the transient crater. In simple craters, the original

excavation cavity is overlain by a lens of collapse breccia, ejecta and melt rock, and a portion of the

central crater floor may sometimes be flat.

Modification and collapse

Identifying impact craters

A meteorite impact crater of relatively small diameter, characterized by a uniformly concave-upward shape and a maximum depth in the center, and lacking a central uplift.

Simple crater

• the collapse and modification of the transient cavity is much more extensive, and the resulting structure is called a complex crater. The collapse of the transient cavity is driven by gravity, and involves both the uplift of the central region and the inward collapse of the rim.

Complex crater

View from the north rim of Meteor Crater(Barringer crater,Arizona)

first-recognized terrestrial impact crater

Panoramic from the lower viewing deck

Closeup of old mine shaft at the bottom of the crater. Note astronaut cutout and flag attached to fence

(inset)

The Canyon Diablo meteorite comprises many fragments of the asteroid that impacted at Barringer Crater (Meteor

Crater), Arizona.

The Chesapeake Bay impact crater was formed by a bolide that impacted the eastern shore of North America about 35 million years ago, in the late Eocene epoch. It is one of the

best-preserved "wet-target" or marine impact craters, and the largest impact crater in the U.S. Continued slumping of

sediments over the rubble of the crater have helped shape Chesapeake Bay.

The collision between Earth and an asteroid a few kilometers in diameter may release as much energy

as several million nuclear weapons detonating simultaneously.

The Chicxulub crater (English is an ancient impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is located near the town of Chicxulub, after which the crater is named. The crater is more than 180 km (110 mi) in diameter,

making the feature one of the largest confirmed impact structures on Earth; the impacting bolide that formed the

crater was at least 10 km (6 mi) in diameter.

Chicxulub, Yucatan Peninsula, Mexico Half of the species on Earth became extinct

including the dinosaurs

Wolfe Creek, Australia 19°10' S, 127° 48' E; rim diameter: 0.875 kilometers (.544 miles);

age: 300,000 years Wolfe Creek is a relatively well-preserved crater that is partly buried under wind blown sand. The crater is situated in the flat desert plains

of north-central Australia. Its crater rim rises ~25 meters (82 feet) above the surrounding plains and the crater floor is ~50 meters (164 feet) below the rim. Oxidized remnants of iron meteoritic material as

well as some impact glass have been found a Wolf Creek. This photograph is a south-looking, oblique aerial view of the crater.

Gosses Bluff, Northern Territory, Australia

Some extraterrestrial impact craters

Mariner 10 photomosaic of Caloris Basin on Mercury,

colour view of the Hellas Planitia region on Mars created from images taken by the Viking orbiters.

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