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The
Alvarez hypothesis claims that the
mass extinctionAn extinction event is a sharp decrease in the number of species in a relatively short period of time. Mass extinctions affect most major taxonomic groups present at the time — birds, mammals, reptiles, amphibians, fish, invertebrates and other simpler life forms...
of the
dinosaur{{Otheruses}}{{pp-semi-protected|small=yes}}{{Otheruses}}{{pp-semi-protected|small=yes}}{{Taxobox|name = Dinosaurs|fossil_range = {{Fossil range|230|65|earliest=230|latest=0|PS=
Descendant taxon Aves survives to present.}}|image = field_dinos_2.jpg...
s and many other living things was caused by the
impactAn impact event is the collision of a large meteorite, asteroid, comet, or other celestial object with the Earth or another planet. Impact events have been a plot and background element in science fiction since knowledge of real impacts became established in the scientific mainstream.-Sizes and...
of a large
asteroidthumb|260px|right|[[253 Mathilde]], a [[C-type asteroid]] measuring about across. Photograph taken in 1997 by the [[NEAR Shoemaker]] probe.Asteroids, sometimes called minor planets or planetoids, are small Solar System bodies in orbit around the Sun, especially in the inner Solar System; they are...
on the
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
sixty-five million years ago, called the
Cretaceous-Tertiary extinction eventThe Cretaceous–Tertiary extinction event, which occurred approximately , was a large-scale mass extinction of animal and plant species in a geologically short period of time. Widely known as the K–T extinction event, it is associated with a geological signature known as the K–T boundary, usually a...
. Evidence indicates that the asteroid fell in the
Yucatán PeninsulaThe Yucatán Peninsula, in southeastern Mexico, separates the Caribbean Sea from the Gulf of Mexico, with the northern coastline on the Yucatán Channel...
,
MexicoThe United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...
. The hypothesis is named after the father-and-son team of scientists Luis and
Walter AlvarezWalter Alvarez is a professor in the Earth and Planetary Science department at the University of California, Berkeley. He is most widely known for the theory that dinosaurs were killed by an asteroid impact, developed in collaboration with his father, Nobel Prize winning physicist Luis...
, who first suggested it in 1980.
In 1980, a team of researchers led by Nobel prize-winning physicist Luis Alvarez, his son geologist
Walter AlvarezWalter Alvarez is a professor in the Earth and Planetary Science department at the University of California, Berkeley. He is most widely known for the theory that dinosaurs were killed by an asteroid impact, developed in collaboration with his father, Nobel Prize winning physicist Luis...
and chemists Frank Asaro and Helen Michels discovered that sedimentary layers found all over the world at the Cretaceous–Tertiary boundary contain a
concentrationIn chemistry, concentration is the measure of how much of a given substance there is mixed with another substance. This can apply to any sort of chemical mixture, but most frequently the concept is limited to homogeneous solutions, where it refers to the amount of solute in the solvent.To...
of
iridiumIridium 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...
hundreds of times greater than normal. Iridium is extremely rare in the Earth's crust because it is very dense, and therefore most of it sank into the Earth's core while the earth was still molten. The Alvarez team suggested that an asteroid struck the earth at the time of the
K–T boundaryThe K–T boundary is a geological signature, usually a thin band, dated to Ma . K is the traditional abbreviation for the Cretaceous period, and T is the abbreviation for the Tertiary period...
. There were other earlier speculations on the possibility of an impact event, but no evidence had been uncovered at that time.
The evidence for the Alvarez impact hypothesis is supported by
chondriticChondrites are stony meteorites that have not been modified due to melting or differentiation of the parent body. They formed when various types of dust and small grains that were present in the early solar system accreted to form primitive asteroids...
meteoriteA meteorite is a natural object originating in outer space that survives an impact with the Earth's surface. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...
s and
asteroidthumb|260px|right|[[253 Mathilde]], a [[C-type asteroid]] measuring about across. Photograph taken in 1997 by the [[NEAR Shoemaker]] probe.Asteroids, sometimes called minor planets or planetoids, are small Solar System bodies in orbit around the Sun, especially in the inner Solar System; they are...
s which contain a much higher iridium concentration than the Earth's crust. The
isotopicIsotopes are different types of atoms of the same chemical element, each having a different number of neutrons. Correspondingly, isotopes differ in mass number but not in atomic number. The difference in the number of nucleons comes from a difference how many neutrons are in the atomic nucleus...
ratio of iridium in asteroids is similar to that of the K–T boundary layer but significantly different from the ratio in the Earth's crust.
ChromiumChromium is a chemical element which has the symbol Cr and atomic number 24, first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odourless, tasteless, and malleable...
isotopic anomalies found in Cretaceous–Tertiary boundary sediments are similar to that of an asteroid or a comet composed of
carbonaceous chondriteCarbonaceous chondrites or C chondrites are a class of chondritic meteorites comprising at least 7 known groups and many ungrouped meteorites. They include some of the most primitive known meteorites...
s.
Shocked quartzShocked 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...
granules, glass spherules and
tektiteTektites are natural glass rocks up to a few centimeters in size, which most scientists argue were formed by the impact of large meteorites on Earth's surface. Tektites are typically black or olive-green, and their shape varies from rounded to irregular.Tektites are among the "driest" rocks, with...
s, indicative of an impact event, are common in the K–T boundary, especially in deposits from around the Caribbean. All of these constituents are embedded in a layer of clay, which the Alvarez team interpreted as the debris spread all over the world by the impact. The location of the impact was unknown when the Alvarez team developed their hypothesis, but later scientists discovered the
Chicxulub CraterThe Chicxulub crater 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 — as well as the rough translation of the Mayan name, "the tail of the devil." The crater is more than...
in the
Yucatán PeninsulaThe Yucatán Peninsula, in southeastern Mexico, separates the Caribbean Sea from the Gulf of Mexico, with the northern coastline on the Yucatán Channel...
, now considered the likely impact site.
Using estimates of the total amount of iridium in the K–T layer, and assuming that the asteroid contained the normal percentage of iridium found in chondrites, the Alvarez team went on to calculate the size of the asteroid. The answer was about 10 kilometers (6 mi) in diameter, about the size of Manhattan. Such a large impact would have had approximately the energy of 1 x 10
8 megatonsTNT equivalent is a method of quantifying the energy released in explosions. The ton of TNT is used as a unit of energy, approximately equivalent to the energy released in the detonation of this amount of TNT....
, i.e. about 2 million times as great as the most powerful thermonuclear bomb ever
testedTsar Bomba , literally "Tsar-bomb", is the nickname for the AN602 hydrogen bomb — the largest, most powerful nuclear weapon ever detonated, and currently the most powerful explosive ever detonated in human history.Developed by the Soviet Union, the bomb was originally designed to have a yield of...
.
The most easily observable consequence of such an impact would be a vast dust cloud which would block sunlight and prevent photosynthesis for a few years. This would account for the extinction of plants and
phytoplanktonPhytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτον , or "plant", and πλαγκτος , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
and of all organisms dependent on them (including predatory animals as well as herbivores). But small creatures whose food chains were based on
detritusDetritus is a biological term used to describe dead or waste organic material.Detritus may also refer to:* Detritus , a geological term used to describe the particles of rock produced by weathering...
would have a reasonable chance of survival. It is estimated that sulfuric acid aerosols were injected into the stratosphere, leading to a 10–20% reduction of solar transmission normal for that period. It would have taken at least ten years for those aerosols to dissipate.
Global firestorms may have resulted as incendiary fragments from the blast fell back to Earth. Analyses of
fluid inclusionsthumb|250px|Trapped in a time capsule the same size as the diameter of a human hair, the ore-forming liquid in this inclusion was so hot and contained so much dissolved solids that when it cooled, crystals of halite, sylvite, gypsum, and hematite formed. As the samples cooled, the fluid shrank more...
in ancient
amberAmber is fossilized tree resin , which has been appreciated for its color and natural beauty since neolithic times. Good quality amber is used for the manufacture of ornamental objects and jewelry...
suggest that the
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
content of the atmosphere was very high (30–35%) during the late Cretaceous. This high O
2 level would have supported intense combustion. The level of atmospheric O
2 plummeted in the early Tertiary Period. If widespread fires occurred, they would have increased the CO
2 content of the atmosphere and caused a temporary
greenhouse effectThe greenhouse effect is the heating of the surface of a planet or moon due to the presence of an atmosphere containing gases that absorb and emit infrared radiation. Thus, greenhouse gases trap heat within the surface-troposphere system...
once the dust cloud settled, and this would have exterminated the most vulnerable survivors of the "long winter".
The impact may also have produced
acid rainAcid rain is rain or any other form of precipitation that is unusually acidic, i.e. elevated levels of hydrogen ions . It has harmful effects on plants, aquatic animals, and infrastructure. Acid rain is mostly caused by emissions of compounds of sulfur, nitrogen, and carbon which react with the...
, depending on what type of rock the asteroid struck. However, recent research suggests this effect was relatively minor. Chemical buffers would have limited the changes, and the survival of animals vulnerable to acid rain effects (such as
frogFrogs are amphibians in the order Anura , formerly referred to as Salientia . Most frogs are characterized by long hind legs, a short body, webbed digits , protruding eyes and the absence of a tail...
s) indicate this was not a major contributor to extinction.
Impact hypotheses can only explain very rapid extinctions, since the dust clouds and possible sulphuric aerosols would wash out of the atmosphere in a fairly short time — possibly under ten years.
Although further studies of the K–T layer consistently show the excess of iridium, the idea that the dinosaurs were exterminated by an asteroid remained a matter of controversy among
geologistFor other uses, see Geologist .A geologist is a scientist who studies the solid and liquid matter that constitutes the Earth and terrestrial planets...
s and paleontologists for more than a decade.