Alamo bolide impact
Encyclopedia
Hypervelocity
The term hypervelocity usually refers to a very high velocity, approximately over 3,000 meters per second . In particular, it refers to velocities so high that the strength of materials upon impact is very small compared to inertial stresses. Thus, even metals behave like fluids under hypervelocity...
objects from space
Space
Space is the boundless, three-dimensional extent in which objects and events occur and have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of a boundless four-dimensional continuum...
slammed into shallow marine waters at a site that is now the Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
Guilmette Formation of the Worthington Mountains
Worthington Mountains
The Worthington Mountains is a mountain range in Lincoln County, Nevada....
and Schell Creek Range
Schell Creek Range
The Schell Creek Range is a linear mountain range in central White Pine County, in east-central Nevada. Its length is approximately in a north-south direction....
of southeastern Nevada
Nevada
Nevada is a state in the western, mountain west, and southwestern regions of the United States. With an area of and a population of about 2.7 million, it is the 7th-largest and 35th-most populous state. Over two-thirds of Nevada's people live in the Las Vegas metropolitan area, which contains its...
; the event is named for breccia
Breccia
Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments....
s of metamorphosed crushed rock deposits, found as far as the town of Alamo, Nevada
Alamo, Nevada
Alamo is an unincorporated community in Lincoln County, Nevada, United States, about north of Las Vegas. Its elevation is 3,449 feet . It is named after the Spanish word for "poplar" due to the presence of that type of tree. Founded in 1901, Alamo lies in the Pahranagat Valley. Its economy is...
(the "Alamo Breccia"). This catastrophic impact event
Impact event
An impact event is the collision of a large meteorite, asteroid, comet, or other celestial object with the Earth or another planet. Throughout recorded history, hundreds of minor impact events have been reported, with some occurrences causing deaths, injuries, property damage or other significant...
resulted in what is one of the best-exposed and has become the most accurately dated impact events; it occurred within the Frasnian
Frasnian
The Frasnian is one of two faunal stages in the Late Devonian epoch. It lasted from 385.3 ± 2.6 million years ago to 374.5 ± 2.6 million years ago. It was preceded by the Givetian stage and followed by the Famennian stage...
age of the Devonian at about 367 Ma, a moment in time that was about 3.5 Ma prior to the Frasnian/Famennian extinction
Late Devonian extinction
The Late Devonian extinction was one of five major extinction events in the history of the Earth's biota. A major extinction, the Kellwasser Event, occurred at the boundary that marks the beginning of the last phase of the Devonian period, the Famennian faunal stage, , about 374 million years ago...
events, which it is unlikely to have affected.
The actual impact site has not yet been significantly documented—even its precise diameter and its internal structural features are not yet clear enough to make speculations on the mass and trajectory genuinely useful. The "tectonic overprint", the subsequent geologic modifications since the event, have distorted the picture; the distribution of the breccia has been compressed and skewed by west-to-east thrust fault
Thrust fault
A thrust fault is a type of fault, or break in the Earth's crust across which there has been relative movement, in which rocks of lower stratigraphic position are pushed up and over higher strata. They are often recognized because they place older rocks above younger...
s across the area. Insufficient detail does not permit a precise reconstruction of the Devonian paleogeography, beyond the fact that it was a "wet-target" impact in a reef front where carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....
s were being built up in marine shallows.
The impact site contains megabreccia
Breccia
Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments....
of gigantic displaced blocks, together with several attributes familiar from other impact sites: shocked quartz
Shocked quartz
Shocked quartz is a form of quartz that has a microscopic structure that is different from normal quartz. Under intense pressure , the crystalline structure of quartz will be deformed along planes inside the crystal...
, elevated iridium
Iridium
Iridium is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second-densest element and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C...
levels, and spherical lapilli
Lapilli
Lapilli is a size classification term for tephra, which is material that falls out of the air during a volcanic eruption or during some meteorite impacts. Lapilli means "little stones" in Latin. They are in some senses similar to ooids or pisoids in calcareous sediments.By definition lapilli range...
. John E. Warme, one of the geologists who first recognized the geologic anomalies as the results of a bolide, estimates that the total volume of limestone reef deposits and bedrock that was smashed, deformed, partially melted or shifted during the Alamo event at 1,000 cubic kilometers. "Ensuing tsunami
Tsunami
A tsunami is a series of water waves caused by the displacement of a large volume of a body of water, typically an ocean or a large lake...
s rearranged much of the debris" he adds.
After initial resistance from the Geological Society of America
Geological Society of America
The Geological Society of America is a nonprofit organization dedicated to the advancement of the geosciences. The society was founded in New York in 1888 by Alexander Winchell, John J. Stevenson, Charles H. Hitchcock, John R. Proctor and Edward Orton and has been headquartered at 3300 Penrose...
, the first paper published concerning the Alamo Breccia was co-authored by John Warme, Brian Ackman, Yarmanto, and Alan Chamberlain in the 1993 Nevada Petroleum Society Field Conference Guidebook.